# The Checker Framework Manual: Custom pluggable types for Java ### https://checkerframework.org ### Version 4.1.0 (2026-05-01)

For the impatient: Section ‍1.3 describes how to install and use pluggable type-checkers.

This manual is also available in PDF.

# Contents

# Chapter ‍1 Introduction

The Checker Framework enhances Java’s type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs.

A “checker” is a compile-time tool that warns you about certain errors or gives you a guarantee that those errors do not occur. The Checker Framework comes with checkers for specific types of errors:

1. Nullness Checker for null pointer errors (see Chapter ‍3)
2. Initialization Checker to ensure all @NonNull fields are set in the constructor (see Chapter ‍3.8)
3. Map Key Checker to track which values are keys in a map (see Chapter ‍4)
4. Optional Checker for errors in using the Optional type (see Chapter ‍5)
5. Interning Checker for errors in equality testing and interning (see Chapter ‍6)
6. Called Methods Checker for the builder pattern (see Chapter ‍7)
7. Resource Leak Checker for ensuring that resources are disposed of properly (see Chapter ‍8)
8. Fake Enum Checker to allow type-safe fake enum patterns and type aliases or typedefs (see Chapter ‍9)
9. Tainting Checker for trust and security errors (see Chapter ‍12)
10. SQL Quotes Checker to mitigate SQL injection attacks (see Chapter ‍13)
11. Lock Checker for concurrency and lock errors (see Chapter ‍10)
12. Index Checker for array accesses (see Chapter ‍11)
13. Regex Checker to prevent use of syntactically invalid regular expressions (see Chapter ‍14)
14. Format String Checker to ensure that format strings have the right number and type of % directives (see Chapter ‍15)
15. Internationalization Format String Checker to ensure that i18n format strings have the right number and type of {} directives (see Chapter ‍16)
16. Property File Checker to ensure that valid keys are used for property files and resource bundles (see Chapter ‍17)
17. Internationalization Checker to ensure that code is properly internationalized (see Chapter ‍17.2)
18. Signature String Checker to ensure that the string representation of a type is properly used, for example in Class.forName (see Chapter ‍18)
19. GUI Effect Checker to ensure that non-GUI threads do not access the UI, which would crash the application (see Chapter ‍19)
20. Units Checker to ensure operations are performed on correct units of measurement (see Chapter ‍20)
21. Signedness Checker to ensure unsigned and signed values are not mixed (see Chapter ‍21)
22. Purity Checker to identify whether methods have side effects (see Chapter ‍22)
23. Constant Value Checker to determine whether an expression’s value can be known at compile time (see Chapter ‍23)
24. Reflection Checker to determine whether an expression’s value (of type Method or Class) can be known at compile time (see Chapter ‍25)
25. Initialized Fields Checker to ensure all fields are set in the constructor (see Chapter ‍3.8)
26. Aliasing Checker to identify whether expressions have aliases (see Chapter ‍27)
27. Must Call Checker to over-approximate the methods that should be called on an object before it is de-allocated (see Chapter ‍28)
28. Subtyping Checker for customized checking without writing any code (see Chapter ‍29)
29. Third-party checkers that are distributed separately from the Checker Framework (see Chapter ‍30)

These checkers are easy to use and are invoked as arguments to javac.

The Checker Framework also enables you to write new checkers of your own; see Chapters ‍29 and ‍36.

## 1.1 How to read this manual

If you wish to get started using some particular type system from the list above, then the most effective way to read this manual is:

- Read all of the introductory material (Chapters ‍1–2).
- Read just one of the descriptions of a particular type system and its checker (Chapters ‍3–30).
- Skim the advanced material that will enable you to make more effective use of a type system (Chapters ‍31–40), so that you will know what is available and can find it later. Skip Chapter ‍36 on creating a new checker.

## 1.2 How it works: Pluggable types

Java’s built-in type-checker finds and prevents many errors — but it doesn’t find and prevent enough errors. The Checker Framework lets you define new type systems and run them as a plug-in to the javac compiler. Your code stays completely backward-compatible: your code compiles with any Java compiler, it runs on any JVM, and your coworkers don’t have to use the enhanced type system if they don’t want to. You can check part of your program, or the whole thing. Type inference tools exist to help you annotate your code; see Section ‍34.

Most programmers will use type systems created by other people, such as those listed at the start of the introduction (Chapter ‍1). Some people, called “type system designers”, create new type systems (Chapter ‍36). The Checker Framework is useful both to programmers who wish to write error-free code, and to type system designers who wish to evaluate and deploy their type systems.

This document uses the terms “checker” and “type-checking compiler plugin” as synonyms.

## 1.3 Installation

This section describes how to install the Checker Framework.

- If you use a build system that automatically downloads dependencies, such as Gradle or Maven, no installation is necessary; just see Chapter ‍38.
- If you wish to try the Checker Framework without installing it, use the Checker Framework Live Demo webpage.
- This section describes how to install the Checker Framework from its distribution. The Checker Framework release contains everything that you need, both to run checkers and to write your own checkers.
- Alternately, you can build the latest development version from source (Section ‍40.3).

Requirement: You must have a JDK (version 17 or later) installed.

The installation process has two required steps and one optional step.

1. Download the Checker Framework distribution: https://checkerframework.org/checker-framework-4.1.0.zip

   For example, on Unix you can run: wget https://checkerframework.org/checker-framework-4.1.0.zip

2. Unzip it to create a checker-framework-4.1.0 directory.

   For example, on Unix you can run: unzip checker-framework-4.1.0.zip

3. Configure your IDE, build system, or command shell to include the Checker Framework on the classpath. Choose the appropriate section of Chapter ‍38.

Now you are ready to start using the checkers.

We recommend that you work through the Checker Framework tutorial, which demonstrates the Nullness, Regex, and Tainting Checkers.

Section ‍1.4 walks you through a simple example. More detailed instructions for using a checker appear in Chapter ‍2.

The Checker Framework is released on a monthly schedule. The minor version (the middle number in the version number) is incremented if there are any incompatibilities with the previous version, including in user-visible behavior or in methods that a checker implementation might call.

## 1.4 Example use: detecting a null pointer bug

This section gives a very simple example of running the Checker Framework. There is also a tutorial that you can work along with.

Let’s consider this very simple Java class. The local variable ref’s type is annotated as @NonNull, indicating that ref must be a reference to a non-null object. Save the file as GetStarted.java.

    import org.checkerframework.checker.nullness.qual.*;

    public class GetStarted {
        void sample() {
            @NonNull Object ref = new Object();
        }
    }

If you run the Nullness Checker (Chapter ‍3), the compilation completes without any errors.

Now, introduce an error. Modify ref’s assignment to:

      @NonNull Object ref = null;

If you run the Nullness Checker again, it emits the following error:

    GetStarted.java:5: incompatible types.
    found   : @Nullable <nulltype>
    required: @NonNull Object
            @NonNull Object ref = null;
                                  ^
    1 error

This is a trivially simple example. Even an unsound bug-finding tool like SpotBugs or Error Prone could have detected this bug. The Checker Framework’s analysis is more powerful than those tools and detects more code defects than they do.

Type qualifiers such as @NonNull are permitted anywhere that you can write a type, including generics and casts; see Section ‍2.1. Here are some examples:

      @Interned String intern() { ... }             // return value
      int compareTo(@NonNull String other) { ... }  // parameter
      @NonNull List<@Interned String> messages;     // non-null list of interned Strings

# Chapter ‍2 Using a checker

A pluggable type-checker enables you to detect certain bugs in your code, or to prove that they are not present. The verification happens at compile time.

Finding bugs, or verifying their absence, with a checker is a two-step process, whose steps are described in Sections ‍2.1 and ‍2.2.

1. The programmer writes annotations, such as @NonNull and @Interned, that specify additional information about Java types. (Or, the programmer uses an inference tool to automatically infer annotations that are consistent with their code: see Section ‍34.) It is possible to annotate only part of your code: see Section ‍2.4.6.
2. The checker reports whether the program contains any erroneous code — that is, code that is inconsistent with the annotations.

This chapter is structured as follows:

- Section ‍2.1: How to write annotations
- Section ‍2.2: How to run a checker
- Section ‍2.3: What the checker guarantees
- Section ‍2.4: Tips about writing annotations

Additional topics that apply to all checkers are covered later in the manual:

- Chapter ‍32: Advanced type system features
- Chapter ‍33: Suppressing warnings
- Chapter ‍35: Annotating libraries
- Chapter ‍36: How to create a new checker
- Chapter ‍38: Integration with external tools

There is a tutorial that walks you through using the Checker Framework on the command line.

## 2.1 Where to write type annotations

You may write a type annotation immediately before any use of a type, including in generics and casts. Because array levels are types and receivers have types, you can also write type annotations on them. Here are a few examples of type annotations:

      @Interned String intern() { ... }               // return value
      int compareTo(@NonNull String other) { ... }    // parameter
      String toString(@Tainted MyClass this) { ... }  // receiver ("this" parameter)
      @NonNull List<@Interned String> messages;       // generics:  non-null list of interned Strings
      @Interned String @NonNull [] messages;          // arrays:  non-null array of interned Strings
      myDate = (@Initialized Date) beingConstructed;  // cast

You only need to write type annotations on method signatures, fields, and some type arguments. Most annotations within method bodies are inferred for you; for more details, see Section ‍32.7.

The Java Language Specification also defines declaration annotations, such as @Deprecated and @Override, which apply to a class, method, or field but do not apply to the method’s return type or the field’s type. They should be written on their own line in the source code, before the method’s signature.

## 2.2 Running a checker

To run a checker, run the compiler javac as usual, but either pass the -processor plugin_class command-line option, or use auto-discovery as described in Section ‍2.2.3. (If your project already uses auto-discovery for some annotation processor, such as AutoValue, then you should use auto-discovery.) Two concretes example of using -processor to run the Nullness Checker are

      javac -processor nullness MyFile.java
      javac -processor org.checkerframework.checker.nullness.NullnessChecker MyFile.java

where javac is as specified in Section ‍38.6.

You can also run a checker from within your favorite IDE or build system. See Chapter ‍38 for details about build tools such as Ant (Section ‍38.3), Buck (Section ‍38.5), Bazel (Section ‍38.4), Gradle (Section ‍38.9), Maven (Section ‍38.13), and sbt (Section ‍38.15); IDEs such as Eclipse (Section ‍38.8), IntelliJ IDEA (Section ‍38.10), NetBeans (Section ‍38.14), and tIDE (Section ‍38.16); and about customizing other IDEs and build tools.

The checker is run on only the Java files that javac compiles. This includes all Java files specified on the command line and those created by another annotation processor. It may also include other of your Java files, if they are more recent than the corresponding .class file. Even when the checker does not analyze a class (say, the class was already compiled, or source code is not available), it does check the uses of those classes in the source code being compiled. Type-checking works modularly and intraprocedurally: when verifying a method, it examines only the signature (including annotations) of other methods, not their implementations. When analyzing a variable use, it relies on the type of the variable, not any dataflow outside the current method that produced the value.

After you compile your code while running a checker, the resulting .class and .jar files can be used for pluggable type-checking of client code.

If you compile code without the -processor command-line option, no checking of the type annotations is performed. Furthermore, only explicitly-written annotations are written to the .class file; defaulted annotations are not, and this will interfere with type-checking of clients that use your code. Therefore, to create .class files that will be distributed or compiled against, you should run the type-checkers for all the annotations that you have written.

### 2.2.1 Using annotated libraries

When your code uses a library that is not currently being compiled, the Checker Framework looks up the library’s annotations in its class files or in a stub file.

Some projects are already distributed with type annotations by their maintainers, so you do not need to do anything special. An example is all the libraries in https://github.com/plume-lib/. Over time, this should become more common.

For some other libraries, the Checker Framework developers have provided an annotated version of the library, either as a stub file or as compiled class files. (If some library is not available in either of these forms, you can contribute by annotating it, which will help you and all other Checker Framework users; see Chapter ‍35.)

Some stub files are used automatically by a checker, without any action on your part. For others, you must pass the -Astubs=... command-line argument. As a special case, if an .astub file appears in checker/src/main/resources/, then pass the command-line option use -Astubs=checker.jar/stubfilename.astub. The “checker.jar” should be literal — don’t provide a path. This special syntax only works for “checker.jar”.

The annotated libraries that are provided as class files appear in the org.checkerframework.annotatedlib group in the Maven Central Repository. The annotated library has identical behavior to the upstream, unannotated version; the source code is identical other than added annotations. (Some of the annotated libraries are bcel, commons-csv, commons-io, guava, and java-getopt.)

To use an annotated library:

- If your project stores .jar files locally, then download the .jar file from the Maven Central Repository.
- If your project manages dependencies using a tool such as Gradle or Maven, then update your buildfile to use the org.checkerframework.annotatedlib group. For example, in build.gradle, change

        api("org.apache.bcel:bcel:6.3.1")
        api("commons-io:commons-io:2.8")

  to

        api("org.checkerframework.annotatedlib:bcel:6.3.1")
        api("org.checkerframework.annotatedlib:commons-io:2.8.0.1")

  Usually use the same version number. (Sometimes you will use a slightly larger number, if the Checker Framework developers have improved the type annotations since the last release by the upstream maintainers.) If a newer version of the upstream library is available but that version is not available in org.checkerframework.annotatedlib, then open an issue requesting that the org.checkerframework.annotatedlib version be updated.

There is one special case. If an .astub file is shipped with the Checker Framework in checker/src/main/resources/, then you can use -Astubs=checker.jar/stubfilename.astub. The “checker.jar” should be literal — don’t provide a path. (This special syntax only works for “checker.jar”.)

### 2.2.2 Summary of command-line options

You can pass command-line arguments to a checker via javac’s standard -A option (“A” stands for “annotation”). All of the distributed checkers support the following command-line options. Each checker may support additional command-line options; see the checker’s documentation.

To pass an option to only a particular checker, prefix the option with the canonical or simple name of a checker, followed by an underscore “_”. Such an option will apply only to a checker with that name or any subclass of that checker. For example, you can use

        -ANullnessChecker_lint=redundantNullComparison
        -Aorg.checkerframework.checker.guieffect.GuiEffectChecker_lint=debugSpew

to pass different lint options to the Nullness and GUI Effect Checkers. A downside is that, in this example, the Nullness Checker will issue a “The following options were not recognized by any processor” warning about the second option and the GUI Effect Checker will issue a “The following options were not recognized by any processor” warning about the first option.

Unsound checking: ignore some errors

- -AsuppressWarnings Suppress all errors and warnings matching the given key; see Section ‍33.3.
- -AskipUses, -AonlyUses Suppress all errors and warnings at all uses of a given class — or at all uses except those of a given class. See Section ‍33.4.
- -AskipDefs, -AonlyDefs Suppress all errors and warnings within the definition of given classes — or everywhere except within the definition of given classes. See Section ‍33.5.
- -AskipFiles, -AonlyFiles Suppress all errors and warnings within given files or directories/folders — or everywhere except within given files or directories/folders. See Section ‍33.6.
- -AassumeSideEffectFree, -AassumeDeterministic, -AassumePure, -AassumePureGetters Unsoundly assume that every method is side-effect-free, deterministic, or both; or that every getter method is pure. See Section ‍22.4 and Section ‍32.7.5.
- -AassumeAssertionsAreEnabled, -AassumeAssertionsAreDisabled Whether to assume that assertions (Java assert statements) are enabled or disabled; see Section ‍32.7.6.
- -AignoreRangeOverflow Ignore the possibility of overflow for range annotations such as @IntRange; see Section ‍23.4.
- -Awarns Treat checker errors as warnings. If you use this, you may wish to also supply -Xmaxwarns 10000, because by default javac prints at most 100 warnings. If you use this, don’t supply -Werror, which is a javac argument to halt compilation if a warning is issued.
- -AignoreInvalidAnnotationLocations Ignore annotations in bytecode that have invalid annotation locations.

More sound (strict) checking: enable errors that are disabled by default

- -AcheckPurityAnnotations Check the bodies of methods marked @SideEffectFree, @Deterministic, and @Pure to ensure the method satisfies the annotation. By default, the Checker Framework unsoundly trusts the method annotation. See Section ‍32.7.5.
- -AinvariantArrays Make array subtyping invariant; that is, two arrays are subtypes of one another only if they have exactly the same element type. By default, the Checker Framework unsoundly permits covariant array subtyping, just as Java does. See Section ‍32.1.
- -AcheckCastElementType In a cast, require that parameterized type arguments and array elements are the same. By default, the Checker Framework unsoundly permits them to differ, just as Java does. See Section ‍31.1.6 and Section ‍32.1.
- -AuseConservativeDefaultsForUncheckedCode Enables conservative defaults, and suppresses all type-checking warnings, in unchecked code. Takes arguments “source,bytecode”. “-source,-bytecode” is the (unsound) default setting.
  - “bytecode” specifies whether the checker should apply conservative defaults to bytecode (that is, to already-compiled libraries); see Section ‍32.5.6.
  - Outside the scope of any relevant @AnnotatedFor annotation, “source” specifies whether conservative default annotations are applied to source code and suppress all type-checking warnings; see Section ‍35.4.
- -AconcurrentSemantics Whether to assume concurrent semantics (field values may change at any time) or sequential semantics; see Section ‍39.4.6.
- -AignoreRawTypeArguments=false Do not ignore subtype tests for type arguments that were inferred for a raw type. See Section ‍31.1.1.
- -processor org.checkerframework.common.initializedfields.InitializedFieldsChecker,... Ensure that all fields are initialized by the constructor. See Chapter ‍26.

Type-checking modes: enable/disable functionality

- -Alint Enable or disable optional checks; see Section ‍33.7.
- -AwarnRedundantAnnotations Warn about redundant annotations. A warning is issued if an explicitly written annotation is the same as the default annotation for that location. This feature does not warn about all redundant annotations, only some.
- -AsuggestPureMethods Suggest methods that could be marked @SideEffectFree, @Deterministic, or @Pure; see Section ‍32.7.5.
- -AresolveReflection Determine the target of reflective calls, and perform more precise type-checking based on that information; see Chapter ‍25. -AresolveReflection=debug causes debugging information to be output.
- -Ainfer=outputformat Output suggested annotations for method signatures and fields. These annotations may reduce the number of type-checking errors on subsequent type-checking runs. This option is typically used by whole-program inference (WPI; see Section ‍34.2) rather than by programmers. Using -Ainfer=jaifs produces .jaif files. Using -Ainfer=stubs produces .astub files. Using -Ainfer=ajava produces .ajava files. You must also supply -Awarns, or the inference output may be incomplete.
- -AinferOutputDirectory The directory into which to write inference output (e.g., .ajava files). It defaults to build/whole-program-inference. When using a build system (e.g., Gradle), the default may be interpreted relative to a build system directory (e.g., $HOME/.gradle/workers) rather than the project root directory, so you may need to provide an absolute path.
- -AinferOutputOriginal When outputting .ajava files when running with -Ainfer=ajava, also output a copy of the original file with no inferred annotations, but with the formatting of a .ajava file, to permit use of diff to view the inferred annotations. Must be combined with -Ainfer=ajava.
- -AshowSuppressWarningsStrings With each warning, show all possible strings to suppress that warning.
- -AwarnUnneededSuppressions Issue a warning if a @SuppressWarnings did not suppress a warning issued by the checker. This only warns about @SuppressWarnings strings that contain a checker name (for syntax, Section ‍33.1.1). The -ArequirePrefixInWarningSuppressions command-line argument ensures that all @SuppressWarnings strings contain a checker name. An unneeded.suppression warning can be suppressed only by @SuppressWarnings("unneeded.suppression") or @SuppressWarnings("checkername:unneeded.suppression"), not by @SuppressWarnings("checkername").
- -AwarnUnneededSuppressionsExceptions=regex disables -AwarnUnneededSuppressions for @SuppressWarnings strings that contain a match for the regular expression. Most users don’t need this.
- -ArequirePrefixInWarningSuppressions Require that the string in a warning suppression annotation begin with a checker name. Otherwise, the suppress warning annotation does not suppress any warnings. For example, if this command-line option is supplied, then @SuppressWarnings("assignment") has no effect, but @SuppressWarnings("nullness:assignment") does.
- -AshowPrefixInWarningMessages When issuing an error or warning, prefix the warning suppression key by the checker name. For instance, output “error: [nullness:assignment] ...” instead of “error: [assignment]”. This makes it easy to tell, from the suppression key only, which checker issued the error or warning.

Partially-annotated libraries

- -Astubs List of stub files or directories; see Section ‍35.5.1.
- -AstubWarnIfNotFound, -AstubNoWarnIfNotFound, -AstubWarnIfNotFoundIgnoresClasses, -AstubWarnIfRedundantWithBytecode, -AstubWarnNote Warn about problems with stub files; see Section ‍35.5.7.
- -AmergeStubsWithSource If both a stub file and a source file for a class are available, trust both and use the greatest lower bound of their annotations. The default behavior (without this flag) is to ignore types from the stub file if source is available. See Section ‍35.5.2.
- -AuseConservativeDefaultsForUncheckedCode=source Outside the scope of any relevant @AnnotatedFor annotation, use conservative default annotations and suppress all type-checking warnings; see Section ‍35.4.

Debugging

- -AprintAllQualifiers, -AprintVerboseGenerics, -Anomsgtext, -Aonelinemsg, -AdumpOnErrors, -AexceptionLineSeparator Amount of detail in messages; see Section ‍36.12.1.
- -Adetailedmsgtext Format of diagnostic messages; see Section ‍36.12.2.
- -Aignorejdkastub, -ApermitMissingJdk, -AparseAllJdk, -AstubDebug Stub and JDK libraries; see Section ‍36.12.3.
- -Afilenames, -Ashowchecks, -AshowWpiFailedInferences Progress tracing; see Section ‍36.12.4.
- -AoutputArgsToFile Output the compiler command-line arguments to a file. Useful when the command line is generated and executed by a tool, such as a build system. This produces a standalone command line that can be executed independently of the tool that generated it (such as a build system). That command line makes it easier to reproduce, report, and debug issues. For example, the command line can be modified to enable attaching a debugger. See Section ‍36.12.5.
- -Aflowdotdir, -Averbosecfg, -Acfgviz Draw a visualization of the CFG (control flow graph); see Section ‍36.12.6.
- -AresourceStats, -AatfDoNotCache, -AatfCacheSize Miscellaneous debugging options; see Section ‍36.12.7.
- -AslowTypecheckingSeconds=N Print a warning for any program construct, such as a method or class, whose type-checking takes more than N seconds (default 45). Often, generic type inference is the slowest part of type-checking, and you can significantly speed up type-checking by explicitly writing a few generic type arguments.
- -Aversion Print the Checker Framework version.
- -AprintGitProperties Print information about the git repository from which the Checker Framework was compiled.

Some checkers support additional options, which are described in that checker’s manual section. For example, -Aquals tells the Subtyping Checker (see Chapter ‍29) and the Fenum Checker (see Chapter ‍9) which annotations to check.

Here are some standard javac command-line options that you may find useful. Many of them contain “processor” or “proc”, because in javac jargon, a checker is an “annotation processor”.

- -processor Names the checker to be run; see Sections ‍2.2 and ‍2.2.4. May be a comma-separated list of multiple checkers. Note that javac stops processing an indeterminate time after detecting an error. When providing multiple checkers, if one checker detects any error, subsequent checkers may not run.
- -processorpath Indicates where to search for the checker. This should also contain any classes used by type-checkers, such as qualifiers used by the Subtyping Checker (see Section ‍29.2) and classes that define statically-executable methods used by the Constant Value Checker (see Section ‍23.2.2).
- -proc:{none,only} Controls whether checking happens; -proc:none means to skip checking; -proc:only means to do only checking, without any subsequent compilation; see Section ‍2.2.3.
- -implicit:class Suppresses warnings about implicitly compiled files (not named on the command line); see Section ‍38.3
- -J Supply an argument to the JVM that is running javac; for example, -J-Xmx4g to increase its maximum heap size.
- -doe To “dump on error”, that is, output a stack trace whenever a compiler warning/error is produced. Useful when debugging the compiler or a checker.

### 2.2.3 Checker auto-discovery

“Auto-discovery” makes the javac compiler always run an annotation processor, such as a checker plugin, without explicitly passing the -processor command-line option. This can make your command line shorter, and it ensures that your code is checked even if you forget the command-line option.

If the javac command line specifies any -processor command-line option, then auto-discovery is disabled. This means that if your project currently uses auto-discovery, you should use auto-discovery for the Checker Framework too. (Alternately, if you prefer to use a -processor command-line argument, you will need to specify all annotation processors, including ones that used to be auto-discovered.)

To enable auto-discovery, place a configuration file named META-INF/services/javax.annotation.processing.Processor in your classpath. The file contains the names of the checkers to be used, listed one per line. For instance, to run the Nullness Checker and the Interning Checker automatically, the configuration file should contain:

      org.checkerframework.checker.nullness.NullnessChecker
      org.checkerframework.checker.interning.InterningChecker

You can disable this auto-discovery mechanism by passing the -proc:none command-line option to javac, which disables all annotation processing including all pluggable type-checking.

### 2.2.4 Shorthand for built-in checkers

Ordinarily, javac’s -processor flag requires fully-qualified class names. When using the Checker Framework javac wrapper (Section ‍38.6), you may omit the package name and the Checker suffix. The following three commands are equivalent:

      javac -processor org.checkerframework.checker.nullness.NullnessChecker MyFile.java
      javac -processor NullnessChecker MyFile.java
      javac -processor nullness MyFile.java

This feature also works when multiple checkers are specified. Their names are separated by commas, with no surrounding space. For example:

      javac -processor NullnessChecker,RegexChecker MyFile.java
      javac -processor nullness,regex MyFile.java

This feature does not apply to javac @argfiles.

## 2.3 What the checker guarantees

A checker guarantees two things: type annotations reflect facts about run-time values, and illegal operations are not performed.

For example, the Nullness Checker (Chapter ‍3) guarantees lack of null pointer exceptions (Java NullPointerException). More precisely, it guarantees that expressions whose type is annotated with @NonNull never evaluate to null, and it forbids other expressions from being dereferenced.

As another example, the Interning Checker (Chapter ‍6) guarantees that correct equality tests are performed. More precisely, it guarantees that every expression whose type is an @Interned type evaluates to an interned value, and it forbids == on other expressions.

The guarantee holds only if you run the checker on every part of your program and the checker issues no warnings anywhere in the code. You can also verify just part of your program.

There are some limitations to the guarantee.

- A compiler plugin can check only those parts of your program that you run it on. If you compile some parts of your program without running the checker, then there is no guarantee that the entire program satisfies the property being checked. Some examples of un-checked code are:
  - Code compiled without the -processor switch. This includes external libraries supplied as a .class file and native methods (because the implementation is not Java code, it cannot be checked).
  - Code compiled with the -AskipUses, -AonlyUses, -AskipDefs, -AonlyDefs, -AskipFiles, or -AonlyFiles command-line arguments (see Chapter ‍33).
  - Dynamically generated code, such as generated by Spring or MyBatis. Its bytecode is directly generated and run, not compiled by javac and not visible to the Checker Framework.

  In each of these cases, any use of the code is checked — for example, a call to a native method must be compatible with any annotations on the native method’s signature. However, the annotations on the un-checked code are trusted; there is no verification that the implementation of the native method satisfies the annotations.

- You can suppress warnings, such as via the @SuppressWarnings annotation (Chapter ‍33). If you do so incorrectly, the checker’s guarantee no longer holds.
- The Checker Framework is, by default, unsound in a few places where a conservative analysis would issue too many false positive warnings. These are listed in Section ‍2.2.2. You can supply a command-line argument to make the Checker Framework sound for each of these cases.
- Specific checkers may have other limitations; see their documentation for details.

In order to avoid a flood of unhelpful warnings, many of the checkers avoid issuing the same warning multiple times. For example, consider this code:

      @Nullable Object x = ...;
      x.toString();                 // warning
      x.toString();                 // no warning

The second call to toString cannot possibly throw a null pointer warning — x is non-null if control flows to the second statement. In other cases, a checker avoids issuing later warnings with the same cause even when later code in a method might also fail. This does not affect the soundness guarantee, but a user may need to examine more warnings after fixing the first ones identified. (Often, a single fix corrects all the warnings.)

If you find that a checker fails to issue a warning that it should, then please report a bug (see Section ‍40.2).

## 2.4 Tips about writing annotations

Section ‍35.1 gives additional tips that are specific to annotating a third-party library.

### 2.4.1 Write annotations before you run a checker

Before you run a checker, annotate the code, based on its documentation. Then, run the checker to uncover bugs in the code or the documentation.

Don’t do the opposite, which is to run the checker and then add annotations according to the warnings issued. This approach is less systematic, so you may overlook some annotations. It often leads to confusion and poor results. It leads users to make changes not for any principled reason, but to “make the type-checker happy”, even when the changes are in conflict with the documentation or the code. Also see “Annotations are a specification”, below.

### 2.4.2 How to get started annotating legacy code

Annotating an entire existing program may seem like a daunting task. But, if you approach it systematically and do a little bit at a time, you will find that it is manageable.

#### Start small

Start small. Focus on one specific property that matters to you; in other words, run just one checker rather than multiple ones. You may choose a different checker for different programs. Focus on the most mission-critical or error-prone part of your code; don’t try to annotate your whole program at first.

It is easiest to add annotations if you know the code or the code contains documentation. While adding annotations, you will spend most of your time understanding the code, and less time actually writing annotations or running the checker.

Don’t annotate the whole program, but work module by module. Start annotating classes at the leaves of the call tree — that is, start with classes/packages that have few dependencies on other code. Annotate supertypes before you annotate classes that extend or implement them. The reason for this rule is that it is easiest to annotate a class if the code it depends on has already been annotated. Sections ‍33.4 and ‍33.5 give ways to skip checking of some files, directories, or packages. Section ‍2.4.6 gives advice about handling calls from annotated code into unannotated code.

When annotating, be systematic; we recommend annotating an entire class or module at a time (not just some of the methods) so that you don’t lose track of your work or redo work. For example, working class-by-class avoids confusion about whether an unannotated type use means you determined that the default is desirable, or it means you didn’t yet examine that type use.

Don’t overuse pluggable type-checking. If the regular Java type system can verify a property using Java subclasses, then that is a better choice than pluggable type-checking (see Section ‍39.1.2).

#### Annotations are a specification

When you write annotations, you are writing a specification, and you should think about them that way. Start out by understanding the program so that you can write an accurate specification. Sections ‍2.4.3 and ‍2.4.4 give more tips about writing specifications.

For each class, read its Javadoc. For instance, if you are adding annotations for the Nullness Checker (Section ‍3), then you can search the documentation for “null” and then add @Nullable anywhere appropriate. Start by annotating signatures and fields, but not method bodies. The only reason to even read the method bodies yet is to determine signature annotations for undocumented methods — for example, if the method returns null, you know its return type should be annotated @Nullable, and a parameter that is compared against null may need to be annotated @Nullable.

The specification should state all facts that are relevant to callees. When checking a method, the checker uses only the specification, not the implementation, of other methods. (Equivalently, type-checking is “modular” or “intraprocedural”.) When analyzing a variable use, the checker relies on the type of the variable, not any dataflow outside the current method that produced the value.

After you have annotated all the signatures, run the checker. Then, fix bugs in code and add/modify annotations as necessary. Don’t get discouraged if you see many type-checker warnings at first. Often, adding just a few missing annotations will eliminate many warnings, and you’ll be surprised how fast the process goes overall (assuming that you understand the code, of course).

It is usually not a good idea to experiment with adding and removing annotations in order to understand their effect. It is better to reason about the desired design. However, to avoid having to manually examine all callees, a more automated approach is to save the checker output before changing an annotation, then compare it to the checker output after changing the annotation.

Chapter ‍35 tells you how to annotate libraries that your code uses. Section ‍2.4.5 and Chapter ‍33 tell you what to do when you are unable to eliminate checker warnings by adding annotations.

#### Write good code

Avoid complex code, which is more error-prone. If you write your code to be simple and clear enough for the type-checker to verify, then it will also be easier for programmers to understand. When you verify your code, a side benefit is improving your code’s structure.

Your code should compile cleanly under the regular Java compiler. As a specific example, your code should not use raw types like List; use parameterized types like List<String> instead (Section ‍31.1.1). If you suppress Java compiler warnings, then the Checker Framework will issue more warnings, and its messages will be more confusing. (Also, if you are not willing to write code that type-checks in Java, then you might not be willing to use an even more powerful type system.)

Do not write unnecessary annotations.

- Do not annotate local variables unless necessary. The checker infers annotations for local variables (see Section ‍32.7). Usually, you only need to annotate fields and method signatures. You should add annotations inside method bodies only if the checker is unable to infer the correct annotation (usually on type arguments or array element types, rather than on top-level types).
- Do not write annotations that are redundant with defaults. For example, when checking nullness (Chapter ‍3), the default annotation is @NonNull, in most locations other than some type bounds (Section ‍32.5.3). When you are starting out, it might seem helpful to write redundant annotations as a reminder, but that’s like when beginning programmers write a comment about every simple piece of code:

      // The below code increments variable i by adding 1 to it.
      i++;

  As you become comfortable with pluggable type-checking, you will find redundant annotations to be distracting clutter, so avoid putting them in your code in the first place.

- Avoid writing @SuppressWarnings annotations unless there is no alternative. It is tempting to think that your code is right and the checker’s warnings are false positives. Sometimes they are, but slow down and convince yourself of that before you dismiss them. Section ‍2.4.5 discusses what to do when a checker issues a warning about your code.

### 2.4.3 Annotations indicate non-exceptional behavior

You should use annotations to specify normal behavior. The annotations indicate all the values that you want to flow to a reference — not every value that might possibly flow there if your program has a bug.

#### Methods that crash when passed certain values

##### Nullness example

As an example, consider the Nullness Checker. Its goal is to guarantee that your program does not crash due to a null value.

This method crashes if null is passed to it:

      /** @throws NullPointerException if arg is null */
      void m1(Object arg) {
        arg.toString();
        ...
      }

Therefore, the type of arg should be @NonNull Object — you can write this as just Object, because @NonNull is the default. The Nullness Checker (Chapter ‍3) prevents null pointer exceptions by warning you whenever a client passes a value that might cause m1 to crash.

Here is another method:

      /** @throws NullPointerException if arg is null */
      void m2(Object arg) {
        Objects.requireNonNull(arg);
        ...
      }

Method m2 behaves just like m1 in that it throws NullPointerException if a client passes null. Therefore, their specifications should be identical (the formal parameter type is annotated with @NonNull), so the checker will issue the same warning if a client might pass null.

The same argument applies to any method that is guaranteed to throw an exception if it receives null as an argument. Examples include:

      com.google.common.base.Preconditions.checkNotNull(Object)
      java.lang.Double.valueOf(String)
      java.lang.Objects.requireNonNull(Object)
      java.lang.String.contains(CharSequence)
      org.junit.Assert.assertNotNull(Object)
      org.junit.jupiter.api.Assert.assertNotNull(Object)

Their formal parameter type is annotated as @NonNull, because otherwise the program might crash. Adding a call to a method like requireNonNull never prevents a crash: your code still crashes, but with a slightly different stack trace. In order to prevent all exceptions in your program caused by null pointers, you need to prevent those thrown by methods including requireNonNull.

(One might argue that the formal parameter should be annotated as @Nullable because passing null has a well-defined semantics (throw an exception) and such an execution may be possible if your program has a bug. However, it is never the programmer’s intent for null to flow there. Preventing such bugs is the purpose of the Nullness Checker.)

A method like requireNonNull is useless for making your code correct, but it does have a benefit: its stack trace may help developers to track down the bug. (For users, the stack trace is scary, confusing, and usually non-actionable.) But if you are using the Checker Framework, you can prevent errors rather than needing extra help in debugging the ones that occur at run time.

##### Optional example

Another example is the Optional Checker (Chapter ‍5) and the orElseThrow() method. The goal of the Optional Checker is to ensure that the program does not crash due to use of a non-present Optional value. Therefore, the receiver of orElseThrow() is annotated as @Present, and the Optional Checker issues an error if the client calls orElseThrow() on a @MaybePresent value. (For details, see Section ‍5.3.)

##### Permitting crashes in some called methods

You can make a checker ignore crashes in library code, such as assertNotNull(), that occur as a result of misuse by your code. This invalidates the checker’s guarantee that your program will not crash. (Programmers and users typically care about all crashes, no matter which method is at the top of the call stack when the exception is thrown.) The checker will still warn you about crashes in your own code.

- The -AskipUses command-line argument (Section ‍33.4) skips checking all method calls to one or more classes.
- A stub file (Section ‍35.5) can override the library’s annotations, for one or more methods.

  As a special case, if you want the Nullness Checker to prevent most null pointer exceptions in your code, but to permit null pointer exceptions at nullness assertion methods, you can pass -Astubs=permit-nullness-assertion-exception.astub.

- Don’t type-check clients of the method. For example, JUnit’s assertNotNull() is typically called only in test code; its clients are the tests. If you type-check only your main program, then the annotation on assertNotNull() is irrelevant.

#### Methods that sometimes crash when passed certain values

If a method can possibly throw an exception because its parameter is null, then that parameter’s type should be @NonNull, which guarantees that the type-checker will issue a warning for every client use that has the potential to cause an exception. Don’t write @Nullable on the parameter just because there exist some executions that don’t necessarily throw an exception.

### 2.4.4 Subclasses must respect superclass annotations

An annotation indicates a guarantee that a client can depend upon. A subclass is not permitted to weaken the contract; for example, if a method accepts null as an argument, then every overriding definition must also accept null. A subclass is permitted to strengthen the contract; for example, if a method does not accept null as an argument, then an overriding definition is permitted to accept null.

As a bad example, consider an erroneous @Nullable annotation in com/google/common/collect/Multiset.java:

    101  public interface Multiset<E> extends Collection<E> {
    ...
    122    /**
    123     * Adds a number of occurrences of an element to this multiset.
    ...
    129     * @param element the element to add occurrences of; may be {@code null} only
    130     *     if explicitly allowed by the implementation
    ...
    137     * @throws NullPointerException if {@code element} is null and this
    138     *     implementation does not permit null elements. Note that if {@code
    139     *     occurrences} is zero, the implementation may opt to return normally.
    140     */
    141    int add(@Nullable E element, int occurrences);

There exist implementations of Multiset that permit null elements, and implementations of Multiset that do not permit null elements. A client with a variable Multiset ms does not know which variety of Multiset ms refers to. However, the @Nullable annotation promises that ms.add(null, 1) is permissible. (Recall from Section ‍2.4.3 that annotations should indicate normal behavior.)

If parameter element on line 141 were to be annotated, the correct annotation would be @NonNull. Suppose a client has a reference to same Multiset ms. The only way the client can be sure not to throw an exception is to pass only non-null elements to ms.add(). A particular class that implements Multiset could declare add to take a @Nullable parameter. That still satisfies the original contract. It strengthens the contract by promising even more: a client with such a reference can pass any non-null value to add(), and may also pass null.

However, the best annotation for line 141 is no annotation at all. The reason is that each implementation of the Multiset interface should specify its own nullness properties when it specifies the type parameter for Multiset. For example, two clients could be written as

      class MyNullPermittingMultiset implements Multiset<@Nullable Object> { ... }
      class MyNullProhibitingMultiset implements Multiset<@NonNull Object> { ... }

or, more generally, as

      class MyNullPermittingMultiset<E extends @Nullable Object> implements Multiset<E> { ... }
      class MyNullProhibitingMultiset<E extends @NonNull Object> implements Multiset<E> { ... }

Then, the specification is more informative, and the Checker Framework is able to do more precise checking, than if line 141 has an annotation.

It is a pleasant feature of the Checker Framework that in many cases, no annotations at all are needed on type parameters such as E in MultiSet.

### 2.4.5 What to do if a checker issues a warning about your code

When you run a type-checker on your code, it is likely to issue warnings or errors. Don’t panic! If you have trouble understanding a Checker Framework warning message, you can search for its text in this manual. There are three general causes for the warnings:

You found a bug
  There is a bug in your code, such as a possible null dereference. Fix your code to prevent that crash.
Wrong annotations
  The annotations are too strong (they are incorrect) or too weak (they are imprecise). Improve the annotations, usually by writing more annotations in order to better express the specification. Only write annotations that accurately describe the intended behavior of the software — don’t write inaccurate annotations just for the purpose of eliminating type-checker warnings.

  Usually you need to improve the annotations in your source code. Sometimes you need to improve annotations in a library that your program uses (see Chapter ‍35).

Type-checker weakness
  There is a weakness in the type-checker. Your code is safe — it never suffers the error at run time — but the checker cannot prove this fact. (Recall that The checker works modularly: when type-checking a method m, it relies on the types and signatures of variables and methods used by m, but not the initialization expressions or the method bodies.)

  If possible, rewrite your code to be simpler for the checker to analyze; this is likely to make it easier for people to understand, too. If that is not possible, suppress the warning (see Chapter ‍33); be sure to include a code comment explaining how you know the code is correct even though the type-checker cannot deduce that fact.

  Do not add an if test that can never fail, just to suppress a warning. Adding a gratuitous if clutters the code and confuses readers. A reader should assume that every if condition can evaluate to true or false. There is one exception to this rule: an if test may have a condition that you think will never evaluate to true, if its body just throws a descriptive error message.

For each warning issued by the checker, you need to determine which of the above categories it falls into. Here is an effective methodology to do so. It relies mostly on manual code examination, but you may also find it useful to write test cases for your code or do other kinds of analysis, to verify your reasoning. (Also see Section ‍40.1.4 and Chapter ‍40, Troubleshooting. In particular, Section ‍40.1.4 explains this same methodology in different words.)

##### Step 1: Explain correctness: write a proof

Write an explanation of why your code is correct and it never suffers the error at run time. In other words, this is an informal proof that the type-checker’s warning is incorrect. Write it in natural language, such as English.

Don’t skip any steps in your proof. (For example, don’t write an unsubstantiated claim such as “x is non-null here”; instead, give a justification.) Don’t let your reasoning rely on facts that you do not write down explicitly. For example, remember that calling a method might change the values of object fields; your proof might need to state that certain methods have no side effects.

If you cannot write a proof, then there is a bug in your code (you should fix the bug) or your code is too complex for you to understand (you should improve its documentation and/or design).

##### Step 2: Translate the proof into annotations.

Here are some examples of the translation.

- If your proof includes “variable x is never null at run time”, then annotate x’s type with @NonNull.
- If your proof includes “method foo always returns a legal regular expression”, then annotate foo’s return type with @Regex.
- If your proof includes “if method join’s first argument is non-null, then join returns a non-null result”, then annotate join’s first parameter and return type with @PolyNull.
- If your proof includes “method processOptions has already been called and it set field tz1”, then annotate processOptions’s declaration with @EnsuresNonNull("tz1").
- If your proof includes “method isEmpty returned false, so its argument must have been non-null”, then annotate isEmpty’s declaration with @EnsuresNonNullIf(expression="#1",result=false).
- If your proof includes “method m has no side effects”, then annotate m’s declaration with @SideEffectFree.
- If your proof includes “each call to method m returns the same value”, then annotate m’s declaration with @Deterministic.

All of these are examples of correcting weaknesses in the annotations you wrote. The Checker Framework provides many other powerful annotations; you may be surprised how many proofs you can express in annotations. If you need to annotate a method that is defined in a library that your code uses, see Chapter ‍35.

Don’t omit any parts of your proof. When the Checker Framework analyzes a method, it examines only the signature/specification (not the implementation) of other methods.

If there are complex facts in your proof that cannot be expressed as annotations, then that is a weakness in the type-checker. For example, the Nullness Checker cannot express “in list lst, elements stored at even indices are always non-null, but elements stored at odd elements might be null.” In this case, you have two choices. First, you can suppress the warning (Chapter ‍33); be sure to write a comment explaining your reasoning for suppressing the warning. You may wish to submit a feature request (Section ‍40.2) asking for annotations that handle your use case. Second, you can rewrite the code to make the proof simpler; in the above example, it might be better to use a list of pairs rather than a heterogeneous list.

##### Step 3: Re-run the checker

At this point, all the steps in your proof have been formalized as annotations. Re-run the checker and repeat the process for any new or remaining warnings.

If every step of your proof can be expressed in annotations, but the checker cannot make one of the deductions (it cannot follow one of the steps), then that is a weakness in the type-checker. First, double-check your reasoning. Then, suppress the warning, along with a comment explaining your reasoning (Chapter ‍33). The comment is an excerpt from your informal proof, and the proof guides you to the best place to suppress the warning. Please submit a bug report so that the checker can be improved in the future (Section ‍40.2).

### 2.4.6 Calls to unannotated code (legacy libraries)

Sometimes, you wish to type-check only part of your program. You might focus on the most mission-critical or error-prone part of your code. When you start to use a checker, you may not wish to annotate your entire program right away. You may not have enough knowledge to annotate poorly-documented libraries that your program uses. Or, the code you are annotating may call into unannotated libraries.

If annotated code uses unannotated code, then the checker may issue warnings. For example, the Nullness Checker (Chapter ‍3) will warn whenever an unannotated method result is used in a non-null context:

      @NonNull myvar = unannotated_method();   // WARNING: unannotated_method may return null

If the call can return null, you should fix the bug in your program by removing the @NonNull annotation in your own program.

If the call never returns null, you have two choices: annotate the library or suppress warnings.

1. To annotate the library:
   - If the unannotated code is in your program, you can write annotations but not type-check them yet. Two ways to prevent the type-checking are via a @SuppressWarnings annotation (Section ‍33.1) or by not running the checker on that file, for example via the -AskipDefs command-line option (Section ‍33.5).
   - To annotate a library whose source code you do not have or cannot change, see Chapter ‍35.
2. To suppress all warnings related to uses of unannotated_method, use the -AskipUses command-line option (Section ‍33.4). Beware: a carelessly-written regular expression may suppress more warnings than you intend.

# Chapter ‍3 Nullness Checker

If the Nullness Checker issues no warnings for a given program, then running that program will never throw a null pointer exception. In other words, the Nullness Checker prevents all NullPointerExceptions. See Section ‍3.1 for more details about the guarantee and what is checked.

The most important annotations supported by the Nullness Checker are @NonNull and @Nullable. @NonNull is rarely written, because it is the default. All of the annotations are explained in Section ‍3.2.

To run the Nullness Checker, supply the -processor org.checkerframework.checker.nullness.NullnessChecker command-line option to javac. For examples, see Section ‍3.5.

The NullnessChecker is actually an ensemble of three pluggable type-checkers that work together: the Nullness Checker proper (which is the main focus of this chapter), the Initialization Checker (Section ‍3.8), and the Map Key Checker (Chapter ‍4). Their type hierarchies are completely independent, but they work together to provide precise nullness checking.

## 3.1 What the Nullness Checker guarantees

If the Nullness Checker type-checks your program without errors, then your program will not crash with a NullPointerException that is caused by misuse of null in checked code. Section ‍2.3 notes some limitations to guarantees made by the Checker Framework.

The checker issues a warning in these cases:

1. When an expression of non-@NonNull type is dereferenced, because it might cause a null pointer exception. Dereferences occur not only when a field is accessed, but when an array is indexed, an exception is thrown, a lock is taken in a synchronized block, and more. For a complete description of all checks performed by the Nullness Checker, see the Javadoc for NullnessVisitor.
2. When an expression of @NonNull type might become null, because it is a misuse of the type: the null value could flow to a dereference that the checker does not warn about.

   As a special case of an of @NonNull type becoming null, the checker also warns whenever a field of @NonNull type is not initialized in a constructor.

This example illustrates the programming errors that the checker detects:

      @Nullable Object   obj;  // might be null
      @NonNull  Object nnobj;  // never null
      ...
      obj.toString()         // checker warning:  dereference might cause null pointer exception
      nnobj = obj;           // checker warning:  nnobj may become null
      if (nnobj == null)     // checker warning:  redundant test

Parameter passing and return values are checked analogously to assignments.

The Nullness Checker also checks the correctness, and correct use, of initialization (see Section ‍3.8) and of map key annotations (see Chapter ‍4).

The checker performs additional checks if certain -Alint command-line options are provided. (See Section ‍33.7 for more details about the -Alint command-line option.)

### 3.1.1 Nullness Checker optional warnings

1. Options that control soundness:
   - If you supply the -Alint=soundArrayCreationNullness command-line option, then the checker warns if it encounters an array creation with a non-null component type. See Section ‍3.3.4 for a discussion.
   - If you supply the -Astubs=collection-object-parameters-may-be-null.astub command-line option, then in JDK collection classes, the checker unsoundly permits null as an argument for any key or value formal parameter whose type is Object (instead of the element type). See Section ‍3.4.2.
   - If you supply the -Alint=trustArrayLenZero command-line option, then the checker will trust @ArrayLen(0) annotations. See Section ‍3.3.5 for a discussion.
   - If you supply the -AassumeKeyFor command-line option, then the checker will unsoundly assume that the argument to Map.get is a key for the receiver map. It will not do any checking of @KeyFor and related qualifiers.
   - If you supply the -AinvocationPreservesArgumentNullness command-line option, the Nullness Checker unsoundly assumes that arguments passed to non-null parameters in an invocation remain non-null after the invocation. This assumption is unsound in general, but holds for most code. For other ways to suppress warnings related to method side effects, see Section ‍32.7.5.
2. Options that warn about poor code style:
   - If you supply the -Alint=redundantNullComparison command-line option, then the checker warns when a null check is performed against a value that is guaranteed to be non-null, as in ("m" == null). Such a check is unnecessary and might indicate a programmer error or misunderstanding. The lint option is disabled by default because sometimes such checks are part of ordinary defensive programming.
3. Options that enable checking modes:
   - If you supply the -Alint=permitClearProperty command-line option, then the checker permits calls to System.setProperties() and calls to System.clearProperty that might clear one of the built-in properties.

     By default, the checker forbids calls to those methods, and also special-cases type-checking of calls to System.getProperty() and System.setProperties(). A call to one of these methods can return null in general, but by default the Nullness Checker treats it as returning non-null if the argument is one of the literal strings listed in the documentation of System.getProperties(). To make this behavior sound, the Nullness Checker forbids calls that might clear any built-in property, as described above.

## 3.2 Nullness annotations

The Nullness Checker uses three separate type hierarchies: one for nullness, one for initialization (Section ‍3.8), and one for map keys (Chapter ‍4) The Nullness Checker has four varieties of annotations: nullness type qualifiers, nullness method annotations, initialization type qualifiers, and map key type qualifiers.

### 3.2.1 Nullness qualifiers

The nullness hierarchy contains these qualifiers:

@Nullable
  indicates a type that includes the null value. For example, the Java type Boolean is nullable: a variable of type Boolean always has one of the values TRUE, FALSE, or null. (Since @NonNull is the default type annotation, you would actually write this type as @Nullable Boolean.)
@NonNull
  indicates a type that does not include the null value. The type boolean is non-null; a variable of type boolean always has one of the values true or false. The type @NonNull Boolean is also non-null: a variable of type @NonNull Boolean always has one of the values TRUE or FALSE — never null. Dereferencing an expression of non-null type can never cause a null pointer exception.

  The @NonNull annotation is rarely written in a program, because it is the default (see Section ‍3.3.2).

@PolyNull
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.
@MonotonicNonNull
  indicates a reference that may be null, but if it ever becomes non-null, then it never becomes null again. This is appropriate for lazily-initialized fields, for field initialization that occurs in a lifecycle method other than the constructor (e.g., an override of android.app.Activity.onCreate), and other uses. @MonotonicNonNull is typically written on field types, but not elsewhere.

  The benefit of @MonotonicNonNull over @Nullable is that after a check of a @MonotonicNonNull field, all subsequent accesses within that method can be assumed to be @NonNull, even after arbitrary external method calls that have access to the given field. By contrast, for a @Nullable field, the Nullness Checker assumes that most method calls might set it to null. (Exceptions are calls to methods that are @SideEffectFree or that have an @EnsuresNonNull or @EnsuresNonNullIf annotation.)

  A @MonotonicNonNull field may be initialized to null, but the field may not be assigned to null anywhere else in the program. If you supply the noInitForMonotonicNonNull lint flag (for example, supply -Alint=noInitForMonotonicNonNull on the command line), then @MonotonicNonNull fields are not allowed to have initializers at their declarations.

  Use of @MonotonicNonNull on a static field is a code smell: it may indicate poor design. You should consider whether it is possible to make the field a member field that is set in the constructor.

  In the type system, @MonotonicNonNull is a supertype of @NonNull and a subtype of @Nullable.

Figure ‍3.1 shows part of the type hierarchy for the Nullness type system. (The annotations exist only at compile time; at run time, Java has no multiple inheritance.)

> Figure 3.1: Partial type hierarchy for the Nullness type system. Java’s Object is expressed as @Nullable Object. Programmers can omit most type qualifiers, because the default annotation (Section ‍3.3.2) is usually correct. The Nullness Checker verifies three type hierarchies: this one for nullness, one for initialization (Section ‍3.8), and one for map keys (Chapter ‍4).

### 3.2.2 Nullness method annotations

The Nullness Checker supports several annotations that specify method behavior. These are declaration annotations, not type annotations: they apply to the method itself rather than to some particular type.

@RequiresNonNull
  indicates a method precondition: The annotated method expects the specified variables to be non-null when the method is invoked. Don’t use this for formal parameters (just annotate their type as @NonNull). @RequiresNonNull is appropriate for a field that is @Nullable in general, but some method requires the field to be non-null.
@EnsuresNonNull
  indicates a method postcondition. With EnsuresNonNull, the successful return (i.e., a non-exceptional return) of the annotated method results in the given expressions being non-null. See the Javadoc for examples of its use.
@EnsuresNonNullIf
  indicates a method postcondition. With @EnsuresNonNull, the given expressions are non-null after the method returns; this is useful for a method that initializes a field, for example. With @EnsuresNonNullIf, if the annotated method returns the given boolean value (true or false), then the given expressions are non-null. See Section ‍3.3.3 and the Javadoc for examples of their use.

### 3.2.3 Initialization qualifiers

The Nullness Checker invokes an Initialization Checker, whose annotations indicate whether an object is fully initialized — that is, whether all of its fields have been assigned.

@Initialized
@UnknownInitialization
@UnderInitialization

Use of these annotations can help you to type-check more code. Figure ‍3.5 shows its type hierarchy. For details, see Section ‍3.8.

### 3.2.4 Map key qualifiers

@KeyFor

indicates that a value is a key for a given map — that is, indicates whether map.containsKey(value) would evaluate to true.

This annotation is checked by a Map Key Checker (Chapter ‍4) that the Nullness Checker invokes. The @KeyFor annotation enables the Nullness Checker to treat calls to Map.get precisely rather than assuming it may always return null. In particular, a call mymap.get(mykey) returns a non-null value if two conditions are satisfied:

1. mymap’s values are all non-null; that is, mymap was declared as Map<KeyType, @NonNull ValueType>. Note that @NonNull is the default type, so it need not be written explicitly.
2. mykey is a key in mymap; that is, mymap.containsKey(mykey) returns true. You express this fact to the Nullness Checker by declaring mykey as @KeyFor("mymap") KeyType mykey. For a local variable, you generally do not need to write the @KeyFor("mymap") type qualifier, because it can be inferred.

If either of these two conditions is violated, then mymap.get(mykey) has the possibility of returning null.

The command-line argument -AassumeKeyFor makes the Nullness Checker not run the Map Key Checker. The Nullness Checker will unsoundly assume that the argument to Map.get is a key for the receiver map. That is, the second condition above is always considered to be true.

## 3.3 Writing nullness annotations

### 3.3.1 Implicit qualifiers

The Nullness Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section ‍32.4). For example, enum types are implicitly non-null, so you never need to write @NonNull MyEnumType.

If you want details about implicitly-added nullness qualifiers, see the implementation of NullnessAnnotatedTypeFactory.

### 3.3.2 Default annotation

Unannotated references are treated as if they had a default annotation. All types default to @NonNull, except that @Nullable is used for casts, locals, instanceof, and implicit bounds (see Section ‍32.5.3). A user can choose a different defaulting rule by writing a @DefaultQualifier annotation on a package, class, or method. In the example below, fields are defaulted to @Nullable instead of @NonNull.

    @DefaultQualifier(value = Nullable.class, locations = TypeUseLocation.FIELD)
    class MyClass {
        Object nullableField = null;
        @NonNull Object nonNullField = new Object();
    }

### 3.3.3 Conditional nullness

The Nullness Checker supports a form of conditional nullness types, via the @EnsuresNonNullIf method annotations. The annotation on a method declares that some expressions are non-null, if the method returns true (false, respectively).

Consider java.lang.Class. Method Class.getComponentType() may return null, but it is specified to return a non-null value if Class.isArray() is true. You could declare this relationship in the following way (this particular example is already done for you in the annotated JDK that comes with the Checker Framework):

      class Class<T> {
        @EnsuresNonNullIf(expression="getComponentType()", result=true)
        public native boolean isArray();

        public native @Nullable Class<?> getComponentType();
      }

A client that checks that a Class reference is indeed that of an array, can then de-reference the result of Class.getComponentType safely without any nullness check. The Checker Framework source code itself uses such a pattern:

        if (clazz.isArray()) {
          // no possible null dereference on the following line
          TypeMirror componentType = typeFromClass(clazz.getComponentType());
          ...
        }

Another example is Queue.peek and Queue.poll, which return non-null if isEmpty returns false.

The argument to @EnsuresNonNullIf is a Java expression, including method calls (as shown above), method formal parameters, fields, etc.; for details, see Section ‍32.8. More examples of the use of these annotations appear in the Javadoc for @EnsuresNonNullIf.

Java programs sometimes contain more complex nullness invariants. When these invariants are more complex than handled by the Nullness Checker, you will need to suppress a warning (see Section ‍3.4).

### 3.3.4 Nullness and array initialization

Suppose that you declare an array to contain non-null elements:

      Object [] oa = new Object[10];

(recall that Object means the same thing as @NonNull Object). By default, the Nullness Checker unsoundly permits this code.

To make the Nullness Checker conservatively reject code that may leave a non-null value in an array, use the command-line option -Alint=soundArrayCreationNullness. The option is currently disabled because it makes the checker issue many false positive errors.

With the option enabled, you can write your code to create a nullable or lazy-nonnull array, initialize each component, and then assign the result to a non-null array:

      @MonotonicNonNull Object [] temp = new @MonotonicNonNull Object[10];
      for (int i = 0; i < temp.length; ++i) {
        temp[i] = new Object();
      }
      @SuppressWarnings("nullness") // temp array is now fully initialized
      @NonNull Object [] oa = temp;

Note that the checker is currently not powerful enough to ensure that each array component was initialized. Therefore, the last assignment needs to be trusted: that is, a programmer must verify that it is safe, then write a @SuppressWarnings annotation.

### 3.3.5 Nullness and conversions from collections to arrays

The nullness semantics of Collection.toArray(T[]) cannot be captured by just the nullness type system, though the Nullness Checker contains special-case code to type-check calls to toArray. Therefore, you will probably have to write @SuppressWarnings("nullness") on any overriding definitions of toArray.

The nullness type of the returned array depends on the size of the passed parameter. In particular, the returned array component is of type @NonNull if the following conditions hold:

- The receiver collection’s type argument (that is, the element type) is @NonNull, and
- The passed array size is less than or equal to the collection size. The Nullness Checker uses these heuristics to handle the most common cases:
  - the argument has length 0:
    - an empty array initializer, e.g. c.toArray(new String[] {}), or
    - array creation tree of size 0, e.g. c.toArray(new String[0]).
  - array creation tree with a collection size() method invocation as argument c.toArray(new String[c.size()]).

Additionally, when you supply the -Alint=trustArrayLenZero command-line option, a call to Collection.toArray will be estimated to return an array with a non-null component type if the argument is a field access where the field declaration has a @ArrayLen(0) annotation. This trusts the @ArrayLen(0) annotation, but does not verify it. Run the Constant Value Checker (see Chapter ‍23) to verify that annotation.

Note: The nullness of the returned array doesn’t depend on the passed array nullness. This is a fact about Collection.toArray(T[]), not a limitation of this heuristic.

### 3.3.6 Run-time checks for nullness

When you perform a run-time check for nullness, such as if (x != null) ..., then the Nullness Checker refines the type of x to @NonNull. The refinement lasts until the end of the scope of the test or until x may be side-effected. For more details, see Section ‍32.7.

### 3.3.7 Inference of @NonNull and @Nullable annotations

It can be tedious to write annotations in your code. Tools exist that can automatically infer annotations and insert them in your source code. (This is different than type qualifier refinement for local variables (Section ‍32.7), which infers a more specific type for local variables and uses them during type-checking but does not insert them in your source code. Type qualifier refinement is always enabled, no matter how annotations on signatures got inserted in your source code.)

Your choice of tool depends on what default annotation (see Section ‍3.3.2) your code uses. You only need one of these tools.

- Inference of @Nullable: If your code uses the standard CLIMB-to-top default (Section ‍32.5.3) or the NonNull default, then use the AnnotateNullable tool of the Daikon invariant detector.
- Inference of @NonNull: If your code uses the Nullable default (this is unusual), use one of these tools:
  - Non-null checker and inferencer of the JastAdd Extensible Compiler.

## 3.4 Suppressing nullness warnings

When the Nullness Checker reports a warning, it’s best to change the code or its annotations, to eliminate the warning. Alternately, you can suppress the warning, which does not change the code but prevents the Nullness Checker from reporting this particular warning to you.

The Checker Framework supplies several ways to suppress warnings, most notably the @SuppressWarnings("nullness") annotation (see Chapter ‍33). An example use is

        // might return null
        @Nullable Object getObject(...) { ... }

        void myMethod() {
          @SuppressWarnings("nullness") // with argument x, getObject always returns a non-null value
          @NonNull Object o2 = getObject(x);
        }

The Nullness Checker supports an additional warning suppression string, nullness:generic.argument. Use of @SuppressWarnings("nullness:generic.argument") causes the Nullness Checker to suppress warnings related to misuse of generic type arguments. One use for this key is when a class is declared to take only @NonNull type arguments, but you want to instantiate the class with a @Nullable type argument, as in List<@Nullable Object>.

The Nullness Checker also permits you to use assertions or method calls to suppress warnings; see below.

### 3.4.1 Suppressing warnings with assertions and method calls

Occasionally, it is inconvenient or verbose to use the @SuppressWarnings annotation. For example, Java does not permit annotations such as @SuppressWarnings to appear on statements, expressions, static initializers, etc. Here are three ways to suppress a warning in such cases:

- Create a local variable to hold a subexpression, and suppress a warning on the local variable declaration.
- Use the @AssumeAssertion string in an assert message (see Section ‍33.2).
- Write a call to the NullnessUtil.castNonNull method.

The rest of this section discusses the castNonNull method. It is useful if you wish to suppress a warning within an expression.

The Nullness Checker considers both the return value, and also the argument, to be non-null after the castNonNull method call. The Nullness Checker issues no warnings in any of the following code:

      // One way to use castNonNull as a cast:
      @NonNull String s = castNonNull(possiblyNull1);

      // Another way to use castNonNull as a cast:
      castNonNull(possiblyNull2).toString();

      // It is possible, but not recommmended, to use castNonNull as a statement:
      // (It would be better to write an assert statement with @AssumeAssertion
      // in its message, instead.)
      castNonNull(possiblyNull3);
      possiblyNull3.toString();

The castNonNull method throws AssertionError if Java assertions are enabled and the argument is null. However, it is not intended for general defensive programming; see Section ‍33.2.1.

To use the castNonNull method, the checker-util.jar file must be on the classpath at run time.

The Nullness Checker introduces a new method, rather than re-using an existing method such as org.junit.Assert.assertNotNull(Object) or com.google.common.base.Preconditions.checkNotNull(Object). Those methods are commonly used for defensive programming, so it is impossible to know the programmer’s intent when writing them. Therefore, it is important to have a method call that is used only for warning suppression. See Section ‍33.2.1 for a discussion of the distinction between warning suppression and defensive programming.

### 3.4.2 Null arguments to collection classes

For collection methods with Object formal parameter type, such as contains, indexOf, and remove, the annotated JDK forbids null as an argument.

The reason is that some implementations (like ConcurrentHashMap) throw NullPointerException if null is passed. It would be unsound to permit null, because it could lead to a false negative: the Checker Framework issuing no warning even though a NullPointerException can occur at run time.

However, many other common implementations permit such calls, so some users may wish to sacrifice soundness for a reduced number of false positive warnings. To permit null as an argument to these methods, pass the command-line argument -Astubs=collection-object-parameters-may-be-null.astub.

### 3.4.3 Conservative nullness annotations on the JDK

The JDK contains nullness annotations that preserve the Nullness Checker’s guarantee (see Section ‍3.1) that your program will not crash with a NullPointerException. In some cases, a formal parameter may be null in some circumstances, but must be non-null in other circumstances, and those circumstances are not expressible using the Nullness Checker’s annotations.

An example is restrictions on collection arguments (see Section ‍3.4.2).

Another example is this WeakReference constructor:

         ...
         * @param q the queue with which the reference is to be registered,
         *          or {@code null} if registration is not required
         */
        public WeakReference(@Nullable T referent, ReferenceQueue<? super T> q) {
          ...

For some calls, q must be non-null. Therefore, q is annotated as @NonNull (which is the default and need not be explicitly written).

These JDK annotations reflect a verification philosophy: a verification tool finds all possible errors, but it sometimes issues a false positive warning. An alternate philosophy is a bug-finding philosophy: permit all calls that might be correct at run time, but sometimes miss a real error. If you wish to use the Checker Framework with the bug-finding philosophy (though the Checker Framework is still much more thorough than other bug-finders), you can do so by passing the command-line argument -Astubs=sometimes-nullable.astub.

## 3.5 Examples

### 3.5.1 Tiny examples

To try the Nullness Checker on a source file that uses the @NonNull qualifier, use the following command (where javac is the Checker Framework compiler that is distributed with the Checker Framework, see Section ‍38.6 for details):

      javac -processor org.checkerframework.checker.nullness.NullnessChecker docs/examples/NullnessExample.java

Compilation will complete without warnings.

To see the checker warn about incorrect usage of annotations (and therefore the possibility of a null pointer exception at run time), use the following command:

      javac -processor org.checkerframework.checker.nullness.NullnessChecker docs/examples/NullnessExampleWithWarnings.java

The compiler will issue two warnings regarding violation of the semantics of @NonNull.

### 3.5.2 Example annotated source code

Some libraries that are annotated with nullness qualifiers are:

- The Nullness Checker itself.
- The Java projects in the plume-lib GitHub organization. Type-checking occurs on each build.
- The Daikon invariant detector. Run the command make check-nullness.

### 3.5.3 Publications

The papers “Practical pluggable types for Java” ‍[PAC+08] (ISSTA 2008, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-issta2008.pdf) and “Building and using pluggable type-checkers” ‍[DDE+11] (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-icse2011.pdf) describe case studies in which the Nullness Checker found previously-unknown errors in real software.

## 3.6 Tips for getting started

Here are some tips about getting started using the Nullness Checker on a legacy codebase. For more generic advice (not specific to the Nullness Checker), see Section ‍2.4.2. Also see the Checker Framework tutorial, which includes an example of using the Nullness Checker.

Your goal is to add @Nullable annotations to the types of any variables that can be null. (The default is to assume that a variable is non-null unless it has a @Nullable annotation.) Then, you will run the Nullness Checker. Each of its errors indicates either a possible null pointer exception, or a wrong/missing annotation. When there are no more warnings from the checker, you are done!

We recommend that you start by searching the code for occurrences of null in the following locations; when you find one, write the corresponding annotation:

- in Javadoc: add @Nullable annotations to method signatures (parameters and return types).
- return null: add a @Nullable annotation to the return type of the given method.
- param == null: when a formal parameter is compared to null, then in most cases you can add a @Nullable annotation to the formal parameter’s type
- TypeName field = null;: when a field is initialized to null in its declaration, then it needs either a @Nullable or a @MonotonicNonNull annotation. If the field is always set to a non-null value in the constructor, then you can just change the declaration to Type field;, without an initializer, and write no type annotation (because the default is @NonNull).
- declarations of contains, containsKey, containsValue, equals, get, indexOf, lastIndexOf, and remove (with Object as the argument type): change the argument type to @Nullable Object; for remove, also change the return type to @Nullable Object.

You should ignore all other occurrences of null within a method body. In particular, you rarely need to annotate local variables (except their type arguments or array element types).

Only after this step should you run the Nullness Checker. The reason is that it is quicker to search for places to change than to repeatedly run the checker and fix the errors it tells you about, one at a time.

Here are some other tips:

- In any file where you write an annotation such as @Nullable, don’t forget to add import org.checkerframework.checker.nullness.qual.*;.
- To indicate an array that can be null, write, for example: int @Nullable [].
  By contrast, @Nullable Object [] means a non-null array that contains possibly-null objects.
- If you know that a particular variable is definitely not null, but the Nullness Checker estimates that the variable might be null, then you can make the Nullness Checker trust your judgment by writing an assertion (see Section ‍33.2):

      assert var != null : "@AssumeAssertion(nullness)";

- To indicate that a routine returns the same value every time it is called, use @Pure (see Section ‍32.7.5).
- To indicate a method precondition (a contract stating the conditions under which a client is allowed to call it), you can use annotations such as @RequiresNonNull (see Section ‍3.2.2).

## 3.7 Other tools for nullness checking

The Checker Framework’s nullness annotations are similar to annotations used in other tools. You might prefer to use the Checker Framework because it has a more powerful analysis that can warn you about more null pointer errors in your code. Most of the other tools are bug-finding tools rather than verification tools, since they give up precision, soundness, or both in favor of being fast and easy to use. Also see Section ‍39.10.1 for a comparison to other tools.

If your code is already annotated with a different nullness annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Nullness Checker, as described in Figures ‍3.2, 3.3, and 3.4. If the other annotation is a declaration annotation, it may be moved; see Section ‍39.6.10.

> ‍android.annotation.NonNull ‍
> ‍android.support.annotation.NonNull ‍
> ‍android.support.annotation.RecentlyNonNull ‍
> ‍androidx.annotation.NonNull ‍
> ‍androidx.annotation.RecentlyNonNull ‍
> ‍com.android.annotations.NonNull ‍
> ‍com.google.firebase.database.annotations.NotNull ‍
> ‍com.google.firebase.internal.NonNull ‍
> ‍com.mongodb.lang.NonNull ‍
> ‍com.sun.istack.NotNull ‍
> ‍com.sun.istack.internal.NotNull ‍
> ‍com.unboundid.util.NotNull ‍
> ‍edu.umd.cs.findbugs.annotations.NonNull ‍
> ‍io.micrometer.core.lang.NonNull ‍
> ‍io.micronaut.core.annotation.NonNull ‍
> ‍io.reactivex.annotations.NonNull ‍
> ‍io.reactivex.rxjava3.annotations.NonNull ‍
> ‍jakarta.annotation.Nonnull ‍
> ‍javax.annotation.Nonnull ‍
> ‍javax.validation.constraints.NotNull ‍
> ‍libcore.util.NonNull ‍
> ‍lombok.NonNull ‍
> ‍net.bytebuddy.agent.utility.nullability.NeverNull ‍
> ‍net.bytebuddy.utility.nullability.NeverNull ‍
> ‍org.antlr.v4.runtime.misc.NotNull ‍
> ‍org.checkerframework.checker.nullness.compatqual.NonNullDecl ‍
> ‍org.checkerframework.checker.nullness.compatqual.NonNullType ‍
> ‍org.chromium.build.annotations.OptimizeAsNonNull ‍
> ‍org.codehaus.commons.nullanalysis.NotNull ‍
> ‍org.eclipse.jdt.annotation.NonNull ‍
> ‍org.eclipse.jgit.annotations.NonNull ‍
> ‍org.eclipse.lsp4j.jsonrpc.validation.NonNull ‍
> ‍org.jetbrains.annotations.NotNull ‍
> ‍org.jmlspecs.annotation.NonNull ‍
> ‍org.jspecify.annotations.NonNull ‍
> ‍org.jspecify.nullness.NonNull ‍
> ‍org.netbeans.api.annotations.common.NonNull ‍
> ‍org.springframework.lang.NonNull ‍
> ‍reactor.util.annotation.NonNull ‍
> | ⇒  ‍org.checkerframework.checker.nullness.qual.NonNull ‍

> Figure 3.2: Correspondence between other nullness annotations and the Checker Framework’s NonNull annotation.

> ‍android.annotation.Nullable ‍
> ‍android.support.annotation.Nullable ‍
> ‍android.support.annotation.RecentlyNullable ‍
> ‍androidx.annotation.Nullable ‍
> ‍androidx.annotation.RecentlyNullable ‍
> ‍com.android.annotations.Nullable ‍
> ‍com.beust.jcommander.internal.Nullable ‍
> ‍com.google.api.server.spi.config.Nullable ‍
> ‍com.google.firebase.database.annotations.Nullable ‍
> ‍com.google.firebase.internal.Nullable ‍
> ‍com.google.gerrit.common.Nullable ‍
> ‍com.google.protobuf.Internal.ProtoMethodAcceptsNullParameter ‍
> ‍com.google.protobuf.Internal.ProtoMethodMayReturnNull ‍
> ‍com.mongodb.lang.Nullable ‍
> ‍com.sun.istack.Nullable ‍
> ‍com.sun.istack.internal.Nullable ‍
> ‍com.unboundid.util.Nullable ‍
> ‍edu.umd.cs.findbugs.annotations.CheckForNull ‍
> ‍edu.umd.cs.findbugs.annotations.Nullable ‍
> ‍edu.umd.cs.findbugs.annotations.PossiblyNull ‍
> ‍edu.umd.cs.findbugs.annotations.UnknownNullness ‍
> ‍io.micrometer.core.lang.Nullable ‍
> ‍io.micronaut.core.annotation.Nullable ‍
> ‍io.reactivex.annotations.Nullable ‍
> ‍io.reactivex.rxjava3.annotations.Nullable ‍
> ‍io.vertx.codegen.annotations.Nullable ‍
> ‍jakarta.annotation.Nullable ‍
> ‍javax.annotation.CheckForNull ‍
> ‍javax.annotation.Nullable ‍
> ‍junitparams.converters.Nullable ‍
> ‍libcore.util.Nullable ‍
> ‍net.bytebuddy.agent.utility.nullability.AlwaysNull ‍
> ‍net.bytebuddy.agent.utility.nullability.MaybeNull ‍
> ‍net.bytebuddy.agent.utility.nullability.UnknownNull ‍
> ‍net.bytebuddy.utility.nullability.AlwaysNull ‍
> ‍net.bytebuddy.utility.nullability.MaybeNull ‍
> ‍net.bytebuddy.utility.nullability.UnknownNull ‍
> ‍org.apache.avro.reflect.Nullable ‍
> ‍org.apache.cxf.jaxrs.ext.Nullable ‍
> ‍org.apache.shindig.common.Nullable ‍
> ‍org.checkerframework.checker.nullness.compatqual.NullableDecl ‍
> ‍org.checkerframework.checker.nullness.compatqual.NullableType ‍
> ‍org.chromium.build.annotations.Nullable ‍
> ‍org.codehaus.commons.nullanalysis.Nullable ‍
> ‍org.eclipse.jdt.annotation.Nullable ‍
> ‍org.eclipse.jgit.annotations.Nullable ‍
> ‍org.jetbrains.annotations.Nullable ‍
> ‍org.jetbrains.annotations.UnknownNullability ‍
> ‍org.jmlspecs.annotation.Nullable ‍
> ‍org.jspecify.annotations.Nullable ‍
> ‍org.jspecify.nullness.Nullable ‍
> ‍org.jspecify.nullness.NullnessUnspecified ‍
> ‍org.netbeans.api.annotations.common.CheckForNull ‍
> ‍org.netbeans.api.annotations.common.NullAllowed ‍
> ‍org.netbeans.api.annotations.common.NullUnknown ‍
> ‍org.springframework.lang.Nullable ‍
> ‍reactor.util.annotation.Nullable ‍
> | ⇒  ‍org.checkerframework.checker.nullness.qual.Nullable ‍

> Figure 3.3: Correspondence between other nullness annotations and the Checker Framework’s Nullable annotation.

> ‍com.google.protobuf.Internal.ProtoPassThroughNullness ‍
> | ⇒  ‍org.checkerframework.checker.nullness.qual.PolyNull ‍

> Figure 3.4: Correspondence between other nullness annotations and the Checker Framework’s PolyNull annotation.

The Checker Framework may issue more or fewer errors than another tool. This is expected, since each tool uses a different analysis. Remember that the Checker Framework aims at soundness: it aims to never miss a possible null dereference, while at the same time limiting false reports. Also, note SpotBugs’s non-standard meaning for @Nullable (Section ‍3.7.2).

Java permits you to import at most one annotation of a given name. For example, if you use both android.annotation.NonNull and lombok.NonNull in your source code, then you must write at least one of them in fully-qualified form, as @android.annotation.NonNull rather than as @NonNull.

### 3.7.1 Which tool is right for you?

Different tools are appropriate in different circumstances. Section ‍39.10.1 compares verification tools like the Checker Framework with bug detectors like SpotBugs and Error Prone. In brief, a bug detector is easier to use because it requires fewer annotations, but it misses lots of real bugs that a verifier finds. You should use whichever tool is appropriate for the importance of your code.

You may also choose to use multiple tools, especially since each tool focuses on different types of errors. If you know that you will eventually want to do verification for some particular task (say, nullness checking), there is little point using the nullness analysis of bug detector such as SpotBugs first. It is easier to go straight to using the Checker Framework.

If some other tool discovers a nullness error that the Checker Framework does not, please report it to us (see Section ‍40.2) so that we can enhance the Checker Framework. For example, SpotBugs might detect an error that the Nullness Checker does not, if you are using an unannotated library (including an unannotated part of the JDK) and running the Checker Framework in an unsound mode (see Section ‍2.2.2).

### 3.7.2 Incompatibility note about FindBugs and SpotBugs @Nullable

FindBugs and SpotBugs have a non-standard definition of @Nullable. This treatment is not documented in its own Javadoc; it is different from the definition of @Nullable in every other tool for nullness analysis; it means the same thing as @NonNull when applied to a formal parameter; and it invariably surprises programmers. Thus, SpotBugs’s @Nullable is detrimental rather than useful as documentation. In practice, your best bet is to not rely on SpotBugs for nullness analysis, even if you find SpotBugs useful for other purposes.

You can skip the rest of this section unless you wish to learn more details.

SpotBugs suppresses all warnings at uses of a @Nullable variable. (You have to use @CheckForNull to indicate a nullable variable that SpotBugs should check.) For example:

         // declare getObject() to possibly return null
         @Nullable Object getObject() { ... }

         void myMethod() {
           @Nullable Object o = getObject();
           // SpotBugs issues no warning about calling toString on a possibly-null reference!
           o.toString();
         }

The Checker Framework does not emulate this non-standard behavior of SpotBugs, even if the code uses FindBugs/SpotBugs annotations.

With SpotBugs, you annotate a declaration, which suppresses checking at all client uses, even the places that you want to check. It is better to suppress warnings at only the specific client uses where the value is known to be non-null; the Checker Framework supports this, if you write @SuppressWarnings at the client uses. The Checker Framework also supports suppressing checking at all client uses, by writing a @SuppressWarnings annotation at the declaration site. Thus, the Checker Framework supports both use cases, whereas SpotBugs supports only one and gives the programmer less flexibility.

In general, the Checker Framework will issue more warnings than SpotBugs, and some of them may be about real bugs in your program. See Section ‍3.4 for information about suppressing nullness warnings.

FindBugs and SpotBugs made a poor choice of names. The choice of names should make a clear distinction between annotations that specify whether a reference is null, and annotations that suppress false warnings. The choice of names should also have been consistent for other tools, and intuitively clear to programmers. The FindBugs/SpotBugs choices make the SpotBugs annotations less helpful to people, and much less useful for other tools.

Another problem is that the SpotBugs @Nullable annotation is a declaration annotation rather than a type annotation. This means that it cannot be written in important locations such as type arguments, and it is misleading when written on a field of array type or a method that returns an array.

Overall, it is best to stay away from the SpotBugs nullness annotations and analysis, and use a tool with a more principled design.

### 3.7.3 Relationship to Optional<T>

Many null pointer exceptions occur because the programmer forgets to check whether a reference is null before dereferencing it. Java 8’s Optional<T> class provides a partial solution: a programmer must call the get method to access the value, and the designers of Optional hope that the syntactic occurrence of the get method will remind programmers to first check that the value is present. This is still easy to forget, however.

The Checker Framework contains an Optional Checker (see Chapter ‍5) that guarantees that programmers use Optional correctly, such as calling isPresent before calling get.

There are some limitations to the utility of Optional, which might lead to you choose to use regular Java references rather than Optional. (For more details, see the article “Nothing is better than the Optional type”.)

- It is still possible to call get on a non-present Optional, leading to a NoSuchElementException. In other words, Optional doesn’t solve the underlying problem — it merely converts a NullPointerException into a NoSuchElementException exception, and in either case your code crashes.
- NullPointerException is still possible in code that uses Optional.
- Optional adds syntactic complexity, making your code longer and harder to read.
- Optional adds time and space overhead.
- Optional does not address important sources of null pointer exceptions, such as partially-initialized objects and calls to Map.get.

The Nullness Checker does not suffer these limitations. Furthermore, it works with existing code and types, it ensures that you check for null wherever necessary, and it infers when the check for null is not necessary based on previous statements in the method.

Java’s Optional class provides utility routines to reduce clutter when using Optional. The Nullness Checker provides an Opt class that provides all the same methods, but written for regular possibly-null Java references. To use the Opt class, the checker-util.jar file must be on the classpath at run time.

## 3.8 Initialization Checker

The Initialization Checker determines whether an object is initialized or not. For any object that is not fully initialized, the Nullness Checker treats its fields as possibly-null — even fields annotated as @NonNull. (The Initialization Checker focuses on @NonNull fields, to detect null pointer exceptions when using them. It does not currently ensure that primitives or @Nullable fields are initialized. Use the Initialized Fields Checker (Chapter ‍26) to check initialization with respect to properties other than nullness.)

Every object’s fields start out as null. By the time the constructor finishes executing, the @NonNull fields have been set to a different value. Your code can suffer a NullPointerException when using a @NonNull field, if your code uses the field during initialization. The Nullness Checker prevents this problem by warning you anytime that you may be accessing an uninitialized field. This check is useful because it prevents errors in your code. However, the analysis can be confusing to understand. If you wish to disable the initialization checks, see Section ‍3.8.8.

An object is partially initialized from the time that its constructor starts until its constructor finishes. This is relevant to the Nullness Checker because while the constructor is executing — that is, before initialization completes — a @NonNull field may be observed to be null, until that field is set. In particular, the Nullness Checker issues a warning for code like this:

      public class MyClass {
        private @NonNull Object f;
        public MyClass(int x) {
          // Error because constructor contains no assignment to this.f.
          // By the time the constructor exits, f must be initialized to a non-null value.
        }
        public MyClass(int x, int y) {
          // Error because this.f is accessed before f is initialized.
          // At the beginning of the constructor's execution, accessing this.f
          // yields null, even though field f has a non-null type.
          this.f.toString();
          f = new Object();
        }
        public MyClass(int x, int y, int z) {
          m();
          f = new Object();
        }
        public void m() {
          // Error because this.f is accessed before f is initialized,
          // even though the access is not in a constructor.
          // When m is called from the constructor, accessing f yields null,
          // even though field f has a non-null type.
          this.f.toString();
        }

When a field f is declared with a @NonNull type, then code can depend on the fact that the field is not null. However, this guarantee does not hold for a partially-initialized object.

The Initialization Checker uses three annotations to indicate whether an object is initialized (all its @NonNull fields have been assigned), under initialization (its constructor is currently executing), or its initialization state is unknown.

These distinctions are mostly relevant within the constructor, or for references to this that escape the constructor (say, by being stored in a field or passed to a method before initialization is complete). Use of initialization annotations is rare in most code.

### 3.8.1 Initialization qualifiers

> Figure 3.5: Partial type hierarchy for the Initialization type system. @UnknownInitialization and @UnderInitialization each take an optional parameter indicating how far initialization has proceeded, and the right side of the figure illustrates its type hierarchy in more detail.

The initialization hierarchy is shown in Figure ‍3.5. The initialization hierarchy contains these qualifiers:

@Initialized
  indicates a type that contains a fully-initialized object. Initialized is the default, so there is little need for a programmer to write this explicitly.
@UnknownInitialization
  indicates a type that may contain a partially-initialized object. In a partially-initialized object, fields that are annotated as @NonNull may be null because the field has not yet been assigned.

  @UnknownInitialization takes a parameter that is the class the object is definitely initialized up to. For instance, the type @UnknownInitialization(Foo.class) denotes an object in which every fields declared in Foo or its superclasses is initialized, but other fields might not be. Just @UnknownInitialization is equivalent to @UnknownInitialization(Object.class).

@UnderInitialization
  indicates a type that contains a partially-initialized object that is under initialization — that is, its constructor is currently executing. It is otherwise the same as @UnknownInitialization. Within the constructor, this has @UnderInitialization type until all the @NonNull fields have been assigned.

A partially-initialized object (this in a constructor) may be passed to a helper method or stored in a variable; if so, the method receiver, or the field, would have to be annotated as @UnknownInitialization or as @UnderInitialization.

If a reference has @UnknownInitialization or @UnderInitialization type, then all of its @NonNull fields are treated as @MonotonicNonNull: when read, they are treated as being @Nullable, but when written, they are treated as being @NonNull.

The initialization hierarchy is orthogonal to the nullness hierarchy. It is legal for a reference to be @NonNull @UnderInitialization, @Nullable @UnderInitialization, @NonNull @Initialized, or @Nullable @Initialized. The nullness hierarchy tells you about the reference itself: might the reference be null? The initialization hierarchy tells you about the @NonNull fields in the referred-to object: might those fields be temporarily null in contravention of their type annotation? Figure ‍3.6 contains some examples.

> Declarations | Expression | Expression’s nullness type, or checker error
>     class C {
>       @NonNull Object f;
>       @Nullable Object g;
>       ...
>     }
> @NonNull @Initialized C a; | a | @NonNull
> | a.f | @NonNull
> | a.g | @Nullable
> @NonNull @UnderInitialization C b; | b | @NonNull
> | b.f | @MonotonicNonNull
> | b.g | @Nullable
> @Nullable @Initialized C c; | c | @Nullable
> | c.f | error: deref of nullable
> | c.g | error: deref of nullable
> @Nullable @UnderInitialization C d; | d | @Nullable
> | d.f | error: deref of nullable
> | d.g | error: deref of nullable

> Figure 3.6: Examples of the interaction between nullness and initialization. Declarations are shown at the left for reference, but the focus of the table is the expressions and their nullness type or error.

### 3.8.2 How an object becomes initialized

Within the constructor, this starts out with @UnderInitialization type. As soon as all of the @NonNull fields in class C have been initialized, then this is treated as @UnderInitialization(C). This means that this is still being initialized, but all initialization of C’s fields is complete, including all fields of supertypes. Eventually, when all constructors complete, the type is @Initialized.

The Initialization Checker issues an error if the constructor fails to initialize any @NonNull field. This ensures that the object is in a legal (initialized) state by the time that the constructor exits. This is different than Java’s test for definite assignment (see JLS ch.16), which does not apply to fields (except blank final ones, defined in JLS §4.12.4) because fields have a default value of null.

All @NonNull fields must either have a default in the field declaration, or be assigned in the constructor or in a helper method that the constructor calls. If your code initializes (some) fields in a helper method, you will need to annotate the helper method with an annotation such as @EnsuresNonNull({"field1", "field2"}) for all the fields that the helper method assigns.

### 3.8.3@UnderInitialization examples

The most common use for the @UnderInitialization annotation is for a helper routine that is called by constructor. For example:

      class MyClass {
        Object field1;
        Object field2;
        Object field3;

        public MyClass(String arg1) {
          this.field1 = arg1;
          init_other_fields();
        }

        // A helper routine that initializes all the fields other than field1.
        @EnsuresNonNull({"field2", "field3"})
        private void init_other_fields(@UnderInitialization(Object.class) MyClass this) {
          field2 = new Object();
          field3 = new Object();
        }

        public MyClass(String arg1, String arg2, String arg3) {
          this.field1 = arg1;
          this.field2 = arg2;
          this.field3 = arg3;
          checkRep();
        }

        // Verify that the representation invariants are satisfied.
        // Works as long as the MyClass fields are initialized, even if the receiver's
        // class is a subclass of MyClass and not all of the subclass fields are initialized.
        private void checkRep(@UnderInitialization(MyClass.class) MyClass this) {
          ...
        }

      }

At the end of the constructor, this is not fully initialized. Rather, it is @UnderInitialization(CurrentClass.class). The reason is that there might be subclasses with uninitialized fields. The following example illustrates this:

    class A {
       @NonNull String a;
       public A() {
           a = "";
           // Now, all fields of A are initialized.
           // However, if this constructor is invoked as part of 'new B()', then
           // the fields of B are not yet initialized.
           // If we would type 'this' as @Initialized, then the following call is valid:
           doSomething();
       }
       void doSomething() {}
    }

    class B extends A {
       @NonNull String b;
       @Override
       void doSomething() {
           // Dereferencing 'b' is ok, because 'this' is @Initialized and 'b' @NonNull.
           // However, when executing 'new B()', this line throws a null-pointer exception.
           b.toString();
       }
    }

### 3.8.4 Partial initialization

So far, we have discussed initialization as if it is an all-or-nothing property: an object is non-initialized until initialization completes, and then it is initialized. The full truth is a bit more complex: during the initialization process an object can be partially initialized, and as the object’s superclass constructors complete, its initialization status is updated. The Initialization Checker lets you express such properties when necessary.

Consider a simple example:

    class A {
      Object aField;
      A() {
        aField = new Object();
      }
    }
    class B extends A {
      Object bField;
      B() {
        super();
        bField = new Object();
      }
    }

Consider what happens during execution of new B().

1. B’s constructor begins to execute. At this point, neither the fields of A nor those of B have been initialized yet.
2. B’s constructor calls A’s constructor, which begins to execute. No fields of A nor of B have been initialized yet.
3. A’s constructor completes. Now, all the fields of A have been initialized, and their invariants (such as that field a is non-null) can be depended on. However, because B’s constructor has not yet completed executing, the object being constructed is not yet fully initialized.
4. B’s constructor completes. The fields declared in A and B are initialized. However, the type system cannot assume that the object is fully initialized — there might be a class C extends B {...}, and B’s constructor might have been invoked from that.

At any moment during initialization, the superclasses of a given class can be divided into those that have completed initialization and those that have not yet completed initialization. More precisely, at any moment there is a point in the class hierarchy such that all the classes above that point are fully initialized, and all those below it are not yet initialized. As initialization proceeds, this dividing line between the initialized and uninitialized classes moves down the type hierarchy.

The Nullness Checker lets you indicate where the dividing line is between the initialized and non-initialized classes. The @UnderInitialization(classliteral) indicates the first class that is known to be fully initialized. When you write @UnderInitialization(OtherClass.class) MyClass x;, that means that variable x is initialized for OtherClass and its superclasses, and x is (possibly) uninitialized for subclasses of OtherClass.

The example above lists 4 moments during construction. At those moments, the type of the object being constructed is:

1. @UnderInitialization B
2. @UnderInitialization A
3. @UnderInitialization(A.class) A
4. @UnderInitialization(B.class) B

Note that neither @UnderInitialization(A.class) A nor @UnderInitialization(A.class) B may be used where <@Initialized A> is required. Permitting that would be unsound. For example, consider this code, where all types are @NonNull because @NonNull is the default annotation:

    class A {
      Object aField;
      A() {
        aField = new Object();
      }
      Object get() {
        return aField;
      }
    }
    class B extends A {
      Object bField;
      B() {
        super();
        bField = new Object();
      }
      @Override
      Object get() {
        return bField;
      }
    }

Given these declarations:

    @Initialized A w;
    @Initialized B x;
    @UnderInitialization(A.class) A y;
    @UnderInitialization(A.class) B z;

the expressions w.get() and x.get() evaluate to a non-null value, but y.get() and z.get() might evaluate to null. (y.get() might evaluate to null because the run-time type of y might be B. That is, y and z might refer to the same value, just as w and x might refer to the same value.)

### 3.8.5 Method calls from the constructor

Consider the following incorrect program.

    class A {
      Object aField;
      A() {
        aField = new Object();
        process(5);  // illegal call
      }
      public void process(int arg) { ... }
    }

The call to process() is not legal. process() is declared to be called on a fully-initialized receiver, which is the default if you do not write a different initialization annotation. At the call to process(), all the fields of A have been set, but this is not fully initialized because fields in subclasses of A have not yet been set. The type of this is @UnderInitialization(A.class), meaning that this is partially-initialized, with the A part of initialization done but the initialization of subclasses not yet complete.

The Initialization Checker output indicates this problem:

    Client.java:7: error: [method.invocation] call to process(int) not allowed on the given receiver.
        process(5);  // illegal call
               ^
      found   : @UnderInitialization(A.class) A
      required: @Initialized A

Here is a subclass and client code that crashes with a NullPointerException.

    class B extends A {
      List<Integer> processed;
      B() {
        super();
        processed = new ArrayList<Integer>();
      }
      @Override
      public void process(int arg) {
        super();
        processed.add(arg);
      }
    }
    class Client {
      public static void main(String[] args) {
        new B();
      }
    }

You can correct the problem in multiple ways.

One solution is to not call methods that can be overridden from the constructor: move the call to process() to after the constructor has completed.

Another solution is to change the declaration of process():

      public void process(@UnderInitialization(A.class) A this, int arg) { ... }

If you choose this solution, you will need to rewrite the definition of B.process() so that it is consistent with the declared receiver type.

A non-solution is to prevent subclasses from overriding process() by using final on the method. This doesn’t work because even if process() is not overridden, it might call other methods that are overridden.

As final classes cannot have subclasses, they can be handled more flexibly: once all fields of the final class have been initialized, this is fully initialized.

### 3.8.6 Initialization of circular data structures

There is one final aspect of the initialization type system to be considered: the rules governing reading and writing to objects that are currently under initialization (both reading from fields of objects under initialization, as well as writing objects under initialization to fields). By default, only fully-initialized objects can be stored in a field of another object. If this was the only option, then it would not be possible to create circular data structures (such as a doubly-linked list) where fields have a @NonNull type. However, the annotation @NotOnlyInitialized can be used to indicate that a field can store objects that are currently under initialization. In this case, the rules for reading and writing to that field become a little bit more interesting, to soundly support circular structures.

The rules for reading from a @NotOnlyInitialized field are summarized in Figure ‍3.7. Essentially, nothing is known about the initialization status of the value returned unless the receiver was @Initialized.

> x.f | f is @NonNull | f is @Nullable
> x is @Initialized | @Initialized @NonNull | @Initialized @Nullable
> x is @UnderInitialization | @UnknownInitialization @Nullable | @UnknownInitialization @Nullable
> x is @UnknownInitialization | @UnknownInitialization @Nullable | @UnknownInitialization @Nullable

> Figure 3.7: Initialization rules for reading a @NotOnlyInitialized field f.

Similarly, Figure ‍3.8 shows under which conditions an assignment x.f = y is allowed for a @NotOnlyInitialized field f. If the receiver x is @UnderInitialization, then any y can be of any initialization state. If y is known to be fully initialized, then any receiver is allowed. All other assignments are disallowed.

> x.f = y | y is @Initialized | y is @UnderInitialization | y is @UnknownInitialization
> x is @Initialized | yes | no | no
> x is @UnderInitialization | yes | yes | yes
> x is @UnknownInitialization | yes | no | no

> Figure 3.8: Rules for deciding when an assignment x.f = y is allowed for a @NotOnlyInitialized field f.

These rules allow for the safe initialization of circular structures. For instance, consider a doubly linked list:

      class List<T> {
        @NotOnlyInitialized
        Node<T> sentinel;

        public List() {
          this.sentinel = new Node<>(this);
        }

        void insert(@Nullable T data) {
          this.sentinel.insertAfter(data);
        }

        public static void main() {
          List<Integer> l = new List<>();
          l.insert(1);
          l.insert(2);
        }
      }

      class Node<T> {
        @NotOnlyInitialized
        Node<T> prev;

        @NotOnlyInitialized
        Node<T> next;

        @NotOnlyInitialized
        List parent;

        @Nullable
        T data;

        // for sentinel construction
        Node(@UnderInitialization List parent) {
          this.parent = parent;
          this.prev = this;
          this.next = this;
        }

        // for data node construction
        Node(Node<T> prev, Node<T> next, @Nullable T data) {
          this.parent = prev.parent;
          this.prev = prev;
          this.next = next;
          this.data = data;
        }

        void insertAfter(@Nullable T data) {
          Node<T> n = new Node<>(this, this.next, data);
          this.next.prev = n;
          this.next = n;
        }
      }

### 3.8.7 How to handle warnings

#### “error: the constructor does not initialize fields: …”

Like any warning, “error: the constructor does not initialize fields: …” indicates that either your annotations are incorrect or your code is buggy. You can fix either the annotations or the code.

- Declare the field as @Nullable. Recall that if you did not write an annotation, the field defaults to @NonNull.
- Declare the field as @MonotonicNonNull. This is appropriate if the field starts out as null but is later set to a non-null value. You may then wish to use the @EnsuresNonNull annotation to indicate which methods set the field, and the @RequiresNonNull annotation to indicate which methods require the field to be non-null.
- Initialize the field in the constructor or in the field’s initializer, if the field should be initialized. (In this case, the Initialization Checker has found a bug!)

  Do not initialize the field to an arbitrary non-null value just to eliminate the warning. Doing so degrades your code: it introduces a value that will confuse other programmers, and it converts a clear NullPointerException into a more obscure error.

#### “call to … is not allowed on the given receiver”

If your code calls an instance method from a constructor, you may see a message such as the following:

    MyFile.java:123: error: call to initHelper() not allowed on the given receiver.
        initHelper();
                  ^
      found   : @UnderInitialization(com.google.Bar.class) @NonNull MyClass
      required: @Initialized @NonNull MyClass

The problem is that the current object (this) is under initialization, but the receiver formal parameter (Section ‍39.6.1) of method initHelper() is implicitly annotated as @Initialized. If initHelper() doesn’t depend on its receiver being initialized — that is, it’s OK to call x.initHelper even if x is not initialized — then you can indicate that by using @UnderInitialization or @UnknownInitialization.

    class MyClass {
      void initHelper(@UnknownInitialization MyClass this, String param1) { ... }
    }

You are likely to want to annotate initHelper() with @EnsuresNonNull as well; see Section ‍3.2.2.

You may get the “call to … is not allowed on the given receiver” error even if your constructor has already initialized all the fields. For this code:

    public class MyClass {
      @NonNull Object field;
      public MyClass() {
        field = new Object();
        helperMethod();
      }
      private void helperMethod() {
      }
    }

the Nullness Checker issues the following warning:

    MyClass.java:7: error: call to helperMethod() not allowed on the given receiver.
        helperMethod();
                    ^
      found   : @UnderInitialization(MyClass.class) @NonNull MyClass
      required: @Initialized @NonNull MyClass
    1 error

The reason is that even though the object under construction has had all the fields declared in MyClass initialized, there might be a subclass of MyClass. Thus, the receiver of helperMethod should be declared as @UnderInitialization(MyClass.class), which says that initialization has completed for all the MyClass fields but may not have been completed overall. If helperMethod had been a public method that could also be called after initialization was actually complete, then the receiver should have type @UnknownInitialization, which is the supertype of @UnknownInitialization and @UnderInitialization.

### 3.8.8 Suppressing warnings

To suppress most warnings related to partially-initialized values, use the warning suppression string “initialization”. You can write @SuppressWarnings("initialization") on a field, constructor, or class, or pass the command-line argument -AsuppressWarnings=initialization when running the Nullness Checker. (For more about suppressing warnings, see Chapter ‍33.) You will no longer get a guarantee of no null pointer exceptions, but you can still use the Nullness Checker to find most of the null-pointer problems in your code.

This suppresses warnings that are specific to the Initialization Checker, but does not suppress warnings issued by the Checker Framework itself, such as about assignments or method overriding.

It is not possible to completely disable initialization checking while retaining nullness checking. That is because of an implementation detail of the Nullness and Initialization Checkers: they are actually the same checker, rather than being two separate checkers that are aggregated together.

### 3.8.9 More details about initialization checking

##### Use of method annotations

A method with a non-initialized receiver may assume that a few fields (but not all of them) are non-null, and it sometimes sets some more fields to non-null values. To express these concepts, use the @RequiresNonNull, @EnsuresNonNull, and @EnsuresNonNullIf method annotations; see Section ‍3.2.2.

##### Source of the type system

The type system enforced by the Initialization Checker is based on “Freedom Before Commitment” ‍[SM11]. Our implementation changes its initialization modifiers (“committed”, “free”, and “unclassified”) to “initialized”, “unknown initialization”, and “under initialization”. Our implementation also has several enhancements. For example, it supports partial initialization (the argument to the @UnknownInitialization and @UnderInitialization annotations). The benefit (in terms of reduced false positive initialization warnings) from supporting partial initialization is greater than the benefit from adopting the Freedom Before Commitment system.

# Chapter ‍4 Map Key Checker

The Map Key Checker tracks which values are keys for which maps. If variable v has type @KeyFor("m")..., then the value of v is a key in Map m. That is, the expression m.containsKey(v) evaluates to true.

Section ‍3.2.4 describes how @KeyFor annotations enable the Nullness Checker (Chapter ‍3) to treat calls to Map.get more precisely by refining its result to @NonNull in some cases.

## 4.1 Invoking the Map Key Checker

You will not typically run the Map Key Checker. It is automatically run by other checkers, in particular the Nullness Checker.

You can unsoundly suppress warnings related to map keys with @SuppressWarnings("keyfor"), or everywhere by using command-line option -AsuppressWarnings=keyfor; see Chapter ‍33.

The command-line argument -AassumeKeyFor makes the Map Key Checker unsoundly assume that the argument to Map.get is a key for the receiver map. This is like declaring the Map.get method as V get(Object key) rather than @Nullable V get(Object key). (Just changing the JDK declaration would not work, however, because the Map Key Checker has special-case logic for Map.get. This is different than suppressing warnings, because it changes a method’s return type. This is not the same as assuming that the return value is @NonNull, because the map’s value type might be @Nullable, as in Map<String, @Nullable Integer>.

## 4.2 Map key annotations

These qualifiers are part of the Map Key type system:

@KeyFor(String[] maps)
  indicates that the value assigned to the annotated variable is a key for at least the given maps.
@UnknownKeyFor
  is used internally by the type system but should never be written by a programmer. It indicates that the value assigned to the annotated variable is not known to be a key for any map. It is the default type qualifier.
@KeyForBottom
  is used internally by the type system but should never be written by a programmer. There are no values of this type (not even null).

The following method annotations can be used to establish a method postcondition that ensures that a certain expression is a key for a map:

@EnsuresKeyFor(String[] value, String[] map)
  When the method with this annotation returns, the expression (or all the expressions) given in the value element is a key for the given maps. More precisely, the expression has the @KeyFor qualifier with the value arguments taken from the targetValue element of this annotation.
@EnsuresKeyForIf(String[] expression, boolean result, String[] map)
  If the method with this annotation returns the given boolean value, then the given expression (or all the given expressions) is a key for the given maps.

> Figure 4.1: The subtyping relationship of the Map Key Checker’s qualifiers. @KeyFor(A) is a supertype of @KeyFor(B) if and only if A is a subset of B. Qualifiers in gray are used internally by the type system but should never be written by a programmer.

## 4.3 Default annotations

The qualifier for the type of the null literal is @UnknownKeyFor. If null were @KeyForBottom, that would mean that null is guaranteed to be a key for every map (which is not necessarily true).

### 4.3.1 Default for lower bounds

Lower bounds are defaulted to @UnknownKeyFor. However, in java.* packages, the default for lower bounds is @KeyForBottom.

It is challenging to choose a default for lower bounds of type variables and wildcards.

Here is a comparison of two choices for lower bounds:

@KeyForBottom default | @UnknownKeyFor default (current choice)
class MyClass1<@UnknownKeyFor T> { | class MyClass1<T> {
‍ ‍T var = null; // OK | ‍ ‍T var = null; // OK
class MyClass2<T> { |
‍ ‍@UnknownKeyFor T var = null; // OK |
class MyClass3<T> { |
‍ ‍T var = null; // ERROR |
| class MySet1<T> implements Set<T> { }
| MySet1<@KeyFor("m") String> s1; // ERROR
class Set<E> { } | class Set<@KeyForBottom E> { }
class MySet2<T> implements Set<T> { } | class MySet2<@KeyForBottom T> implements Set<T> { }
MySet2<@KeyFor("m") String> s2; // OK ‍ ‍ ‍ | MySet2<@KeyFor("m") String> s2; // OK

If lower bounds are defaulted to @KeyForBottom (which is not currently the case), then whenever null is assigned to a variable whose type is a type variable, programmers must write an @UnknownKeyFor annotation on either the type variable declaration or on variable declarations, as shown in MyClass1 and MyClass2. A disadvantage of this default is that the Map Key checker may issue warnings in code that has nothing to do with map keys, and in which no map key annotations are used.

If lower bounds are defaulted to @UnknownKeyFor (which is currently the case), then whenever a client might use a @KeyFor type argument, programmers must write a @KeyForBottom annotation on the type parameter, as in MySet2 (and Set).

### 4.3.2 Diagnosing the need for explicit @KeyFor on lower bounds

Under the current defaulting (lower bounds default to @UnknownKeyFor), suppose you write this code:

    public class Graph<N> {
      Map<N, Integer> nodes = new HashMap<>();
    }

    class Client {
      @Nullable Graph<@KeyFor("g.nodes") String> g;
    }

The Nullness Checker issues this error message:

    Graph.java:14: error: [type.argument] incompatible types in type argument.
      @Nullable Graph<@KeyFor("g.nodes") String> g;
                      ^
      found   : @KeyFor("this.g.nodes") String
      required: [extends @UnknownKeyFor Object super @UnknownKeyFor null]

Note that the upper and lower bounds are both @UnknownKeyFor. You can make the code type-check by writing a lower bound, which is written before the type variable name (Section ‍31.1.2):

    public class Graph<@KeyForBottom N> {
    ...

## 4.4 Examples

The Map Key Checker keeps track of which variables reference keys to which maps. A variable annotated with @KeyFor(mapSet) can only contain a value that is a key for all the maps in mapSet. For example:

    Map<String,Date> m, n;
    @KeyFor("m") String km;
    @KeyFor("n") String kn;
    @KeyFor({"m", "n"}) String kmn;
    km = kmn;   // OK - a key for maps m and n is also a key for map m
    km = kn;    // error: a key for map n is not necessarily a key for map m

As with any annotation, use of the @KeyFor annotation may force you to slightly refactor your code. For example, this would be illegal:

    Map<String,Object> m;
    Collection<@KeyFor("m") String> coll;
    coll.add(x);   // error: element type is @KeyFor("m") String, but x does not have that type
    m.put(x, ...);

The example type-checks if you reorder the two calls:

    Map<String,Object> m;
    Collection<@KeyFor("m") String> coll;
    m.put(x, ...);    // after this statement, x has type @KeyFor("m") String
    coll.add(x);      // OK

## 4.5 Local inference of @KeyFor annotations

Within a method body, you usually do not have to write @KeyFor explicitly (except sometimes on type arguments), because the checker infers it based on usage patterns. When the Map Key Checker encounters a run-time check for map keys, such as “if (m.containsKey(k)) ...”, then the Map Key Checker refines the type of k to @KeyFor("m") within the scope of the test (or until k is side-effected within that scope). The Map Key Checker also infers @KeyFor annotations based on iteration over a map’s key set or calls to put or containsKey. For more details about type refinement, see Section ‍32.7.

Suppose we have these declarations:

    Map<String,Date> m = new Map<>();
    String k = "key";
    @KeyFor("m") String km;

Ordinarily, the following assignment does not type-check:

    km = k;   // Error since k is not known to be a key for map m.

The following examples show cases where the Map Key Checker infers a @KeyFor annotation for variable k based on usage patterns, enabling the km = k assignment to type-check.

    m.put(k, ...);
    // At this point, the type of k is refined to @KeyFor("m") String.
    km = k;   // OK

    if (m.containsKey(k)) {
        // At this point, the type of k is refined to @KeyFor("m") String.
        km = k;   // OK
        ...
    } else {
        km = k;   // Error since k is not known to be a key for map m.
        ...
    }

The following example shows a case where the Map Key Checker resets its assumption about the type of a field used as a key because that field may have been side-effected.

    class MyClass {
        private Map<String,Object> m;
        private String k;   // The type of k defaults to @UnknownKeyFor String
        private @KeyFor("m") String km;

        public void myMethod() {
            if (m.containsKey(k)) {
                km = k;   // OK: the type of k is refined to @KeyFor("m") String

                sideEffectFreeMethod();
                km = k;   // OK: the type of k is not affected by the method call
                          // and remains @KeyFor("m") String

                otherMethod();
                km = k;   // error: At this point, the type of k is once again
                          // @UnknownKeyFor String, because otherMethod might have
                          // side-effected k such that it is no longer a key for map m.
            }
        }

        @SideEffectFree
        private void sideEffectFreeMethod() { ... }

        private void otherMethod() { ... }
    }

# Chapter ‍5 Optional Checker for possibly-present data

Use of the Optional Checker guarantees that your program will not suffer a NoSuchElementException when calling methods on an expression of Optional type. The Optional Checker also enforces Stuart Marks’s style guidelines (see below).

Java 8 introduced the Optional class, a container that is either empty or contains a non-null value.

Using Optional is intended to help programmers remember to check whether data is present or not. However, Optional itself is prone to misuse. The article Nothing is better than the Optional type gives reasons to use regular nullable references rather than Optional. However, if you do use Optional, then the Optional Checker will help you avoid Optional’s pitfalls. Most notably, the Optional Checker guarantees your code will not suffer a NoSuchElementException due to use of an empty Optional.

Stuart Marks gave 7 rules to avoid problems with Optional:

1. Never, ever, use null for an Optional variable or return value.
2. Never use Optional.get() unless you can prove that the Optional is present.
3. Prefer alternative APIs over Optional.isPresent() and Optional.get().
4. It’s generally a bad idea to create an Optional for the specific purpose of chaining methods from it to get a value.
5. If an Optional chain has a nested Optional chain, or has an intermediate result of Optional, it’s probably too complex.
6. Avoid using Optional in fields, method parameters, and collections.
7. Don’t use an Optional to wrap any collection type (List, Set, Map). Instead, use an empty collection to represent the absence of values.

Rule #1 is guaranteed by the Nullness Checker (Chapter ‍3). Rules #2–#7 are guaranteed by the Optional Checker, described in this chapter. (Exception: Rule #5 is not yet implemented and will be checked by the Optional Checker in the future.)

## 5.1 How to run the Optional Checker

The standard way to run the Optional Checker is one of these command lines:

    javac -processor optional MyFile.java ...
    javac -processor org.checkerframework.checker.optional.OptionalChecker MyFile.java ...

## 5.2 Optional annotations

These qualifiers make up the Optional type system:

@MaybePresent
  The annotated Optional container may or may not contain a value. This is the default type, so programmers do not have to write it.
@Present
  The annotated Optional container definitely contains a (non-null) value.
@PolyPresent
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.

The subtyping hierarchy of the Optional Checker’s qualifiers is shown in Figure ‍5.1.

> Figure 5.1: The subtyping relationship of the Optional Checker’s qualifiers.

### 5.2.1 Optional method annotations

The Optional Checker supports several annotations that specify method behavior. These are declaration annotations, not type annotations: they apply to the method itself rather than to some particular type.

@RequiresPresent
  indicates a method precondition: The annotated method expects the specified expressions to be a present Optional when this method is invoked. @RequiresPresent is a useful annotation for a method that requires a @MaybePresent field to be @Present.
@EnsuresPresent
  indicates a method postcondition. The successful return (i.e., a non-exceptional return) of the annotated method results in the given Optional expression being present. See the Javadoc for examples of its use.
@EnsuresPresentIf
  indicates a method postcondition. With @EnsuresPresent, the given Optional expression is present after the method returns. With @EnsuresPresentIf, if the annotated method returns the given boolean value (true or false), then the given Optional expression is present. See the Javadoc for examples of their use.

## 5.3 What the Optional Checker guarantees

The Optional Checker guarantees that your code will not throw a NoSuchElementException exception due to use of an absent Optional where a present Optional is needed. More specifically, the Optional Checker will issue an error if you call get or orElseThrow() on a @MaybePresent Optional receiver, because each of these methods throws a NoSuchElementException exception if the receiver is a possibly-absent Optional.

By contrast, the Optional Checker does not issue an error if you call orElseThrow(Supplier) with a possibly-absent Optional. That method call does not throw NoSuchElementException. The Optional Checker assumes that the programmer has mechanisms in place to handle whatever exception it throws. If you wish for the Optional Checker to warn about calling orElseThrow(Supplier) on a possibly-absent Optional, then you can use a stub file (Section ‍35.5) to annotate its receiver as @Present.

The Optional Checker does not check nullness properties, such as requiring that the argument to of is non-null or guaranteeing that the result of get is non-null. To obtain such a guarantee, run both the Optional Checker and the Nullness Checker (Chapter ‍3).

As with any checker, the guarantee is subject to certain limitations (see Section ‍2.3).

## 5.4 Suppressing optional warnings

It is often best to change the code or annotations when the Optional Checker reports a warning. Alternatively, you might choose to suppress the warning. This does not change the code but prevents the warning from being presented to you.

The Checker Framework supplies several ways to suppress warnings. The @SuppressWarnings("optional") annotation is specific to warnings raised by the Optional Checker. See Chapter ‍33 for additional usages. An example use is

        // might return a possibly-empty Optional
        Optional<T> wrapWithOptional(...) { ... }

        void myMethod() {
          @SuppressWarnings("optional") // with argument x, wrapWithOptional always returns a present Optional
          @Present Optional<T> optX = wrapWithOptional(x);
        }

The Optional Checker also permits the use of method calls and assertions to suppress warnings; see immediately below.

### 5.4.1 Suppressing warnings with assertions and method calls

Occasionally, it is inconvenient or verbose to use the @SuppressWarnings annotation. For example, Java does not permit annotations such as @SuppressWarnings to appear on statements, expressions, static initializers, etc. Here are three ways to suppress a warning in such cases:

- Create a local variable to hold a subexpression, and suppress a warning on the local variable declaration.
- Use the @AssumeAssertion string in an assert message (see Section ‍33.2).
- Write a call to the OptionalUtil.castPresent method.

The rest of this section discusses the castPresent method. It is useful if you wish to suppress a warning within an expression.

The Optional Checker considers both the return value, and also the argument, to be an instance of a present Optional after the castPresent method call. The Optional Checker issues no warnings in any of the following code:

      // One way to use castPresent as a cast:
      @Present Optional<String> optString = castPresent(possiblyEmpty1);

      // Another way to use castPresent as a cast:
      castPresent(possiblyEmpty2).toString();

      // It is possible, but not recommmended, to use castPresent as a statement:
      // (It would be better to write an assert statement with @AssumeAssertion
      // in its message, instead.)
      castPresent(possiblyEmpty3);
      possiblyEmpty3.toString();

The castPresent method throws AssertionError if Java assertions are enabled and the argument is an empty Optional. However, it is not intended for general defensive programming; see Section ‍33.2.1.

To use the castPresent method, the checker-util.jar file must be on the classpath at run time.

# Chapter ‍6 Interning Checker

If the Interning Checker issues no errors for a given program, then all reference equality tests (i.e., all uses of “==”) are proper; that is, == is not misused where equals() should have been used instead.

Interning is a design pattern in which the same object is used whenever two different objects would be considered equal. Interning is also known as canonicalization or hash-consing, and it is related to the flyweight design pattern. Interning has two benefits: it can save memory, and it can speed up testing for equality by permitting use of ==.

The Interning Checker prevents two types of problems in your code. First, it prevents using == on non-interned values, which can result in subtle bugs. For example:

      Integer x = new Integer(22);
      Integer y = new Integer(22);
      System.out.println(x == y);  // prints false!

Second, the Interning Checker helps to prevent performance problems that result from failure to use interning. (See Section ‍2.3 for caveats to the checker’s guarantees.)

Interning is such an important design pattern that Java builds it in for these types: String, Boolean, Byte, Character, Integer, Short. Every string literal in the program is guaranteed to be interned (JLS §3.10.5), and the String.intern() method performs interning for strings that are computed at run time. The valueOf methods in wrapper classes always (Boolean, Byte) or sometimes (Character, Integer, Short) return an interned result (JLS §5.1.7). Users can also write their own interning methods for other types.

It is a proper optimization to use ==, rather than equals(), whenever the comparison is guaranteed to produce the same result — that is, whenever the comparison is never provided with two different objects for which equals() would return true. Here are three reasons that this property could hold:

1. Interning. A factory method ensures that, globally, no two different interned objects are equals() to one another. (For some classes, every instance is interned; however, in other cases it is possible for two objects of the class to be equals() to one another, even if one of them is interned.) Interned objects should always be immutable.
2. Global control flow. The program’s control flow is such that the constructor for class C is called a limited number of times, and with specific values that ensure the results are not equals() to one another. Objects of class C can always be compared with ==. Such objects may be mutable or immutable.
3. Local control flow. Even though not all objects of the given type may be compared with ==, the specific objects that can reach a given comparison may be.
   - When searching for an element (say, in a collection), == may be appropriate.
   - Some routines return either their argument, or a modified version of it. Your code might compare s == s.toLowerCase() to see whether a string contained any upper-case characters.

To eliminate Interning Checker errors, you will need to annotate the declarations of any expression used as an argument to ==. Thus, the Interning Checker could also have been called the Reference Equality Checker.

To run the Interning Checker, supply the -processor org.checkerframework.checker.interning.InterningChecker command-line option to javac. For examples, see Section ‍6.5.

## 6.1 Interning annotations

### 6.1.1 Interning qualifiers

These qualifiers are part of the Interning type system:

@Interned
  indicates a type that includes only interned values (no non-interned values).
@InternedDistinct
  indicates that the value is not equals() to any other Java value. This is a stronger (more restrictive) property than @Interned, but is a weaker property than writing @Interned on a class declaration. For details, see Section ‍6.3.3.
@UnknownInterned
  indicates a type whose values might or might not be interned. It is used internally by the type system and is not written by programmers.
@PolyInterned
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.

### 6.1.2 Interning method and class annotations

@UsesObjectEquals
  is a class annotation (not a type annotation) that indicates that this class’s equals method is the same as that of Object. Since Object.equals uses reference equality, this means that for such a class, == and equals are equivalent, and so the Interning Checker does not issue errors or warnings for either one.

  Two ways to satisfy this annotation are: (1) neither this class nor any of its superclasses overrides the equals method, or (2) this class defines equals with body return this == o;.

@InternMethod
  is a method declaration annotation that indicates that this method returns an interned object and may be invoked on an uninterned object. See Section ‍6.3.1 for more details.
@EqualsMethod
  is a method declaration annotation that indicates that this method has a specification like equals(). The Interning Checker permits use of this == arg within the body.
@CompareToMethod
  is a method declaration annotation that indicates that this method has a specification like compareTo(). The Interning Checker permits use of if (arg1 == arg2) { return 0; } within the body.
@FindDistinct
  is a formal parameter declaration annotation that indicates that this method uses == to perform comparisons against the annotated formal parameter. A common reason is that the method searches for the formal parameter in some data structure, using ==. Any value may be passed to the method.

## 6.2 Annotating your code with @Interned

> Figure 6.1: Type hierarchy for the Interning type system.

In order to perform checking, you must annotate your code with the @Interned type annotation. A type annotated with @Interned contains the canonical representation of an object:

                String s1 = ...;  // type is (uninterned) "String"
      @Interned String s2 = ...;  // type is "@Interned String"

The Interning Checker ensures that only interned values can be assigned to s2.

## 6.3 Interned classes

An interned annotation on a class declaration indicates that all objects of a type are interned except for newly created objects. That means that all uses of such types are @Interned by default and the type @UnknownInterned MyClass is an invalid type.

An exception is constructor results. Constructor results and this within the body of the constructor are @UnknownInterned by default. Although @UnknownInterned InternClass is not a legal type, no “type.invalid” error is issued at constructor declarations. Instead, an “interned.object.creation” error is issued at the invocation of the constructor. The user should inspect this location and suppress the warning if the newly created object is interned.

For example:

    @Interned class InternedClass {
      @UnknownInterned InternedClass() {
        // error, "this" is @UnknownInterned.
        @Interned InternedClass that = this;
      }

      @SuppressWarnings("intern") // Only creation of an InternedClass object.
      static final InternedClass ONE = new InternedClass();
    }

### 6.3.1 The intern() methods

Some interned classes use an intern() method to look up the interned version of the object. These methods must be annotated with the declaration annotation @InternMethod. This allows the checker to verify that a newly created object is immediately interned and therefore not issue an interned object creation error.

    new InternedClass().intern() // no error

Because an intern method is expected to be called on uninterned objects, the type of this in intern is implicitly @UnknownInterned. This will cause an error if this is used someplace where an interned object is expected. Some of these warnings will be false positives that should be suppressed by the user.

    @InternMethod
    public InternedClass intern() {
      // Type of "this" inside an @InternMethod is @UnknownInterned
      @Interned InternedClass that = this; // error

      if (!pool.contains(this)) {
        @SuppressWarnings("interning:assignment")
        @Interned InternedClass internedThis = this;
        pool.add(internedThis);
      }
      return pool.get(this);
    }

Some interned classes do not use an intern method to ensure that every object of that class is interned. For these classes, the user will have to manually inspect every constructor invocation and suppress the “interned.object.creation” error.

If every invocation of a constructor is guaranteed to be interned, then the user should annotate the constructor result with @Interned and suppress a warning at the constructor.

    @Interned class AnotherInternedClass {
      // manually verified that all constructor invocations used such that all
      // new objects are interned
      @SuppressWarnings("super.invocation")
      @Interned AnotherInternedClass() {}
    }

### 6.3.2 Default qualifiers and qualifiers for literals

The Interning Checker adds qualifiers to unannotated types, reducing the number of annotations that must appear in your code (see Section ‍32.4).

For a complete description of all defaulting rules for interning qualifiers, see the Javadoc for InterningAnnotatedTypeFactory.

### 6.3.3 InternedDistinct: values not equals() to any other value

The @InternedDistinct annotation represents values that are not equals() to any other value. Suppose expression e has type @InternedDistinct. Then e.equals(x) == (e == x). Therefore, it is legal to use == whenever at least one of the operands has type @InternedDistinct.

@InternedDistinct is stronger (more restrictive) than @Interned. For example, consider these variables:

    @Interned String i = "22";
              String s = new Integer(22).toString();

The variable i is not @InternedDistinct because i.equals(s) is true.

@InternedDistinct is not as restrictive as stating that all objects of a given Java type are interned.

The @InternedDistinct annotation is rarely used, because it arises from coding paradigms that are tricky to reason about. One use is on static fields that hold canonical values of a type. Given this declaration:

    class MyType {
      final static @InternedDistinct MyType SPECIAL = new MyType(...);
      ...
    }

it would be legal to write myValue == MyType.SPECIAL rather than myValue.equals(MyType.SPECIAL).

The @InternedDistinct is trusted (not verified), because it would be too complex to analyze the equals() method to ensure that no other value is equals() to a @InternedDistinct value. You will need to manually verify that it is only written in locations where its contract is satisfied. For example, here is one set of guidelines that you could check manually:

- The constructor is private.
- The factory method (whose return type is annotated with @InternedDistinct returns the canonical version for certain values.
- The class is final, so that subclasses cannot violate these properties.

The result of a constructor call (that is, new MyClass(...)) is not necessarily @InternedDistinct because the equals() method of MyClass might return true for two distinct objects.

## 6.4 What the Interning Checker checks

Objects of an @Interned type may be safely compared using the “==” operator.

The checker issues an error in two cases:

1. When a reference (in)equality operator (“==” or “!=”) has an operand of non-@Interned type. As a special case, the operation is permitted if either argument is of @InternedDistinct type
2. When a non-@Interned type is used where an @Interned type is expected.

This example shows both sorts of problems:

                        Date  date;
              @Interned Date idate;
      @InternedDistinct Date ddate;
      ...
      if (date == idate) ...  // error: reference equality test is unsafe
      idate = date;           // error: idate's referent might no longer be interned
      ddate = idate;          // error: idate's referent might be equals() to some other value

The Interning Checker’s warnings look like

    MyFile.java:716: error: [interning:not.interned] attempting to use a non-@Interned comparison operand
            if (date == idate)
                ^

To resolve a not.interned error, you should change the argument that is passed to ==, or use .equals() instead of ==, or suppress the warning.

The checker also issues a warning when .equals is used where == could be safely used. You can disable this behavior via the javac -Alint=-dotequals command-line option.

For a complete description of all checks performed by the checker, see the Javadoc for InterningVisitor.

To restrict which types the checker should type-check, pass a canonical name (fully-qualified name) using the -Acheckclass option. For example, to find only the interning errors related to uses of String, you can pass -Acheckclass=java.lang.String. The Interning Checker always checks all subclasses and superclasses of the given class.

### 6.4.1 Imprecision (false positive warnings) of the Interning Checker

The Interning Checker conservatively assumes that the Character, Integer, and Short valueOf methods return a non-interned value. In fact, these methods sometimes return an interned value and sometimes a non-interned value, depending on the run-time argument (JLS §5.1.7). If you know that the run-time argument to valueOf implies that the result is interned, then you will need to suppress an error. (The Interning Checker should make use of the Value Checker to estimate the upper and lower bounds on char, int, and short values so that it can more precisely determine whether the result of a given valueOf call is interned.)

## 6.5 Examples

To try the Interning Checker on a source file that uses the @Interned qualifier, use the following command:

      javac -processor org.checkerframework.checker.interning.InterningChecker docs/examples/InterningExample.java

Compilation will complete without errors or warnings.

To see the checker warn about incorrect usage of annotations, use the following command:

      javac -processor org.checkerframework.checker.interning.InterningChecker docs/examples/InterningExampleWithWarnings.java

The compiler will issue an error regarding violation of the semantics of @Interned.

The Daikon invariant detector (https://plse.cs.washington.edu/daikon/) is also annotated with @Interned. From directory java/, run make check-interning.

The paper “Building and using pluggable type-checkers” ‍[DDE+11] (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-icse2011.pdf) describes case studies in which the Interning Checker found previously-unknown errors in real software.

## 6.6 Other interning annotations

The Checker Framework’s interning annotations are similar to annotations used elsewhere.

If your code is already annotated with a different interning annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Interning Checker, as described in Figure ‍6.2. If the other annotation is a declaration annotation, it may be moved; see Section ‍39.6.10.

> ‍com.sun.istack.internal.Interned ‍
> | ⇒  ‍org.checkerframework.checker.interning.qual.Interned ‍

> Figure 6.2: Correspondence between other interning annotations and the Checker Framework’s annotations.

# Chapter ‍7 Called Methods Checker for the builder pattern and more

The Called Methods Checker tracks the names of methods that have definitely been called on an object. This checker is useful for checking any property of the form “call method A before method B”. For the purpose of this checker, a method has “definitely been called” if it is invoked: a method that might never return or that might throw an exception has definitely been called on every path after the call, including exceptional paths. The checker also assumes that the program is free of null-pointer dereferences. You can verify that all pointer dereferences are safe by running the Nullness Checker (Section ‍3).

The Called Methods Checker provides built-in support for one such property: that clients of the builder pattern for object construction always provide all required arguments before calling build(). The builder pattern is a flexible and readable way to construct objects, but it is error-prone. Failing to provide a required argument causes a run-time error that manifests during testing or in the field, instead of at compile time as for regular Java constructors. The Called Methods Checker verifies at compile time that your code correctly uses the builder pattern, never omitting a required argument. The Called Methods Checker has built-in support for Lombok (see the caveats about Lombok in Section ‍7.2) and AutoValue.

You can verify other builders, or verify other properties of the form “foo() must be called before bar()”, by writing method specifications. Section ‍7.5 describes another example related to a security property.

If the checker issues no warnings, then you have a guarantee that your code supplies all the required information to the builder. The checker might yield a false positive warning when your code is too tricky for it to verify. Please submit an issue if you discover this.

## 7.1 How to run the Called Methods Checker

    javac -processor calledmethods MyFile.java ...
    javac -processor org.checkerframework.checker.calledmethods.CalledMethodsChecker MyFile.java ...

The Called Methods Checker supports the following optional command-line arguments:

- The -ACalledMethodsChecker_disableBuilderFrameworkSupports option disables automatic annotation inference for builder frameworks. Section ‍7.4 describes its syntax. Supply this if you are uninterested in errors in the use of builders, but are using the Called Methods Checker to detect errors in other types of code.
- The -ACalledMethodsChecker_disableReturnsReceiver option disables the Returns Receiver Checker (Chapter ‍24), which ordinarily runs as a subchecker of the Called Methods Checker. If the code being checked does not use fluent APIs, then you can supply this option and the Called Methods Checker will run much faster. This option is the default when the Called Methods Checker is run as part of the Resource Leak Checker (Chapter ‍8).
- The -ACalledMethodsChecker_useValueChecker option improves precision when analyzing code that uses the AWS SDK’s DescribeImageRequest API. See Section ‍7.5.

## 7.2 For Lombok users

The Called Methods Checker supports projects that use Lombok via the io.freefair.lombok Gradle plugin automatically. However, note that the checker’s error messages refer to Lombok’s output, which is a variant of your source code that appears in a delombok directory. To fix issues, you should edit your original source code, not the files in the checker’s error messages.

If you use Lombok with a build system other than Gradle, you must configure it to do two tasks. If either of these is not done, the checker will not issue any errors on Lombok code.

- set Lombok configuration option lombok.addLombokGeneratedAnnotation = true
- delombok the code before passing it to the checker

## 7.3 Specifying your code

The Called Methods Checker reads method specifications (contracts) that state what a method requires when it is called. It warns if method arguments do not satisfy the method’s specification.

If you use AutoValue or Lombok, most specifications are automatically inferred by the Called Methods Checker, from field annotations such as @Nullable and field types such as Optional. Section ‍7.4 gives defaulting rules for Lombok and AutoValue.

> Figure 7.1: The type hierarchy for the Called Methods type system, for an object with two methods: a() and b(). Types displayed in gray should rarely be written by the programmer.

In some cases, you may need to specify your code. You do so by writing one of the following type annotations (Figure ‍7.1):

@CalledMethods(String[] methodNames)
  The annotated type represents values on which all the given method were definitely called. (Other methods might also have been called.) @CalledMethods(), with no arguments, is the default annotation.

  Suppose that the method build is annotated as

        class MyObjectBuilder {
          MyObject build(@CalledMethods({"setX", "setY"}) MyObjectBuilder this) { ... }
        }

  Then the receiver for any call to build() must have had setX() and setY() called on it.

  A typical case for Lombok users for manually writing this annotation is when performing extra builder input validation. Performing validation can be done through subclassing generated builders and overriding the generated build() method as follows.

          class MyObjectSubBuilder extends MyObjectBuilder {
            @Override
            MyObject build(@CalledMethods({"setX", "setY"}) MyObjectSubBuilder this) {
              MyObject o = super.build();
              if ((o.getX() == null) != (o.getY() == null)) {
                throw new IllegalArgumentException("Nullness for x and y should be equal");
              }
              return o;
            }
          }

@CalledMethodsPredicate(String expression)
  The boolean expression specifies the required method calls. The string is a boolean expression composed of method names, disjunction (||), conjunction (&&), not (!), and parentheses.

  For example, the annotation @CalledMethodsPredicate("x && y || z") on a type represents objects such that either both the x() and y() methods have been called on the object, or the z() method has been called on the object.

  A note on the not operator (!): the annotation @CalledMethodsPredicate("!m") means “it is not true m was definitely called”; equivalently “there is some path on which m was not called”. The annotation @CalledMethodsPredicate("!m") does not mean “m was not called”.

  The Called Methods Checker does not have a way of expressing that a method must never be called. You can do unsound bug-finding for such a property by using the ! operator. The Called Methods Checker will detect if the method was always called, but will silently approve the code if the method is called on some but not all paths.

@This
  @This may only be written on a method return type, and means that the method returns its receiver. This is helpful when type-checking fluent APIs. This annotation is defined by the Returns Receiver Checker (Chapter ‍24), but is particularly useful for the Called Methods Checker because many builders are fluent APIs.

  For Lombok users it is important to (manually) add @This to any custom method that calls any other generated builder method. For instance, in the following example, it is important to annotate setXMod42() with @This, since the added method calls setX() (which returns the current builder instance).

            class MyObjectBuilder {

              @This
              MyObjectBuilder setXMod42(int x) {
                return setX(x % 42);
              }
            }

@CalledMethodsBottom
  The bottom type for the Called Methods hierarchy. Conceptually, this annotation means that all possible methods have been called on the object. Programmers should rarely, if ever, need to write this annotation—write an appropriate @CalledMethods annotation instead. The type of null is @CalledMethodsBottom.

There are also method annotations:

@EnsuresCalledMethods
  This declaration annotation specifies a post-condition on a method, indicating the methods it guarantees to be called. This annotation is repeatable, meaning that you can write multiple copies of it (with different arguments) on the same method, and the checker will check all of them.

  For example, this specification:

          @EnsuresCalledMethods(value = "#1", methods = {"x", "y"})
          void m(Param p) { ... }

  guarantees that p.x() and p.y() will always be called before m returns. The body of m must satisfy that property, and clients of m can depend on the property.

  Sometimes, you need to provide information to enable the Called Methods Checker to verify the property. Consider this example:

        @EnsuresCalledMethods(value="#1", methods="close")
        public void closeSocket(Socket sock) throws IOException {
            sock.close();
            m();
        }

  If m() might have side-effects (i.e., it is not annotated as @SideEffectFree or @Pure), then the Called Methods Checker issues an error because it cannot make any assumptions about the call to m(), and therefore assumes the worst: that all information it knows about in-scope variables (including that close() was called on sock) is stale and must be discarded. There are two possible fixes:

  - add a @SideEffectFree or @Pure annotation to m(), if m() is in fact side-effect free or pure; or
  - re-order the calls to sock.close() and m() so that the call to sock.close() appears last in closeSocket().
@EnsuresCalledMethodsIf
  This declaration annotation specifies a post-condition on a method, indicating the methods it guarantees to be called if it returns a given result.

  For example, this specification:

          @EnsuresCalledMethodsIf(expression = "#1", methods = {"x", "y"}, result=true)
          boolean m(Param p) { ... }

  guarantees that p.x() and p.y() will always be called if m returns true. The body of m must satisfy that property, and clients of m can depend on the property.

@EnsuresCalledMethodsVarargs
  This version of @EnsuresCalledMethods always applies to the varargs parameter of the annotated method. It has only one argument, which is the list of methods that are guaranteed to be called on the varargs parameter’s elements before the method returns. This annotation currently cannot be verified, and a ensuresvarargs.unverified error is always issued when it is used. When annotating a method as @EnsuresCalledMethodsVarargs, you should verify that the named methods are actually called on every element of the varargs parameter via some other method (such as manual inspection) and then suppress the warning.
@EnsuresCalledMethodsOnException
  This declaration annotation specifies a post-condition on a method, indicating the methods it guarantees to call when it throws an exception. Like most post-condition annotations, the other Called Methods post-condition annotations (@EnsuresCalledMethods, @EnsuresCalledMethodsIf, and @EnsuresCalledMethodsVarargs) only specify behavior for normal returns. The annotation @EnsuresCalledMethodsOnException allows you to write stronger specifications indicating that a method always calls the given methods:

          @EnsuresCalledMethods(expression = "#1", methods = {"close"})
          @EnsuresCalledMethodsOnException(expression = "#1", methods = {"close"})
          void closeIfNonNull(Closeable p) {
            if (p != null) {
              p.close();
            }
          }

  @EnsuresCalledMethodsOnException can also be used together with the Resource Leak Checker’s @Owning annotation. See ‍8.4.1.

@RequiresCalledMethods
  This declaration annotation specifies a pre-condition on a method, indicating that the expressions in its value argument must have called-methods types that include all the methods named in its methods argument. If the expression is a parameter of the annotated method, you should use an @CalledMethods annotation on the parameter instead.

## 7.4 Default handling for Lombok and AutoValue

This section explains how the Called Methods Checker infers types for code that uses the Lombok and AutoValue frameworks. Most readers can skip these details.

You can disable the builder framework support by specifying them in a comma-separated lowercase list to the command-line flag disableBuilderFrameworkSupports. For example, to disable both Lombok and AutoValue support, use:
-ACalledMethodsChecker_disableBuilderFrameworkSupports=autovalue,lombok

The Called Methods Checker automatically assumes default annotations for code that uses builders generated by Lombok and AutoValue. There are three places annotations are usually assumed:

- A @CalledMethods annotation is placed on the receiver of the build() method, indicating the setter methods that must be invoked on the builder before calling build(). For Lombok, this annotation’s argument is the set of @lombok.NonNull fields that do not have default values. For AutoValue, it is the set of fields that are not @Nullable, Optional, or a Guava Immutable Collection.
- The return type of a toBuilder() method (for example, if the toBuilder = true option is passed to Lombok’s @Builder annotation) is annotated with the same @CalledMethods annotation as the receiver of build(), using the same rules as above.
- A @This annotation is placed on the return type of each setter in the builder’s implementation.

If your program directly defines any of these methods (for example, by adding your own setters to a Lombok builder), you may need to write the annotations manually.

Minor notes/caveats on these rules:

- Lombok fields annotated with @Singular will be treated as defaulted (i.e., not required), because Lombok will set them to empty collections if the corresponding setter is not called.
- If you manually provide defaults to a Lombok builder (for example, by defining the builder yourself and assigning a default value to the builder’s field), the checker will treat that field as defaulted most of the time. In particular, it will not treat it as defaulted if it is defined in bytecode rather than in source code.

## 7.5 Using the Called Methods Checker for properties unrelated to builders

The Called Methods Checker can be used to verify any property of the form “always call A before B”, even if the property is unrelated to builders.

For example, consider the AWS EC2 describeImages API, which clients use during the process of initializing a new cloud instance. CVE-2018-15869 describes how an improperly-configured request to this API can make the requesting client vulnerable to a “machine-image sniping” attack that would allow a malicious third-party to control the operating system image used to initialize the machine. To prevent this attack, clients must specify some trusted source for the image by calling the withOwners or withImageIds methods on the request prior to sending it to AWS. Using a stub file for the describeImages API (DescribeImages.astub), the Called Methods Checker can prove that a client is not vulnerable to such an attack.

To improve precision, you can specify the -ACalledMethodsChecker_useValueChecker command-line option, which instructs the checker to treat provably-safe calls to the withFilters method of a DescribeImagesRequest as equivalent to the withOwners or withImageIds methods.

## 7.6 More information

The paper “Verifying Object Construction” ‍[KRS+20] (ICSE 2020, https://homes.cs.washington.edu/~mernst/pubs/object-construction-icse2020-abstract.html) gives more information about the Called Methods Checker, such as theoretical underpinnings and results of experiments. (The paper uses an earlier name, “Object Construction Checker”.)

# Chapter ‍8 Resource Leak Checker for must-call obligations

The Resource Leak Checker guarantees that every resource is closed rather than leaked. Examples of resources are a channel, executor, ExecutionControl, file, FileLock, Formatter, reader, Scanner, socket, stream, writer, etc.

More precisely, the Resource Leak Checker guarantees that the program fulfills every object’s must-call obligations before the object is de-allocated.

A resource leak occurs when a program does not explicitly dispose of some finite underlying resource, such as a socket, file descriptor, or database connection. To dispose of the resource, the program should call some method on an object. (De-allocating or garbage-collecting the object is not sufficient.) For example, the program must call close() on every object that implements the interface java.io.Closeable.

The Resource Leak Checker can check any property of the form “the programmer must call each method in a set of methods M at least once on object O before O is de-allocated”. For resource leaks, by default M is the set containing close() and O is any object that implements the interface java.io.Closeable. You can extend this guarantee to other types and methods by writing @MustCall or @InheritableMustCall annotations, as described in Section ‍28.1.

The Resource Leak Checker works in three stages:

1. The Must Call Checker (Chapter ‍28) over-approximates each expression’s must-call methods as a @MustCall type.
2. The Called Methods Checker (Chapter ‍7) under-approximates each expression’s definitely-called methods as a @CalledMethods type.
3. When any program element goes out of scope (i.e., it is ready to be de-allocated), the Resource Leak Checker compares the types @MustCall(MC) and @CalledMethods(CM). It reports an error if there exists some method in MC that is not in CM.

## 8.1 How to run the Resource Leak Checker

Run one of these lines:

    javac -processor resourceleak MyFile.java ...
    javac -processor org.checkerframework.checker.resourceleak.ResourceLeakChecker MyFile.java ...

The Resource Leak Checker supports all the command-line arguments listed in Section ‍7.1 for the Called Methods Checker, plus four others:

-ApermitStaticOwning
  See Section ‍8.4.2.
-AresourceLeakIgnoredExceptions=...
  See Section ‍8.7.
-ApermitInitializationLeak
  See Section ‍8.8.
-AenableReturnsReceiverForRlc
  Enables the Returns Receiver Checker (Section ‍24) when running this checker. By default, the Returns Receiver checker is disabled when running the Called Methods Checker as part of the RLC, because it has a significant runtime cost but provides little benefit in the common case, since resources are rarely combined with fluent builders.
-AenableWpiForRlc
  See Section ‍8.11.

If you are running the Resource Leak Checker, then there is no need to run the Must Call Checker (Chapter ‍28), because the Resource Leak Checker does so automatically.

## 8.2 Resource Leak Checker annotations

The Resource Leak Checker relies on the type qualifiers of two other checkers: the Must Call Checker (Section ‍28.1) and the Called Methods Checker (Section ‍7.3). You might need to write qualifiers from either type hierarchy. The most common annotations from these checkers that you might need to write are:

@MustCall(String[] value)
  for example on an element with compile-time type Object that might contain a Socket. See Section ‍28.1.
@InheritableMustCall(String[] value)
  on any classes defined in your program that have must-call obligations. See Section ‍28.2.
@EnsuresCalledMethods and/or @EnsuresCalledMethodsOnException
  on a method in your code that fulfills a must-call obligation of one of its parameters or of a field. See Section ‍7.3.

The Resource Leak Checker supports annotations that express aliasing patterns related to resource leaks:

@Owning
@NotOwning
  expresses ownership. When two aliases exist to the same Java object, @Owning and @NotOwning indicate which of the two is responsible for fulfilling must-call obligations. Constructor results are always @Owning. Method returns default to @Owning. Formal parameters and fields default to @NotOwning. For more details, see Section ‍8.4.
@MustCallAlias
  represents a “resource-aliasing” relationship. Resource aliases are distinct Java objects that control the same resource(s): fulfilling the must-call obligations of one object also fulfills the obligations of the other object. For details, see Section ‍8.5.

The Resource Leak Checker also supports an annotation to permit re-assigning fields or re-opening resources:

@CreatesMustCallFor(String value)
  is a declaration annotation that indicates that after a call to a method with this annotation none of the must-call obligations of the in-scope, owning expression listed in value have been met. In other words, the annotated method “resets” the must-call obligations of the expression. Multiple @CreatesMustCallFor annotations can be written on the same method. Section ‍8.6 explains how this annotation permits re-assignment of owning fields or the re-opening of resources.

## 8.3 Example of how safe resource usage is verified

Consider the following example of safe use of a Socket, in which the comments indicate the inferred Must Call and Called Methods type qualifiers for s:

    {
      Socket s = null;
      // 1. @MustCall({}) @CalledMethodsBottom
      try {
        s = new Socket(myHost, myPort);
        // 2. @MustCall("close") @CalledMethods({})
      } catch (Exception e) {
        // do nothing
      } finally {
        if (s != null) {
          s.close();
          // 3. @MustCall("close") @CalledMethods("close")
        } else {
          // do nothing
          // 4. @MustCall("close") @CalledMethodsBottom
        }
        // 5. @MustCall("close") @CalledMethods("close")
      }
      // 6. @MustCall("close") @CalledMethods("close")
    }

At point 1, s’s type qualifiers are the type qualifiers of null: null has no must-call obligations (@MustCall({})), and methods cannot be called on it (@CalledMethodsBottom).

At point 2, s is a new Socket object, which has a must-call obligation (@MustCall("close")) and has had no methods called on it (@CalledMethods({})).

At point 3, close() has definitely been called on s, so s’s Called Methods type is updated. Note that the Must Call type does not change.

At point 4, s is definitely null and its type is adjusted accordingly.

At point 5, s’s type is the least upper bound of the types at points 3 and 4.

At point 6, s goes out of scope. The Resource Leak Checker reports a required.method.not.called error if the Must Call set contains any element that the Called Methods set does not.

## 8.4 Aliased references and ownership transfer

Resource leak checking is complicated by aliasing. Multiple expressions may evaluate to the same Java object, but each object only needs to be closed once. (Section ‍8.5 describes a related situation called “resource aliasing”, when multiple Java objects refer to the same underlying resource.)

For example, consider the following code that safely closes a Socket:

      void example(String myHost, int myPort) throws IOException {
        Socket s = new Socket(myHost, myPort);
        closeSocket(s);
      }
      void closeSocket(@Owning @MustCall("close") Socket t) {
        try {
          t.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }

There are two aliases for a socket object: s in example() and t in closeSocket(). Ordinarily, the Resource Leak Checker requires that close() is called on every expression of type Socket, but that is not necessary here. The Resource Leak Checker should not warn when s goes out of scope in example(), because closeSocket() takes ownership of the socket — that is, closeSocket() takes responsibility for closing it. The @Owning annotation on t’s declaration expresses this fact; it tells the Resource Leak Checker that t is the reference that must be closed, and its alias s need not be closed.

Constructor returns are always @Owning. Method returns default to @Owning, and parameters and fields default to @NotOwning. This treatment of parameter and return types ensures sound handling of unannotated third-party libraries: any object returned from such a library will be tracked by default, and the checker never assumes that passing an object to an unannotated library will satisfy its obligations.

@Owning and @NotOwning always transfer must-call obligations: must-call obligations are conserved (i.e., neither created nor destroyed) by ownership annotations. Writing @Owning or @NotOwning can never make the checker unsound: a real warning can never be hidden by them. As with any annotation, incorrect or missing annotations can lead to false positive warnings.

### 8.4.1 Owning parameters and exceptions

When @Owning is written on a method parameter, the method only takes ownership of the parameter when it returns normally. In this example, the Resource Leak Checker will report an error in the example method and allow the definition of closeSocket:

      void example(String myHost, int myPort) throws Exception {
        // Error: `s` is not closed on all paths
        Socket s = new Socket(myHost, myPort);

        // `closeSocket` does not have to close `s` when it throws IOException.
        // Instead, this method has to catch the exception and close `s`.
        closeSocket(s);
      }

      void closeSocket(@Owning Socket t) throws IOException {
        throw new IOException();
      }

Sometimes a method really does promise to call some methods on an @Owning parameter, even if it throws an exception. The annotation @EnsuresCalledMethodsOnException can overcome this limitation. For example, a constructor that throws an exception might choose to close an @Owning parameter instead of letting ownership remain with the caller:

      @EnsuresCalledMethodsOnException(value = "#1", methods = "close")
      public Constructor(@Owning Closeable resource) {
        this.resource = resource;
        try {
          initialize();
        } catch (Exception e) {
          resource.close();
          throw e;
        }
      }

### 8.4.2 Owning fields

Unannotated fields are treated as non-owning.

For final, non-static owning fields, the Resource Leak Checker enforces the “resource acquisition is initialization (RAII)” programming idiom. Some destructor-like method d() must satisfy the field’s must-call obligation (and this fact must be expressed via a @EnsuresCalledMethods annotation on d()), and the enclosing class must have a @MustCall("d") obligation to ensure the destructor is called. In addition to the @EnsuresCalledMethods annotation, which guarantees that the field(s) it references have their must-call obligations satisfied on non-exceptional paths, the Resource Leak Checker requires those fields to have their must-call obligations satisfied on all paths in (only) the destructor, and will issue a destructor.exceptional.postcondition error if they are not satisfied. Resolve this error by ensuring that the required methods are called on all exceptional paths.

Non-final, non-static owning fields usually require one or more @CreatesMustCallFor annotations when they might be re-assigned. See Section ‍8.6 for more details on how to annotate a non-final, non-static owning field.

Owning fields are treated slightly differently in constructors versus normal methods. In normal methods, assigning a value to an owning field always satisfies the object’s must-call obligations. However, within a constructor, those obligations are only satisfied if the constructor returns normally. If the constructor throws an exception, the constructed object will not be accessible afterward, and therefore its fields need to be closed before it exits.

This constructor safely closes the object it allocates before throwing an exception:

      private final @Owning Socket socket;

      public ConstructorThatCanThrow() throws IOException {
        Socket s = new Socket(myHost, myPort);
        try {
          initialize(s); // may throw IOException
        } catch (Exception e) {
          s.close();
          throw e;
        }
        this.socket = s;
      }

An assignment to a static owning field does not satisfy a must-call obligation; for example,

      static @Owning PrintWriter debugLog = new PrintWriter("debug.log");

The Resource Leak Checker issues a warning about every assignment of an object with a must-call annotation into a static owning field, indicating that the obligation of the field’s content might not be satisfied. When those fields are used throughout execution, until the program exits, there is no good place to dispose of them, so these warnings might not be useful. The -ApermitStaticOwning command-line argument suppresses warnings related to static owning fields. This can help in checking legacy code. It permits only a small number of resource retained throughout execution, related to the number of such fields and assignments to them.

## 8.5 Resource aliasing

A resource alias set is a set of Java objects that correspond to the same underlying system resource. Calling a must-call method on any member of a resource-alias set fulfills that obligation for all members of the set. Members of the set may have different Java types.

Programmers most often encounter resource aliasing when using wrapper types. For example, the Java BufferedOutputStream wrapper adds buffering to a delegate stream. The wrapper’s close() method invokes close() on the delegate. Calling close() on either object has the same effect: it closes the underlying resource.

A resource aliasing relationship is expressed in source code via a pair of @MustCallAlias annotations: one on a parameter of a method or constructor, and another on its return type. For example, the annotated JDK contains this constructor of BufferedOutputStream:

    @MustCallAlias BufferedOutputStream(@MustCallAlias OutputStream out);

When a pair of @MustCallAlias annotations is written on a method or constructor m’s return type and its parameter p, the Resource Leak Checker requires one of the following:

1. p is passed to another method or constructor (including super) in a @MustCallAlias position, and m returns that method’s result, or
2. p is stored in the only @Owning field of the enclosing class (a class with more than one @Owning field cannot have a resource alias relationship).

### 8.5.1 A complete wrapper type example

Here is a complete example of a type InputStreamWrapper that wraps an InputStream as a resource alias. Defining a wrapper type typically involves combined usage of @InheritableMustCall, @EnsuresCalledMethods, an @Owning field, and @MustCallAlias. The test method shows that the checker is able to verify code that releases an InputStream using either the InputStream directly or a wrapping InputStreamWrapper.

    @InheritableMustCall("dispose")
    class InputStreamWrapper {
      private final @Owning InputStream stream;

      @MustCallAlias InputStreamWrapper(@MustCallAlias InputStream stream) {
        this.stream = stream;
      }

      @EnsuresCalledMethods(value = "this.stream", methods = "close")
      public void dispose() throws IOException {
        this.stream.close();
      }

      /** Shows that either the stream or the wrapper can be closed. */
      static void test(@Owning InputStream stream, boolean b) throws IOException {
        InputStreamWrapper wrapper = new InputStreamWrapper(stream);
        if (b) {
          stream.close();
        } else {
          wrapper.dispose();
        }
      }
    }

## 8.6 Creating obligations (how to re-assign a non-final owning field)

Consider a class that has must-call obligations; that is, the class declaration is annotated with @MustCall(...). Every constructor implicitly creates obligations for the newly-created object. Non-constructor methods may also create obligations when re-assigning non-final owning fields or allocating new system-level resources.

A post-condition annotation, @CreatesMustCallFor, indicates for which expression an obligation is created. If you write @CreatesMustCallFor(T) on a method N that overrides a method M, then M must also be annotated as @CreatesMustCallFor(T). (M may also have other @CreatesMustCallFor annotations that N does not.)

@CreatesMustCallFor allows the Resource Leak Checker to verify uses of non-final fields that contain a resource, even if they are re-assigned. Consider the following example:

      @MustCall("close") // default qualifier for uses of SocketContainer
      class SocketContainer {
        private @Owning Socket sock;

        public SocketContainer() { sock = ...; }

        void close() { sock.close() };

        @CreatesMustCallFor("this")
        void reconnect() {
          if (!sock.isClosed()) {
            sock.close();
          }
          sock = ...;
        }
      }

In the lifetime of a SocketContainer object, sock might be re-assigned arbitrarily many times: once at each call to reconnect(). This code is safe, however: reconnect() ensures that sock is closed before re-assigning it.

Sections ‍8.6.1 and ‍8.6.2 explain how the Resource Leak Checker verifies uses and declarations of methods annotated with @CreatesMustCallFor.

### 8.6.1 Requirements at a call site of a @CreatesMustCallFor method

At a call site to a method annotated as @CreatesMustCallFor(expr), the Resource Leak Checker:

1. Treats any existing @MustCall obligations of expr as satisfied,
2. Creates a fresh obligation to check, as if expr was assigned to a newly-allocated object (i.e. as if expr were a constructor result).
3. Un-refines the type in the Called Methods Checker’s type hierarchy for expr to @CalledMethods({}), if it had any other Called Methods type.
4. Requires that the expression corresponding to expr (that is, expr viewpoint-adapted to the method call site) is owned; that is, it is annotated or defaulted as @Owning. Otherwise, the checker will issue a reset.not.owning error at the call-site. You can avoid this error by extracting expr into a new local variable (because locals are @Owning by default) and replacing all instances of expr in the call with references to the new local variable.

Treating the obligation before the call as satisfied is sound: the checker creates a new obligation for calls to @CreatesMustCallFor methods, and the Must Call Checker (Chapter ‍28) ensures the @MustCall type for the target expression will have a superset of any methods present before the call. Intuitively, calling an @CreatesMustCallFor method “resets” the obligations of the target expression, so whether they were satisfied before the call or not is irrelevant.

If an @CreatesMustCallFor method n is invoked within a method m that has an @CreatesMustCallFor annotation, and the @CreatesMustCallFor annotations on n and m have the same target expression—imposing the obligation produced by calling n on the caller of m—then the newly-created obligation is treated as satisfied immediately at the call-site of n in the body of m (because it is imposed at call-sites of m instead).

### 8.6.2 Requirements at a declaration of a @CreatesMustCallFor method

Any method that re-assigns a non-final, owning field of some object obj must be annotated @CreatesMustCallFor("obj"). Other methods may also be annotated with @CreatesMustCallFor.

The Resource Leak Checker enforces two rules to ensure that re-assignments to non-final, owning fields (like sock in method reconnect above) are sound:

- any method that re-assigns a non-final, owning field of an object must be annotated with a @CreatesMustCallFor annotation whose expression is a reference to that object.
- when a non-final, owning field f is re-assigned at statement s, at the program point before s, f’s must-call obligations must have been satisfied.

The first rule ensures that close() is called after the last call to reconnect(), and the second rule ensures that reconnect() safely closes sock before re-assigning it. Because the Called Methods Checker treats calls to an @CreatesMustCallFor method like reconnect() as if the call might cause arbitrary side-effects, after such a call the only method known to have been definitely called is the @CreatesMustCallFor method: previous called methods (including close()) do not appear in the @CalledMethods type qualifier.

## 8.7 Ignored exception types

The Resource Leak Checker checks that an element’s must-call obligations are fulfilled when that element may go out of scope: at the end of its lexical scope or when control may be transferred to the end of its lexical scope, such as via a break or continue statement or via throwing an exception. As an example of an exception, consider the following method:

      void foo() {
        Socket s = ...;
        bar();
        s.close();
      }

If bar is declared to throw an exception, the Resource Leak Checker warns that a Socket may be leaked. If bar throws an exception, the only reference to s is lost, which could lead to a resource leak.

The Resource Leak Checker ignores control flow due to some exceptions.

- By default the Resource Leak Checker ignores run-time errors that can occur unpredictably at most points in the program. For example, the JVM can throw an OutOfMemoryError on any allocation. Similarly, ClassCircularityError, ClassFormatError, and NoClassDefFoundError may occur at any reference to a class. Such exceptions usually terminate the program, and in that case unclosed resources do not matter. Furthermore, any method can throw RuntimeException, and the Checker Framework pessimistically assumes one can be thrown at every call site. Accounting for such exceptions would lead to vast numbers of false positive warnings, so the Resource Leak Checker assumes they are never thrown. Strictly speaking, this is an unsoundness: it can lead to false negatives (missed resource leaks) if the programmer catches these exceptions, which is a discouraged practice.
- The Resource Leak Checker also ignores exception types that can be verified to never occur. In particular, the Resource Leak Checker ignores NullPointerExceptions (use the Nullness Checker, Chapter ‍3) and ArrayIndexOutOfBoundsExceptions and NegativeArraySizeExceptions (use the Index Checker, Chapter ‍11). Other exception types may be added to this list in the future. Please let us know if there is a type that you think should be ignored by filing an issue listing both the exception type and the verification tool.

The set of ignored exception types can be controlled with the option -AresourceLeakIgnoredExceptions=.... The option takes a comma-separated list of fully-qualified exception types. A type can be prefixed with = to ignore exactly that type and not its subclasses. For example, for a very pedantic set of ignored exceptions use:

      -AresourceLeakIgnoredExceptions=java.lang.Error, =java.lang.NullPointerException

which ignores java.lang.Error (and all its subclasses) as well as java.lang.NullPointerException (but not its subclasses).

The keyword default will expand to the default set of ignored exceptions. So, to add an additional exception to the set of ignored exceptions, use:

      -AresourceLeakIgnoredExceptions=default,package.MyCustomException

## 8.8 Errors about field initialization

The Resource Leak Checker warns about re-assignments to owning fields, because the value that was overwritten might not have had its obligations satisfied. Such a warning is not necessary on the first assignment to a field, since the field had no content before the assignment. Sometimes, the Resource Leak Checker is unable to determine that an assignment is the first one, so it conservatively assumes the assignment is a re-assignment and issues an error.

One way to prevent this false positive warning is to declare the field as final.

Alternately, to suppress all warnings related to field assignments in the constructor and in initializer blocks, pass the -ApermitInitializationLeak command-line argument. This makes the checker unsound: the Resource Leak Checker will not warn if the constructor and initializers set a field more than once. The amount of leakage is limited to how many times the field is set.

## 8.9 Errors about unknown must call obligations

The Resource Leak Checker issues a required.method.not.known error when a variable with the type @MustCallUnknown has a must call obligation. @MustCallUnknown rarely occurs, but if you encounter this error usually the right thing to do is to write an explicit @MustCall annotation on the indicated expression (e.g., as a cast), because the Must Call Checker will only use @MustCallUnknown as a default when encountering a language feature that it is unable to reason about.

## 8.10 Collections of resources

The Resource Leak Checker cannot verify code that stores a collection of resources in a generic collection (e.g., java.util.List) and then resolves the obligations of each element of the collection at once (e.g., by iterating over the List). In the future, the checker will support this. The remainder of this section explains the implementation issues; most users can skip it.

The first implementation issue is that @Owning and @NotOwning are declaration annotations rather than type qualifiers, so they cannot be written on type arguments. It is possible under the current design to have an @Owning List<Socket>, but not a List<@Owning Socket>. It would be better to make @Owning a type annotation, but this is a challenging design problem.

The second implementation issue is the defaulting rule for @MustCall on type variable upper bounds. Currently, this default is @MustCall({}), which prevents many false positives in code with type variables that makes no use of resources — an important design principle. However, this defaulting rule does have an unfortunate consequence: it is an error to write List<Socket> or any other type with a concrete type argument where the type argument itself isn’t @MustCall({}). A programmer who needs to write such a type while using the Resource Leak Checker has a few choices, all of which have some downsides:

- Write List<? extends Socket>. This rejects calls to add() or other methods that require an instance of the type variable, but it preserves some of the behavior (e.g., calls to remove() are permitted). This is the best choice most of the time if the List is not intended to be owning.
- Write List<@MustCall Socket>. This makes it an error to add a Socket to the list, since the type of the Socket is @MustCall("close") but the list requires @MustCall().
- Suppress one or more warnings.

The recommended way to use the Resource Leak Checker in this situation is to rewrite the code to avoid a List of owning resources. If rewriting is not possible, the programmer will probably need to suppress a warning and then verify the code using a method other than the Resource Leak Checker.

## 8.11 Further reading

The paper “Lightweight and Modular Resource Leak Verification” ‍[KSSE21] (ESEC/FSE 2021, https://homes.cs.washington.edu/~mernst/pubs/resource-leak-esecfse2021-abstract.html) gives more details about the Resource Leak Checker.

The Resource Leak Checker uses a specialized algorithm to infer annotations when whole program inference (WPI, Section ‍34.2) is enabled. The algorithm is described in the paper “Inference of Resource Management Specifications” ‍[SGT+23] (OOPSLA 2023, https://homes.cs.washington.edu/~mernst/pubs/resource-inference-oopsla2023-abstract.html).

# Chapter ‍9 Fake Enum Checker for fake enumerations

The Fake Enum Checker, or Fenum Checker, enables you to define a type alias or typedef, in which two different sets of values have the same representation (the same Java type) but are not allowed to be used interchangeably. It is also possible to create a typedef using the Subtyping Checker (Chapter ‍29), and that approach is sometimes more appropriate.

One common use for the Fake Enum Checker is the fake enumeration pattern (Section ‍9.6). For example, consider this code adapted from Android’s IntDef documentation:

    @NavigationMode int NAVIGATION_MODE_STANDARD = 0;
    @NavigationMode int NAVIGATION_MODE_LIST = 1;
    @NavigationMode int NAVIGATION_MODE_TABS = 2;

    @NavigationMode int getNavigationMode();

    void setNavigationMode(@NavigationMode int mode);

The Fake Enum Checker can issue a compile-time warning if the programmer ever tries to call setNavigationMode with an int that is not a @NavigationMode int.

The Fake Enum Checker gives the same safety guarantees as a true enumeration type or typedef, but retaining backward-compatibility with interfaces that use existing Java types. You can apply fenum annotations to any Java type, including all primitive types and also reference types. Thus, you could use it (for example) to represent floating-point values between 0 and 1, or Strings with some particular characteristic. (Note that the Fake Enum Checker does not let you create a shorter alias for a long type name, as a real typedef would if Java supported it.)

As explained in Section ‍9.1, you can either define your own fenum annotations, such as @NavigationMode above, or you can use the @Fenum type qualifier with a string argument. Figure ‍9.1 shows part of the type hierarchy for the Fenum type system.

> Figure 9.1: Partial type hierarchy for the Fenum type system. There are two forms of fake enumeration annotations — above, illustrated by @Fenum("A") and @FenumC. See Section ‍9.1 for descriptions of how to introduce both types of fenums. The type qualifiers in gray (@FenumTop, @FenumUnqualified, and @FenumBottom) should never be written in source code; they are used internally by the type system. @FenumUnqualified is the default qualifier for unannotated types, except for upper bounds which default to @FenumTop.

## 9.1 Fake enum annotations

The Fake Enum Checker supports two ways to introduce a new fake enum (fenum):

1. Introduce your own specialized fenum annotation with code like this in file MyFenum.java:

       package myPackage.qual;

       import java.lang.annotation.Documented;
       import java.lang.annotation.ElementType;
       import java.lang.annotation.Retention;
       import java.lang.annotation.RetentionPolicy;
       import java.lang.annotation.Target;
       import org.checkerframework.checker.fenum.qual.FenumTop;
       import org.checkerframework.framework.qual.SubtypeOf;

       @Documented
       @Retention(RetentionPolicy.RUNTIME)
       @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
       @SubtypeOf(FenumTop.class)
       public @interface MyFenum {}

   You only need to adapt the italicized package, annotation, and file names in the example.

   Note that all custom annotations must have the @Target({ElementType.TYPE_USE}) meta-annotation. See section 36.5.1.

2. Use the provided @Fenum annotation, which takes a String argument to distinguish different fenums or type aliases. For example, @Fenum("A") and @Fenum("B") are two distinct type qualifiers.

The first approach allows you to define a short, meaningful name suitable for your project, whereas the second approach allows quick prototyping.

## 9.2 What the Fenum Checker checks

The Fenum Checker ensures that unrelated types are not mixed. All types with a particular fenum annotation, or @Fenum(...) with a particular String argument, are disjoint from all unannotated types and from all types with a different fenum annotation or String argument.

The checker ensures that only compatible fenum types are used in comparisons and arithmetic operations (if applicable to the annotated type).

It is the programmer’s responsibility to ensure that fields with a fenum type are properly initialized before use. Otherwise, one might observe a null reference or zero value in the field of a fenum type. (The Nullness Checker (Chapter ‍3) can prevent failure to initialize a reference variable.)

## 9.3 Running the Fenum Checker

The Fenum Checker can be invoked by running the following commands.

- If you define your own annotation(s), provide the name(s) of the annotation(s) through the -Aquals option, using a comma-no-space-separated notation:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.checker.fenum.FenumChecker \
              -Aquals=myPackage.qual.MyFenum MyFile.java ...

  The annotations listed in -Aquals must be accessible to the compiler during compilation. Before you run the Fenum Checker with javac, they must be compiled and on the same path (the classpath or processorpath) as the Checker Framework. It is not sufficient to supply their source files on the command line.

  You can also provide the fully-qualified paths to a set of directories that contain the annotations through the -AqualDirs option, using a colon-no-space-separated notation. For example, if the Checker Framework is on the classpath rather than the processorpath:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.checker.fenum.FenumChecker \
              -AqualDirs=/full/path/to/myProject/bin:/full/path/to/myLibrary/bin MyFile.java ...

  Note that in these two examples, the compiled class file of the myPackage.qual.MyFenum annotation must exist in either the myProject/bin directory or the myLibrary/bin directory. The following placement of the class file will work with the above commands:

        .../myProject/bin/myPackage/qual/MyFenum.class

  The two options can be used at the same time to provide groups of annotations from directories, and individually named annotations.

- If your code uses the @Fenum annotation, you do not need the -Aquals or -AqualDirs option:

        javac -processor org.checkerframework.checker.fenum.FenumChecker MyFile.java ...

For an example of running the Fake Enum Checker on Android code, see https://github.com/karlicoss/checker-fenum-android-demo.

## 9.4 Suppressing warnings

One example of when you need to suppress warnings is when you initialize the fenum constants to literal values. To remove this warning message, add a @SuppressWarnings annotation to either the field or class declaration, for example:

    @SuppressWarnings("fenum:assignment") // initialization of fake enums
    class MyConsts {
      public static final @Fenum("A") int ACONST1 = 1;
      public static final @Fenum("A") int ACONST2 = 2;
    }

## 9.5 Example

The following example introduces two fenums in class TestStatic and then performs a few typical operations.

    @SuppressWarnings("fenum:assignment")   // initialization of fake enums
    public class TestStatic {
      public static final @Fenum("A") int ACONST1 = 1;
      public static final @Fenum("A") int ACONST2 = 2;

      public static final @Fenum("B") int BCONST1 = 4;
      public static final @Fenum("B") int BCONST2 = 5;
    }

    class FenumUser {
      @Fenum("A") int state1 = TestStatic.ACONST1;     // ok
      @Fenum("B") int state2 = TestStatic.ACONST1;     // Incompatible fenums forbidden!

      void fenumArg(@Fenum("A") int p) {}

      void fenumTest() {
        state1 = 4;                     // Direct use of value forbidden!
        state1 = TestStatic.BCONST1;    // Incompatible fenums forbidden!
        state1 = TestStatic.ACONST2;    // ok

        fenumArg(5);                    // Direct use of value forbidden!
        fenumArg(TestStatic.BCONST1);   // Incompatible fenums forbidden!
        fenumArg(TestStatic.ACONST1);   // ok
      }
     }

Also, see the example project in the docs/examples/fenum-extension directory.

The paper “Building and using pluggable type-checkers” ‍[DDE+11] (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-icse2011.pdf) describes case studies of the Fake Enum Checker.

## 9.6 The fake enumeration pattern

Java’s enum keyword lets you define an enumeration type: a finite set of distinct values that are related to one another but are disjoint from all other types, including other enumerations. Before enums were added to Java, there were two ways to encode an enumeration, both of which are error-prone:

the fake enum pattern
  a set of int or String constants (as often found in older C code).
the typesafe enum pattern
  a class with private constructor.

Sometimes you need to use the fake enum pattern, rather than a real enum or the typesafe enum pattern. One reason is backward-compatibility. A public API that predates Java’s enum keyword may use int constants; it cannot be changed, because doing so would break existing clients. For example, Java’s JDK still uses int constants in the AWT and Swing frameworks, and Android also uses int constants rather than Java enums. Another reason is performance, especially in environments with limited resources. Use of an int instead of an object can reduce code size, memory requirements, and run time.

In cases when code has to use the fake enum pattern, the Fake Enum Checker, or Fenum Checker, gives the same safety guarantees as a true enumeration type. The developer can introduce new types that are distinct from all values of the base type and from all other fake enums. Fenums can be introduced for primitive types as well as for reference types.

## 9.7 References

- Case studies of the Fake Enum Checker:
  “Building and using pluggable type-checkers” ‍[DDE+11] (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-icse2011.pdf#page=3)
- Java Language Specification on enums:
  https://docs.oracle.com/javase/specs/jls/se17/html/jls-8.html#jls-8.9
- Tutorial trail on enums:
  https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
- Java Tip 122: Beware of Java typesafe enumerations:
  https://www.infoworld.com/article/2077487/java-tip-122--beware-of-java-typesafe-enumerations.html

# Chapter ‍10 Lock Checker

The Lock Checker prevents certain concurrency errors by enforcing a locking discipline. A locking discipline indicates which locks must be held when a given operation occurs. You express the locking discipline by declaring a variable’s type to have the qualifier @GuardedBy("lockexpr"). This indicates that the variable’s value may be dereferenced only if the given lock is held.

To run the Lock Checker, supply the -processor org.checkerframework.checker.lock.LockChecker command-line option to javac. The -AconcurrentSemantics command-line option is always enabled for the Lock Checker (see Section ‍39.4.6).

## 10.1 What the Lock Checker guarantees

The Lock Checker gives the following guarantee. Suppose that expression e has type @GuardedBy({"x", "y.z"}). Then the value computed for e is only dereferenced by a thread when the thread holds locks x and y.z. Dereferencing a value is reading or writing one of its fields. The guarantee about e’s value holds not only if the expression e is dereferenced directly, but also if the value was first copied into a variable, returned as the result of a method call, etc. Copying a reference is always permitted by the Lock Checker, regardless of which locks are held.

A lock is held if it has been acquired but not yet released. Java has two types of locks. A monitor lock is acquired upon entry to a synchronized method or block, and is released on exit from that method or block. An explicit lock is acquired by a method call such as Lock.lock(), and is released by another method call such as Lock.unlock(). The Lock Checker enforces that any expression whose type implements Lock is used as an explicit lock, and all other expressions are used as monitor locks.

Ensuring that your program obeys its locking discipline is an easy and effective way to eliminate a common and important class of errors. If the Lock Checker issues no warnings, then your program obeys its locking discipline. However, your program might still have other types of concurrency errors. For example, you might have specified an inadequate locking discipline because you forgot some @GuardedBy annotations. Your program might release and re-acquire the lock, when correctness requires it to hold it throughout a computation. And, there are other concurrency errors that cannot, or should not, be solved with locks.

## 10.2 Lock annotations

This section describes the lock annotations you can write on types and methods.

### 10.2.1 Type qualifiers

@GuardedBy(exprSet)
  If a variable x has type @GuardedBy("expr"), then a thread may dereference the value referred to by x only when the thread holds the lock that expr currently evaluates to.

  The @GuardedBy annotation can list multiple expressions, as in @GuardedBy({"expr1", "expr2"}), in which case the dereference is permitted only if the thread holds all the locks.

  Section ‍32.8 explains which expressions the Lock Checker is able to analyze as lock expressions. These include <self>, the value of the annotated reference (non-primitive) variable. For example, @GuardedBy("<self>") Object o indicates that the value referenced by o is guarded by the intrinsic (monitor) lock of the value referenced by o.

  @GuardedBy({}), which means the value is always allowed to be dereferenced, is the default type qualifier that is used for all locations where the programmer does not write an explicit locking type qualifier (except all CLIMB-to-top locations other than upper bounds and exception parameters — see Section ‍32.5.3). (Section ‍10.5.4 discusses this choice.) It is also the conservative default type qualifier for method parameters in unannotated libraries (see Chapter ‍35).

@GuardedByUnknown
  If a variable x has type @GuardedByUnknown, then it is not known which locks protect x’s value. Those locks might even be out of scope (inaccessible) and therefore unable to be written in the annotation. The practical consequence is that the value referred to by x can never be dereferenced.

  Any value can be assigned to a variable of type @GuardedByUnknown. In particular, if it is written on a formal parameter, then any value, including one whose locks are not currently held, may be passed as an argument.

  @GuardedByUnknown is the conservative default type qualifier for method receivers in unannotated libraries (see Chapter ‍35).

@NewObject
  This type represents a newly-constructed value; such as the result of calling a constructor or factory method. The client can use the value as any @GuardedBy type.
@GuardedByBottom
  If a variable x has type @GuardedByBottom, then the value referred to by x is null and can never be dereferenced.

> Figure 10.1: The subtyping relationship of the Lock Checker’s qualifiers. @GuardedBy({}) is the default type qualifier for unannotated types (except all CLIMB-to-top locations other than upper bounds and exception parameters — see Section ‍32.5.3).

Figure ‍10.1 shows the type hierarchy of these qualifiers. All @GuardedBy annotations are incomparable: if exprSet1 ≠ exprSet2, then @GuardedBy(exprSet1) and @GuardedBy(exprSet2) are siblings in the type hierarchy. You might expect that @GuardedBy({"x", "y"}) T is a subtype of @GuardedBy({"x"}) T. The first type requires two locks to be held, and the second requires only one lock to be held and so could be used in any situation where both locks are held. The type system conservatively prohibits this in order to prevent type-checking loopholes that would result from aliasing and side effects — that is, from having two mutable references, of different types, to the same data. See Section ‍10.4.2 for an example of a problem that would occur if this rule were relaxed.

##### Polymorphic type qualifiers

@GuardSatisfied(index)
  If a variable x has type @GuardSatisfied, then all lock expressions for x’s value are held.

  As with other qualifier-polymorphism annotations (Section ‍31.2), the index argument indicates when two values are guarded by the same (unknown) set of locks.

  @GuardSatisfied is only allowed in method signatures: on formal parameters (including the receiver) and return types. It may not be written on fields. Also, it is a limitation of the current design that @GuardSatisfied may not be written on array elements or on local variables.

  A return type can only be annotated with @GuardSatisfied(index), not @GuardSatisfied.

  See Section ‍10.4.6 for an example of a use of @GuardSatisfied.

### 10.2.2 Declaration annotations

The Lock Checker supports several annotations that specify method behavior. These are declaration annotations, not type annotations: they apply to the method itself rather than to some particular type.

##### Method pre-conditions and post-conditions

@Holding(String[] locks)
  All the given lock expressions are held at the method call site.
@EnsuresLockHeld(String[] locks)
  The given lock expressions are locked upon method return if the method terminates successfully. This is useful for annotating a method that acquires a lock such as ReentrantLock.lock().
@EnsuresLockHeldIf(String[] locks, boolean result)
  If the annotated method returns the given boolean value (true or false), the given lock expressions are locked upon method return if the method terminates successfully. This is useful for annotating a method that conditionally acquires a lock. See Section ‍10.4.4 for examples.

##### Side effect specifications

@LockingFree
  The method does not acquire or release locks, directly or indirectly. The method is not synchronized, it contains no synchronized blocks, it contains no calls to lock or unlock methods, and it contains no calls to methods that are not themselves @LockingFree.

  Since @SideEffectFree implies @LockingFree, if both are applicable then you only need to write @SideEffectFree.

@ReleasesNoLocks
  The method maintains a strictly nondecreasing lock hold count on the current thread for any locks that were held prior to the method call. The method might acquire locks but then release them, or might acquire locks but not release them (in which case it should also be annotated with @EnsuresLockHeld or @EnsuresLockHeldIf).

  This is the default for methods being type-checked that have no @LockingFree, @MayReleaseLocks, @SideEffectFree, or @Pure annotation.

@MayReleaseLocks
  The method may release locks that were held prior to the method being called. You can write this when you are certain the method releases locks, or when you don’t know whether the method releases locks. This is the conservative default for methods in unannotated libraries (see Chapter ‍35).

## 10.3 Type-checking rules

In addition to the standard subtyping rules enforcing the subtyping relationship described in Figure ‍10.1, the Lock Checker enforces the following additional rules.

### 10.3.1 Polymorphic qualifiers

@GuardSatisfied

  The overall rules for polymorphic qualifiers are given in Section ‍31.2.

  Here are additional constraints for (pseudo-)assignments:

  - If the left-hand side has type @GuardSatisfied (with or without an index), then all locks mentioned in the right-hand side’s @GuardedBy type must be currently held.
  - A formal parameter with type qualifier @GuardSatisfied without an index cannot be assigned to.
  - If the left-hand side is a formal parameter with type @GuardSatisfied(index), the right-hand-side must have identical @GuardSatisfied(index) type.

  If a formal parameter type is annotated with @GuardSatisfied without an index, then that formal parameter type is unrelated to every other type in the @GuardedBy hierarchy, including other occurrences of @GuardSatisfied without an index.

  @GuardSatisfied may not be used on formal parameters, receivers, or return types of a method annotated with @MayReleaseLocks.

### 10.3.2 Dereferences

@GuardedBy
  An expression of type @GuardedBy(eset) may be dereferenced only if all locks in eset are held.
@GuardSatisfied
  An expression of type @GuardSatisfied may be dereferenced.
Not @GuardedBy or @GuardSatisfied
  An expression whose type is not annotated with @GuardedBy or @GuardSatisfied may not be dereferenced.

### 10.3.3 Primitive types, boxed primitive types, and Strings

Primitive types, boxed primitive types (such as java.lang.Integer), and type java.lang.String are annotated with @GuardedBy({}). It is an error for the programmer to annotate any of these types with an annotation from the @GuardedBy type hierarchy, including @GuardedBy({}).

### 10.3.4 Overriding

Overriding methods annotated with @Holding
  If class B overrides method m from class A, then the expressions in B’s @Holding annotation must be a subset of or equal to that of A’s @Holding annotation.
Overriding methods annotated with side effect annotations
  If class B overrides method m from class A, then the side effect annotation on B’s declaration of m must be at least as strong as that in A’s declaration of m. From weakest to strongest, the side effect annotations processed by the Lock Checker are:

        @MayReleaseLocks
        @ReleasesNoLocks
        @LockingFree
        @SideEffectFree
        @Pure

### 10.3.5 Side effects

Releasing explicit locks
  Any method that releases an explicit lock must be annotated with @MayReleaseLocks. The Lock Checker issues a warning if it encounters a method declaration annotated with @MayReleaseLocks and having a formal parameter or receiver annotated with @GuardSatisfied. This is because the Lock Checker cannot guarantee that the guard will be satisfied throughout the body of a method if that method may release a lock.
No side effects on lock expressions
  If expression expr is used to acquire a lock, then expr must evaluate to the same value, starting from when expr is used to acquire a lock until expr is used to release the lock. An expression is used to acquire a lock if it is the receiver at a call site of a synchronized method, is the expression in a synchronized block, or is the argument to a lock method.
Locks are released after possible side effects
  After a call to a method annotated with @LockingFree, @ReleasesNoLocks, @SideEffectFree, or @Pure, the Lock Checker’s estimate of held locks after a method call is the same as that prior to the method call. After a call to a method annotated with @MayReleaseLocks, the estimate of held locks is conservatively reset to the empty set, except for those locks specified to be held after the call by an @EnsuresLockHeld or @EnsuresLockHeldIf annotation on the method. Assignments to variables also cause the estimate of held locks to be conservatively reduced to a smaller set if the Checker Framework determines that the assignment might have side-effected a lock expression. For more information on side effects, please refer to Section ‍32.7.5.

## 10.4 Examples

The Lock Checker guarantees that a value that was computed from an expression of @GuardedBy type is dereferenced only when the current thread holds all the expressions in the @GuardedBy annotation.

### 10.4.1 Examples of @GuardedBy

The following example demonstrates the basic type-checking rules.

    class MyClass {
      final ReentrantLock lock; // Initialized in the constructor

      @GuardedBy("lock") Object x = new Object();
      @GuardedBy("lock") Object y = x; // OK, since dereferences of y will require "lock" to be held.
      @GuardedBy({}) Object z = x; // ILLEGAL since dereferences of z don't require "lock" to be held.
      @GuardedBy("lock") Object myMethod() { // myMethod is implicitly annotated with @ReleasesNoLocks.
         return x; // OK because the return type is annotated with @GuardedBy("lock")
      }

      [...]

      void exampleMethod() {
         x.toString(); // ILLEGAL because the lock is not known to be held
         y.toString(); // ILLEGAL because the lock is not known to be held
         myMethod().toString(); // ILLEGAL because the lock is not known to be held
         lock.lock();
         x.toString();  // OK: the lock is known to be held
         y.toString();  // OK: the lock is known to be held, and toString() is annotated with @SideEffectFree.
         myMethod().toString(); // OK: the lock is known to be held, since myMethod
                                // is implicitly annotated with @ReleasesNoLocks.
      }
    }

Note that the expression new Object() is inferred to have type @GuardedBy("lock") because it is immediately assigned to a newly-declared variable having type annotation @GuardedBy("lock"). You could explicitly write new @GuardedBy("lock") Object() but it is not required.

The following example demonstrates that using <self> as a lock expression allows a guarded value to be dereferenced even when the original variable name the value was originally assigned to falls out of scope.

    class MyClass {
      private final @GuardedBy("<self>") Object x = new Object();
      void method() {
        x.toString(); // ILLEGAL because x is not known to be held.
        synchronized(x) {
          x.toString(); // OK: x is known to be held.
        }
      }

      public @GuardedBy("<self>") Object get_x() {
        return x; // OK, since the return type is @GuardedBy("<self>").
      }
    }

    class MyOtherClass {
      void method() {
        MyClass m = new MyClass();
        final @GuardedBy("<self>") Object o = m.get_x();
        o.toString(); // ILLEGAL because o is not known to be held.
        synchronized(o) {
          o.toString(); // OK: o is known to be held.
        }
      }
    }

### 10.4.2 @GuardedBy({“a”, “b”}) is not a subtype of @GuardedBy({“a”})

@GuardedBy(exprSet)

The following example demonstrates the reason the Lock Checker enforces the following rule: if exprSet1 ≠ exprSet2, then @GuardedBy(exprSet1) and @GuardedBy(exprSet2) are siblings in the type hierarchy.

    class MyClass {
        final Object lockA = new Object();
        final Object lockB = new Object();
        @GuardedBy("lockA") Object x = new Object();
        @GuardedBy({"lockA", "lockB"}) Object y = new Object();
        void myMethod() {
            y = x;      // ILLEGAL; if legal, later statement x.toString() would cause trouble
            synchronized(lockA) {
              x.toString();  // dereferences y's value without holding lock lockB
            }
        }
    }

If the Lock Checker permitted the assignment y = x;, then the undesired dereference would be possible.

### 10.4.3 Examples of @Holding

The following example shows the interaction between @GuardedBy and @Holding:

      void helper1(@GuardedBy("myLock") Object a) {
        a.toString(); // ILLEGAL: the lock is not held
        synchronized(myLock) {
          a.toString();  // OK: the lock is held
        }
      }
      @Holding("myLock")
      void helper2(@GuardedBy("myLock") Object b) {
        b.toString(); // OK: the lock is held
      }
      void helper3(@GuardedBy("myLock") Object d) {
        d.toString(); // ILLEGAL: the lock is not held
      }
      void myMethod2(@GuardedBy("myLock") Object e) {
        helper1(e);  // OK to pass to another routine without holding the lock
                     // (but helper1's body has an error)
        e.toString(); // ILLEGAL: the lock is not held
        synchronized (myLock) {
          helper2(e); // OK: the lock is held
          helper3(e); // OK, but helper3's body has an error
        }
      }

### 10.4.4 Examples of @EnsuresLockHeld and @EnsuresLockHeldIf

@EnsuresLockHeld and @EnsuresLockHeldIf are primarily intended for annotating JDK locking methods, as in:

    package java.util.concurrent.locks;

    class ReentrantLock {

        @EnsuresLockHeld("this")
        public void lock();

        @EnsuresLockHeldIf(expression="this", result=true)
        public boolean tryLock();

        ...
    }

They can also be used to annotate user methods, particularly for higher-level lock constructs such as a Monitor, as in this simplified example:

    public class Monitor {

        private final ReentrantLock lock; // Initialized in the constructor

        ...

        @EnsuresLockHeld("lock")
        public void enter() {
           lock.lock();
        }

        ...
    }

### 10.4.5 Example of @LockingFree, @ReleasesNoLocks, and @MayReleaseLocks

@LockingFree is useful when a method does not make any use of synchronization or locks but causes other side effects (hence @SideEffectFree is not appropriate). @SideEffectFree implies @LockingFree, therefore if both are applicable, you should only write @SideEffectFree. @ReleasesNoLocks has a weaker guarantee than @LockingFree, and @MayReleaseLocks provides no guarantees.

    private Object myField;
    private final ReentrantLock lock; // Initialized in the constructor
    private @GuardedBy("lock") Object x; // Initialized in the constructor

    [...]

    // This method does not use locks or synchronization, but it cannot
    // be annotated as @SideEffectFree since it alters myField.
    @LockingFree
    void myMethod() {
      myField = new Object();
    }

    @SideEffectFree
    int mySideEffectFreeMethod() {
      return 0;
    }

    @MayReleaseLocks
    void myUnlockingMethod() {
      lock.unlock();
    }

    @ReleasesNoLocks
    void myLockingMethod() {
      lock.lock();
    }

    @MayReleaseLocks
    void clientMethod() {
      if (lock.tryLock()) {
        x.toString(); // OK: the lock is held
        myMethod();
        x.toString(); // OK: the lock is still held since myMethod is locking-free
        mySideEffectFreeMethod();
        x.toString(); // OK: the lock is still held since mySideEffectFreeMethod is side-effect-free
        myUnlockingMethod();
        x.toString(); // ILLEGAL: myUnlockingMethod may have released a lock
      }
      if (lock.tryLock()) {
        x.toString(); // OK: the lock is held
        myLockingMethod();
        x.toString(); // OK: the lock is held
      }
      if (lock.isHeldByCurrentThread()) {
        x.toString(); // OK: the lock is known to be held
      }
    }

### 10.4.6 Polymorphism and method formal parameters with unknown guards

The polymorphic @GuardSatisfied type annotation allows a method body to dereference the method’s formal parameters even if the @GuardedBy annotations on the actual parameters are unknown at the method declaration site.

The declaration of StringBuffer.append(String str) is annotated as:

    @LockingFree
    public @GuardSatisfied(1) StringBuffer append(@GuardSatisfied(1) StringBuffer this,
                                                  @GuardSatisfied(2) String str)

The method manipulates the values of its arguments, so all their locks must be held. However, the declaration does not know what those are and they might not even be in scope at the declaration. Therefore, the declaration cannot use @GuardedBy and must use @GuardSatisfied. The arguments to @GuardSatisfied indicate that the receiver and result (which are the same value) are guarded by the same (unknown, possibly empty) set of locks, and the str parameter may be guarded by a different set of locks.

The @LockingFree annotation indicates that this method makes no use of locks or synchronization.

Given these annotations on append, the following code type-checks:

    final ReentrantLock lock1, lock2; // Initialized in the constructor
    @GuardedBy("lock1") StringBuffer filename;
    @GuardedBy("lock2") StringBuffer extension;
    ...
    lock1.lock();
    lock2.lock();
    filename = filename.append(extension);

## 10.5 More locking details

This section gives some details that are helpful for understanding how Java locking and the Lock Checker works.

The paper “Locking discipline inference and checking” ‍[ELM+16] (ICSE 2016, https://homes.cs.washington.edu/~mernst/pubs/locking-inference-checking-icse2016-abstract.html) gives additional details.

### 10.5.1 Two types of locking: monitor locks and explicit locks

Java provides two types of locking: monitor locks and explicit locks.

- A synchronized(E) block acquires the lock on the value of E; similarly, a method declared using the synchronized method modifier acquires the lock on the method receiver when called. (More precisely, the current thread locks the monitor associated with the value of E; see JLS §17.1.) The lock is automatically released when execution exits the block or the method body, respectively. We use the term “monitor lock” for a lock acquired using a synchronized block or synchronized method modifier.
- A method call, such as Lock.lock(), acquires a lock that implements the Lock interface. The lock is released by another method call, such as Lock.unlock(). We use the term “explicit lock” for a lock expression acquired in this way.

You should not mix the two varieties of locking, and the Lock Checker enforces this. To prevent an object from being used both as a monitor and an explicit lock, the Lock Checker issues a warning if a synchronized(E) block’s expression E has a type that implements Lock.

### 10.5.2 Held locks and held expressions; aliasing

Whereas Java locking is defined in terms of values, Java programs are written in terms of expressions. We say that a lock expression is held if the value to which the expression currently evaluates is held.

The Lock Checker conservatively estimates the expressions that are held at each point in a program. The Lock Checker does not track aliasing (different expressions that evaluate to the same value); it only considers the exact expression used to acquire a lock to be held. After any statement that might side-effect a held expression or a lock expression, the Lock Checker conservatively considers the expression to be no longer held.

Section ‍32.8 explains which Java expressions the Lock Checker is able to analyze as lock expressions.

The @LockHeld and @LockPossiblyHeld type qualifiers are used internally by the Lock Checker and should never be written by the programmer. If you see a warning mentioning @LockHeld or @LockPossiblyHeld, please contact the Checker Framework developers as it is likely to indicate a bug in the Checker Framework.

### 10.5.3 Run-time checks for locking

When you perform a run-time check for locking, such as if (explicitLock.isHeldByCurrentThread()){...} or if (Thread.holdsLock(monitorLock)){...}, then the Lock Checker considers the lock expression to be held within the scope of the test. For more details, see Section ‍32.7.

### 10.5.4 Discussion of default qualifier

The default qualifier for unannotated types is @GuardedBy({}). This default forces you to write explicit @GuardSatisfied in method signatures in the common case that clients ensure that all locks are held.

It might seem that @GuardSatisfied would be a better default for method signatures, but such a default would require even more annotations. The reason is that @GuardSatisfied cannot be used on fields. If @GuardedBy({}) is the default for fields but @GuardSatisfied is the default for parameters and return types, then getters, setters, and many other types of methods do not type-check without explicit lock qualifiers.

### 10.5.5 Discussion of @Holding

A programmer might choose to use the @Holding method annotation in two different ways: to specify correctness constraints for a synchronization protocol, or to summarize intended usage. Both of these approaches are useful, and the Lock Checker supports both.

##### Synchronization protocol

@Holding can specify a synchronization protocol that is not expressible as locks over the parameters to a method. For example, a global lock or a lock on a different object might need to be held. By requiring locks to be held, you can create protocol primitives without giving up the benefits of the annotations and checking of them.

##### Method summary that simplifies reasoning

@Holding can be a method summary that simplifies reasoning. In this case, the @Holding doesn’t necessarily introduce a new correctness constraint; the program might be correct even if the lock were not already acquired.

Rather, here @Holding expresses a fact about execution: when execution reaches this point, the following locks are known to be already held. This fact enables people and tools to reason intra- rather than inter-procedurally.

In Java, it is always legal to re-acquire a lock that is already held, and the re-acquisition always works. Thus, whenever you write

      @Holding("myLock")
      void myMethod() {
        ...
      }

it would be equivalent, from the point of view of which locks are held during the body, to write

      void myMethod() {
        synchronized (myLock) {   // no-op:  re-acquire a lock that is already held
          ...
        }
      }

It is better to write a @Holding annotation rather than writing the extra synchronized block. Here are reasons:

- The annotation documents the fact that the lock is intended to already be held; that is, the method’s contract requires that the lock be held when the method is called.
- The Lock Checker enforces that the lock is held when the method is called, rather than masking a programmer error by silently re-acquiring the lock.
- The version with a synchronized statement can deadlock if, due to a programmer error, the lock is not already held. The Lock Checker prevents this type of error.
- The annotation has no run-time overhead. The lock re-acquisition consumes time, even if it succeeds.

## 10.6 Other lock annotations

The Checker Framework’s lock annotations are similar to annotations used elsewhere.

If your code is already annotated with a different lock annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Lock Checker, as described in Figure ‍10.2. If the other annotation is a declaration annotation, it may be moved; see Section ‍39.6.10.

> ‍net.jcip.annotations.GuardedBy ‍
> ‍javax.annotation.concurrent.GuardedBy ‍
> | ⇒  ‍org.checkerframework.checker.lock.qual.GuardedBy (for fields) or …Holding (for methods) ‍

> Figure 10.2: Correspondence between other lock annotations and the Checker Framework’s annotations.

### 10.6.1 Relationship to annotations in Java Concurrency in Practice

The book Java Concurrency in Practice‍[GPB+06] defines a @GuardedBy annotation that is the inspiration for ours. The book’s @GuardedBy serves two related but distinct purposes:

- When applied to a field, it means that the given lock must be held when accessing the field. The lock acquisition and the field access may occur arbitrarily far in the future.
- When applied to a method, it means that the given lock must be held by the caller at the time that the method is called — in other words, at the time that execution passes the @GuardedBy annotation.

The Lock Checker renames the method annotation to @Holding, and it generalizes the @GuardedBy annotation into a type annotation that can apply not just to a field but to an arbitrary type (including the type of a parameter, return value, local variable, generic type parameter, etc.). Another important distinction is that the Lock Checker’s annotations express and enforce a locking discipline over values, just like the JLS expresses Java’s locking semantics; by contrast, JCIP’s annotations express a locking discipline that protects variable names and does not prevent race conditions. This makes the annotations more expressive and also more amenable to automated checking. It also accommodates the distinct meanings of the two annotations, and resolves ambiguity when @GuardedBy is written in a location that might apply to either the method or the return type.

(The JCIP book gives some rationales for reusing the annotation name for two purposes. One rationale is that there are fewer annotations to learn. Another rationale is that both variables and methods are “members” that can be “accessed” and @GuardedBy creates preconditions for doing so. Variables can be accessed by reading or writing them (putfield, getfield), and methods can be accessed by calling them (invokevirtual, invokeinterface). This informal intuition is inappropriate for a tool that requires precise semantics.)

## 10.7 Possible extensions

The Lock Checker validates some uses of locks, but not all. It would be possible to enrich it with additional annotations. This would increase the programmer annotation burden, but would provide additional guarantees.

Lock ordering: Specify that one lock must be acquired before or after another, or specify a global ordering for all locks. This would prevent deadlock.

Not-holding: Specify that a method must not be called if any of the listed locks are held.

These features are supported by Clang’s thread-safety analysis.

# Chapter ‍11 Index Checker for sequence bounds (arrays and strings)

The Index Checker warns about potentially out-of-bounds accesses to sequence data structures, such as arrays and strings.

The Index Checker prevents IndexOutOfBoundsExceptions that result from an index expression that might be negative or might be equal to or larger than the sequence’s length. It also prevents NegativeArraySizeExceptions that result from a negative array dimension in an array creation expression. (A caveat: the Index Checker does not check for arithmetic overflow. If an expression overflows, the Index Checker might fail to warn about a possible exception. This is unlikely to be a problem in practice unless you have an array whose length is Integer.MAX_VALUE.)

The programmer can write annotations that indicate which expressions are indices for which sequences. The Index Checker prohibits any operation that may violate these properties, and the Index Checker takes advantage of these properties when verifying indexing operations. Typically, a programmer writes few annotations, because the Index Checker infers properties of indexes from the code around them. For example, it will infer that x is positive within the then block of an if (x > 0) statement. The programmer does need to write field types and method pre-conditions or post-conditions. For instance, if a method’s formal parameter is used as an index for myArray, the programmer might need to write an @IndexFor("myArray") annotation on the formal parameter’s types.

The Index Checker checks fixed-size data structures, whose size is never changed after creation. A fixed-size data structure has no add or remove operation. Examples are strings and arrays, and you can add support for other fixed-size data structures (see Section ‍11.9).

To run the Index Checker, run either of these commands:

      javac -processor index MyJavaFile.java
      javac -processor org.checkerframework.checker.index.IndexChecker MyJavaFile.java

Recall that in Java, type annotations are written before the type; in particular, array annotations appear immediately before “[]”. Here is how to declare a length-9 array of positive integers:

      @Positive int @ArrayLen(9) []

Multi-dimensional arrays are similar. Here is how to declare a length-2 array of length-4 arrays:

      String @ArrayLen(2) [] @ArrayLen(4) []

## 11.1 Index Checker structure and annotations

Internally, the Index Checker computes information about integers that might be indices:

- the lower bound on an integer, such as whether it is known to be positive (Section ‍11.2)
- the upper bound on an integer, such as whether it is less than the length of a given sequence (Section ‍11.3)
- whether an integer came from calling the JDK’s binary search routine on an array (Section ‍11.6)
- whether an integer came from calling a string search routine (Section ‍11.7)

and about sequence lengths:

- the minimum length of a sequence, such “myArray contains at least 3 elements” (Section ‍11.4)
- whether two sequences have the same length (Section ‍11.5)

The Index Checker checks of all these properties at once, but this manual discusses each type system in a different section. There are some annotations that are shorthand for writing multiple annotations, each from a different type system:

@IndexFor(String[] names)
  The value is a valid index for the named sequences. For example, the String.charAt(int) method is declared as

        class String {
          char charAt(@IndexFor("this") int index) { ... }
        }

  More generally, a variable declared as @IndexFor("someArray") int i has type @IndexFor("someArray") int and its run-time value is guaranteed to be non-negative and less than the length of someArray. You could also express this as @NonNegative @LTLengthOf("someArray") int i, but @IndexFor("someArray") int i is more concise.

@IndexOrHigh(String[] names)
  The value is non-negative and is less than or equal to the length of each named sequence. This type combines @NonNegative and @LTEqLengthOf.

  For example, the Arrays.fill method is declared as

        class Arrays {
          void fill(Object[] a, @IndexFor("#1") int fromIndex, @IndexOrHigh("#1") int toIndex, Object val)
        }

@LengthOf(String[] names)
  The value is exactly equal to the length of the named sequences. In the implementation, this type aliases @IndexOrHigh, so writing it only adds documentation (although future versions of the Index Checker may use it to improve precision).
@IndexOrLow(String[] names)
  The value is -1 or is a valid index for each named sequence. This type combines @GTENegativeOne and @LTLengthOf.
@PolyIndex
  indicates qualifier polymorphism. This type combines @PolyLowerBound and @PolyUpperBound. For a description of qualifier polymorphism, see Section ‍31.2.
@PolyLength
  is a special polymorphic qualifier that combines @PolySameLen and @PolyValue from the Constant Value Checker (see Chapter ‍23). @PolyLength exists as a shorthand for these two annotations, since they often appear together.

## 11.2 Lower bounds

The Index Checker issues an error when a sequence is indexed by an integer that might be negative. The Lower Bound Checker uses a type system (Figure ‍11.1) with the following qualifiers:

@Positive
  The value is 1 or greater, so it is not too low to be used as an index. Note that this annotation is trusted by the Constant Value Checker, so if the Constant Value Checker is run on code containing this annotation, the Lower Bound Checker must be run on the same code in order to guarantee soundness.
@NonNegative
  The value is 0 or greater, so it is not too low to be used as an index.
@GTENegativeOne
  The value is -1 or greater. It may not be used as an index for a sequence, because it might be too low. (“GTE” stands for “Greater Than or Equal to”.)
@PolyLowerBound
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.
@LowerBoundUnknown
  There is no information about the value. It may not be used as an index for a sequence, because it might be too low.
@LowerBoundBottom
  The value cannot take on any integral types. The bottom type, which should not need to be written by the programmer.

> ‍ ‍ ‍ ‍ ‍ ‍ ‍ ‍

> Figure 11.1: The two type hierarchies for integer types used by the Index Checker. On the left is a type system for lower bounds. On the right is a type system for upper bounds. Qualifiers written in gray should never be written in source code; they are used internally by the type system. In the Upper Bound type system, subtyping rules depend on both the array name ("myArray", in the figure) and on the offset (which is 0, the default, in the figure). Another qualifier is @UpperBoundLiteral, whose subtyping relationships depend on its argument and on offsets for other qualifiers.

## 11.3 Upper bounds

The Index Checker issues an error when a sequence index might be too high. To do this, it maintains information about which expressions are safe indices for which sequences. The length of a sequence is arr.length for arrays and str.length() for strings. It uses a type system (Figure ‍11.1) with the following qualifiers:

It issues an error when a sequence arr is indexed by an integer that is not of type @LTLengthOf("arr") or @LTOMLengthOf("arr").

@LTLengthOf(String[] names, String[] offset)
  An expression with this type has value less than the length of each sequence listed in names. The expression may be used as an index into any of those sequences, if it is non-negative. For example, an expression of type @LTLengthOf("a") int might be used as an index to a. The type @LTLengthOf({"a", "b"}) is a subtype of both @LTLengthOf("a") and @LTLengthOf("b"). (“LT” stands for “Less Than”.)

  @LTLengthOf takes an optional offset element, meaning that the annotated expression plus the offset is less than the length of the given sequence. For example, suppose expression e has type @LTLengthOf(value = {"a", "b"}, offset = {"-1", "x"}). Then e - 1 is less than a.length, and e + x is less than b.length. This helps to make the checker more precise. Programmers rarely need to write the offset element.

@LTEqLengthOf(String[] names)
  An expression with this type has value less than or equal to the length of each sequence listed in names. It may not be used as an index for these sequences, because it might be too high. @LTEqLengthOf({"a", "b"}) is a subtype of both @LTEqLengthOf("a") and @LTEqLengthOf("b"). (“LTEq” stands for “Less Than or Equal to”.)

  @LTEqLengthOf({"a"}) = @LTLengthOf(value={"a"}, offset=-1), and
  @LTEqLengthOf(value={"a"}, offset=x) = @LTLengthOf(value={"a"}, offset=x-1) for any x.

@LTOMLengthOf(String[] names)
  An expression with this type has value at least 2 less than the length of each sequence listed in names. It may always used as an index for a sequence listed in names, if it is non-negative.

  This type exists to allow the checker to infer the safety of loops of the form:

        for (int i = 0; i < array.length - 1; ++i) {
          arr[i] = arr[i+1];
        }

  This annotation should rarely (if ever) be written by the programmer; usually @LTLengthOf(String[] names) should be written instead. @LTOMLengthOf({"a", "b"}) is a subtype of both @LTOMLengthOf("a") and @LTOMLengthOf("b"). (“LTOM” stands for “Less Than One Minus”, because another way of saying “at least 2 less than a.length” is “less than a.length-1”.)

  @LTOMLengthOf({"a"}) = @LTLengthOf(value={"a"}, offset=1), and
  @LTOMLengthOf(value={"a"}, offset=x) = @LTLengthOf(value={"a"}, offset=x+1) for any x.

@UpperBoundLiteral
  represents a constant value, typically a literal written in source code. Its subtyping relationship is: @UpperBoundLiteral(lit) <: LTLengthOf(value="myArray", offset=off) if lit+offset ≤ -1.
@PolyUpperBound
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.
@UpperBoundUnknown
  There is no information about the upper bound on the value of an expression with this type. It may not be used as an index for a sequence, because it might be too high. This type is the top type, and should never need to be written by the programmer.
@UpperBoundBottom
  This is the bottom type for the upper bound type system. It should never need to be written by the programmer.

The following method annotations can be used to establish a method postcondition that ensures that a certain expression is a valid index for a sequence:

@EnsuresLTLengthOf(String[] value, String[] targetValue, String[] offset)
  When the method with this annotation returns, the expression (or all the expressions) given in the value element is less than the length of the given sequences with the given offsets. More precisely, the expression has the @LTLengthOf qualifier with the value and offset arguments taken from the targetValue and offset elements of this annotation.
@EnsuresLTLengthOfIf(String[] expression, boolean result, String[] targetValue, String[] offset)
  If the method with this annotation returns the given boolean value, then the given expression (or all the given expressions) is less than the length of the given sequences with the given offsets.

There is one declaration annotation that indicates the relationship between two sequences:

@HasSubsequence(String[] value, String[] from, String[] to)
  indicates that a subsequence (from from to to) of the annotated sequence is equal to some other sequence, named by value).

  For example, to indicate that shorter is a subsequence of longer:

        int start;
        int end;
        int[] shorter;
        @HasSubsequence(value="shorter", from="this.start", to="this.end")
        int[] longer;

  Thus, a valid index into shorter is also a valid index (between start and end-1 inclusive) into longer. More generally, if x is @IndexFor("shorter") in the example above, then start + x is @IndexFor("longer"). If y is @IndexFor("longer") and @LessThan("end"), then y - start is @IndexFor("shorter"). Finally, end - start is @IndexOrHigh("shorter").

  This annotation is in part checked and in part trusted. When an array is assigned to longer, three facts are checked: that start is non-negative, that start is less than or equal to end, and that end is less than or equal to the length of longer. This ensures that the indices are valid. The programmer must manually verify that the value of shorter equals the subsequence that they describe.

## 11.4 Sequence minimum lengths

The Index Checker estimates, for each sequence expression, how long its value might be at run time by computing a minimum length that the sequence is guaranteed to have. This enables the Index Checker to verify indices that are compile-time constants. For example, this code:

      String getThirdElement(String[] arr) {
        return arr[2];
      }

is legal if arr has at least three elements, which can be indicated in this way:

      String getThirdElement(String @MinLen(3) [] arr) {
        return arr[2];
      }

When the index is not a compile-time constant, as in arr[i], then the Index Checker depends not on a @MinLen annotation but on i being annotated as @LTLengthOf("arr").

The MinLen type qualifier is implemented in practice by the Constant Value Checker, using @ArrayLenRange annotations (see Chapter ‍23). This means that errors related to the minimum lengths of arrays must be suppressed using the "value" argument to @SuppressWarnings. @ArrayLenRange and @ArrayLen annotations can also be used to establish the minimum length of a sequence, if a more precise estimate of length is known. For example, if arr is known to have exactly three elements:

      String getThirdElement(String @ArrayLen(3) [] arr) {
        return arr[2];
      }

The following type qualifiers (from Chapter ‍23) can establish the minimum length of a sequence:

@MinLen(int value)
  The value of an expression of this type is a sequence with at least value elements. The default annotation is @MinLen(0), and it may be applied to non-sequences. @MinLen(x) is a subtype of @MinLen(x−1). An @MinLen annotation is treated internally as an @ArrayLenRange with only its from field filled.
@ArrayLen(int[] value)
  The value of an expression of this type is a sequence whose length is exactly one of the integers listed in its argument. The argument can contain at most ten integers; larger collections of integers are converted to @ArrayLenRange annotations. The minimum length of a sequence with this annotation is the smallest element of the argument.
@ArrayLenRange(int from, int to)
  The value of an expression of this type is a sequence whose length is bounded by its arguments, inclusive. The minimum length of a sequence with this annotation is its from argument.

> Figure 11.2: The type hierarchy for arrays of equal length ("a" and "b" are assumed to be in-scope sequences). Qualifiers written in gray should never be written in source code; they are used internally by the type system.

The following method annotation can be used to establish a method postcondition that ensures that a certain sequence has a minimum length:

@EnsuresMinLenIf(String[] expression, boolean result, int targetValue)
  If the method with this annotation returns the given boolean value, then the given expression (or all the given expressions) is a sequence with at least targetValue elements.

## 11.5 Sequences of the same length

The Index Checker determines whether two or more sequences have the same length. This enables it to verify that all the indexing operations are safe in code like the following:

      boolean lessThan(double[] arr1, double @SameLen("#1") [] arr2) {
        for (int i = 0; i < arr1.length; i++) {
          if (arr1[i] < arr2[i]) {
            return true;
          } else if (arr1[i] > arr2[i]) {
            return false;
          }
        }
        return false;
      }

When needed, you can specify which sequences have the same length using the following type qualifiers (Figure ‍11.2):

@SameLen(String[] names)
  An expression with this type represents a sequence that has the same length as the other sequences named in names. In general, @SameLen types that have non-intersecting sets of names are not subtypes of each other. However, if at least one sequence is named by both types, the types are actually the same, because all the named sequences must have the same length.
@PolySameLen
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.
@SameLenUnknown
  No information is known about which other sequences have the same length as this one. This is the top type, and programmers should never need to write it.
@SameLenBottom
  This is the bottom type, and programmers should rarely need to write it. null has this type.

## 11.6 Binary search indices

The JDK’s Arrays.binarySearch method returns either where the value was found, or a negative value indicating where the value could be inserted. The Search Index Checker represents this concept.

> Figure 11.3: The type hierarchy for the Index Checker’s internal type system that captures information about the results of calls to Arrays.binarySearch.

The Search Index Checker’s type hierarchy (Figure ‍11.3) has four type qualifiers:

@SearchIndexFor(String[] names)
  An expression with this type represents an integer that could have been produced by calling Arrays.binarySearch: for each array a specified in the annotation, the annotated integer is between -a.length-1 and a.length-1, inclusive
@NegativeIndexFor(String[] names)
  An expression with this type represents a “negative index” that is between a.length-1 and -1, inclusive; that is, a value that is both a @SearchIndex and is negative. Applying the bitwise complement operator (`~`) to an expression of this type produces an expression of type @IndexOrHigh.
@SearchIndexBottom
  This is the bottom type, and programmers should rarely need to write it.
@SearchIndexUnknown
  No information is known about whether this integer is a search index. This is the top type, and programmers should rarely need to write it.

## 11.7 Substring indices

The methods String.indexOf and String.lastIndexOf return an index of a given substring within a given string, or -1 if no such substring exists. The index i returned from receiver.indexOf(substring) satisfies the following property, which is stated here in three equivalent ways:

     i == -1 || ( i >= 0       && i <= receiver.length() - substring.length()                  )
     i == -1 || ( @NonNegative && @LTLengthOf(value="receiver", offset="substring.length()-1") )
     @SubstringIndexFor(value="receiver", offset="substring.length()-1")

The return type of methods String.indexOf and String.lastIndexOf has the annotation @SubstringIndexFor(value="this", offset="#1.length()-1")). This allows writing code such as the following with no warnings from the Index Checker:

      public static String removeSubstring(String original, String removed) {
        int i = original.indexOf(removed);
        if (i != -1) {
          return original.substring(0, i) + original.substring(i + removed.length());
        }
        return original;
      }

> Figure 11.4: The type hierarchy for the Substring Index Checker, which captures information about the results of calls to String.indexOf and String.lastIndexOf.

The @SubstringIndexFor annotation is implemented in a Substring Index Checker that runs together with the Index Checker and has its own type hierarchy (Figure ‍11.4) with three type qualifiers:

@SubstringIndexFor(String[] value, String[] offset)
  An expression with this type represents an integer that could have been produced by calling String.indexOf: the annotated integer is either -1, or it is non-negative and is less than or equal to receiver.length - offset (where the sequence receiver and the offset offset are corresponding elements of the annotation’s arguments).
@SubstringIndexBottom
  This is the bottom type, and programmers should rarely need to write it.
@SubstringIndexUnknown
  No information is known about whether this integer is a substring index. This is the top type, and programmers should rarely need to write it.

### 11.7.1 The need for the @SubstringIndexFor annotation

No other annotation supported by the Index Checker precisely represents the possible return values of methods String.indexOf and String.lastIndexOf. The reason is the methods’ special cases for empty strings and for failed matches.

Consider the result i of receiver.indexOf(substring):

- i is @GTENegativeOne, because i >= -1.
- i is @LTEqLengthOf("receiver"), because i <= receiver.length().
- i is not @IndexOrLow("receiver"), because for receiver = "", substring = "", i = 0, the property i >= -1 && i < receiver.length() does not hold.
- i is not @IndexOrHigh("receiver"), because for receiver = "", substring = "b", i = -1, the property i >= 0 && i <= receiver.length() does not hold.
- i is not @LTLengthOf(value = "receiver", offset = "substring.length()-1"), because for receiver = "", substring = "abc", i = -1, the property i + substring.length() - 1 < receiver.length() does not hold.

The last annotation in the list above, @LTLengthOf(value = "receiver", offset = "substring.length()-1"), is the correct and precise upper bound for all values of i except -1. The offset expresses the fact that we can add substring.length() to this index and still get a valid index for receiver. That is useful for type-checking code that adds the length of the substring to the found index, in order to obtain the rest of the string. However, the upper bound applies only after the index is explicitly checked not to be -1:

      int i = receiver.indexOf(substring);
      // i is @GTENegativeOne and @LTEqLengthOf("receiver")
      // i is not @LTLengthOf(value = "receiver", offset = "substring.length()-1")
      if (i != -1) {
        // i is @NonNegative and @LTLengthOf(value = "receiver", offset = "substring.length()-1")
        int j = i + substring.length();
        // j is @IndexOrHigh("receiver")
        return receiver.substring(j); // this call is safe
      }

The property of the result of indexOf cannot be expressed by any combination of lower-bound (Section ‍11.2) and upper-bound (Section ‍11.3) annotations, because the upper-bound annotations apply independently of the lower-bound annotations, but in this case, the upper bound i <= receiver.length() - substring.length() holds only if i >= 0. Therefore, to express this property and make the example type-check without false positives, a new annotation such as @SubstringIndexFor(value = "receiver", offset = "substring.length()-1") is necessary.

## 11.8 Inequalities

The Index Checker estimates which expression’s values are less than other expressions’ values.

@LessThan(String[] values)
  An expression with this type has a value that is less than the value of each expression listed in values. The expressions in values must be composed of final or effectively final variables and constants.
@LessThanUnknown
  There is no information about the value of an expression this type relative to other expressions. This is the top type, and should not be written by the programmer.
@LessThanBottom
  This is the bottom type for the less than type system. It should never need to be written by the programmer.

## 11.9 Annotating your own fixed-size datatypes

The Index Checker has built-in support for Strings and arrays. You can add support for additional fixed-size data structures by writing annotations. This allows the Index Checker to typecheck the data structure’s implementation and to typecheck uses of the class.

This section gives an example: a fixed-length collection.

    /** ArrayWrapper is a fixed-size generic collection. */
    public class ArrayWrapper<T> {
        private final Object @SameLen("this") [] delegate;

        @SuppressWarnings("index") // constructor creates object of size @SameLen(this) by definition
        ArrayWrapper(@NonNegative int size) {
            delegate = new Object[size];
        }

        public @LengthOf("this") int size() {
            return delegate.length;
        }

        public void set(@IndexFor("this") int index, T obj) {
            delegate[index] = obj;
        }

        @SuppressWarnings("unchecked") // required for normal Java compilation due to unchecked cast
        public T get(@IndexFor("this") int index) {
            return (T) delegate[index];
        }
    }

The Index Checker treats methods annotated with @LengthOf("this") as the length of a sequence like arr.length for arrays and str.length() for strings.

With these annotations, client code like the following typechecks with no warnings:

      public static void clearIndex1(ArrayWrapper<? extends Object> a, @IndexFor("#1") int i) {
        a.set(i, null);
      }

      public static void clearIndex2(ArrayWrapper<? extends Object> a, int i) {
        if (0 <= i && i < a.size()) {
          a.set(i, null);
        }
      }

## 11.10 Technical papers

The paper “Lightweight Verification of Array Indexing” (ISSTA 2018, https://homes.cs.washington.edu/~mernst/pubs/array-indexing-issta2018-abstract.html) gives more details about the Index Checker. “Enforcing correct array indexes with a type system” ‍[San16] (FSE 2016) describes an earlier version.

# Chapter ‍12 Tainting Checker

The Tainting Checker prevents certain kinds of trust errors. A tainted, or untrusted, value is one that comes from an arbitrary, possibly malicious source, such as user input or unvalidated data. In certain parts of your application, using a tainted value can compromise the application’s integrity, causing it to crash, corrupt data, leak private data, etc.

For example, a user-supplied pointer, handle, or map key should be validated before being dereferenced. As another example, a user-supplied string should not be concatenated into a SQL query, lest the program be subject to a SQL injection attack. A location in your program where malicious data could do damage is called a sensitive sink.

A program must “sanitize” or “untaint” an untrusted value before using it at a sensitive sink. There are two general ways to untaint a value: by checking that it is innocuous/legal (e.g., it contains no characters that can be interpreted as SQL commands when pasted into a string context), or by transforming the value to be legal (e.g., quoting all the characters that can be interpreted as SQL commands). A correct program must use one of these two techniques so that tainted values never flow to a sensitive sink. The Tainting Checker ensures that your program does so.

If the Tainting Checker issues no warning for a given program, then no tainted value ever flows to a sensitive sink. However, your program is not necessarily free from all trust errors. As a simple example, you might have forgotten to annotate a sensitive sink as requiring an untainted type, or you might have forgotten to annotate untrusted data as having a tainted type.

To run the Tainting Checker, supply the -processor TaintingChecker or -processor org.checkerframework.checker.tainting.TaintingChecker command-line option to javac.

## 12.1 Tainting annotations

The Tainting type system uses the following annotations:

@Untainted
  indicates a type that includes only untainted (trusted) values.
@Tainted
  indicates a type that may include tainted (untrusted) or untainted (trusted) values. @Tainted is a supertype of @Untainted. It is the default qualifier.
@PolyTainted
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.

## 12.2 Tips on writing @Untainted annotations

Most programs are designed with a boundary that surrounds sensitive computations, separating them from untrusted values. Outside this boundary, the program may manipulate malicious values, but no malicious values ever pass the boundary to be operated upon by sensitive computations.

In some programs, the area outside the boundary is very small: values are sanitized as soon as they are received from an external source. In other programs, the area inside the boundary is very small: values are sanitized only immediately before being used at a sensitive sink. Either approach can work, so long as every possibly-tainted value is sanitized before it reaches a sensitive sink.

Once you determine the boundary, annotating your program is easy: put @Tainted outside the boundary, @Untainted inside, and @SuppressWarnings("tainting") at the validation or sanitization routines that are used at the boundary.

The Tainting Checker’s standard default qualifier is @Tainted (see Section ‍32.5 for overriding this default). This is the safest default, and the one that should be used for all code outside the boundary (for example, code that reads user input). You can set the default qualifier to @Untainted in code that may contain sensitive sinks.

The Tainting Checker does not know the intended semantics of your program, so it cannot warn you if you mis-annotate a sensitive sink as taking @Tainted data, or if you mis-annotate external data as @Untainted. So long as you correctly annotate the sensitive sinks and the places that untrusted data is read, the Tainting Checker will ensure that all your other annotations are correct and that no undesired information flows exist.

As an example, suppose that you wish to prevent SQL injection attacks. You would start by annotating the Statement class to indicate that the execute operations may only operate on untainted queries (Chapter ‍35 describes how to annotate external libraries):

      public boolean execute(@Untainted String sql) throws SQLException;
      public boolean executeUpdate(@Untainted String sql) throws SQLException;

## 12.3@Tainted and @Untainted can be used for many purposes

The @Tainted and @Untainted annotations have only minimal built-in semantics. In fact, the Tainting Checker provides only a small amount of functionality beyond the Subtyping Checker (Chapter ‍29). This lack of hard-coded behavior has two consequences. The first consequence is that the annotations can serve many different purposes, such as:

- Prevent SQL injection attacks: @Tainted is external input, @Untainted has been checked for SQL syntax.
- Prevent cross-site scripting attacks: @Tainted is external input, @Untainted has been checked for JavaScript syntax.
- Prevent information leakage: @Tainted is secret data, @Untainted may be displayed to a user.

The second consequence is that the Tainting Checker is not useful unless you annotate the appropriate sources, sinks, and untainting/sanitization routines.

If you want more specialized semantics, or you want to annotate multiple types of tainting (for example, HTML and SQL) in a single program, then you can copy the definition of the Tainting Checker to create a new annotation and checker with a more specific name and semantics. You will change the copy to rename the annotations, and you will annotate libraries and/or your code to identify sources, sinks, and validation/sanitization routines. See Chapter ‍36 for more details.

## 12.4 A caution about polymorphism and side effects

Misuse of polymorphism can lead to unsoundness with the Tainting Checker and other similar information flow checkers. To understand the potential problem, consider the append function in java.lang.StringBuffer:

      public StringBuffer append(StringBuffer this, String toAppend);

Given these declarations:

      @Tainted StringBuffer tsb;
      @Tainted String ts;
      @Untainted StringBuffer usb;
      @Untainted String us;

both of these invocations should be legal:

      tsb.append(ts);
      usb.append(us);

That suggests that perhaps the function should be annotated as polymorphic:

      // UNSOUND annotation -- do not do this!
      public @PolyTainted StringBuffer append(@PolyTainted StringBuffer this, @PolyTainted String toAppend);

The problem with the above annotation is that it permits the undesirable invocation:

      usb.append(ts); // illegal invocation

This invocation is permitted because, in the expression, all @PolyTainted annotations on formal parameters are instantiated to @Tainted, the top annotation, and each argument is a subtype of the corresponding formal parameter.

Beware this problem both in code you write, and also in annotated libraries (such as stub files). The correct way to annotate this class is to add a class qualifier parameter; see Section ‍31.3.

(Side note: if append were purely functional (had no side effects and returned a new StringBuffer) the method call would be acceptable, because the return type is instantiated to @Tainted StringBuffer for the expression usb.append(ts). However, the append method works via side-effect, and only returns a reference to the buffer as a convenience for writing “fluent” client code.)

# Chapter ‍13 SQL Quotes Checker

The SQL Quotes Checker helps prevent SQL injection vulnerabilities. Many SQL injection attacks involve malicious user input containing unescaped special characters such as the single quote, which can change whether subsequent concatenations are interpreted as SQL command code or as SQL query values.

The SQL Quotes Checker determines whether a string contains an unescaped single quote character. Furthermore, it marks user-supplied Strings as unsafe for SQL query use, because their quoting is unknown; the program must sanitize or validate all such values before concatenating them to a SQL query for execution. Sanitization can be done by quoting user input as necessary to ensure special characters are properly escaped. The SQL Quotes Checker follows the ANSI standard and considers two consecutive single quote characters as one escaped single quote. (Some databases support the use of the backslash as an escape character for single quotes. The SQL Quotes Checker does not support that syntax because it is not safe for all SQL database implementations.)

The SQL Quotes Checker guarantees that no unchecked values are passed to a SQL query execution statement. It does not guarantee that SQL queries written by the code author are safe for execution; e.g., the checker will not warn at compile time about syntax errors or improper uses of DELETE or DROP statements.

When possible, it is better code style to use a prepared statement or other mechanisms that automatically handle quoting without the need for the programmer to do any work. The SQL Quotes Checker is useful for the large quantity of code that still uses string concatenation to create SQL queries.

To run the SQL Quotes Checker, supply the -processor SqlQuotesChecker or -processor org.checkerframework.checker.sqlquotes.SqlQuotesChecker command-line option to javac.

## 13.1 SQL Quotes annotations

The SQL Quotes type system uses the following annotations:

@SqlEvenQuotes
  indicates a String literal that contains an even number of single quotes (possibly zero).
@SqlOddQuotes
  indicates a String literal that contains an odd number of single quotes.
@SqlQuotesUnknown
  indicates a String whose quoting is unknown. It is the default qualifier for non-literal Strings.
@SqlQuotesBottom
  is the bottom qualifier. Programmers rarely need to write it.

The subtyping hierarchy of the SQL Quotes Checker’s qualifiers is shown in Figure ‍13.1.

> Figure 13.1: The subtyping relationship of the SQL Quotes Checker’s qualifiers. The type qualifiers are applicable to Strings. Qualifiers in gray are used internally by the type system but should never be written by a programmer.

## 13.2 What the SQL Quotes Checker checker

Concatenation of @SqlEvenQuotes and @SqlOddQuotes Strings parallels integer parity over addition. In other words, concatenation of @SqlOddQuotes and @SqlEvenQuotes Strings are most narrowly typed as follows:

        @SqlOddQuotes + @SqlOddQuotes = @SqlEvenQuotes
        @SqlOddQuotes + @SqlEvenQuotes = @SqlOddQuotes
        @SqlEvenQuotes + @SqlOddQuotes = @SqlOddQuotes
        @SqlEvenQuotes + @SqlEvenQuotes = @SqlEvenQuotes

## 13.3 Library annotations

The programmer needs to write trusted annotations on SQL-related methods. This is already done for you, for commonly-used libraries such as java.sql and org.springframework.security.crypto.bcrypt. (If you annotate other libraries, please contribute them to the Checker Framework project. Thanks!)

A SQL query execution method such as Statement.executeQuery must be annotated to take a @SqlEvenQuotes string as a parameter.

A sanitization or quoting method should be annotated to take a @SqlQuotesUnknown parameter and to return a @SqlEvenQuotes result.

Given such annotations, the type system will issue an error if an un-sanitized value can ever flow into a database query. (You might need to write some annotations in your own code as well.)

# Chapter ‍14 Regex Checker for regular expression syntax

The Regex Checker prevents, at compile-time, use of syntactically invalid regular expressions and access of invalid capturing groups.

A regular expression, or regex, is a pattern for matching certain strings of text. In Java, a programmer writes a regular expression as a string. The syntax of regular expressions is complex, so it is easy to make a mistake. It is also easy to accidentally use a regex feature from another language that is not supported by Java (see section “Comparison to Perl 5” in the Pattern Javadoc). These problems cause run-time errors.

Regular expressions in Java also have capturing groups, which are delimited by parentheses and allow for extraction from text. If a programmer uses an incorrect index (larger than the number of capturing groups), an IndexOutOfBoundsException is thrown.

The Regex Checker warns about these problems at compile time, guaranteeing that your program does not crash due to incorrect use of regular expressions.

For further details, including case studies, see the paper “A type system for regular expressions” ‍[SDE12] (FTfJP 2012, https://homes.cs.washington.edu/~mernst/pubs/regex-types-ftfjp2012-abstract.html).

To run the Regex Checker, supply the -processor org.checkerframework.checker.regex.RegexChecker command-line option to javac.

## 14.1 Regex annotations

These qualifiers make up the Regex type system:

@Regex
  indicates that the run-time value is a valid regular expression String. If the optional parameter is supplied to the qualifier, then the number of capturing groups in the regular expression is at least that many. If not provided, the parameter defaults to 0. For example, if an expression’s type is @Regex(1) String, then its run-time value could be "colo(u?)r" or "(brown|beige)" but not "colou?r" nor a non-regex string such as "1) first point".
@PolyRegex
  indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section ‍31.2.

The subtyping hierarchy of the Regex Checker’s qualifiers is shown in Figure ‍14.1.

> Figure 14.1: The subtyping relationship of the Regex Checker’s qualifiers. The type qualifiers are applicable to CharSequence and its subtypes. Because the parameter to a @Regex qualifier is at least the number of capturing groups in a regular expression, a @Regex qualifier with more capturing groups is a subtype of a @Regex qualifier with fewer capturing groups. Qualifiers in gray are used internally by the type system but should never be written by a programmer.

## 14.2 Annotating your code with @Regex

### 14.2.1 Implicit qualifiers

The Regex Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section ‍32.4). If a String literal is a valid regex, the checker implicitly adds the @Regex qualifier with the argument set to the correct number of capturing groups. The Regex Checker allows the null literal to be assigned to any type qualified with the Regex qualifier.

### 14.2.2 Capturing groups

The Regex Checker validates that a legal capturing group number is passed to Matcher’s group, start and end methods. To do this, the type of Matcher must be qualified with a @Regex annotation with the number of capturing groups in the regular expression. This is handled implicitly by the Regex Checker for local variables (see Section ‍32.7), but you may need to add @Regex annotations with a capturing group count to Pattern and Matcher fields and parameters.

### 14.2.3 Concatenation of partial regular expressions

>     public @Regex String parenthesize(@Regex String regex) {
>         return "(" + regex + ")"; // Even though the parentheses are not @Regex Strings,
>                                   // the whole expression is a @Regex String
>     }

> Figure 14.2: An example of the Regex Checker’s support for concatenation of non-regular-expression Strings to produce valid regular expression Strings.

In general, concatenating a non-regular-expression String with any other string yields a non-regular-expression String. The Regex Checker can sometimes determine that concatenation of non-regular-expression Strings will produce valid regular expression Strings. For an example see Figure ‍14.2.

### 14.2.4 Testing whether a string is a regular expression

Sometimes, the Regex Checker cannot infer whether a particular expression is a regular expression — and sometimes your code cannot either! In these cases, you can use the isRegex method to perform such a test, and other helper methods to provide useful error messages. A common use is for user-provided regular expressions (such as ones passed on the command-line). Figure ‍14.3 gives an example of the intended use of the RegexUtil methods.

RegexUtil.isRegex
  returns true if its argument is a valid regular expression.
RegexUtil.regexError
  returns a String error message if its argument is not a valid regular expression, or null if its argument is a valid regular expression.
RegexUtil.regexException
  returns the PatternSyntaxException that Pattern.compile(String) throws when compiling an invalid regular expression. It returns null if its argument is a valid regular expression.

An additional version of each of these methods is also provided that takes an additional group count parameter. The RegexUtil.isRegex method verifies that the argument has at least the given number of groups. The RegexUtil.regexError and RegexUtil.regexException methods return a String error message and PatternSyntaxException, respectively, detailing why the given String is not a syntactically valid regular expression with at least the given number of capturing groups.

If you detect that a String is not a valid regular expression but would like to report the error higher up the call stack (potentially where you can provide a more detailed error message) you can throw a RegexUtil.CheckedPatternSyntaxException. This exception is functionally the same as a PatternSyntaxException except it is checked to guarantee that the error will be handled up the call stack. For more details, see the Javadoc for RegexUtil.CheckedPatternSyntaxException.

To use the RegexUtil class, the checker-util.jar file must be on the classpath at run time.

>     String regex = getRegexFromUser();
>     if (! RegexUtil.isRegex(regex)) {
>        throw new RuntimeException("Error parsing regex " + regex, RegexUtil.regexException(regex));
>     }
>     Pattern p = Pattern.compile(regex);

> Figure 14.3: Example use of RegexUtil methods.

### 14.2.5 Suppressing warnings

If you are positive that a particular string that is being used as a regular expression is syntactically valid, but the Regex Checker cannot conclude this and issues a warning about possible use of an invalid regular expression, then you can use the RegexUtil.asRegex method to suppress the warning.

You can think of this method as a cast: it returns its argument unchanged, but with the type @Regex String if it is a valid regular expression. It throws an error if its argument is not a valid regular expression, but you should only use it when you are sure it will not throw an error.

There is an additional RegexUtil.asRegex method that takes a capturing group parameter. This method works the same as described above, but returns a @Regex String with the parameter on the annotation set to the value of the capturing group parameter passed to the method.

The use case shown in Figure ‍14.3 should support most cases so the asRegex method should be used rarely.

# Chapter ‍15 Format String Checker

The Format String Checker prevents use of incorrect format strings in format methods such as System.out.printf and String.format.

The Format String Checker warns you if you write an invalid format string, and it warns you if the other arguments are not consistent with the format string (in number of arguments or in their types). Here are examples of errors that the Format String Checker detects at compile time. Section ‍15.3 provides more details.

      String.format("%y", 7);           // error: invalid format string

      String.format("%d", "a string");  // error: invalid argument type for %d

      String.format("%d %s", 7);        // error: missing argument for %s
      String.format("%d", 7, 3);        // warning: unused argument 3
      String.format("{0}", 7);          // warning: unused argument 7, because {0} is wrong syntax

To run the Format String Checker, supply the -processor org.checkerframework.checker.formatter.FormatterChecker command-line option to javac.

The paper “A type system for format strings” ‍[WKSE14] (ISSTA 2014, https://homes.cs.washington.edu/~mernst/pubs/format-string-issta2014-abstract.html) gives more details about the Format String Checker and the Internationalization Format String Checker (Chapter ‍16);

## 15.1 Formatting terminology

Printf-style formatting takes as an argument a format string and a list of arguments. It produces a new string in which each format specifier has been replaced by the corresponding argument. The format specifier determines how the format argument is converted to a string. A format specifier is introduced by a % character. For example, String.format("The %s is %d.","answer",42) yields "The answer is 42.". "The %s is %d." is the format string, "%s" and "%d" are the format specifiers; "answer" and 42 are format arguments.

## 15.2 Format String Checker annotations

The @Format qualifier on a string type indicates a valid format string. A programmer rarely writes the @Format annotation, as it is inferred for string literals. A programmer may need to write it on fields and on method signatures.

The @Format qualifier is parameterized with a list of conversion categories that impose restrictions on the format arguments. Conversion categories are explained in more detail in Section ‍15.2.1. The type qualifier for "%d %f" is for example @Format({INT, FLOAT}).

Consider the below printFloatAndInt method. Its parameter must be a format string that can be used in a format method, where the first format argument is “float-like” and the second format argument is “integer-like”. The type of its parameter, @Format({FLOAT, INT}) String, expresses that contract.

        void printFloatAndInt(@Format({FLOAT, INT}) String fs) {
            System.out.printf(fs, 3.1415, 42);
        }

        printFloatAndInt("Float %f, Number %d");  // OK
        printFloatAndInt("Float %f");             // error

> Figure 15.1: The Format String Checker type qualifier hierarchy. The type qualifiers are applicable to CharSequence and its subtypes. The figure does not show the subtyping rules among different @Format(...) qualifiers; see Section ‍15.2.2.

Figure ‍15.1 shows all the type qualifiers. The annotations other than @Format are only used internally and cannot be written in your code. @InvalidFormat indicates an invalid format string — that is, a string that cannot be used as a format string. For example, the type of "%y" is @InvalidFormat String. @FormatBottom is the type of the null literal. @UnknownFormat is the default that is applied to strings that are not literals and on which the user has not written a @Format annotation.

There is also a @FormatMethod annotation; see Section ‍15.5.

### 15.2.1 Conversion Categories

Given a format specifier, only certain format arguments are compatible with it, depending on its “conversion” — its last, or last two, characters. For example, in the format specifier "%d", the conversion d restricts the corresponding format argument to be “integer-like”:

        String.format("%d", 5);         // OK
        String.format("%d", "hello");   // error

Many conversions enforce the same restrictions. A set of restrictions is represented as a conversion category. The “integer like” restriction is for example the conversion category INT. The following conversion categories are defined in the ConversionCategory enumeration:

  GENERAL imposes no restrictions on a format argument’s type. Applicable for conversions b, B, h, H, s, S.
  CHAR requires that a format argument represents a Unicode character. Specifically, char, Character, byte, Byte, short, and Short are allowed. int or Integer are allowed if Character.isValidCodePoint(argument) would return true for the format argument. (The Format String Checker permits any int or Integer without issuing a warning or error — see Section ‍15.3.2.) Applicable for conversions c, C.
  INT requires that a format argument represents an integral type. Specifically, byte, Byte, short, Short, int and Integer, long, Long, and BigInteger are allowed. Applicable for conversions d, o, x, X.
  FLOAT requires that a format argument represents a floating-point type. Specifically, float, Float, double, Double, and BigDecimal are allowed. Surprisingly, integer values are not allowed. Applicable for conversions e, E, f, g, G, a, A.
  TIME requires that a format argument represents a date or time. Specifically, long, Long, Calendar, and Date are allowed. Applicable for conversions t, T.
  UNUSED imposes no restrictions on a format argument. This is the case if a format argument is not used as replacement for any format specifier. "%2$s" for example ignores the first format argument.

All conversion categories accept null. Furthermore, null is always a legal argument array, because it is treated as supplying null to each format specifer. For example, String.format("%d %f %s", (Object[]) null) evaluates to "null null null".

The same format argument may serve as a replacement for multiple format specifiers. Until now, we have assumed that the format specifiers simply consume format arguments left to right. But there are two other ways for a format specifier to select a format argument:

- n$ specifies a one-based index n. In the format string "%2$s", the format specifier selects the second format argument.
- The < flag references the format argument that was used by the previous format specifier. In the format string "%d %<d" for example, both format specifiers select the first format argument.

In the following example, the format argument must be compatible with both conversion categories, and can therefore be neither a Character nor a long.

        format("Char %1$c, Int %1$d", (int)42);            // OK
        format("Char %1$c, Int %1$d", new Character(42));  // error
        format("Char %1$c, Int %1$d", (long)42);           // error

Only three additional conversion categories are needed represent all possible intersections of previously-mentioned conversion categories:

  NULL is used if no object of any type can be passed as parameter. In this case, the only legal value is null. For example, the format string "%1$f %1$c" requires that the first format argument be null. Passing either 4 or 4.2 would lead to an exception.
  CHAR_AND_INT is used if a format argument is restricted by a CHAR and a INT conversion category (CHAR ∩ INT).
  INT_AND_TIME is used if a format argument is restricted by an INT and a TIME conversion category (INT ∩ TIME).

All other intersections lead to already existing conversion categories. For example, GENERAL ∩ CHAR = CHAR and UNUSED ∩ GENERAL = GENERAL.

Figure ‍15.2 summarizes the subset relationship among all conversion categories.

> Figure 15.2: The subset relationship among conversion categories.

### 15.2.2 Subtyping rules for @Format

Here are the subtyping rules among different @Format qualifiers. It is legal to:

- use a format string with a weaker (less restrictive) conversion category than required.
- use a format string with fewer format specifiers than required. Although this is legal a warning is issued because most occurrences of this are due to programmer error.

The following example shows the subtyping rules in action:

        @Format({FLOAT, INT}) String f;

        f = "%f %d";       // OK
        f = "%s %d";       // OK, %s is weaker than %f
        f = "%f";          // warning: last argument is ignored
        f = "%f %d %s";    // error: too many arguments
        f = "%d %d";       // error: %d is not weaker than %f

        String.format(f, 0.8, 42);

## 15.3 What the Format String Checker checks

If the Format String Checker issues no errors, it provides the following guarantees:

1. The following guarantees hold for every format method invocation:
   1. The format method’s first parameter (or second if a Locale is provided) is a valid format string (or null).
   2. A warning is issued if one of the format string’s conversion categories is UNUSED.
   3. None of the format string’s conversion categories is NULL.
2. If the format arguments are passed to the format method as varargs, the Format String Checker guarantees the following additional properties:
   1. No fewer format arguments are passed than required by the format string.
   2. A warning is issued if more format arguments are passed than required by the format string.
   3. Every format argument’s type satisfies its conversion category’s restrictions.
3. If the format arguments are passed to the format method as an array, a warning is issued by the Format String Checker.

Following are examples for every guarantee:

        String.format("%d", 42);                      // OK
        String.format(Locale.GERMAN, "%d", 42);       // OK
        String.format(new Object());                  // error (1a)
        String.format("%y");                          // error (1a)
        String.format("%2$s", "unused", "used");      // warning (1b)
        String.format("%1$d %1$f", 5.5);              // error (1c)
        String.format("%1$d %1$f %d", null, 6);       // error (1c)
        String.format("%s");                          // error (2a)
        String.format("%s", "used", "ignored");       // warning (2b)
        String.format("%c",4.2);                      // error (2c)
        String.format("%c", (String)null);            // error (2c)
        String.format("%1$d %1$f", new Object[]{1});  // warning (3)
        String.format("%s", new Object[]{"hello"});   // warning (3)

### 15.3.1 Possible false alarms

There are three cases in which the Format String Checker may issue a warning or error, even though the code cannot fail at run time. (These are in addition to the general conservatism of a type system: code may be correct because of application invariants that are not captured by the type system.) In each of these cases, you can rewrite the code, or you can manually check it and write a @SuppressWarnings annotation if you can reason that the code is correct.

Case 1(b): Unused format arguments. It is legal to provide more arguments than are required by the format string; Java ignores the extras. However, this is an uncommon case. In practice, a mismatch between the number of format specifiers and the number of format arguments is usually an error.

Case 1(c): Format arguments that can only be null. It is legal to write a format string that permits only null arguments and throws an exception for any other argument. An example is String.format("%1$d %1$f", null). The Format String Checker forbids such a format string. If you should ever need such a format string, simply replace the problematic format specifier with "null". For example, you would replace the call above by String.format("null null").

Case 3: Array format arguments. The Format String Checker performs no analysis of arrays, only of varargs invocations. It is better style to use varargs when possible.

### 15.3.2 Possible missed alarms

The Format String Checker helps prevent bugs by detecting, at compile time, which invocations of format methods will fail. While the Format String Checker finds most of these invocations, there are cases in which a format method call will fail even though the Format String Checker issued neither errors nor warnings. These cases are:

1. The format string is null. Use the Nullness Checker to prevent this.
2. A format argument’s toString method throws an exception.
3. A format argument implements the Formattable interface and throws an exception in the formatTo method.
4. A format argument’s conversion category is CHAR or CHAR_AND_INT, and the passed value is an int or Integer, and Character.isValidCodePoint(argument) returns false.

The following examples illustrate these limitations:

        class A {
            public String toString() {
                throw new Error();
            }
        }

        class B implements Formattable {
            public void formatTo(Formatter fmt, int f,
                    int width, int precision) {
                throw new Error();
            }
        }

        // The checker issues no errors or warnings for the
        // following illegal invocations of format methods.
        String.format(null);          // NullPointerException (1)
        String.format("%s", new A()); // Error (2)
        String.format("%s", new B()); // Error (3)
        String.format("%c", (int)-1); // IllegalFormatCodePointException (4)

## 15.4 Implicit qualifiers

The Format String Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section ‍32.4). The checker implicitly adds the @Format qualifier with the appropriate conversion categories to any String literal that is a valid format string.

## 15.5 @FormatMethod

Your project may contain methods that forward their arguments to a format method. Consider for example the following log method:

    @FormatMethod
    void log(String format, Object... args) {
        if (enabled) {
            logfile.print(indent_str);
            logfile.printf(format , args);
        }
    }

You should annotate such a method with the @FormatMethod annotation, which indicates that the String argument is a format string for the remaining arguments.

## 15.6 Testing whether a format string is valid

The Format String Checker automatically determines whether each String literal is a valid format string or not. When a string is computed or is obtained from an external resource, then the string must be trusted or tested.

One way to test a string is to call the FormatUtil.asFormat method to check whether the format string is valid and its format specifiers match certain conversion categories. If this is not the case, asFormat raises an exception. Your code should catch this exception and handle it gracefully.

The following code examples may fail at run time, and therefore they do not type check. The type-checking errors are indicated by comments.

    Scanner s = new Scanner(System.in);
    String fs = s.next();
    System.out.printf(fs, "hello", 1337);          // error: fs is not known to be a format string

    Scanner s = new Scanner(System.in);
    @Format({GENERAL, INT}) String fs = s.next();  // error: fs is not known to have the given type
    System.out.printf(fs, "hello", 1337);          // OK

The following variant does not throw a run-time error, and therefore passes the type-checker:

    Scanner s = new Scanner(System.in);
    String format = s.next()
    try {
        format = FormatUtil.asFormat(format, GENERAL, INT);
    } catch (IllegalFormatException e) {
        // Replace this by your own error handling.
        System.err.println("The user entered the following invalid format string: " + format);
        System.exit(2);
    }
    // fs is now known to be of type: @Format({GENERAL, INT}) String
    System.out.printf(format, "hello", 1337);

To use the FormatUtil class, the checker-util.jar file must be on the classpath at run time.

# Chapter ‍16 Internationalization Format String Checker (I18n Format String Checker)

The Internationalization Format String Checker, or I18n Format String Checker, prevents use of incorrect i18n format strings.

If the I18n Format String Checker issues no warnings or errors, then MessageFormat.format will raise no error at run time. “I18n” is short for “internationalization” because there are 18 characters between the “i” and the “n”.

Here are the examples of errors that the I18n Format Checker detects at compile time.

      // Warning: the second argument is missing.
      MessageFormat.format("{0} {1}", 3.1415);
      // String argument cannot be formatted as Time type.
      MessageFormat.format("{0, time}", "my string");
      // Invalid format string: unknown format type: thyme.
      MessageFormat.format("{0, thyme}", new Date());
      // Invalid format string: missing the right brace.
      MessageFormat.format("{0", new Date());
      // Invalid format string: the argument index is not an integer.
      MessageFormat.format("{0.2, time}", new Date());
      // Invalid format string: "#.#.#" subformat is invalid.
      MessageFormat.format("{0, number, #.#.#}", 3.1415);

For instructions on how to run the Internationalization Format String Checker, see Section ‍16.6.

The Internationalization Checker or I18n Checker (Chapter ‍17.2) has a different purpose. It verifies that your code is properly internationalized: any user-visible text should be obtained from a localization resource and all keys exist in that resource.

The paper “A type system for format strings” ‍[WKSE14] (ISSTA 2014, https://homes.cs.washington.edu/~mernst/pubs/format-string-issta2014-abstract.html) gives more details about the Internationalization Format String Checker and the Format String Checker (Chapter ‍15);

## 16.1 Internationalization Format String Checker annotations

> Figure 16.1: The Internationalization Format String Checker type qualifier hierarchy. The type qualifiers are applicable to CharSequence and its subtypes. The figure does not show the subtyping rules among different @I18nFormat(...) qualifiers; see Section ‍16.2. All @I18nFormatFor annotations are unrelated by subtyping, unless they are identical. The qualifiers in gray are used internally by the checker and should never be written by a programmer.

The MessageFormat documentation specifies the syntax of the i18n format string.

These are the qualifiers that make up the I18n Format String type system. Figure ‍16.1 shows their subtyping relationships.

@I18nFormat
  represents a valid i18n format string. For example, @I18nFormat({GENERAL, NUMBER, UNUSED, DATE}) is a legal type for "{0}{1, number} {3, date}", indicating that when the format string is used, the first argument should be of GENERAL conversion category, the second argument should be of NUMBER conversion category, and so on. Conversion categories such as GENERAL are described in Section ‍16.2.
@I18nFormatFor
  indicates that the qualified type is a valid i18n format string for use with some array of values. For example, @I18nFormatFor("#2") indicates that the string can be used to format the contents of the second parameter array. The argument is a Java expression whose syntax is explained in Section ‍32.8. An example of its use is:

        static void method(@I18nFormatFor("#2") String format, Object... args) {
          // the body may use the parameters like this:
          MessageFormat.format(format, args);
        }

        method("{0, number} {1}", 3.1415, "A string");  // OK
        // error: The string "hello" cannot be formatted as a Number.
        method("{0, number} {1}", "hello", "goodbye");

@I18nInvalidFormat
  represents an invalid i18n format string. Programmers are not allowed to write this annotation. It is only used internally by the type checker.
@I18nUnknownFormat
  represents any string. The string might or might not be a valid i18n format string. Programmers are not allowed to write this annotation.
@I18nFormatBottom
  indicates that the value is definitely null. Programmers are not allowed to write this annotation.

## 16.2 Conversion categories

In a message string, the optional second element within the curly braces is called a format type and must be one of number, date, time, and choice. These four format types correspond to different conversion categories. date and time correspond to DATE in the conversion categories figure. choice corresponds to NUMBER. The format type restricts what arguments are legal. For example, a date argument is not compatible with the number format type, i.e., MessageFormat.format("{0, number}", new Date()) will throw an exception.

The I18n Checker represents the possible arguments via conversion categories. A conversion category defines a set of restrictions or a subtyping rule.

Figure ‍16.2 summarizes the subset relationship among all conversion categories.

> Figure 16.2: The subset relationship among i18n conversion categories.

## 16.3 Subtyping rules for @I18nFormat

Here are the subtyping rules among different @I18nFormat qualifiers. It is legal to:

- use a format string with a weaker (less restrictive) conversion category than required.
- use a format string with fewer format specifiers than required. Although this is legal a warning is issued because most occurrences of this are due to programmer error.

The following example shows the subtyping rules in action:

      @I18nFormat({NUMBER, DATE}) String f;

      f = "{0, number, #.#} {1, date}"; // OK
      f = "{0, number} {1}";            // OK, GENERAL is weaker (less restrictive) than DATE
      f = "{0} {1, date}";              // OK, GENERAL is weaker (less restrictive) than NUMBER
      f = "{0, number}";                // warning: last argument is ignored
      f = "{0}";                        // warning: last argument is ignored
      f = "{0, number} {1, number}";    // error: NUMBER is stronger (more restrictive) than DATE
      f = "{0} {1} {2}";                // error: too many arguments

The conversion categories are:

UNUSED
  indicates an unused argument. For example, in MessageFormat.format("{0, number} {2, number}", 3.14, "Hello", 2.718) , the second argument Hello is unused. Thus, the conversion categories for the format, 0, number 2, number, is (NUMBER, UNUSED, NUMBER).
GENERAL
  means that any value can be supplied as an argument.
DATE
  is applicable for date, time, and number types. An argument needs to be of Date, Time, or Number type or a subclass of them, including Timestamp and the classes listed immediately below.
NUMBER
  means that the argument needs to be of Number type or a subclass: Number, AtomicInteger, AtomicLong, BigDecimal, BigInteger, Byte, Double, Float, Integer, Long, Short.

## 16.4 What the Internationalization Format String Checker checks

The Internationalization Format String Checker checks calls to the i18n formatting method MessageFormat.format and guarantees the following:

1. The checker issues a warning for the following cases:
   1. There are missing arguments from what is required by the format string.

      MessageFormat.format("{0, number} {1, number}", 3.14); // Output: 3.14 {1}

   2. More arguments are passed than what is required by the format string.

      MessageFormat.format("{0, number}", 1, new Date());

      MessageFormat.format("{0, number} {0, number}", 3.14, 3.14);

      This does not cause an error at run time, but it often indicates a programmer mistake. If it is intentional, then you should suppress the warning (see Chapter ‍33).

   3. Some argument is an array of objects.

      MessageFormat.format("{0, number} {1}", array);

      The checker cannot verify whether the format string is valid, so the checker conservatively issues a warning. This is a limitation of the Internationalization Format String Checker.

2. The checker issues an error for the following cases:
   1. The format string is invalid.
      - Unmatched braces.

        MessageFormat.format("{0, time", new Date());

      - The argument index is not an integer or is negative.

        MessageFormat.format("{0.2, time}", new Date());

        MessageFormat.format("{-1, time}", new Date());

      - Unknown format type.

        MessageFormat.format("{0, foo}", 3.14);

      - Missing a format style required for choice format.

        MessageFormat.format("{0, choice}", 3.14);

      - Wrong format style.

        MessageFormat.format("{0, time, number}", 3.14);

      - Invalid subformats.

        MessageFormat.format("{0, number, #.#.#}", 3.14)

   2. Some argument’s type doesn’t satisfy its conversion category.

      MessageFormat.format("{0, number}", new Date());

The Checker also detects illegal assignments: assigning a non-format-string or an incompatible format string to a variable declared as containing a specific type of format string. For example,

      @I18nFormat({GENERAL, NUMBER}) String format;
      // OK.
      format = "{0} {1, number}";
      // OK, GENERAL is weaker (less restrictive) than NUMBER.
      format = "{0} {1}";
      // OK, it is legal to have fewer arguments than required (less restrictive).
      // But the warning will be issued instead.
      format = "{0}";

      // Error, the format string is stronger (more restrictive) than the specifiers.
      format = "{0} {1} {2}";
      // Error, the format string is more restrictive. NUMBER is a subtype of GENERAL.
      format = "{0, number} {1, number}";

## 16.5 Resource files

A programmer rarely writes an i18n format string literally. (The examples in this chapter show that for simplicity.) Rather, the i18n format strings are read from a resource file. The program chooses a resource file at run time depending on the locale (for example, different resource files for English and Spanish users).

For example, suppose that the resource1.properties file contains

      key1 = The number is {0, number}.

Then code such as the following:

      String formatPattern = ResourceBundle.getBundle("resource1").getString("key1");
      System.out.println(MessageFormat.format(formatPattern, 2.2361));

will output “The number is 2.2361.” A different resource file would contain key1 = El número es {0, number}.

When you run the I18n Format String Checker, you need to indicate which resource file it should check. If you change the resource file or use a different resource file, you should re-run the checker to ensure that you did not make an error. The I18n Format String Checker supports two types of resource files: ResourceBundles and property files. The example above shows use of resource bundles. For more about checking property files, see Chapter ‍17.

## 16.6 Running the Internationalization Format Checker

The checker can be invoked by running one of the following commands (with the whole command on one line).

- Using ResourceBundles:

  javac -processor org.checkerframework.checker.i18nformatter.I18nFormatterChecker -Abundlenames=MyResource MyFile.java

- Using property files:

  javac -processor org.checkerframework.checker.i18nformatter.I18nFormatterChecker -Apropfiles=MyResource.properties MyFile.java

- Not using a property file. Use this if the programmer hard-coded the format patterns without loading them from a property file.

  javac -processor org.checkerframework.checker.i18nformatter.I18nFormatterChecker MyFile.java

## 16.7 Testing whether a string has an i18n format type

In the case that the checker cannot infer the i18n format type of a string, you can use the I18nFormatUtil.hasFormat method to define the type of the string in the scope of a conditional statement.

I18nFormatUtil.hasFormat
  returns true if the given string has the given i18n format type.

For an example, see Section ‍16.8.

To use the I18nFormatUtil class, the checker-util.jar file must be on the classpath at run time.

## 16.8 Examples of using the Internationalization Format Checker

- Using MessageFormat.format.

            // suppose the bundle "MyResource" contains:  key1={0, number} {1, date}
            String value = ResourceBundle.getBundle("MyResource").getString("key1");
            MessageFormat.format(value, 3.14, new Date());  // OK
            // error: incompatible types in argument; found String, expected number
            MessageFormat.format(value, "Text", new Date());

- Using the I18nFormatUtil.hasFormat method to check whether a format string has particular conversion categories.

            void test1(String format) {
              if (I18nFormatUtil.hasFormat(format, I18nConversionCategory.GENERAL,
                                                   I18nConversionCategory.NUMBER)) {
                MessageFormat.format(format, "Hello", 3.14);  // OK
                // error: incompatible types in argument; found String, expected number
                MessageFormat.format(format, "Hello", "Bye");
                // error: missing arguments; expected 2 but 1 given
                MessageFormat.format(format, "Bye");
                // error: too many arguments; expected 2 but 3 given
                MessageFormat.format(format, "A String", 3.14, 3.14);
              }
            }

- Using @I18nFormatFor to ensure that an argument is a particular type of format string.

            static void method(@I18nFormatFor("#2") String f, Object... args) {...}

            // OK, MessageFormat.format(...) would return "3.14 Hello greater than one"
            method("{0, number} {1} {2, choice,0#zero|1#one|1<greater than one}",
                   3.14, "Hello", 100);

            // error: incompatible types in argument; found String, expected number
            method("{0, number} {1}", "Bye", "Bye");

- Annotating a string with @I18nFormat.

            @I18nFormat({I18nConversionCategory.DATE}) String;
            s1 = "{0}";
            s1 = "{0, number}";        // error: incompatible types in assignment

# Chapter ‍17 Property File Checker

The Property File Checker ensures that a property file or resource bundle (both of which act like maps from keys to values) is only accessed with valid keys. Accesses without a valid key either return null or a default value, which can lead to a NullPointerException or hard-to-trace behavior. The Property File Checker (Section 17.1) ensures that the used keys are found in the corresponding property file or resource bundle.

We also provide two specialized checkers. An Internationalization Checker (Section 17.2) verifies that code is properly internationalized. A Compiler Message Key Checker (Section 17.3) verifies that compiler message keys used in the Checker Framework are declared in a property file. This is an example of a simple specialization of the property file checker, and the Checker Framework source code shows how it is used.

It is easy to customize the property key checker for other related purposes. Take a look at the source code of the Compiler Message Key Checker and adapt it for your purposes.

## 17.1 General Property File Checker

The general Property File Checker ensures that a resource key is located in a specified property file or resource bundle.

The annotation @PropertyKey indicates that the qualified CharSequence is a valid key found in the property file or resource bundle. You do not need to annotate String literals. The checker looks up every String literal in the specified property file or resource bundle, and adds annotations as appropriate.

If you pass a CharSequence (including String) variable to be eventually used as a key, you also need to annotate all these variables with @PropertyKey.

The checker can be invoked by running the following command:

      javac -processor org.checkerframework.checker.propkey.PropertyKeyChecker
            -Abundlenames=MyResource MyFile.java ...

You must specify the resources, which map keys to strings. The checker supports two types of resource: resource bundles and property files. You can specify one or both of the following two command-line options:

1. -Abundlenames=resource_name

   resource_name is the name of the resource to be used with ResourceBundle.getBundle(). The checker uses the default Locale and ClassLoader in the compilation system. (For a tutorial about ResourceBundles, see https://docs.oracle.com/javase/tutorial/i18n/resbundle/concept.html.) Multiple resource bundle names are separated by colons ’:’.

2. -Apropfiles=prop_file

   prop_file is the name of a properties file that maps keys to values. The file format is described in the Javadoc for Properties.load(). Multiple files are separated by Java’s File.pathSeparator (semicolon “;” on Windows, colon “:” on Linux and Mac).

## 17.2 Internationalization Checker (I18n Checker)

The Internationalization Checker, or I18n Checker, verifies that your code is properly internationalized. Internationalization is the process of designing software so that it can be adapted to different languages and locales without needing to change the code. Localization is the process of adapting internationalized software to specific languages and locales.

Internationalization is sometimes called i18n, because the word starts with “i”, ends with “n”, and has 18 characters in between. Localization is similarly sometimes abbreviated as l10n.

The checker focuses on one aspect of internationalization: user-visible text should be presented in the user’s own language, such as English, French, or German. This is achieved by looking up keys in a localization resource, which maps keys to user-visible text. For instance, one version of a resource might map "CANCEL_STRING" to "Cancel", and another version of the same resource might map "CANCEL_STRING" to "Abbrechen".

There are other aspects to localization, such as formatting of dates (3/5 vs. ‍5/3 for March 5), that the checker does not check.

The Internationalization Checker verifies these two properties:

1. Any user-visible text should be obtained from a localization resource. For example, String literals should not be output to the user.
2. When looking up keys in a localization resource, the key should exist in that resource. This check catches incorrect or misspelled localization keys.

If you use the Internationalization Checker, you may want to also use the Internationalization Format String Checker, or I18n Format String Checker (Chapter ‍16). It verifies that internationalization format strings are well-formed and used with arguments of the proper type, so that MessageFormat.format does not fail at run time.

### 17.2.1 Internationalization annotations

The Internationalization Checker supports two type systems:

@Localized
  indicates that the qualified CharSequence is a message that has been localized and/or formatted with respect to the used locale. It is a subtype of @UnknownLocalized, which is the default and which programmers do not need to write.
@LocalizableKey
  indicates that the qualified CharSequence or Object is a valid key found in the localization resource. This annotation is a specialization of the @PropertyKey annotation, that gets checked by the general Property Key Checker. It is a subtype of @UnknownLocalizableKey, which is the default and which programmers do not need to write.

You may need to add the @Localized annotation to more methods in the JDK or other libraries, or in your own code.

### 17.2.2 Running the Internationalization Checker

The Internationalization Checker can be invoked by running the following command:

      javac -processor org.checkerframework.checker.i18n.I18nChecker -Abundlenames=MyResource MyFile.java ...

You must specify the localization resource, which maps keys to user-visible text. Like the general Property Key Checker, the Internationalization Checker supports two types of localization resource: ResourceBundles using the -Abundlenames=resource_name option or property files using the -Apropfiles=prop_file option.

## 17.3 Compiler Message Key Checker

The Checker Framework uses compiler message keys to output error messages. These keys are substituted by localized strings for user-visible error messages. Using keys instead of the localized strings in the source code enables easier testing, as the expected message keys can stay unchanged while the localized strings can still be modified. We use the Compiler Message Key Checker to ensure that all internal keys are correctly localized. Instead of using the Property File Checker, we use a specialized checker, giving us more precise documentation of the intended use of Strings.

The single annotation used by this checker is @CompilerMessageKey. The Checker Framework is completely annotated; for example, class org.checkerframework.framework.source.Result uses @CompilerMessageKey in methods failure and warning. For most users of the Checker Framework there will be no need to annotate any Strings, as the checker looks up all String literals and adds annotations as appropriate.

The Compiler Message Key Checker can be invoked by running the following command:

      javac -processor org.checkerframework.checker.compilermsgs.CompilerMessagesChecker
            -Apropfiles=messages.properties MyFile.java ...

You must specify the resource, which maps compiler message keys to user-visible text. The checker supports the same options as the general property key checker. Within the Checker Framework we only use property files, so the -Apropfiles=prop_file option should be used.

# Chapter ‍18 Signature String Checker for string representations of types

The Signature String Checker, or Signature Checker for short, verifies that string representations of types and signatures are used correctly.

Java defines multiple different string representations for types (see Section ‍18.1), and it is easy to misuse them or to miss bugs during testing. Using the wrong string format leads to a run-time exception or an incorrect result. This is a particular problem for fully qualified and binary names, which are nearly the same — they differ only for nested classes and arrays.

The paper “Building and using pluggable type-checkers” ‍[DDE+11] (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/pluggable-checkers-icse2011.pdf) describes case studies of the Signature String Checker.

## 18.1 Signature annotations

Java defines six formats for the string representation of a type. There is an annotation for each of these representations. Figure ‍18.1 shows how they are related; examples appear in a table below.

> Figure 18.1: Partial type hierarchy for the Signature type system, showing string representations of a Java type. The type qualifiers are applicable to CharSequence and its subtypes. Programmers usually only need to write the boldfaced qualifiers. The other qualifiers (and some not shown) are included to improve the internal handling of String literals.

@FullyQualifiedName
  A fully qualified name (JLS §6.7), such as mypackage.Outer.Inner, is used in Java code and in messages to the user.
@ClassGetName
  The type representation used by the Class.getName(), Class.forName(String), and Class.forName(String, boolean, ClassLoader) methods. This format is: for any non-array type, the binary name; and for any array type, a format like the FieldDescriptor field descriptor, but using “.” ‍where the field descriptor uses “/”. See examples below.
@FieldDescriptor
  A field descriptor (JVMS §4.3.2), such as Lmypackage/Outer$Inner;, is used in a .class file’s constant pool, for example to refer to other types. It abbreviates primitive types and array types. It uses internal form (binary names, but with / instead of .; see JVMS §4.2) for class names. See examples below.
@BinaryName
  A binary name (JLS §13.1), such as mypackage.Outer$Inner, is the conceptual name of a type in its own .class file.
@InternalForm
  The internal form (JVMS §4.2), such as mypackage/Outer$Inner, is how a class name is actually represented in its own .class file. It is also known as the “syntax of binary names that appear in class file structures”. It is the same as the binary name, but with periods (.) replaced by slashes (/). Programmers more often use the binary name, leaving the internal form as a JVM implementation detail.
@ClassGetSimpleName
  The type representation returned by the Class.getSimpleName() method. This format is not required by any method in the JDK, so you will rarely write it in source code. The string can be empty. This is not the same as the “simple name” defined in (JLS §6.2), which is the same as @Identifier.
@FqBinaryName
  An extension of binary name format to represent primitives and arrays. It is like @FullyQualifiedName, but using “$” instead of “.” to separate nested classes from their enclosing classes. For example, "pkg.Outer$Inner" or "pkg.Outer$Inner[][]" or "int[]".
@CanonicalName
  Syntactically identical to @FullyQualifiedName, but some classes have multiple fully-qualified names, only one of which is canonical (see JLS §6.7).

Other type qualifiers are the intersection of two or more qualifiers listed above; for example, a @DotSeparatedIdentifiers is a string that is a valid fully-qualified name and a valid binary name. A programmer should never or rarely use these qualifiers, and you can ignore them as implementation details of the Signature Checker, though you might occasionally see them in an error message. These qualifiers exist to give literals sufficiently precise types that they can be used in any appropriate context.

Java also defines other string formats for a type, notably qualified names (JLS §6.2). The Signature Checker does not include annotations for these.

Here are examples of the supported formats:

fully qualified name | Class.getName | field descriptor | binary name | internal form | Class.getSimpleName
int | int | I | n/a for primitive type | n/a for primitive type | int
int[][] | [[I | [[I | n/a for array type | n/a for array type | int[][]
MyClass | MyClass | LMyClass; | MyClass | MyClass | MyClass
MyClass[] | [LMyClass; | [LMyClass; | n/a for array type | n/a for array type | MyClass[]
n/a for anonymous class | MyClass$22 | LMyClass$22; | MyClass$22 | MyClass$22 | (empty string)
n/a for array of anon. ‍class | [LMyClass$22; | [LMyClass$22; | n/a for array type | n/a for array type | []
java.lang.Integer | java.lang.Integer | Ljava/lang/Integer; | java.lang.Integer | java/lang/Integer | Integer
java.lang.Integer[] | [Ljava.lang.Integer; | [Ljava/lang/Integer; | n/a for array type | n/a for array type | Integer[]
pkg.Outer.Inner | pkg.Outer$Inner | Lpkg/Outer$Inner; | pkg.Outer$Inner | pkg/Outer$Inner | Inner
pkg.Outer.Inner[] | [Lpkg.Outer$Inner; | [Lpkg/Outer$Inner; | n/a for array type | n/a for array type | Inner[]
n/a for anonymous class | pkg.Outer$22 | Lpkg/Outer$22; | pkg.Outer$22 | pkg/Outer$22 | (empty string)
n/a for array of anon. ‍class | [Lpkg.Outer$22; | [Lpkg/Outer$22; | n/a for array type | n/a for array type | []

Java defines one format for the string representation of a method signature:

@MethodDescriptor
  A method descriptor (JVMS §4.3.3) identifies a method’s signature (its parameter and return types), just as a field descriptor identifies a type. The method descriptor for the method

          Object mymethod(int i, double d, Thread t)

  is

          (IDLjava/lang/Thread;)Ljava/lang/Object;

### 18.1.1 How to choose which annotation to use

Sometimes, there are multiple valid annotations for a value. As an example, a non-primitive non-array type is represented identically by @BinaryName, @ClassGetName, and @FqBinaryName. Using the lowest type in the type hierarchy (in this case, @BinaryName) has two advantages. First, it acts as documentation that the value is never a primitive or array. Second, it permits the value to be used in any of the three contexts: as a @BinaryName, a @ClassGetName, or a @FqBinaryName.

Casting to @BinaryName from one of the other types adds clutter if it is not necessary. Suppose that a method returns a @ClassGetName and the value will only be used in contexts that require a @ClassGetName (say, it is passed to a method that requires an argument of that type). Then there is no point in casting to @BinaryName in between, even if you know the type being represented is not a primitive or an array.

## 18.2 What the Signature Checker checks

Certain methods in the JDK, such as Class.forName, are annotated indicating the type they require. The Signature Checker ensures that clients call them with the proper arguments. The Signature Checker does not reason about string operations such as concatenation, substring, parsing, etc.

To run the Signature Checker, supply the -processor org.checkerframework.checker.signature.SignatureChecker command-line option to javac.

# Chapter ‍19 GUI Effect Checker

One of the most prevalent GUI-related bugs is invalid UI update or invalid thread access: accessing the UI directly from a background thread.

Most GUI frameworks (including Android, AWT, Swing, and SWT) create a single distinguished thread — the UI event thread — that handles all GUI events and updates. To keep the interface responsive, any expensive computation should be offloaded to background threads (also called worker threads). If a background thread accesses a UI element such as a JPanel (by calling a JPanel method or reading/writing a field of JPanel), the GUI framework raises an exception that terminates the program. To fix the bug, the background thread should send a request to the UI thread to perform the access on its behalf.

It is difficult for a programmer to remember which methods may be called on which thread(s). The GUI Effect Checker solves this problem. The programmer annotates each method to indicate whether:

- It accesses no UI elements (and may run on any thread); such a method is said to have the “safe effect”.
- It may access UI elements (and must run on the UI thread); such a method is said to have the “UI effect”.

The GUI Effect Checker verifies these effects and statically enforces that UI methods are only called from the correct thread. A method with the safe effect is prohibited from calling a method with the UI effect.

For example, the effect system can reason about when method calls must be dispatched to the UI thread via a message such as Display.syncExec.

    @SafeEffect
    public void calledFromBackgroundThreads(JLabel l) {
        l.setText("Foo");       // Error: calling a @UIEffect method from a @SafeEffect method
        Display.syncExec(new Runnable {
            @UIEffect // inferred by default
            public void run() {
                l.setText("Bar");  // OK: accessing JLabel from code run on the UI thread
            }
        });

    }

The GUI Effect Checker’s annotations fall into three categories:

- effect annotations on methods (Section ‍19.1),
- class or package annotations controlling the default effect (Section ‍19.4), and
- effect-polymorphism: code that works for both the safe effect and the UI effect (Section ‍19.5).

## 19.1 GUI effect annotations

There are two primary GUI effect annotations:

@SafeEffect
  is a method annotation marking code that must not access UI objects.
@UIEffect
  is a method annotation marking code that may access UI objects. Most UI object methods (e.g., methods of JPanel) are annotated as @UIEffect.

@SafeEffect is a sub-effect of @UIEffect, in that it is always safe to call a @SafeEffect method anywhere it is permitted to call a @UIEffect method. We write this relationship as

@SafeEffect ≺ @UIEffect

## 19.2 What the GUI Effect Checker checks

The GUI Effect Checker ensures that only the UI thread accesses UI objects. This prevents GUI errors such as invalid UI update and invalid thread access.

The GUI Effect Checker issues errors in the following cases:

- A @UIEffect method is invoked by a @SafeEffect method.
- Method declarations violate subtyping restrictions: a supertype declares a @SafeEffect method, and a subtype annotates an overriding version as @UIEffect.

Additionally, if a method implements or overrides a method in two supertypes (two interfaces, or an interface and parent class), and those supertypes give different effects for the methods, the GUI Effect Checker issues a warning (not an error).

## 19.3 Running the GUI Effect Checker

The GUI Effect Checker can be invoked by running the following command:

      javac -processor org.checkerframework.checker.guieffect.GuiEffectChecker MyFile.java ...

## 19.4 Annotation defaults

The default method annotation is @SafeEffect, since most code in most programs is not related to the UI. This also means that typically, code that is unrelated to the UI need not be annotated at all.

The GUI Effect Checker provides three primary ways to change the default method effect for a class or package:

@UIType
  is a class annotation that makes the effect for unannotated methods in that class default to @UIEffect. (See also @UI in Section ‍19.5.2.)
@UIPackage
  is a package annotation, that makes the effect for unannotated methods in that package default to @UIEffect. It is not transitive; a package nested inside a package marked @UIPackage does not inherit the changed default.
@SafeType
  is a class annotation that makes the effect for unannotated methods in that class default to @SafeEffect. Because @SafeEffect is already the default effect, @SafeType is only useful for class types inside a package marked @UIPackage.

There is one other place where the default annotation is not automatically @SafeEffect: anonymous inner classes. Since anonymous inner classes exist primarily for brevity, it would be unfortunate to spoil that brevity with extra annotations. By default, an anonymous inner class method that overrides or implements a method of the parent type inherits that method’s effect. For example, an anonymous inner class implementing an interface with method @UIEffect void m() need not explicitly annotate its implementation of m(); the implementation will inherit the parent’s effect. Methods of the anonymous inner class that are not inherited from a parent type follow the standard defaulting rules.

## 19.5 Polymorphic effects

Sometimes a type is reused for both UI-specific and background-thread work. A good example is the Runnable interface, which is used both for creating new background threads (in which case the run() method must have the @SafeEffect) and for sending code to the UI thread to execute (in which case the run() method may have the @UIEffect). But the declaration of Runnable.run() may have only one effect annotation in the source code. How do we reconcile these conflicting use cases?

Effect-polymorphism permits a type to be used for both UI and non-UI purposes. It is similar to Java’s generics in that you define, then use, the effect-polymorphic type. Recall that to define a generic type, you write a type parameter such as <T> and use it in the body of the type definition; for example, class List<T> { ... T get() {...} ... }. To instantiate a generic type, you write its name along with a type argument; for example, List<Date> myDates;.

### 19.5.1 Defining an effect-polymorphic type

To declare that a class is effect-polymorphic, annotate its definition with @PolyUIType. To use the effect variable in the class body, annotate a method with @PolyUIEffect. It is an error to use @PolyUIEffect in a class that is not effect-polymorphic.

Consider the following example:

    @PolyUIType
    public interface Runnable {
        @PolyUIEffect
        void run();
    }

This declares that class Runnable is parameterized over one generic effect, and that when Runnable is instantiated, the effect argument will be used as the effect for the run method.

### 19.5.2 Using an effect-polymorphic type

To instantiate an effect-polymorphic type, write one of these three type qualifiers before a use of the type:

@AlwaysSafe
  instantiates the type’s effect to @SafeEffect.
@UI
  instantiates the type’s effect to @UIEffect. Additionally, it changes the default method effect for the class to @UIEffect.
@PolyUI
  instantiates the type’s effect to @PolyUIEffect for the same instantiation as the current (containing) class. For example, this is the qualifier of the receiver this inside a method of a @PolyUIType class, which is how one method of an effect-polymorphic class may call an effect-polymorphic method of the same class.

As an example:

    @AlwaysSafe Runnable s = ...;    s.run();    // s.run() is @SafeEffect
    @PolyUI Runnable p = ...;        p.run();    // p.run() is @PolyUIEffect (context-dependent)
    @UI Runnable u = ...;            u.run();    // u.run() is @UIEffect

It is an error to apply an effect instantiation qualifier to a type that is not effect-polymorphic.

Note that no annotation is required on the anonymous class declaration itself (e.g. new Runnable(){...} does not require a type use annotation, although the variable, field, or argument it ends up being assigned to might). Instead, the GUI Effect Checker will infer the effect qualifier based on the method being called from within the members of that specific anonymous class.

### 19.5.3 Subclassing a specific instantiation of an effect-polymorphic type

Sometimes you may wish to subclass a specific instantiation of an effect-polymorphic type, just as you may extend List<String>.

To do this, simply place the effect instantiation qualifier by the name of the type you are defining, e.g.:

    @UI
    public class UIRunnable extends Runnable {...}
    @AlwaysSafe
    public class SafeRunnable extends Runnable {...}

The GUI Effect Checker will automatically apply the qualifier to all classes and interfaces the class being defined extends or implements. (This means you cannot write a class that is a subtype of a @AlwaysSafe Foo and a @UI Bar, but this has not been a problem in our experience.)

### 19.5.4 Subtyping with polymorphic effects

With three effect annotations, we must extend the static sub-effecting relationship:

@SafeEffect ≺ @PolyUIEffect ≺ @UIEffect

This is the correct sub-effecting relation because it is always safe to call a @SafeEffect method (whether from an effect-polymorphic method or a UI method), and a @UIEffect method may safely call any other method.

This induces a subtyping hierarchy on type qualifiers:

@AlwaysSafe ≺ @PolyUI ≺ @UI

This is sound because a method instantiated according to any qualifier will always be safe to call in place of a method instantiated according to one of its super-qualifiers. This allows clients to pass “safer” instances of some object type to a given method.

#### Effect polymorphism and arguments

Sometimes it is useful to have @PolyUI parameters on a method. As a trivial example, this permits us to specify an identity method that works for both @UI Runnable and @AlwaysSafe Runnable:

    public @PolyUI Runnable id(@PolyUI Runnable r) {
        return r;
    }

This use of @PolyUI will be handled as is standard for polymorphic qualifiers in the Checker Framework (see Section 31.2).

@PolyUIEffect methods should generally not use @PolyUI arguments: it is permitted by the framework, but its interaction with inheritance is subtle, and may not behave as you would hope.

The shortest explanation is this: @PolyUI arguments may only be overridden by @PolyUI arguments, even though the implicitly @PolyUI receiver may be overridden with a @AlwaysSafe receiver.

As noted earlier (Section 31.1.6), Java’s generics are invariant — A<X> is a subtype of B<Y> only if X is identical to Y. Class-level use of @PolyUI behaves slightly differently. Marking a type declaration @PolyUIType is conceptually equivalent to parameterizing the type by some E extends Effect. But in this view, Runnable<SafeEffect> (really @AlwaysSafe Runnable) would be considered a subtype of Runnable<UIEffect> (really @UI Runnable), as explained earlier in this section. Java’s generics do not permit this, which is called covariant subtyping in the effect parameter. Permitting it for all generics leads to problems where a type system can miss errors. Java solves this by making all generics invariant, which rejects more programs than strictly necessary, but leads to an easy-to-explain limitation. For this checker, covariant subtyping of effect parameters is very important: being able to pass an @AlwaysSafe Runnable in place of a @UI Runnable is extremely useful. Since we need to allow some cases for flexibility, but need to reject other cases to avoid missing errors, the distinction is a bit more subtle for this checker.

Consider this small example (warning: the following is rejected by the GUI Effect Checker):

    @PolyUIType
    public interface Dispatcher {
        @PolyUIEffect
        void dispatch(@PolyUI Runnable toRun);
    }
    @SafeType
    public class SafeDispatcher implements Dispatcher {
        @Override
        public void dispatch(@AlwaysSafe Runnable toRun) {
            runOnBackgroundThread(toRun);
        }
    }

This may initially seem like harmless code to write, which simply specializes the implicit effect parameter from Dispatcher in the SafeDispatcher implementation. However, the way method effect polymorphism is implemented is by implicitly making the receiver of a @PolyUIEffect method — the object on which the method is invoked — @PolyUI. So if the definitions above were permitted, the following client code would be possible:

    @AlwaysSafe SafeDispatcher s = ...;
    @UI Runnable uitask = ...;
    s.dispatch(uitask);

At the call to dispatch, the Checker Framework is free to consider s as its supertype, @UI SafeDispatcher. This permits the framework to choose the same qualifier for both the (implicit) receiver use of @PolyUI and the toRun argument to Dispatcher.dispatch, passing the checker. But this code would then pass a UI-thread-only task to a method which should only accept background thread tasks — exactly what the checker should prevent!

To resolve this, the GUI Effect Checker rejects the definitions above, specifically the @AlwaysSafe on SafeDispatcher.dispatch’s parameter, which would need to remain @PolyUI.

A subtlety of the code above is that the receiver for SafeDispatcher.dispatch is also overridden, switching from a @PolyUI receiver to a @Safe receiver. That change is permissible. But when that is done, the polymorphic arguments may no longer be interchangeable with the receiver.

## 19.6 References

The ECOOP 2013 paper “JavaUI: Effects for controlling UI object access” ‍[GDEG13] (ECOOP 2013, https://homes.cs.washington.edu/~mernst/pubs/gui-thread-ecoop2013-abstract.html) includes some case studies on the checker’s efficacy, including descriptions of the relatively few false warnings we encountered. It also contains a more formal description of the effect system.

# Chapter ‍20 Units Checker

For many applications, it is important to use the correct units of measurement for primitive types. For example, NASA’s Mars Climate Orbiter (cost: $327 million) was lost because of a discrepancy between use of the metric unit Newtons and the imperial measure Pound-force.

The Units Checker ensures consistent usage of units. For example, consider the following code:

    @m int meters = 5 * UnitsTools.m;
    @s int secs = 2 * UnitsTools.s;
    @mPERs int speed = meters / secs;

Due to the annotations @m and @s, the variables meters and secs are guaranteed to contain only values with meters and seconds as units of measurement. The assignment of an unqualified value to meters, as in meters = 99, will be flagged as an error by the Units Checker. Utility class UnitsTools provides constants that you can multiply with unqualified integer are multiplied to get values of the corresponding unit; for example, meters = 99 * UnitsTools.m is legal, or just meters = 99 * m if the file contains import static org.checkerframework.checker.units.util.UnitsTools.*;. To use the UnitsTools class, the checker-util.jar file must be on the classpath at run time.

The division meters/secs takes the types of the two operands into account and determines that the result is of type meters per second, signified by the @mPERs qualifier. We provide an extensible framework to define the result of operations on units.

## 20.1 Units annotations

The checker currently supports three varieties of units annotations: kind annotations (@Length, @Mass, …), the SI units (@m, @kg, …), and polymorphic annotations (@PolyUnit).

Kind annotations can be used to declare what the expected unit of measurement is, without fixing the particular unit used. For example, one could write a method taking a @Length value, without specifying whether it will take meters or kilometers. The following kind annotations are defined:

@Acceleration
@Angle
@Area
@Current
@Force
@Length
@Luminance
@Mass
@Speed
@Substance
@Temperature
@Time
@Volume

For each kind of unit, the corresponding SI unit of measurement is defined:

1. For @Acceleration: Meter Per Second Square @mPERs2
2. For @Angle: Radians @radians, and the derived unit Degrees @degrees
3. For @Area: the derived units square millimeters @mm2, square meters @m2, and square kilometers @km2
4. For @Current: Ampere @A
5. For @Force: Newton @N and the derived unit kilonewton @kN
6. For @Length: Meters @m and the derived units millimeters @mm and kilometers @km
7. For @Luminance: Candela @cd
8. For @Mass: kilograms @kg and the derived units grams @g and metric tons @t
9. For @Speed: meters per second @mPERs and kilometers per hour @kmPERh
10. For @Substance: Mole @mol
11. For @Temperature: Kelvin @K and the derived unit Celsius @C
12. For @Time: seconds @s and the derived units minutes @min and hours @h
13. For @Volume: the derived units cubic millimeters @mm3, cubic meters @m3, and cubic kilometers @km3

You may specify SI unit prefixes, using enumeration Prefix. The basic SI units (@s, @m, @g, @A, @K, @mol, @cd) take an optional Prefix enum as argument. For example, to use nanoseconds as unit, you could use @s(Prefix.nano) as a unit type. You can sometimes use a different annotation instead of a prefix; for example, @mm is equivalent to @m(Prefix.milli).

Class UnitsTools contains a constant for each SI unit. To create a value of the particular unit, multiply an unqualified value with one of these constants. By using static imports, this allows very natural notation; for example, after statically importing UnitsTools.m, the expression 5 * m represents five meters. As all these unit constants are public, static, and final with value one, the compiler will optimize away these multiplications. To use the UnitsTools class, the checker-util.jar file must be on the classpath at run time.

The polymorphic annotation @PolyUnit enables you to write a method that takes an argument of any unit type and returns a result of that same type. For more about polymorphic qualifiers, see Section ‍31.2. For an example of its use, see the @PolyUnit Javadoc.

## 20.2 Extending the Units Checker

You can create new kind annotations and unit annotations that are specific to the particular needs of your project. An easy way to do this is by copying and adapting an existing annotation. (In addition, search for all uses of the annotation’s name throughout the Units Checker implementation, to find other code to adapt; read on for details.)

Here is an example of a new unit annotation.

    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf({Time.class})
    @UnitsMultiple(quantity=s.class, prefix=Prefix.nano)
    public @interface ns {}

The @SubtypeOf meta-annotation specifies that this annotation introduces an additional unit of time. The @UnitsMultiple meta-annotation specifies that this annotation should be a nano multiple of the basic unit @s: @ns and @s(Prefix.nano) behave equivalently and interchangeably. Most annotation definitions do not have a @UnitsMultiple meta-annotation.

Note that all custom annotations must have the @Target(ElementType.TYPE_USE) meta-annotation. See section 36.5.1.

To take full advantage of the additional unit qualifier, you need to do two additional steps. (1) ‍Provide constants that convert from unqualified types to types that use the new unit. See class UnitsTools for examples (you will need to suppress a checker warning in just those few locations). (2) ‍Put the new unit in relation to existing units. Provide an implementation of the UnitsRelations interface as a meta-annotation to one of the units.

See demonstration docs/examples/units-extension/ for an example extension that defines Hertz (hz) as scalar per second, and defines an implementation of UnitsRelations to enforce it.

## 20.3 What the Units Checker checks

The Units Checker ensures that unrelated types are not mixed.

All types with a particular unit annotation are disjoint from all unannotated types, from all types with a different unit annotation, and from all types with the same unit annotation but a different prefix.

Subtyping between the units and the unit kinds is taken into account, as is the @UnitsMultiple meta-annotation.

Multiplying a scalar with a unit type results in the same unit type.

The division of a unit type by the same unit type results in the unqualified type.

Multiplying or dividing different unit types, for which no unit relation is known to the system, will result in a MixedUnits type, which is separate from all other units. If you encounter a MixedUnits annotation in an error message, ensure that your operations are performed on correct units or refine your UnitsRelations implementation.

The Units Checker does not change units based on multiplication; for example, if variable mass has the type @kg double, then mass * 1000 has that same type rather than the type @g double. (The Units Checker has no way of knowing whether you intended a conversion, or you were computing the mass of 1000 items. You need to make all conversions explicit in your code, and it’s good style to minimize the number of conversions.)

## 20.4 Running the Units Checker

The Units Checker can be invoked by running the following commands.

- If your code uses only the SI units that are provided by the framework, simply invoke the checker:

        javac -processor org.checkerframework.checker.units.UnitsChecker MyFile.java ...

- If you define your own units, provide the fully-qualified class names of the annotations through the -Aunits option, using a comma-no-space-separated notation:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.checker.units.UnitsChecker \
              -Aunits=myPackage.qual.MyUnit,myPackage.qual.MyOtherUnit MyFile.java ...

  The annotations listed in -Aunits must be accessible to the compiler during compilation. Before you run the Units Checker with javac, they must be compiled and on the same path (the classpath or processorpath) as the Checker Framework. It is not sufficient to supply their source files on the command line.

- You can also provide the fully-qualified paths to a set of directories that contain units qualifiers through the -AunitsDirs option, using a colon-no-space-separated notation. For example, if the Checker Framework is on the classpath rather than the processorpath:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.checker.units.UnitsChecker \
              -AunitsDirs=/full/path/to/myProject/bin:/full/path/to/myLibrary/bin MyFile.java ...

  Note that in these two examples, the compiled class file of the myPackage.qual.MyUnit and myPackage.qual.MyOtherUnit annotations must exist in either the myProject/bin directory or the myLibrary/bin directory. The following placement of the class files will work with the above commands:

        .../myProject/bin/myPackage/qual/MyUnit.class
        .../myProject/bin/myPackage/qual/MyOtherUnit.class

  The two options can be used at the same time to provide groups of annotations from directories, and individually named annotations.

Also, see the example project in the docs/examples/units-extension directory.

## 20.5 Suppressing warnings

One example of when you need to suppress warnings is when you initialize a variable with a unit type by a literal value. To remove this warning message, it is best to introduce a constant that represents the unit and to add a @SuppressWarnings annotation to that constant. For examples, see class UnitsTools.

## 20.6 References

- The GNU Units tool provides a comprehensive list of units:
  https://www.gnu.org/software/units/
- The F# units of measurement system inspired some of our syntax:
  https://en.wikibooks.org/wiki/F_Sharp_Programming/Units_of_Measure

# Chapter ‍21 Signedness Checker

The Signedness Checker guarantees that signed and unsigned integral values are not mixed together in a computation. In addition, it prohibits meaningless operations, such as division on an unsigned value.

Recall that a computer represents a number as a sequence of bits. Signedness indicates how to interpret the most significant bit. For example, the bits 10000010 ordinarily represent the value -126, but when interpreted as unsigned, those bits represent the value 130. The bits 01111110 represent the value 126 in signed and in unsigned interpretation. The range of signed byte values is -128 to 127. The range of unsigned byte values is 0 to 255.

Signedness is only applicable to the integral types byte, short, int, and long and their boxed variants Byte, Short, Integer, and Long. char and Character are always unsigned. Floating-point types float, double, Float, and Double are always signed.

Signedness is primarily about how the bits of the representation are interpreted, not about the values that it can represent. An unsigned value is always non-negative, but just because a variable’s value is non-negative does not mean that it should be marked as @Unsigned. If variable v will be compared to a signed value, or used in arithmetic operations with a signed value, then v should have signed type. To indicate the range of possible values for a variable, use the @NonNegative annotation of the Index Checker (see Chapter ‍11) or the @IntRange annotation of the Constant Value Checker (see Chapter ‍23).

Additional details appear in the paper “Preventing signedness errors in numerical computations in Java” ‍[Mac16] (FSE 2016).

To run the Signedness Checker, run javac with -processor org.checkerframework.checker.signedness.SignednessChecker.

## 21.1 Annotations

The Signedness Checker uses type annotations to indicate the signedness that the programmer intends an expression to have.

> Figure 21.1: The type qualifier hierarchy of the signedness annotations. Qualifiers in gray are used internally by the type system but should never be written by a programmer.

These are the qualifiers in the signedness type system:

@Unsigned
  indicates that the programmer intends the value to be interpreted as unsigned. That is, if the most significant bit in the bitwise representation is set, then the bits should be interpreted as a large positive value.
@Signed
  indicates that the programmer intends the value to be interpreted as signed. That is, if the most significant bit in the bitwise representation is set, then the bits should be interpreted as a negative value. This is the default annotation.
@SignedPositive
  indicates that a value is known at compile time to be in the non-negative signed range, so it has the same interpretation as signed or unsigned and may be used with either interpretation. (Equivalently, the most significant bit is guaranteed to be 0.) Programmers should usually write @Signed or @Unsigned instead.
@SignednessGlb
  indicates that a value may be interpreted as unsigned or signed. It covers the same cases as @SignedPositive, plus manifest literals, to prevent the programmer from having to annotate them all explicitly. Programmers should rarely write this annotation, except on fields whose value is a negative manifest literal.
@PolySigned
  indicates qualifier polymorphism. When two formal parameter types are annotated with @PolySigned, the two arguments at a call site must have the same signedness type annotation. (This differs from the standard rule for polymorphic qualifiers.) For a description of qualifier polymorphism, see Section ‍31.2.
@UnknownSignedness
  indicates that a value’s type is not relevant or known to this checker. This annotation is used internally, and should not be written by the programmer.
@SignednessBottom
  indicates that the value is null. This annotation is used internally, and should not be written by the programmer.

### 21.1.1 Default qualifiers

The only type qualifier that the programmer should need to write is @Unsigned. When a programmer leaves an expression unannotated, the Signedness Checker treats it in one of the following ways:

- All byte, short, int, and long literals default to @SignednessGlb.
- All char and Character expressions are @Unsigned; this cannot be changed.
- All char and Character variables are @Unsigned; this cannot be changed.
- All other expressions default to @Signed.

## 21.2 Prohibited operations

The Signedness Checker prohibits the following uses of operators:

- Division (/) or modulus (%) with an @Unsigned operand.
- Signed right shift (`>>`) with an @Unsigned left operand.
- Unsigned right shift (`>>>`) with a @Signed left operand.
- Greater/less than (or equal) comparators (<, <=, >, >=) with an @Unsigned operand.
- Any other binary operator with one @Unsigned operand and one @Signed operand, with the exception of left shift (`<<`).

There are some special cases where these operations are permitted; see Section ‍21.2.2.

Like every type-checker built with the Checker Framework, the Signedness Checker ensures that assignments and pseudo-assignments have consistent types. For example, it is not permitted to assign a @Signed expression to an @Unsigned variable or vice versa.

### 21.2.1 Rationale

The Signedness Checker prevents misuse of unsigned values in Java code. Most Java operations interpret operands as signed. If applied to unsigned values, those operations would produce unexpected, incorrect results.

Consider the following Java code:

    public class SignednessManualExample {

        int s1 = -2;
        int s2 = -1;

        @Unsigned int u1 = 2147483646; // unsigned: 2^32 - 2, signed: -2
        @Unsigned int u2 = 2147483647; // unsigned: 2^32 - 1, signed: -1

        void m() {
            int w = s1 / s2; // OK: result is 2, which is correct for -2 / -1
            int x = u1 / u2; // ERROR: result is 2, which is incorrect for (2^32 - 2) / (2^32 - 1)
        }

        int s3 = -1;
        int s4 = 5;

        @Unsigned int u3 = 2147483647; // unsigned: 2^32 - 1, signed: -1
        @Unsigned int u4 = 5;

        void m2() {
            int y = s3 % s4; // OK: result is -1, which is correct for -1 % 5
            int z = u3 % u4; // ERROR: result is -1, which is incorrect for (2^32 - 1) % 5 = 2
        }
    }

These examples illustrate why division and modulus with an unsigned operand are illegal. Other uses of operators are prohibited for similar reasons.

### 21.2.2 Permitted shifts

As exceptions to the rules given above, the Signedness Checker permits certain right shifts which are immediately followed by a cast or masking operation.

For example, right shift by 8 then mask by 0xFF evaluates to the same value whether the argument is interpreted as signed or unsigned. Thus, the Signedness Checker permits both `((myInt >> 8) & 0xFF)` and `((myInt >>> 8) & 0xFF)`, regardless of the qualifier on the type of myInt.

Likewise, right shift by 8 then cast to byte evaluates to the same value whether the argument is interpreted as signed or unsigned, so the Signedness Checker permits both `(byte) (myInt >> 8)` and `(byte) (myInt >>> 8)`, regardless of the type of myInt.

## 21.3 Utility routines for manipulating unsigned values

Class SignednessUtil provides static utility methods for working with unsigned values. They are properly annotated with @Unsigned where appropriate, so using them may reduce the number of annotations that you need to write. To use the SignednessUtil class, the checker-util.jar file must be on the classpath at run time.

Class SignednessUtilExtra contains more utility methods that reference packages not included in Android. This class is not included in checker-util.jar, so you may want to copy the methods to your code.

## 21.4 Local type refinement

Local type refinement/inference (Section ‍32.7) may be surprising for the Signedness type system. Ordinarily, an expression with unsigned type may not participate in a division, as shown in Sections ‍21.2 and ‍21.2.1. However, if a constant is assigned to a variable that was declared with @Unsigned type, then — just like the constant — the variable may be treated as either signed or unsigned, due to local type refinement (Section ‍32.7). For example, it can participate in division.

        void useLocalVariables() {

            int s1 = -2;
            int s2 = -1;

            @Unsigned int u1 = 2147483646; // unsigned: 2^32 - 2, signed: -2
            @Unsigned int u2 = 2147483647; // unsigned: 2^32 - 1, signed: -1

            int w = s1 / s2; // OK: result is 2, which is correct for -2 / -1
            int x = u1 / u2; // OK; computation over constants, interpreted as signed; result is signed
        }

To prevent local type refinement, use a cast:

            @Unsigned int u1 = (@Unsigned int) 2147483646;

Note that type-checking produces a different result for int x = u1 / u2; here than in the similar example in Section ‍21.2.1. In Section ‍21.2.1, the method is reading fields, and all it knows is the declared type of the field. In 20.4, the method is reading a local variable, and dataflow (that is, flow-sensitive type refinement) refines the types of local variables.

## 21.5 Instantiating polymorphism

When calling a method with formal parameters annotated as @PolySigned, all arguments for @PolySigned formal parameters must have comparable types. (One way to do this is for all types to be the same.) This is different than the usual rules for polymorphic qualifiers. If you violate this rule, then the Signedness Checker’s error messages can be obscure, because they are about @SignednessBottom. You can fix the signedness error messages by casting the arguments.

## 21.6 Other signedness annotations

The Checker Framework’s signedness annotations are similar to annotations used elsewhere.

If your code is already annotated with a different annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Signedness Checker, as described in Figure ‍21.2.

> ‍jdk.jfr.Unsigned ‍
> | ⇒  ‍org.checkerframework.checker.signedness.qual.Unsigned ‍

> Figure 21.2: Correspondence between other signedness annotations and the Checker Framework’s annotations.

# Chapter ‍22 Purity Checker

The Purity Checker identifies methods that have no side effects, that return the same value each time they are called on the same argument, or both.

Purity analysis aids type refinement (Section ‍32.7).

All checkers utilize purity annotations on called methods. You do not need to run the Purity Checker directly. However you may run just the Purity Checker by supplying the following command-line options to javac: -processor org.checkerframework.framework.util.PurityChecker.

The Checker Framework can infer purity annotations. If you supply the command-line option -AsuggestPureMethods, then the Checker Framework will suggest methods that can be marked as @SideEffectFree, @Deterministic, or @Pure. In addition, such suggestions are output by -Ainfer and when using whole-program inference.

## 22.1 Purity annotations

@SideEffectFree
  indicates that the method has no externally-visible side effects.
@Deterministic
  indicates that if the method is called multiple times with identical arguments, then it returns the identical result according to == (not just according to equals()).
@Pure
  indicates that the method is both @SideEffectFree and @Deterministic.

## 22.2 Purity annotations are trusted

By default, purity annotations are trusted. Purity annotations on called methods affect type-checking of client code. However, you can make a mistake by writing @SideEffectFree on the declaration of a method that is not actually side-effect-free or by writing @Deterministic on the declaration of a method that is not actually deterministic.

To enable checking of the annotations, supply the command-line option -AcheckPurityAnnotations. It is not enabled by default because of a high false positive rate. In the future, after a new purity-checking analysis is implemented, the Checker Framework will default to checking purity annotations.

## 22.3 Overriding methods must respect specifications in superclasses

If a method in a superclass has a purity annotation, then every overriding definition must also have that purity annotation (or a stronger one).

Here is an example error if this requirement is violated:

    MyClass.java:1465: error: int hashCode() in MyClass cannot override int hashCode(Object this) in java.lang.Object;
    attempting to use an incompatible purity declaration
        public int hashCode() {
                   ^
      found   : []
      required: [SIDE_EFFECT_FREE, DETERMINISTIC]

The reason for the error is that the Object class is annotated as:

    class Object {
      ...
      @Pure int hashCode() { ... }
    }

(where @Pure means both @SideEffectFree and @Deterministic). Every overriding definition, including those in your program, must use be at least as strong a specification. For hashCode, every overriding definition must be annotated as @Pure.

You can fix the definition by adding @Pure to your method definition. Alternately, you can suppress the warning. You can suppress each such warning individually using @SuppressWarnings("purity.overriding"), or you can use the -AsuppressWarnings=purity.overriding command-line argument to suppress all such warnings. In the future, the Checker Framework will support inheriting annotations from superclass definitions.

## 22.4 Suppressing warnings

The command-line options -AassumeSideEffectFree, -AassumeDeterministic, and -AassumePure make the Checker Framework unsoundly assume that every called method is side-effect-free, is deterministic, or is both, respectively.

The command-line option -AassumePureGetters makes the Checker Framework unsoundly assume that every getter method is side-effect-free and deterministic. For the purposes of -AassumePureGetters, a getter method is defined as an instance method with no formal parameters, whose name starts with “get”, “is”, “not”, or “has” followed by an uppercase letter.

This can make flow-sensitive type refinement much more effective, since method calls will not cause the analysis to discard information that it has learned. However, this option can mask real errors. It is most appropriate when one of the following is true:

- You are starting out annotating a project.
- You are using the Checker Framework to find bugs but not to give a guarantee that no more errors exist of the given type.
- You are working with an unannotated library that makes the given assumptions, in which case (say) using -AassumePureGetters is easier than writing stub files for all the library’s getter methods.

# Chapter ‍23 Constant Value Checker

The Constant Value Checker is a constant propagation analysis: for each variable, it determines whether that variable’s value can be known at compile time.

There are two ways to run the Constant Value Checker.

- Typically, it is automatically run by another type checker.
- Alternately, you can run just the Constant Value Checker, by supplying the following command-line options to javac: -processor org.checkerframework.common.value.ValueChecker

## 23.1 Annotations

The Constant Value Checker uses type annotations to indicate the value of an expression (Section ‍23.1.1), and it uses method annotations to indicate methods that the Constant Value Checker can execute at compile time (Section ‍23.2.2).

### 23.1.1 Type Annotations

Typically, the programmer does not write any type annotations. Rather, the type annotations are inferred by the Constant Value Checker. The programmer is also permitted to write type annotations. This is only necessary in locations where the Constant Value Checker does not infer annotations: on fields and method signatures.

The main type annotations are @BoolVal, @IntVal, @IntRange, @DoubleVal, @StringVal, @MatchesRegex, @DoesNotMatchRegex, and @EnumVal. Additional type annotations for arrays and strings are @ArrayLen, @ArrayLenRange, and @MinLen. A polymorphic qualifier (@PolyValue) is also supported (see Section ‍31.2). In addition, there are separate checkers for @ClassVal and @MethodVal annotations (see Section ‍25.2).

Each *Val type annotation takes as an argument a set of values, and its meaning is that at run time, the expression evaluates to one of the values. For example, an expression of type @StringVal("a", "b") evaluates to one of the values "a", "b", or null. The set is limited to 10 entries; if a variable could be more than 10 different values, the Constant Value Checker gives up and its type becomes @IntRange for integral types, @ArrayLenRange for array types, @MatchesRegex, @DoesNotMatchRegex, @ArrayLen, or @ArrayLenRange for String, and @UnknownVal for all other types. The @ArrayLen annotation means that at run time, the expression evaluates to an array or a string whose length is one of the annotation’s arguments.

In the case of too many strings in @StringVal, the values are forgotten and just the lengths are used in @ArrayLen. If this would result in too many lengths, only the minimum and maximum lengths are used in @ArrayLenRange, giving a range of possible lengths of the string.

The @StringVal, @MatchesRegex, and @DoesNotMatchRegex annotations may be applied to char arrays. Although byte arrays are often converted to/from strings, these annotations may not be applied to them. This is because the conversion depends on the platform’s character set.

The @MatchesRegex and @DoesNotMatchRegex annotations use the standard Java regular expression syntax. @MatchesRegex(A) is only a subtype of @MatchesRegex(B) if the set of regular expressions A is a subset of the set of regular expressions B. An @StringVal annotation is a subtype of an @MatchesRegex annotation if each string matches at least one of the regular expressions. @DoesNotMatchRegex(A) is only a subtype of @MatchesRegex(B) if the set of regular expressions A is a superset of the set of regular expressions B. Matching is done via the java.lang.String#matches method, which matches against the entire string (it does not look for a matching substring).

The @EnumVal annotation’s argument is the names of the enum constants that the type might evaluate to. (Java syntax does not allow the enum constants themselves to be arguments.) @EnumVal is treated identically to @StringVal by the checker internally, so @StringVal may appear in error messages related to enums.

@IntRange takes two arguments — a lower bound and an upper bound. Its meaning is that at run time, the expression evaluates to a value between the bounds (inclusive). For example, an expression of type @IntRange(from=0, to=255) evaluates to 0, 1, 2, …, 254, or 255. An @IntVal and @IntRange annotation that represent the same set of values are semantically identical and interchangeable: they have exactly the same meaning, and using either one has the same effect. @ArrayLenRange has the same relationship to @ArrayLen that @IntRange has to @IntVal. The @MinLen annotation is an alias for @ArrayLenRange (meaning that every @MinLen annotation is automatically converted to an @ArrayLenRange annotation) that only takes one argument, which is the lower bound of the range. The upper bound of the range is the maximum integer value.

Figure ‍23.1 shows the subtyping relationship among the type annotations. For two annotations of the same type, subtypes have a smaller set of possible values, as also shown in the figure. Because int can be casted to double, an @IntVal annotation is a subtype of a @DoubleVal annotation with the same values.

> Figure 23.1: At the top, the type qualifier hierarchy of the Constant Value Checker annotations. The first four qualifiers are applicable to primitives and their wrappers; the next to Strings (it can also be written as @EnumVal for enumeration constants), and the final two to arrays. Qualifiers in gray are used internally by the type system but should never be written by a programmer. At the bottom are examples of additional subtyping relationships that depend on the annotations’ arguments.

Figure ‍23.2 illustrates how the Constant Value Checker infers type annotations (using flow-sensitive type qualifier refinement, Section ‍32.7).

>     public void flowSensitivityExample(boolean b) {
>         int i = 1;     // i has type:  @IntVal({1}) int
>         if (b) {
>             i = 2;     // i now has type:  @IntVal({2}) int
>         }
>                        // i now has type:  @IntVal({1,2}) int
>         i = i + 1;     // i now has type:  @IntVal({2,3}) int
>     }

> Figure 23.2: The Constant Value Checker infers different types for a variable on different lines of the program.

## 23.2 Other constant value annotations

The Checker Framework’s constant value annotations are similar to annotations used elsewhere.

If your code is already annotated with a different constant value or range annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Constant Value Checker, as described in Figure ‍23.3. If the other annotation is a declaration annotation, it may be moved; see Section ‍39.6.10.

> ‍android.support.annotation.IntRange ‍
> | ⇒  ‍org.checkerframework.checker.common.value.qual.IntRange ‍

> Figure 23.3: Correspondence between other constant value and range annotations and the Checker Framework’s annotations.

The Constant Value Checker trusts the @Positive, @NonNegative, and @GTENegativeOne annotations. If your code contains any of these annotations, then in order to guarantee soundness, you must run the Index Checker whenever you run the Constant Value Checker.

### 23.2.1 Compile-time execution of expressions

Whenever all the operands of an expression are compile-time constants (that is, their types have constant-value type annotations), the Constant Value Checker attempts to execute the expression. This is independent of any optimizations performed by the compiler and does not affect the code that is generated.

The Constant Value Checker statically executes (at compile time) operators that do not throw exceptions (e.g., +, -, <<, !=).

### 23.2.2@StaticallyExecutable methods and the classpath

The Constant Value Checker statically executes (at compile time) methods annotated with @StaticallyExecutable.

>     @StaticallyExecutable @Pure
>     public static int myAdd(int a, int b) {
>         return a + b;
>     }

>     public void bar() {
>         int a = 5;            // a has type:  @IntVal({5}) int
>         int b = 4;            // b has type:  @IntVal({4}) int
>         int c = myAdd(a, b);  // c has type:  @IntVal({9}) int
>     }

> Figure 23.4: The @StaticallyExecutable annotation enables constant propagation through method calls.

The static execution feature has some requirements:

- A @StaticallyExecutable method must be @Pure (side-effect-free and deterministic).
- The Constant Value Checker must have an estimate for all the arguments at a call site.

  This means that @StaticallyExecutable is not applicable to user-written instance methods. It is only applicable to instance methods whose receiver is a compile-time constant, such as a primitive wrapper or an array.

- The @StaticallyExecutable method and any method it calls must be on the same path (the classpath or the processorpath) as the Checker Framework. This is because the Constant Value Checker reflectively calls these methods at compile time.

  To use @StaticallyExecutable on methods in your own code, you should first compile the code without the Constant Value Checker and then add the location of the resulting .class files to the classpath or processorpath, whichever is appropriate. For example, the command-line arguments to the Checker Framework might include:

        -processor org.checkerframework.common.value.ValueChecker
        -classpath $CLASSPATH:MY_PROJECT/build/

  or

        -processor org.checkerframework.common.value.ValueChecker
        -processorpath ${CHECKERFRAMEWORK}/checker/build/libs/checker-3.6.1-SNAPSHOT.jar:MY_PROJECT/build/

## 23.3 Warnings

If the option -AreportEvalWarns options is used, the Constant Value Checker issues a warning if it cannot load and run, at compile time, a method marked as @StaticallyExecutable. If it issues such a warning, then the return value of the method will be @UnknownVal instead of being able to be resolved to a specific value annotation. Some examples of these:

- [class.find.failed] Failed to find class named Test.

  The checker could not find the class specified for resolving a @StaticallyExecutable method. Typically this means that the path that contains the Checker Framework (the classpath or the processorpath) lacks the given classfile.

- [method.find.failed] Failed to find a method named foo with argument types [@IntVal(3) int].

  The checker could not find the method foo(int) specified for resolving a @StaticallyExecutable method, but could find the class. This is usually due to providing an outdated version of the classfile that does not contain the method that was annotated as @StaticallyExecutable.

- [method.evaluation.exception] Failed to evaluate method public static int Test.foo(int) because it threw an exception: java.lang.ArithmeticException: / by zero.

  An exception was thrown when trying to statically execute (at compile time) the method. In this case it was a divide-by-zero exception. If the arguments to the method each only had one value in their annotations then this exception will always occur when the program is actually run as well. If there are multiple possible values then the exception might not be thrown on every execution, depending on the run-time values.

There are some other situations in which the Constant Value Checker produces a warning message:

- [too.many.values.given] The maximum number of arguments permitted is 10.

  The Constant Value Checker only tracks up to 10 possible values for an expression. If you write an annotation with more values than will be tracked, the annotation is replaced with @IntRange, @ArrayLen, @ArrayLenRange, or @UnknownVal.

## 23.4 Unsoundly ignoring overflow

The Constant Value Checker takes Java’s overflow rules into account when computing the possible values of expressions. The -AignoreRangeOverflow command-line option makes it ignore the possibility of overflow for range annotations @IntRange and @ArrayLenRange. Figure ‍23.5 gives an example of behavior with and without the -AignoreRangeOverflow command-line option.

>       ...
>       if (i > 5) {
>         // i now has type:  @IntRange(from=5, to=Integer.MAX_VALUE)
>         i = i + 1;
>         // If i started out as Integer.MAX_VALUE, then i is now Integer.MIN_VALUE.
>         // i's type is now @IntRange(from=Integer.MIN_VALUE, to=Integer.MAX_VALUE).
>         // When ignoring overflow, i's type is now @IntRange(from=6, to=Integer.MAX_VALUE).
>       }

> Figure 23.5: With the -AignoreRangeOverflow command-line option, the Constant Value Checker ignores overflow for range types, which gives smaller ranges to range types.

As with any unsound behavior in the Checker Framework, this option reduces the number of warnings and errors produced, and may reduce the number of @IntRange qualifiers that you need to write in the source code. However, it is possible that at run time, an expression might evaluate to a value that is not in its @IntRange qualifier. You should either accept that possibility, or verify the lack of overflow using some other tool or manual analysis.

## 23.5 Strings can be null in concatenations

By default, the Constant Value Checker is sound with respect to string concatenation and nullness. It assumes that, in a string concatenation, every non-primitive argument might be null, except for String literals and compile-time constants. It ignores Nullness Checker annotations. (This behavior is conservative but sound.)

Consider a variable declared as @StringVal("a", "b") String x;. At run time, x evaluates to one of the values "a", "b", or null. Therefore, the type of “x + "c"” is @StringVal("ac", "bc", "nullc") String.

The -AnonNullStringsConcatenation command-line option makes the Constant Value Checker unsoundly assume that no arguments in a string concatenation are null. With the command-line argument, the type of “x + "c"” is @StringVal("ac", "bc") String.

# Chapter ‍24 Returns Receiver Checker

The Returns Receiver Checker enables documenting and checking that a method returns its receiver (i.e., the this parameter).

There are two ways to run the Returns Receiver Checker.

- Typically, it is automatically run by another checker.

  If the code being checked does not use fluent APIs, you can pass the -AdisableReturnsReceiver command-line option. This disables the Returns Receiver Checker and makes the other checker run faster.

- Alternately, you can run just the Returns Receiver Checker, by supplying the following command-line options to javac: -processor org.checkerframework.common.returnsreceiver.ReturnsReceiverChecker

## 24.1 Annotations

The qualifier @This on the return type of a method indicates that the method returns its receiver. Methods that return their receiver are common in so-called “fluent” APIs. Here is an example:

    class MyBuilder {
      @This MyBuilder setName(String name) {
        this.name = name;
        return this;
      }
    }

An @This annotation can only be written on a return type, a receiver type, or in a downcast.

As is standard, the Returns Receiver Checker has a top qualifier, @UnknownThis, and a bottom qualifier, @BottomThis. Programmers rarely need to write these annotations.

Here are additional details. @This is a polymorphic qualifier rather than a regular type qualifier (see Section ‍31.2). Conceptually, a receiver type always has an @This qualifier. When a method return type also has an @This qualifier, the presence of the polymorphic annotation on both the method’s return and receiver type forces their type qualifiers to be equal. Hence, the method will only pass the type checker if it returns its receiver argument, achieving the desired checking.

## 24.2 AutoValue and Lombok Support

>     @AutoValue
>     abstract class Animal {
>       abstract String name();
>       abstract int numberOfLegs();
>       static Builder builder() {
>         return new AutoValue_Animal.Builder();
>       }
>       @AutoValue.Builder
>       abstract static class Builder {
>         abstract Builder setName(String value);       // @This is automatically added here
>         abstract Builder setNumberOfLegs(int value);  // @This is automatically added here
>         abstract Animal build();
>       }
>     }

> Figure 24.1: User-written code that uses the @AutoValue.Builder annotation. Given this code, (1) AutoValue automatically generates a concrete subclass of Animal.Builder, see Figure ‍24.2, and (2) the Returns Receiver Checker automatically adds @This annotations on setters in both user-written and automatically-generated code.

>         class AutoValue_Animal {
>           static final class Builder extends Animal.Builder {
>             private String name;
>             private Integer numberOfLegs;
>             @This Animal.Builder setName(String name) {
>               this.name = name;
>               return this;
>             }
>             @This Animal.Builder setNumberOfLegs(int numberOfLegs) {
>               this.numberOfLegs = numberOfLegs;
>               return this;
>             }
>             @Override
>             Animal build() {
>               return new AutoValue_Animal(this.name, this.numberOfLegs);
>             }
>           }
>         }

> Figure 24.2: Code generated by AutoValue for the example of Figure ‍24.1, including the @This annotations added by the Returns Receiver Checker.

The AutoValue and Lombok projects both support automatic generation of builder classes, which enable flexible object construction. For code using these two frameworks, the Returns Receiver Checker automatically adds @This annotations to setter methods in builder classes. All the @This annotations in Figures ‍24.1 and ‍24.2 are automatically added by the Returns Receiver Checker.

# Chapter ‍25 Reflection resolution

A call to Method.invoke might reflectively invoke any method. That method might place requirements on its formal parameters, and it might return any value. To reflect these facts, the annotated JDK contains conservative annotations for Method.invoke. These conservative library annotations often cause a checker to issue false positive warnings when type-checking code that uses reflection.

If you supply the -AresolveReflection command-line option, the Checker Framework attempts to resolve reflection. At each call to Method.invoke or Constructor.newInstance, the Checker Framework first soundly estimates which methods might be invoked at run time. When type-checking the call, the Checker Framework uses the library annotations for the possibly-invoked methods, rather than the imprecise one for Method.invoke.

If the estimate of invoked methods is small, the checker issues fewer false positive warnings. If the estimate of invoked methods is large, these types may be no better than the conservative library annotations.

Reflection resolution is disabled by default, because it increases the time to type-check a program. You should enable reflection resolution with the -AresolveReflection command-line option if, for some call site of Method.invoke or Constructor.newInstance in your program:

1. the conservative library annotations on Method.invoke or Constructor.newInstance cause false positive warnings,
2. the set of possibly-invoked methods or constructors can be known at compile time, and
3. the reflectively invoked methods/constructors are on the class path at compile time.

Reflection resolution does not change your source code or generated code. In particular, it does not replace the Method.invoke or Constructor.newInstance calls.

The command-line option -AresolveReflection=debug outputs verbose information about the reflection resolution process, which may be useful for debugging.

Section ‍25.1 gives an example of reflection resolution. Then, Section ‍25.2 describes the MethodVal and ClassVal Checkers, which reflection resolution uses internally. The paper “Static analysis of implicit control flow: Resolving Java reflection and Android intents” ‍[BJM+15] (ASE 2015, https://homes.cs.washington.edu/~mernst/pubs/implicit-control-flow-ase2015-abstract.html) gives further details.

## 25.1 Reflection resolution example

Consider the following example, in which the Nullness Checker employs reflection resolution to avoid issuing a false positive warning.

    public class LocationInfo {
        @NonNull Location getCurrentLocation() {  ...  }
    }

    public class Example {
        LocationInfo privateLocation = ... ;
        String getCurrentCity() throws Exception {
            Method getCurrentLocationObj = LocationInfo.class.getMethod("getCurrentLocation");
            Location currentLocation = (Location) getCurrentLocationObj.invoke(privateLocation);
            return currentLocation.nameOfCity();
        }
    }

When reflection resolution is not enabled, the Nullness Checker uses conservative annotations on the Method.invoke method signature:

@Nullable Object invoke(@NonNull Object recv, @NonNull Object ... args)

This causes the Nullness Checker to issue the following warning even though currentLocation cannot be null.

    error: [dereference.of.nullable] dereference of possibly-null reference currentLocation
            return currentLocation.nameOfCity();
                   ^
    1 error

When reflection resolution is enabled, the MethodVal Checker infers that the @MethodVal annotation for getCurrentLocationObj is:

@MethodVal(className="LocationInfo", methodName="getCurrentLocation", params=0)

Based on this @MethodVal annotation, the reflection resolver determines that the reflective method call represents a call to getCurrentLocation in class LocationInfo. The reflection resolver uses this information to provide the following precise procedure summary to the Nullness Checker, for this call site only:

@NonNull Object invoke(@NonNull Object recv, @Nullable Object ... args)

Using this more precise signature, the Nullness Checker does not issue the false positive warning shown above.

## 25.2 MethodVal and ClassVal Checkers

The implementation of reflection resolution internally uses the ClassVal Checker (Section ‍25.2.1) and the MethodVal Checker (Section ‍25.2.2). They are similar to the Constant Value Checker (Section ‍23) in that their annotations estimate the run-time value of an expression.

In some cases, you may need to write annotations such as @ClassVal, @MethodVal, @StringVal, and @ArrayLen to aid in reflection resolution. Often, though, these annotations can be inferred (Section ‍25.2.3).

### 25.2.1 ClassVal Checker

The ClassVal Checker defines the following annotations:

@ClassVal(String[] value)
  If an expression has @ClassVal type with a single argument, then its exact run-time value is known at compile time. For example, @ClassVal("java.util.HashMap") indicates that the Class object represents the java.util.HashMap class.

  If multiple arguments are given, then the expression’s run-time value is known to be in that set.

  Each argument is a “fully-qualified binary name” (@FqBinaryName): a primitive or binary name (JLS §13.1), possibly followed by array brackets.

@ClassBound(String[] value)
  If an expression has @ClassBound type, then its run-time value is known to be upper-bounded by that type. For example, @ClassBound("java.util.HashMap") indicates that the Class object represents java.util.HashMap or a subclass of it.

  If multiple arguments are given, then the run-time value is equal to or a subclass of some class in that set.

  Each argument is a “fully-qualified binary name” (@FqBinaryName): a primitive or binary name (JLS §13.1), possibly followed by array brackets.

@UnknownClass
  Indicates that there is no compile-time information about the run-time value of the class — or that the Java type is not Class. This is the default qualifier, and it may not be written in source code.
@ClassValBottom
  Type given to the null literal. It may not be written in source code.

> Figure 25.1: Partial type hierarchy for the ClassVal type system. The type qualifiers in gray (@UnknownClass and @ClassValBottom) should never be written in source code; they are used internally by the type system.

#### Subtyping rules

Figure ‍25.1 shows part of the type hierarchy of the ClassVal type system. @ClassVal(A) is a subtype of @ClassVal(B) if A is a subset of B. @ClassBound(A) is a subtype of @ClassBound(B) if A is a subset of B. @ClassVal(A) is a subtype of @ClassBound(B) if A is a subset of B.

### 25.2.2 MethodVal Checker

The MethodVal Checker defines the following annotations:

@MethodVal(String[] className, String[] methodName, int[] params)
  Indicates that an expression of type Method or Constructor has a run-time value in a given set. If the set has size n, then each of @MethodVal’s arguments is an array of size n, and the ith method in the set is represented by { className[i], methodName[i], params[i] }. For a constructor, the method name is “<init>”.

  Consider the following example:

      @MethodVal(className={"java.util.HashMap", "java.util.HashMap"},
                 methodName={"containsKey", "containsValue"},
                 params={1, 1})

  This @MethodVal annotation indicates that the Method is either HashMap.containsKey with 1 formal parameter or HashMap.containsValue with 1 formal parameter.

  The @MethodVal type qualifier indicates the number of parameters that the method takes, but not their type. This means that the Checker Framework’s reflection resolution cannot distinguish among overloaded methods.

@UnknownMethod
  Indicates that there is no compile-time information about the run-time value of the method — or that the Java type is not Method or Constructor. This is the default qualifier, and it may not be written in source code.
@MethodValBottom
  Type given to the null literal. It may not be written in source code.

> Figure 25.2: Partial type hierarchy for the MethodVal type system. The type qualifiers in gray (@UnknownMethod and @MethodValBottom) should never be written in source code; they are used internally by the type system.

#### Subtyping rules

Figure ‍25.2 shows part of the type hierarchy of the MethodVal type system. @MethodVal(classname=CA, methodname=MA, params=PA) is a subtype of @MethodVal(classname=CB, methodname=MB, params=PB) if

∀ indexesi∃ an indexj:CA[i] =CB[j],MA[i] =MA[j],and PA[i] =PB[j]

where CA, MA, and PA are lists of equal size and CB, MB, and PB are lists of equal size.

### 25.2.3 MethodVal and ClassVal inference

The developer rarely has to write @ClassVal or @MethodVal annotations, because the Checker Framework infers them according to Figure ‍25.3. Most readers can skip this section, which explains the inference rules.

> Figure 25.3: Example inference rules for @ClassVal, @ClassBound, and @MethodVal. Additional rules exist for expressions with similar semantics but that call methods with different names or signatures.

The ClassVal Checker infers the exact class name (@ClassVal) for a Class literal (C.class), and for a static method call (e.g., Class.forName(arg), ClassLoader.loadClass(arg), ...) if the argument is a statically computable expression. In contrast, it infers an upper bound (@ClassBound) for instance method calls (e.g., obj.getClass()).

The MethodVal Checker infers @MethodVal annotations for Method and Constructor types that have been created using a method call to Java’s Reflection API:

- Class.getMethod(String name, Class<?>... ‍paramTypes)
- Class.getConstructor(Class<?>... ‍paramTypes)

Note that an exact class name is necessary to precisely resolve reflectively-invoked constructors since a constructor in a subclass does not override a constructor in its superclass. This means that the MethodVal Checker does not infer a @MethodVal annotation for Class.getConstructor if the type of that class is @ClassBound. In contrast, either an exact class name or a bound is adequate to resolve reflectively-invoked methods because of the subtyping rules for overridden methods.

# Chapter ‍26 Initialized Fields Checker

The Initialized Fields Checker warns if a constructor does not initialize a field.

## 26.1 Running the Initialized Fields Checker

An example invocation is

    javac -processor org.checkerframework.common.initializedfields.InitializedFieldsChecker MyFile.java

If you run it together with other checkers, then it issues warnings only if the default value assigned by Java (0, false, or null) is not consistent with the field’s annotation, for the other checkers. An example invocation is

    javac -processor ValueChecker,InitializedFieldsChecker MyFile.java

## 26.2 Motivation: uninitialized fields

Without the Initialized Fields Checker, every type system is unsound with respect to fields that are never set. (Exception: The Nullness Checker (Chapter ‍3) is sound. Also, a type system is sound if every annotation is consistent with 0, false, and null.) Consider the following code:

    import org.checkerframework.checker.index.qual.Positive;

    class MyClass {
      @Positive int x;
      MyClass() {
        // empty body
      }

      @Positive int getX() {
        return x;
      }
    }

Method getX is incorrect because it returns 0, which is not positive. However, the code type-checks because there is never an assignment to x whose right-hand side is not positive. If you run the Index Checker together with the Initialized Fields Checker, then the code correctly does not type-check.

#### Remaining unsoundness

Even with the Initialized Fields Checker, every type system (except the Nullness Checker, Chapter ‍3) is unsound with respect to partially-initialized fields. Consider the following code:

    import org.checkerframework.checker.index.qual.Positive;

    class MyClass {
      @Positive int x;
      MyClass() {
        foo(this);
        x = 1;
      }

      @Positive int foo() {
        // ... use x, expecting it to be positive ...
      }
    }

Within method foo, x can have the value 0 even though the type of x is @Positive int.

## 26.3 Example

As an example, consider the following code:

    import org.checkerframework.checker.index.qual.Positive;

    class MyClass {

      @Positive int x;
      @Positive int y;
      int z;

      // Warning: field y is not initialized
      MyClass() {
        x = 1;
      }
    }

When run by itself, the Initialized Fields Checker warns that fields y and field z are not set.

When run together with the Index Checker, the Initialized Fields Checker warns that field y is not set. It does not warn about field z, because its default value (0) is consistent with its annotations.

## 26.4 Annotations

The Initialized Fields type system uses the following type annotations:

@InitializedFields
  indicates which fields have definitely been initialized so far.
@InitializedFieldsBottom
  is the type of null. Programmers rarely write this type.
@PolyInitializedFields
  is a qualifier that is polymorphic over field initialization. For a description of qualifier polymorphism, see Section ‍31.2.

> Figure 26.1: The type qualifier hierarchy of the Initialized Fields Checker. @InitializedFieldsBottom is rarely written by a programmer.

Figure ‍26.1 shows the subtyping relationships among the type qualifiers.

There is also a method declaration annotation:

@EnsuresInitializedFields
  indicates which fields the method sets. Use this for helper methods that are called from a constructor.

## 26.5 Comparison to the Initialization Checker

The Initialized Fields Checker is a lightweight version of the Initialization Checker (Section ‍3.8). Here is a comparison between them.

| Initialization Checker | Initialized Fields Checker
superclasses | tracks initialization of supertype fields | checks one class at a time
partial initialization | changes the types of fields that are not initialized | unsound treatment of partially-initialized objects (*)
type systems | works only with the Nullness Checker (**) | works for any type system
disabling | always runs with the Nullness Checker | can be enabled/disabled per run

* See Section ‍26.2 for an example.
** The Initialization Checker could be made to work with any type system, but doing so would require changing the implementation of both the type system and the Initialization Checker.

# Chapter ‍27 Aliasing Checker

The Aliasing Checker identifies expressions that definitely have no aliases.

Two expressions are aliased when they have the same non-primitive value; that is, they are references to the identical Java object in the heap. Another way of saying this is that two expressions, exprA and exprB, are aliases of each other when exprA == exprB at the same program point.

Assigning to a variable or field typically creates an alias. For example, after the statement a = b;, the variables a and b are aliased.

Knowing that an expression is not aliased permits more accurate reasoning about how side effects modify the expression’s value.

To run the Aliasing Checker, supply the -processor org.checkerframework.common.aliasing.AliasingChecker command-line option to javac. However, a user rarely runs the Aliasing Checker directly. This type system is mainly intended to be used together with other type systems. For example, the SPARTA information flow type-checker (Section ‍30.20) uses the Aliasing Checker to improve its type refinement — if an expression has no aliases, a more refined type can often be inferred, otherwise the type-checker makes conservative assumptions.

## 27.1 Aliasing annotations

> Figure 27.1: Type hierarchy for the Aliasing type system. These qualifiers are applicable to any reference (non-primitive) type.

There are two possible types for an expression:

@MaybeAliased
  is the type of an expression that might have an alias. This is the default, so every unannotated type is @MaybeAliased. (This includes the type of null.)
@Unique
  is the type of an expression that has no aliases.

  The @Unique annotation is only allowed at local variables, method parameters, constructor results, and method returns. A constructor’s result should be annotated with @Unique only if the constructor’s body does not create an alias to the constructed object.

There are also two annotations, which are currently trusted instead of verified, that can be used on formal parameters (including the receiver parameter, this):

@NonLeaked
  identifies a formal parameter that is not leaked nor returned by the method body. For example, the formal parameter of the String copy constructor, String(String s), is @NonLeaked because the body of the method only makes a copy of the parameter.
@LeakedToResult
  is used when the parameter may be returned, but it is not otherwise leaked. For example, the receiver parameter of StringBuffer.append(StringBuffer this, String s) is @LeakedToResult, because the method returns the updated receiver.

## 27.2 Leaking contexts

This section lists the expressions that create aliases. These are also called “leaking contexts”.

Assignments
  After an assignment, the left-hand side and the right-hand side are typically aliased. (The only counterexample is when the right-hand side is a fresh expression; see Section ‍27.4.)

        @Unique Object u = ...;
        Object o = u;                    // (not.unique) type-checking error!

  If this example type-checked, then u and o would be aliased. For this example to type-check, either the @Unique annotation on the type of u, or the o = u; assignment, must be removed.

Method calls and returns (pseudo-assignments)
  Passing an argument to a method is a “pseudo-assignment” because it effectively assigns the argument to the formal parameter. Return statements are also pseudo-assignments. As with assignments, the left-hand side and right-hand side of pseudo-assignments are typically aliased.

  Here is an example for argument-passing:

        void mightDoAnything(Object o) { ... }

        @Unique Object u = ...;
        mightDoAnything(u);  // type-checking error, because the callee may create an alias of the passed argument

  Passing a non-aliased reference to a method does not necessarily create an alias. However, the body of the method might create an alias or leak the reference. Thus, the Aliasing Checker always treats a method call as creating aliases for each argument unless the corresponding formal parameter is marked as @NonLeaked or @LeakedToResult.

  Here is an example for a return statement:

      Object id(@Unique Object p) {
          return p;     // (not.unique) type-checking error!
      }

  If this code type-checked, then it would be possible for clients to write code like this:

      @Unique Object u = ...;
      Object o = id(u);

  after which there is an alias to u even though it is declared as @Unique.

  However, it is permitted to write

      Object id(@LeakedToResult Object p) {
          return p;
      }

  after which the following code type-checks:

      @Unique Object u = ...;
      id(u);                   // method call result is not used
      Object o1 = ...;
      Object o2 = id(o1);      // argument is not @Unique

Throws
  A thrown exception can be captured by a catch block, which creates an alias of the thrown exception.

      void aliasInCatchBlock() {
          @Unique Exception uex = new Exception();
          try {
              throw uex;    // (not.unique) type-checking error!
          } catch (Exception ex) {
              // uex and ex refer to the same object here.
          }
      }

Array initializers

  Array initializers assign the elements in the initializers to corresponding indexes in the array, therefore expressions in an array initializer are leaked.

      void aliasInArrayInitializer() {
          @Unique Object o = new Object();
          Object[] ar = new Object[] { o };  // (not.unique) type-checking error!
          // The expressions o and ar[0] are now aliased.
      }

## 27.3 Restrictions on where @Unique may be written

The @Unique qualifier may not be written on locations such as fields, array elements, and type parameters.

As an example of why @Unique may not be written on a field’s type, consider the following code:

    class MyClass {
        @Unique Object field;
        void makesAlias() {
            MyClass myClass2 = this;
            // this.field is now an alias of myClass2.field
        }
    }

That code must not type-check, because field is declared as @Unique but has an alias. The Aliasing Checker solves the problem by forbidding the @Unique qualifier on subcomponents of a structure, such as fields. Other solutions might be possible; they would be more complicated but would permit more code to type-check.

@Unique may not be written on a type parameter for similar reasons. The assignment

    List<@Unique Object> l1 = ...;
    List<@Unique Object> l2 = l1;

must be forbidden because it would alias l1.get(0) with l2.get(0) even though both have type @Unique. The Aliasing Checker forbids this code by rejecting the type List<@Unique Object>.

## 27.4 Aliasing type refinement

Type refinement enables a type checker to treat an expression as a subtype of its declared type. For example, even if you declare a local variable as @MaybeAliased (or don’t write anything, since @MaybeAliased is the default), sometimes the Aliasing Checker can determine that it is actually @Unique. For more details, see Section ‍32.7.

The Aliasing Checker treats type refinement in the usual way, except that at (pseudo-)assignments the right-hand-side (RHS) may lose its type refinement, before the left-hand-side (LHS) is type-refined. The RHS always loses its type refinement (it is widened to @MaybeAliased, and its declared type must have been @MaybeAliased) except in the following cases:

- The RHS is a fresh expression — an expression that returns a different value each time it is evaluated. In practice, this is only method/constructor calls with @Unique return type. A variable/field is not fresh because it can return the same value when evaluated twice.
- The LHS is a @NonLeaked formal parameter and the RHS is an argument in a method call or constructor invocation.
- The LHS is a @LeakedToResult formal parameter, the RHS is an argument in a method call or constructor invocation, and the method’s return value is discarded — that is, the method call or constructor invocation is written syntactically as a statement rather than as a part of a larger expression or statement.

A consequence of the above rules is that most method calls are treated conservatively. If a variable with declared type @MaybeAliased has been refined to @Unique and is used as an argument of a method call, it usually loses its @Unique refined type.

Figure ‍27.2 gives an example of the Aliasing Checker’s type refinement rules.

>     // Annotations on the StringBuffer class, used in the examples below.
>     // class StringBuffer {
>     //  @Unique StringBuffer();
>     //  StringBuffer append(@LeakedToResult StringBuffer this, @NonLeaked String s);
>     // }

>     void foo() {
>         StringBuffer sb = new StringBuffer();    // sb is refined to @Unique.

>         StringBuffer sb2 = sb;                   // sb loses its refinement.
>         // Both sb and sb2 have aliases and because of that have type @MaybeAliased.
>     }

>     void bar() {
>         StringBuffer sb = new StringBuffer();     // sb is refined to @Unique.

>         sb.append("someString");
>         // sb stays @Unique, as no aliases are created.

>         StringBuffer sb2 = sb.append("someString");
>         // sb is leaked and becomes @MaybeAliased.

>         // Both sb and sb2 have aliases and because of that have type @MaybeAliased.
>     }

> Figure 27.2: Example of the Aliasing Checker’s type refinement rules.

# Chapter ‍28 Must Call Checker

The Must Call Checker conservatively over-approximates the set of methods that an object should call before it is de-allocated. The checker does not enforce any rules other than subtyping; in particular, it does not enforce that the methods are called before objects are de-allocated. The Must Call Checker is intended to be run as a subchecker of another checker. The primary client of the Must Call Checker is the Resource Leak Checker (Section ‍8), which enforces that every method in a must-call obligation for an expression is called before that expression is de-allocated.

For example, consider a java.io.OutputStream. This OutputStream might have an obligation to call the close() method, to release an underlying file resource. The type of this OutputStream is @MustCall({"close"}). Or, the OutputStream might not have such an obligation, if the underlying resource is a byte array. The type of this OutputStream is @MustCall({}). For an arbitrary OutputStream, the Must Call Checker over-approximates the methods that it should call by assigning it the type @MustCall({"close"}) OutputStream, which can be read as “an OutputStream that might need to call close() (but no other methods) before it is de-allocated”.

If you are running the Resource Leak Checker (Chapter ‍8), then there is no need to run the Must Call Checker, because the Resource Leak Checker does so automatically. Running both may lead to warning suppressions for the Must Call Checker not working.

## 28.1 Must Call annotations

The Must Call Checker supports these type qualifiers:

@MustCall(String[] value)
  gives a superset of the methods that must be called before the expression’s value is de-allocated. The annotation is not refined after a method is called; an expression of type @MustCall("foo") might or might not have already had foo() called on it. The default type qualifier for an unannotated type is @MustCall({}).
@MustCallUnknown
  represents a value with an unknown or infinite set of must-call obligations. It is used internally by the type system but should never be written by a programmer.
@PolyMustCall
  is the polymorphic annotation for the Must Call type system. For a description of polymorphic qualifiers, see Section ‍31.2.
@InheritableMustCall(String[] value)
  is an alias of @MustCall that can only be written on a class declaration. It applies both to the class declaration on which it is written and also to all subclasses. This frees the user of the need to write the @MustCall annotation on every subclass. See Section ‍28.2 for details.

> Figure 28.1: Part of the Must Call Checker’s type qualifier hierarchy. The full hierarchy contains one @MustCall annotation for every combination of methods. Qualifiers in gray are used internally by the type system but should never be written by a programmer.

Here are some facts about the type qualifier hierarchy, which is shown in Figure ‍28.1. Any expression of type @MustCall({}) Object also has type @MustCall({"foo"}) Object. The type @MustCall({"foo"}) Object contains objects that need to call foo and objects that need to call nothing, but the type does not contain an object that needs to call bar (or both foo and bar). @MustCall({"foo", "bar"}) Object represents all objects that need to call foo, bar, both, or neither; it cannot not represent an object that needs to call some other method.

## 28.2 Writing @MustCall/@InheritableMustCall on a class

As explained in Section ‍32.3, a type annotation on a type declaration means that every use of the type has that annotation by default. If a class A’s declaration has a @MustCall annotation, then A’s subclasses usually should too. To do so, a programmer can either write a @MustCall annotation on every subclass, or can write @InheritableMustCall on only A, which will cause the checker to treat every subclass as if it has an identical @MustCall annotation. The latter is the preferred style.

For example, given the following class annotation:

        package java.net;

        import org.checkerframework.checker.mustcall.qual.InheritableMustCall;

        @InheritableMustCall({"close"}) class Socket { }

any use of Socket or a subclass of Socket defaults to @MustCall({"close"}) Socket.

The @InheritableMustCall annotation is necessary because type annotations cannot be made inheritable. @InheritableMustCall is a declaration annotation.

## 28.3 Relationship to lightweight ownership

The Must Call Checker interacts with the @Owning and @NotOwning annotations of the Resource Leak Checker (Section ‍8). In particular, any location that the Resource Leak Checker would consider “non-owning” (e.g., unannotated parameters, @NotOwning returns, etc.) is treated as @MustCall() by the Must Call Checker. This can lead to surprising results when running the Must Call Checker alone (which is not recommended). For example, consider the following program:

      @MustCall({"toString"}) String foo(@MustCall({"hashCode"}) String arg) {
        return arg;
      }

The Must Call Checker will not issue a warning about this code, because arg is non-owning (and therefore treated as @MustCall()).

## 28.4 Assumptions about reflection

The Must Call Checker is unsound with respect to reflection: it assumes that objects instantiated reflectively do not have must-call obligations (i.e., that their type is @MustCall({}). If the checker is run with -AresolveReflection, then this assumption applies only to types that cannot be resolved.

## 28.5 Type parameter bounds often need to be annotated

In an unannotated program, there may be mismatches between defaulted type qualifiers that lead to type-checking errors. That is, the defaulted annotations at two different locations may be different and in conflict. A specific example is in code that contains a mix of explicit upper bounds with an extends clause and implicit upper bounds without an extends clause.

For example, consider the following example (from plume-util) of an interface with explicit upper bounds and a client with implicit upper bounds:

      interface Partition<ELEMENT extends @Nullable Object, CLASS extends @Nullable Object> {}

      class MultiRandSelector<T> {
        private Partition<T, T> eq;
      }

The code contains no @MustCall annotations. Running the Must Call Checker on this code produces an error at each use of T in MultiRandSelector:

    error: [type.argument] incompatible type argument for type parameter ELEMENT of Partitioner.
            private Partitioner<T, T> eq;
                                ^
      found   : T extends @MustCallUnknown Object
      required: @MustCall Object
    error: [type.argument] incompatible type argument for type parameter CLASS of Partitioner.
            private Partitioner<T, T> eq;
                                   ^
      found   : T extends @MustCallUnknown Object
      required: @MustCall Object

The defaulted Must Call annotations differ. Partitioner has an explicit bound, which uses the usual default of @MustCall({}). MultiRandSelector has an implicit bound, which defaults to top (that is, @MustCallUnknown), as explained in Section ‍32.5.3.

In many cases, including this one, you can eliminate the false positive warning without writing any @MustCall annotations. You can either:

- add an explicit bound to MultiRandSelector, changing its declaration to class MultiRandSelector<T extends Object>, or
- remove the explicit bound from Partition, changing its declaration to interface Partition<ELEMENT, CLASS>.

# Chapter ‍29 Subtyping Checker

The Subtyping Checker enforces only subtyping rules. It operates over annotations specified by a user on the command line. Thus, users can create a simple type-checker without writing any code beyond definitions of the type qualifier annotations.

The Subtyping Checker can accommodate all of the type system enhancements that can be declaratively specified (see Chapter ‍36). This includes type introduction rules via the @QualifierForLiterals meta-annotation, and other features such as type refinement (Section ‍32.7) and qualifier polymorphism (Section ‍31.2).

The Subtyping Checker is also useful to type system designers who wish to experiment with a checker before writing code; the Subtyping Checker demonstrates the functionality that a checker inherits from the Checker Framework.

If you need typestate analysis, then you can extend a typestate checker. For more details (including a definition of “typestate”), see Chapter ‍30.30. See Section ‍39.7.1 for a simpler alternative.

For type systems that require special checks (e.g., warning about dereferences of possibly-null values), you will need to write code and extend the framework as discussed in Chapter ‍36.

## 29.1 Using the Subtyping Checker

The Subtyping Checker is used in the same way as other checkers (using the -processor org.checkerframework.common.subtyping.SubtypingChecker option; see Chapter ‍2), except that it requires an additional annotation processor argument via the standard “-A” switch. You must provide one or both of the two following command-line arguments:

- Provide the fully-qualified class name(s) of the annotation(s) in the custom type system through the -Aquals option, using a comma-no-space-separated notation:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.common.subtyping.SubtypingChecker \
              -Aquals=myPackage.qual.MyQual,myPackage.qual.OtherQual MyFile.java ...

- Provide the fully-qualified paths to a set of directories that contain the annotations in the custom type system through the -AqualDirs option, using a colon-no-space-separated notation. For example, if the Checker Framework is on the classpath rather than the processorpath:

        javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
              -processor org.checkerframework.common.subtyping.SubtypingChecker \
              -AqualDirs=/full/path/to/myProject/bin:/full/path/to/myLibrary/bin MyFile.java

If the Checker Framework is on the processorpath, place the annotations on the processorpath instead of on the classpath.

### 29.1.1 Compiling your qualifiers and your project

The annotations listed in -Aquals or -AqualDirs must be accessible to the compiler during compilation. Before you run the Subtyping Checker with javac, they must be compiled and on the same path (the classpath or processorpath) as the Checker Framework. It is not sufficient to supply their source files on the command line.

### 29.1.2 Suppressing warnings from the Subtyping Checker

When suppressing a warning issued by the Subtyping Checker, as the “checker name” you may use the unqualified, uncapitalized name of any of the annotations passed to -Aquals. (See Section ‍33.1.1 for details about the @SuppressWarnings syntax.) As a matter of style, you should choose one of the annotations as the “checker name” part of the @SuppressWarnings string and use it consistently; this avoids confusion and makes it easier to search for uses.

## 29.2 Subtyping Checker example

Consider a hypothetical Encrypted type qualifier, which denotes that the representation of an object (such as a String, CharSequence, or byte[]) is encrypted. To use the Subtyping Checker for the Encrypted type system, follow three steps.

(This example also appears in the Checker Framework Tutorial, in more detail.)

1. Define two annotations for the Encrypted and PossiblyUnencrypted qualifiers:

       package myPackage.qual;

       import org.checkerframework.framework.qual.DefaultFor;
       import org.checkerframework.framework.qual.SubtypeOf;
       import java.lang.annotation.ElementType;
       import java.lang.annotation.Target;

       /**
        * Denotes that the representation of an object is encrypted.
        */
       @SubtypeOf(PossiblyUnencrypted.class)
       @DefaultFor({TypeUseLocation.LOWER_BOUND})
       @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
       public @interface Encrypted {}

   ‍

       package myPackage.qual;

       import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
       import org.checkerframework.framework.qual.SubtypeOf;
       import java.lang.annotation.ElementType;
       import java.lang.annotation.Target;

       /**
        * Denotes that the representation of an object might not be encrypted.
        */
       @DefaultQualifierInHierarchy
       @SubtypeOf({})
       @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
       public @interface PossiblyUnencrypted {}

   Note that all custom annotations must have the @Target(ElementType.TYPE_USE) meta-annotation. See Section ‍36.5.1.

   Don’t forget to compile these classes:

       $ javac myPackage/qual/Encrypted.java myPackage/qual/PossiblyUnencrypted.java

   The resulting .class files should either be on the same path (the classpath or processor path) as the Checker Framework.

2. Write @Encrypted annotations in your program (say, in file YourProgram.java):

       import myPackage.qual.Encrypted;

       ...

       public @Encrypted String encrypt(String text) {
           // ...
       }

       // Only send encrypted data!
       public void sendOverInternet(@Encrypted String msg) {
           // ...
       }

       void sendText() {
           // ...
           @Encrypted String ciphertext = encrypt(plaintext);
           sendOverInternet(ciphertext);
           // ...
       }

       void sendPassword() {
           String password = getUserPassword();
           sendOverInternet(password);
       }

   You may also need to add @SuppressWarnings annotations to the encrypt and decrypt methods. Analyzing them is beyond the capability of any realistic type system.

3. Invoke the compiler with the Subtyping Checker, specifying the @Encrypted annotation using the -Aquals option. You should add the Encrypted classfile to the processor classpath:

         javac -processorpath myqualpath -processor org.checkerframework.common.subtyping.SubtypingChecker
               -Aquals=myPackage.qual.Encrypted,myPackage.qual.PossiblyUnencrypted YourProgram.java

       YourProgram.java:42: incompatible types.
       found   : @myPackage.qual.PossiblyUnencrypted java.lang.String
       required: @myPackage.qual.Encrypted java.lang.String
           sendOverInternet(password);
                            ^

4. You can also provide the fully-qualified paths to a set of directories that contain the qualifiers using the -AqualDirs option, and add the directories to the boot classpath, for example:

         javac -classpath /full/path/to/myProject/bin:/full/path/to/myLibrary/bin \
               -processor org.checkerframework.common.subtyping.SubtypingChecker \
               -AqualDirs=/full/path/to/myProject/bin:/full/path/to/myLibrary/bin YourProgram.java

   Note that in these two examples, the compiled class file of the myPackage.qual.Encrypted and myPackage.qual.PossiblyUnencrypted annotations must exist in either the myProject/bin directory or the myLibrary/bin directory. The following placement of the class files will work with the above commands:

         .../myProject/bin/myPackage/qual/Encrypted.class
         .../myProject/bin/myPackage/qual/PossiblyUnencrypted.class

Also, see the example project in the docs/examples/subtyping-extension directory.

## 29.3 Type aliases and typedefs

A type alias or typedef is a type that shares the same representation as another type but is conceptually distinct from it. For example, some strings in your program may be street addresses; others may be passwords; and so on. You wish to indicate, for each string, which one it is, and to avoid mixing up the different types of strings. Likewise, you could distinguish integers that are offsets from those that are absolute values.

Creating a new type makes your code easier to understand by conveying the intended use of each variable. It also prevents errors that come from using the wrong type or from mixing incompatible types in an operation.

If you want to create a type alias or typedef, you have multiple options: a regular Java subtype, the Units Checker (Chapter ‍20), the Fake Enum Checker (Chapter ‍9), or the Subtyping Checker.

A Java subtype is easy to create and does not require a tool such as the Checker Framework; for instance, you would declare class Address extends String. There are a number of limitations to this “pseudo-typedef”, however ‍[Goe06]. Primitive types and final types (including String) cannot be extended. Equality and identity tests can return incorrect results when a wrapper object is used. Existing return types in code would need to be changed, which is easy with an annotation but disruptive to change the Java type. Therefore, it is best to avoid the pseudo-typedef antipattern.

The Units Checker (Chapter ‍20) is useful for the particular case of units of measurement, such as kilometers verses miles.

The Fake Enum Checker (Chapter ‍9) builds in a set of assumptions. If those fit your use case, then it’s easiest to use the Fake Enum Checker (though you can achieve them using the Subtyping Checker). The Fake Enum Checker forbids mixing of fenums of different types, or fenums and unannotated types. For instance, binary operations other than string concatenations are forbidden, such as NORTH+1, NORTH+MONDAY, and NORTH==MONDAY. However, NORTH+SOUTH is permitted.

By default, the Subtyping Checker does not forbid any operations.

If you choose to use the Subtyping Checker, then you have an additional design choice to make about the type system. In the general case, your type system will look something like Figure ‍29.1.

> Figure 29.1: Type system for a type alias or typedef type system. The type system designer may choose to omit some of these types, but this is the general case. The type system designer’s choice of defaults affects the interpretation of unannotated code, which affects the guarantees given for unannotated code.

References whose type is @MyType are known to store only values from your new type. There is no such guarantee for @MyTypeUnknown and @NotMyType, but those types mean different things. An expression of type @NotMyType is guaranteed never to evaluate to a value of your new type. An expression of type @MyTypeUnknown may evaluate to any value — including values of your new type and values not of your new type. (@MyTypeBottom is the type of null and is also used for dead code and erroneous situations; it can be ignored for this discussion.)

A key choice for the type system designer is which type is the default. That is, if a programmer does not write @MyType on a given type use, should that type use be interpreted as @MyTypeUnknown or as @NotMyType?

- If unannotated types are interpreted as @NotMyType, then the type system enforces very strong separation between your new type and all other types. Values of your type will never mix with values of other types. If you don’t see @MyType written explicitly on a type, you will know that it does not contain values of your type.
- If unannotated types are interpreted as @MyTypeUnknown, then a generic, unannotated type may contain a value of your new type. In this case, @NotMyType does not need to exist, and @MyTypeBottom may or may not exist in your type system.

A downside of the stronger guarantee that comes from using @NotMyType as the default is the need to write additional annotations. For example, if @NotMyType is the default, this code does not typecheck:

    void method(Object o) { ... }
    <U> void use(List<U> list) {
      method(list.get(0));
    }

Because (implicit) upper bounds are interpreted as the top type (see Section ‍31.1.2), this is interpreted as

    void method(@NotMyType Object o) { ... }
    <@U extends @MyTypeUnknown Object> void use(List<U> list) {
      // type error: list.get(0) has type @MyTypeUnknown, method expects @NotMyType
      method(list.get(0));
    }

To make the code type-check, it is necessary to write an explicit annotation, either to restrict use’s argument or to expand method’s parameter type.

# Chapter ‍30 Third-party checkers

The Checker Framework has been used to build other checkers that are not distributed together with the framework. This chapter gives an incomplete list of them. (If you know of others, or if you want this chapter to reference your checker, please send us a link and a short description.) Many of the publications in Section ‍40.7 provide implementations, which are not necessarily listed here yet.

These tools are externally-maintained, so if you have problems or questions, you should contact their maintainers rather than the Checker Framework maintainers.

This list is in reverse chronological order; newer tools appear first and older ones appear last.

## 30.1 Determinism checker

The Determinism Checker ensures that a program is deterministic across executions. A determinismic program is easier to test, and it is easier to debug (such as comparing executions).

The Determinism Checker focuses on sequential programs. It detects determinism due to the iteration order of a hash table (or on any other property of hash codes), default formatting (such as Java’s Object.toString(), which includes a memory address), random, date-and-time functions, and accessing system properties such as the file system or environment variables.

The paper “Verifying determinism in sequential programs”‍[MWME21] describes the Determinism Checker.

## 30.2 Constant Value Inference (Interval Inference)

Interval analysis estimates the run-time values of numerical expressions in the source code by computing a lower bound and an upper bound. The Constant Value Inference project is a whole-program inference approach for integral range analysis (interval analysis).

The thesis “Interval Type Inference: Improvements and Evaluations”‍[Wan21] describes Constant Value Inference.

## 30.3 Crypto Checker

The Crypto Checker can help you find whether there are any weak or unsupported crypto algorithms and the unsupported algorithm providers being used in your program. If the Crypto Checker issues no warnings for a given program, then you have a guarantee that your program at runtime will never have these issues. This is similar to the earlier AWS Crypto Policy Compliance Checker (Section ‍30.4).

The paper “Ensuring correct cryptographic algorithm and provider usage at compile time”‍[XCD21] (FTFjP 2021) and the thesis “Light-weight verification of cryptographic API usage”‍[Xin20] describe the Crypto Checker.

## 30.4 AWS crypto policy compliance checker

The AWS crypto policy compliance checker checks that no weak cipher algorithms are used with the Java crypto API.

The paper “Continuous Compliance”‍[KSTE20] (ASE 2020) describes several custom Checker Framework checkers in the context of a compliance regime at Amazon Web Services.

## 30.5 AWS KMS compliance checker

The AWS KMS compliance checker extends the Constant Value Checker (see Chapter ‍23) to enforce that calls to Amazon Web Services’ Key Management System only request 256-bit (or longer) data keys. This checker can be used to help enforce a compliance requirement (such as from SOC or PCI-DSS) that customer data is always encrypted with 256-bit keys.

The KMS compliance checker is available in Maven Central. To use it in build.gradle, add the following dependency:

    compile("software.amazon.checkerframework:aws-kms-compliance-checker:1.0.2")

Other build systems are similar.

The paper “Continuous Compliance”‍[KSTE20] (ASE 2020) describes several custom Checker Framework checkers in the context of a compliance regime at Amazon Web Services.

## 30.6 PUnits units of measurement

The PUnits system can check the correctness of a program or annotate a program with units of measurement. It is described in the paper “Precise inference of expressive units of measurement types” (OOPSLA 2020) ‍[XLD20].

## 30.7 JaTyC typestate checker

JaTyC, Java Typestate Checker, ensures that methods are called in the correct order. The sequences of method calls allowed are specified in a protocol file which is associated with a Java class by adding a @Typestate annotation to the class. It is described in “Behavioural Types for Memory and Method Safety in a Core Object-Oriented Language”‍[BFG+20], and the implementation is an extension of Mungo (https://www.dcs.gla.ac.uk/research/mungo/index.html).

## 30.8 NullAway

NullAway is a fast type-checker for only nullness properties. It is built on top of Error Prone and the Checker Framework’s Dataflow Framework. See Section ‍39.10.3 for a comparison between NullAway and the Nullness Checker. NullAway is described in the paper “NullAway: Practical type-based null safety for Java”‍[BCS19].

## 30.9 Nullness Rawness Checker

The Nullness Rawness Checker is a variant of the Nullness Checker that uses a different type system for initialization. It was distributed with the Checker Framework through release 2.9.0 (dated 3 July 2019). If you wish to use it, install Checker Framework version 2.9.0.

The paper “Inference of field initialization”‍[SE11] describes inference for the Rawness Initialization Checker.

## 30.10 UI Thread Checker for ReactiveX

The Rx Thread & Effect Checker enforces UI Thread safety properties for stream-based Android applications. The paper “Safe Stream-Based Programming with Refinement Types”‍[SCSC18] describes the Rx Thread & Effect Checker.

## 30.11 Practical Immutability For Classes And Objects (PICO)

Practical Immutability For Classes And Objects (PICO) is a type system that supports class level and object level immutability based on Checker Framework. The implementation is available at https://github.com/opprop/immutability, and a docker image is available at https://hub.docker.com/repository/docker/lnsun/pico.

The theses “Context Sensitive Typechecking And Inference: Ownership And Immutability”‍[Ta18] and “An Immutability Type System for Classes and Objects: Improvements, Experiments, and Comparisons”‍[Sun21] describe PICO.

## 30.12 Read Checker and Cast Checker for ensuring that EOF is recognized

The InputStream.read() and Reader.read() methods read a byte (or character) from a stream. The methods return an int, using -1 to indicate the end of the stream. It is an error to cast the value to a byte (or character) without first checking the value against -1. This is rule CERT-FIO08-J. The Read Checker and/or Cast Checker enforces it.

The paper “Don’t miss the end: Preventing unsafe end-of-file comparisons” ‍[CD18] and the thesis “Pluggable Properties for Program Understanding: Ontic Type Checking and Inference”‍[Che18] describes the Read Checker.

## 30.13 Ontology type system

The Ontology type system groups related types into coarser concepts. For example, the SEQUENCE concept includes Java’s Array, List, and their subtypes. Other examples are VELOCITY and FORCE.

The thesis “Pluggable Properties for Program Understanding: Ontic Type Checking and Inference”‍[Che18] describes the Ontic type system.

## 30.14 Glacier: Class immutability

Glacier enforces transitive class immutability in Java. According to its webpage:

- Transitive: if a class is immutable, then every field must be immutable. This means that all reachable state from an immutable object’s fields is immutable.
- Class: the immutability of an object depends only on its class’s immutability declaration.
- Immutability: state in an object is not changable through any reference to the object.

Glacier is described by the paper “Glacier: Transitive class immutability for Java” (ICSE 2017) ‍[CNA+17] and the PhD thesis “User-Centered Design of Principled Programming Languages”‍[Cob20].

## 30.15 SQL checker that supports multiple dialects

jOOQ is a database API that lets you build typesafe SQL queries. jOOQ version 3.0.9 and later ships with a SQL checker that provides even more safety: it ensures that you don’t use SQL features that are not supported by your database implementation. You can learn about the SQL checker at https://blog.jooq.org/jsr-308-and-the-checker-framework-add-even-more-typesafety-to-jooq-3-9/.

## 30.16 Immutability checkers: IGJ, OIGJ, and Javari

Javari ‍[TE05], IGJ ‍[ZPA+07], and OIGJ ‍[ZPL+10] are type systems that enforce immutability constraints. Type-checkers for all three type systems were distributed with the Checker Framework through release 1.9.13 (dated 1 April 2016). If you wish to use them, install Checker Framework version 1.9.13.

They were removed from the main distribution on June 1, 2016 because the implementations were not being maintained as the Checker Framework evolved. The type systems are valuable, and some people found the type-checkers useful. However, we wanted to focus on distributing checkers that are currently being maintained.

IGJ and OIGJ are described in the papers “Object and reference immutability using Java generics”‍[ZPA+07] (ESEC/FSE 2007) and “Ownership and immutability in generic Java”‍[ZPL+10] (OOPSLA 2010). The Javari type system is described in “Javari: Adding reference immutability to Java”‍[TE05] (OOPSLA 2005); for inference, see “Inference of reference immutability”‍[QTE08] (ECOOP 2008) and “Parameter reference immutability: Formal definition, inference tool, and comparison”‍[AQKE09] (J.ASE 2009). The paper “Practical pluggable types for Java”‍[PAC+08] (ISSTA 2008) describes case studies in which the Javari and IGJ Checkers found previously-unknown errors in real software.

## 30.17 JCrypt: computation over encrypted data

JCrypt is a static program analysis which transforms a Java program into an equivalent one, so that it performs computation over encrypted data and preserves data confidentiality. It is described in the paper “JCrypt: Towards computation over encrypted data”‍[DMD16a].

## 30.18 DroidInfer: information flow

DroidInfer determines the Android library sources (e.g., location access, phone state) and sinks (e.g., Internet access) used by a program, and determines whether there is impermissible information flow between them. It is described in the paper “Scalable and Precise Taint Analysis for Android” (ISSTA 2015) ‍[HDMD15].

## 30.19 Error Prone linter

Error Prone is a linter or bug detector for Java. It reports violations of style rules. It is built on top of the Checker Framework’s Dataflow Framework. See Section ‍39.10.1 for a comparison between Error Prone and the Nullness Checker.

## 30.20 SPARTA information flow type-checker for Android

SPARTA is a security toolset aimed at preventing malware from appearing in an app store. SPARTA provides an information-flow type-checker that is customized to Android but can also be applied to other domains. The paper “Collaborative verification of information flow for a high-assurance app store”‍[EJM+14] (CCS 2014) describes the SPARTA toolset and information flow type-checker.

## 30.21 SFlow type system for information flow

SFlow is a context-sensitive type system for secure information flow. It contains two variants, SFlow/Integrity and SFlow/Confidentiality. SFlowInfer is its worst-case-cubic inference analysis.

The paper “Type-based taint analysis for Java web applications” (FASE 2014) and the thesis “An Inference And Checking Framework For Context-Sensitive Pluggable Types” (PhD thesis, 2014) describe SFlow and SFlowInfer.

## 30.22 CheckLT taint checker

CheckLT uses taint tracking to detect illegal information flows, such as unsanitized data that could result in a SQL injection attack.

## 30.23 EnerJ checker

EnerJ is an extension to Java that exposes hardware faults in a safe, principled manner to save energy with only slight sacrifices to the quality of service. More details appear in the paper “EnerJ: Approximate Data Types for Safe and General Low-Power Computation”‍[SDF+11] (PLDI 2011).

## 30.24 ReIm immutability

A checker and inference tool for ReIm ‍[HMDE12, Hua14], an immutability type system, is available at https://github.com/proganalysis/type-inference.

## 30.25 SFlow x ReIm for information flow and reference immutability

The “SFlow × ReIm” system combines information flow and reference immutability. It is described in the paper “Composing polymorphic information flow systems with reference immutability”‍[MH13].

## 30.26 Generic Universe Types checker

A checker for Generic Universe Types ‍[DEM11], a lightweight ownership type system, is available from https://ece.uwaterloo.ca/~wdietl/ownership/ and https://github.com/opprop/universe.

The paper “Tunable static inference for Generic Universe Types”‍[DEM11] (ECOOP 2011) describes inference for the Generic Universe Types type system. Another implementation of Universe Types and ownership types is described in “Inference and checking of object ownership”‍[HDME12] (ECOOP 2012).

## 30.27 Safety-Critical Java checker

A checker for Safety-Critical Java (SCJ, JSR 302) ‍[TPV10, TPNV11] is available at https://sss.cs.purdue.edu/projects/oscj/checker/checker.html. Developer resources are available at the project page https://code.google.com/archive/p/scj-jsr302/.

## 30.28 Thread locality checker

Loci is a checker for thread locality. For more details, see the paper “Loci: Simple thread-locality for Java”‍[WPM+09] (ECOOP 2009) and the thesis “The Design, Implementation and Evaluation of a Pluggable Type Checker for Thread-Locality in Java”‍[She11].

## 30.29 Units and dimensions checker

A checker for units and dimensions was available at https://www.lexspoon.org/expannots/.

Unlike the Units Checker that is distributed with the Checker Framework (see Section ‍20), this checker includes dynamic checks and permits annotation arguments that are Java expressions. This added flexibility, however, requires that you use a special version both of the Checker Framework and of the javac compiler.

## 30.30 Typestate checkers

In a regular type system, a variable has the same type throughout its scope. In a typestate system, a variable’s type can change as operations are performed on it.

The most common example of typestate is for a File object. Assume a file can be in two states, @Open and @Closed. Calling the close() method changes the file’s state. Any subsequent attempt to read, write, or close the file will lead to a run-time error. It would be better for the type system to warn about such problems, or guarantee their absence, at compile time.

Accumulation analysis (Chapter ‍37) is a special case of typestate analysis. One instantiation of it is the Called Methods Checker (Chapter ‍7), which can check any property of the form “call method A before method B”. It also ensures that builders are used correctly.

JaTyC (Section ‍30.7) is a Java typestate checker.

### 30.30.1 Comparison to flow-sensitive type refinement

The Checker Framework’s flow-sensitive type refinement (Section ‍32.7) implements a form of typestate analysis. For example, after code that tests a variable against null, the Nullness Checker (Chapter ‍3) treats the variable’s type as @NonNull T, for some T.

For many type systems, flow-sensitive type refinement is sufficient. But sometimes, you need full typestate analysis. This section compares the two. (Unused variables (Section ‍32.10) also have similarities with typestate analysis and can occasionally substitute for it. For brevity, this discussion omits them.)

A typestate analysis is easier for a user to create or extend. Flow-sensitive type refinement is built into the Checker Framework and is optionally extended by each checker. Modifying the rules requires writing Java code in your checker. By contrast, it is possible to write a simple typestate checker declaratively, by writing annotations on the methods (such as close()) that change a reference’s typestate.

A typestate analysis can change a reference’s type to something that is not consistent with its original definition. For example, suppose that a programmer decides that the @Open and @Closed qualifiers are incomparable — neither is a subtype of the other. A typestate analysis can specify that the close() operation converts an @Open File into a @Closed File. By contrast, flow-sensitive type refinement can only give a new type that is a subtype of the declared type — for flow-sensitive type refinement to be effective, @Closed would need to be a child of @Open in the qualifier hierarchy (and close() would need to be treated specially by the checker).

# Chapter ‍31 Generics and polymorphism

Section ‍31.1 describes support for Java generics (also known as “parametric polymorphism”). Section ‍31.2 describes polymorphism over type qualifiers for methods. Section ‍31.3 describes polymorphism over type qualifiers for classes.

## 31.1 Generics (parametric polymorphism or type polymorphism)

The Checker Framework fully supports type-qualified Java generic types and methods (also known as “parametric polymorphism”). When instantiating a generic type, clients supply the qualifier along with the type argument, as in List<@NonNull String>. When using a type variable T within the implementation of a generic type, typically no type qualifier is written (see Section ‍31.1.3); rather, the instantiation of the type parameter is restricted (see Section ‍31.1.2).

### 31.1.1 Raw types

Before running any pluggable type-checker, we recommend that you eliminate raw types from your code (e.g., your code should use List<...> as opposed to List). Your code should compile without warnings when using the standard Java compiler and the -Xlint:unchecked -Xlint:rawtypes command-line options. Using generics helps prevent type errors just as using a pluggable type-checker does, and makes the Checker Framework’s warnings easier to understand.

If your code uses raw types, then the Checker Framework will do its best to infer the Java type arguments and the type qualifiers. By default these inferred types are ignored in subtyping checks. If you supply the command-line option -AignoreRawTypeArguments=false you will see errors from raw types.

### 31.1.2 Restricting instantiation of a generic class

When you define a generic class in Java, the extends clause of the generic type parameter (known as the “upper bound”) requires that the corresponding type argument must be a subtype of the bound. For example, given the definition `class G<T extends Number> {...}`, the upper bound is Number and a client can instantiate it as G<Number> or G<Integer> but not G<Date>.

You can write a type qualifier on the extends clause to make the upper bound a qualified type. For example, you can declare that a generic list class can hold only non-null values:

      class MyList<T extends @NonNull Object> {...}

      MyList<@NonNull String> m1;       // OK
      MyList<@Nullable String> m2;      // error

That is, in the above example, all arguments that replace T in MyList<T> must be subtypes of @NonNull Object.

#### Syntax for upper and lower bounds

Conceptually, each generic type parameter has two bounds — a lower bound and an upper bound — and at instantiation, the type argument must be within the bounds. Java only allows you to specify the upper bound; the lower bound is implicitly the bottom type void. The Checker Framework gives you more power: you can specify both an upper and lower bound for type parameters. Write the upper bound on the extends clause, and write the lower bound on the type variable.

      class MyList<@LowerBound T extends @UpperBound Object> { ... }

You may omit either the upper or the lower bound, and the Checker Framework will use a default.

For a discussion of wildcards, see Section ‍31.1.4.

For a concrete example, consider the type system of the Regex Checker (see Figure ‍14.1) in which @Regex(0) :> @Regex(1) :> @Regex(2) :> @Regex(3) :> ….

      class MyRegexes<@Regex(5) T extends @Regex(1) String> { ... }

      MyRegexes<@Regex(0) String> mu;   // error - @Regex(0) is not a subtype of @Regex(1)
      MyRegexes<@Regex(1) String> m1;   // OK
      MyRegexes<@Regex(3) String> m3;   // OK
      MyRegexes<@Regex(5) String> m5;   // OK
      MyRegexes<@Regex(6) String> m6;   // error - @Regex(6) is not a supertype of @Regex(5)

The above declaration states that the upper bound of the type variable is @Regex(1) String and the lower bound is @Regex(5) void. That is, arguments that replace T in MyList<T> must be subtypes of @Regex(1) String and supertypes of @Regex(5) void. Since void cannot be used to instantiate a generic class, MyList may be instantiated with @Regex(1) String through @Regex(5) String.

To specify an exact bound, place the same annotation on both bounds. For example:

      class MyListOfNonNulls<@NonNull T extends @NonNull Object> { ... }
      class MyListOfNullables<@Nullable T extends @Nullable Object> { ... }

      MyListOfNonNulls<@NonNull Number> v1;      // OK
      MyListOfNonNulls<@Nullable Number> v2;     // error
      MyListOfNullables<@NonNull Number> v4;     // error
      MyListOfNullables<@Nullable Number> v3;    // OK

It is an error if the lower bound is not a subtype of the upper bound.

      class MyClass<@Nullable T extends @NonNull Object>  // error: @Nullable is not a subtype of @NonNull

#### Defaults

A generic type parameter or wildcard is written as class MyClass<@LowerBound T extends @UpperBound JavaUpperBound> or as MyClass<@UpperBound ? super @LowerBound JavaLowerBound>, where “@LowerBound” and “@UpperBound” are type qualifiers.

For lower bounds: If no type annotation is written in front of ?, then the lower bound defaults to @BottomType void.

For upper bounds:

- If the extends clause is omitted, then the upper bound defaults to @TopType Object.
- If the extends clause is written but contains no type qualifier, then the normal defaulting rules apply to the type in the extends clause (see Section ‍32.5.3).

The upper-bound rules mean that even though in Java the following two declarations are equivalent:

      class MyClass<T>
      class MyClass<T extends Object>

they specify different type qualifiers on the upper bound, if the type system’s default annotation is not its top annotation.

The Nullness type system is an example.

      class MyClass<T>                 ==  class MyClass<T extends @Nullable Object>
      class MyClass<T extends Object>  ==  class MyClass<T extends @NonNull Object>

The rationale for this choice is:

- The “<T>” in MyClass<T> means “fully unconstrained”, and the rules maintain that, without the need for a programmer to change existing code.
- The “Object” in MyClass<T extends Object> is treated exactly like every other occurrence of Object in the program — it would be confusing for different occurrences of Object to mean different annotated types.

Here are some style guidelines:

- Use “<T>” when there are no constraints on the type qualifiers. This is short and is what already appears in source code.
- Whenever you write an extends clause, write an explicit type annotation on it. For example, for the Nullness Checker, write class MyClass<T> rather than class MyClass<T extends @Nullable Object>, and write class MyClass<T extends @NonNull Object> rather than class MyClass<T extends Object>.

For further discussion, see Section ‍39.7.2.

### 31.1.3 Type annotations on a use of a generic type variable

A type annotation on a use of a generic type variable overrides/ignores any type qualifier (in the same type hierarchy) on the corresponding actual type argument. For example, suppose that T is a formal type parameter. Then using @Nullable T within the scope of T applies the type qualifier @Nullable to the (unqualified) Java type of T. This feature is sometimes useful, but more often the implementation of a generic type just uses the type variable T, whose instantiation is restricted (see Section ‍31.1.2).

Here is an example of applying a type annotation to a generic type variable:

      class MyClass2<T> {
        ...
        @Nullable T myField = null;
        ...
      }

The type annotation does not restrict how MyClass2 may be instantiated. In other words, both MyClass2<@NonNull String> and MyClass2<@Nullable String> are legal, and in both cases @Nullable T means @Nullable String. In MyClass2<@Interned String>, @Nullable T means @Nullable @Interned String.

Defaulting never affects a use of a type variable, even if the type variable use has no explicit annotation. Defaulting helps to choose a single type qualifier for a concrete Java class or interface. By contrast, a type variable use represents a set of possible types.

### 31.1.4 Annotations on wildcards

At an instantiation of a generic type, a Java wildcard indicates that some constraints are known on the type argument, but the type argument is not known exactly. For example, you can indicate that the type parameter for variable ls is some unknown subtype of CharSequence:

      List<? extends CharSequence> ls;
      ls = new ArrayList<String>();      // OK
      ls = new ArrayList<Integer>();     // error: Integer is not a subtype of CharSequence

For more details about wildcards, see the Java tutorial on wildcards or JLS §4.5.1.

You can write a type annotation on the bound of a wildcard:

      List<? extends @NonNull CharSequence> ls;
      ls = new ArrayList<@NonNull String>();    // OK
      ls = new ArrayList<@Nullable String>();   // error: @Nullable is not a subtype of @NonNull

Conceptually, every wildcard has two bounds — an upper bound and a lower bound. Java only permits you to write one bound. You can specify the upper bound with <? extends SomeType>, in which case the lower bound is implicitly the bottom type void. You can specify the lower bound (with <? super OtherType>), in which case the upper bound is implicitly the top type Object. The Checker Framework is more flexible: it lets you similarly write annotations on both the upper and lower bound.

To annotate the implicit bound, write the type annotation before the ?. For example:

      List<@LowerBound ? extends @UpperBound CharSequence> lo;
      List<@UpperBound ? super @NonNull Number> ls;

For an unbounded wildcard (<?>, with neither bound specified), the annotation in front of a wildcard applies to both bounds. The following three declarations are equivalent (except that you cannot write the bottom type void; note that Void does not denote the bottom type):

      List<@NonNull ?> lnn;
      List<@NonNull ? extends @NonNull Object> lnn;
      List<@NonNull ? super @NonNull void> lnn;

Note that the annotation in front of a type parameter always applies to its lower bound, because type parameters can only be written with extends and never super.

The defaulting rules for wildcards also differ from those of type parameters (see Section ‍32.5.5).

### 31.1.5 Examples of qualifiers on a type parameter

Recall that @Nullable X is a supertype of @NonNull X, for any X. Most of the following types mean different things:

      class MyList1<@Nullable T> { ... }
      class MyList1a<@Nullable T extends @Nullable Object> { ... } // same as MyList1
      class MyList2<@NonNull T extends @NonNull Object> { ... }
      class MyList2a<T extends @NonNull Object> { ... } // same as MyList2
      class MyList3<T extends @Nullable Object> { ... }

MyList1 and MyList1a must be instantiated with a nullable type. The implementation of MyList1 must be able to consume (store) a null value and produce (retrieve) a null value.

MyList2 and MyList2a must be instantiated with non-null type. The implementation of MyList2 has to account for only non-null values — it does not have to account for consuming or producing null.

MyList3 may be instantiated either way: with a nullable type or a non-null type. The implementation of MyList3 must consider that it may be instantiated either way — flexible enough to support either instantiation, yet rigorous enough to impose the correct constraints of the specific instantiation. It must also itself comply with the constraints of the potential instantiations.

One way to express the difference among MyList1, MyList2, and MyList3 is by comparing what expressions are legal in the implementation of the list — that is, what expressions may appear in the ellipsis in the declarations above, such as inside a method’s body. Suppose each class has, in the ellipsis, these declarations:

      T t;
      @Nullable T nble;      // Section "Type annotations on a use of a generic type variable", below,
      @NonNull T nn;         // further explains the meaning of "@Nullable T" and "@NonNull T".
      void add(T arg) {}
      T get(int i) {}

Then the following expressions would be legal, inside a given implementation — that is, also within the ellipses. (Compilable source code appears as file checker-framework/checker/tests/nullness/generics/GenericsExample.java.)

| MyList1 | MyList2 | MyList3
t = null; | OK | error | error
t = nble; | OK | error | error
nble = null; | OK | OK | OK
nn = null; | error | error | error
t = this.get(0); | OK | OK | OK
nble = this.get(0); | OK | OK | OK
nn = this.get(0); | error | OK | error
this.add(t); | OK | OK | OK
this.add(nble); | OK | error | error
this.add(nn); | OK | OK | OK

The differences are more significant when the qualifier hierarchy is more complicated than just @Nullable and @NonNull.

### 31.1.6 Covariant type parameters

Java types are invariant in their type parameter. This means that A<X> is a subtype of B<Y> only if X is identical to Y. For example, ArrayList<Number> is a subtype of List<Number>, but neither ArrayList<Integer> nor List<Integer> is a subtype of List<Number>. (If they were, there would be a loophole in the Java type system.) For the same reason, type parameter annotations are treated invariantly. For example, List<@Nullable String> is not a subtype of List<String>.

When a type parameter is used in a read-only way — that is, when clients read values of that type from the class but never pass values of that type to the class — then it is safe for the type to be covariant in the type parameter. Use the @Covariant annotation to indicate this. When a type parameter is covariant, two instantiations of the class with different type arguments have the same subtyping relationship as the type arguments do.

For example, consider Iterator. A client can read elements but not write them, so Iterator<@Nullable String> can be a subtype of Iterator<String> without introducing a hole in the type system. Therefore, its type parameter is annotated with @Covariant. The first type parameter of Map.Entry is also covariant. Another example would be the type parameter of a hypothetical class ImmutableList.

The @Covariant annotation is trusted but not checked. If you incorrectly specify as covariant a type parameter that can be written (say, the class supports a set operation or some other mutation on an object of that type), then you have created an unsoundness in the type system. For example, it would be incorrect to annotate the type parameter of ListIterator as covariant, because ListIterator supports a set operation.

### 31.1.7 Method type argument inference and type qualifiers

Sometimes method type argument inference does not interact well with type qualifiers. In such situations, you might need to provide explicit method type arguments, for which the syntax is as follows:

        Collections.<@MyTypeAnnotation Object>sort(l, c);

This uses Java’s existing syntax for specifying a method call’s type arguments.

### 31.1.8 The Bottom type

Many type systems have a *Bottom type that is used only for the null value, dead code, and some erroneous situations. A programmer should rarely write the bottom type.

One use is on a lower bound, to indicate that any type qualifier is permitted. A lower-bounded wildcard indicates that a consumer method can accept a collection containing any Java type above some Java type, and you can add the bottom type qualifier as well:

    public static void addNumbers(List<? super @SignednessBottom Integer> list) { ... }

## 31.2 Qualifier polymorphism for methods

Type qualifier polymorphism permits a single method to have multiple different qualified type signatures.

Here is where a polymorphic qualifier (e.g., @PolyNull) can be used:

- Polymorphic qualifiers are most often used in method signatures. See the examples below in Section ‍31.2.1.
- Polymorphic qualifiers can also be written in method bodies (implementations).
- If you can use generics, you typically do not need to use a polymorphic qualifier. Do not write a polymorphic qualifier on a type variable declaration.
- Polymorphic qualifiers may not be used on a class declaration. To apply qualifier polymorphism to classes, use a class qualifier parameter; see Section ‍31.3.
- A polymorphic qualifier may be used on a field declaration only in a class with a class qualifier parameter; see Section ‍31.3.2.
- If a class has a class qualifier parameter, then a polymorphic qualifier written on a method in the class has a slightly different meaning, see Section ‍31.3.1.

Section ‍36.5.2 explains how to define a polymorphic qualifier.

### 31.2.1 Using polymorphic qualifiers in a method signature

A method whose signature has a polymorphic qualifier (such as @PolyNull) conceptually has multiple versions, somewhat like the generics feature of Java or a template in C++. In each version, each instance of the polymorphic qualifier has been replaced by the same other qualifier from the hierarchy.

The method body must type-check with all signatures. A method call is type-correct if it type-checks under any one of the signatures. If a call matches multiple signatures, then the compiler uses the most specific matching signature for the purpose of type-checking. This is the same as Java’s rule for resolving overloaded methods.

As an example of the use of @PolyNull, method Class.cast returns null if and only if its argument is null:

      @PolyNull T cast(@PolyNull Object obj) { ... }

This is like writing:

       @NonNull T cast( @NonNull Object obj) { ... }
      @Nullable T cast(@Nullable Object obj) { ... }

except that the latter is not legal Java, since it defines two methods with the same Java signature.

As another example, consider

      // Returns null if either argument is null.
      @PolyNull T max(@PolyNull T x, @PolyNull T y);

which is like writing

       @NonNull T max( @NonNull T x,  @NonNull T y);
      @Nullable T max(@Nullable T x, @Nullable T y);

At a call site, the most specific applicable signature is selected.

Another way of thinking about which one of the two max variants is selected is that the nullness annotations of (the declared types of) both arguments are unified to a type that is a supertype of both, also known as the least upper bound or lub. If both arguments are @NonNull, their unification (lub) is @NonNull, and the method return type is @NonNull. But if even one of the arguments is @Nullable, then the unification (lub) is @Nullable, and so is the return type.

### 31.2.2 Relationship to subtyping and generics

Qualifier polymorphism has the same purpose and plays the same role as Java’s generics. You use them for the similar reasons, such as:

- A method operates on collections with different types of elements.
- Two different arguments have the same type, without constraining them to be one specific type.
- A method returns a value of the same type as its argument.

If a method is written using Java generics, it usually does not need qualifier polymorphism. If you can use Java’s generics, then that is often better. On the other hand, if you have legacy code that is not written generically, and you cannot change it to use generics, then you can use qualifier polymorphism to achieve a similar effect, with respect to type qualifiers only. The Java compiler still treats the base Java types non-generically.

In some cases, you don’t need qualifier polymorphism because subtyping already provides the needed functionality. String is a supertype of @Interned String, so a method toUpperCase that is declared to take a String parameter can also be called on an @Interned String argument.

### 31.2.3 Using multiple polymorphic qualifiers in a method signature

Usually, it does not make sense to write only a single instance of a polymorphic qualifier in a method definition: if you write one instance of (say) @PolyNull, then you should use at least two. The main benefit of polymorphic qualifiers comes when one is used multiple times in a method, since then each instance turns into the same type qualifier. (Section ‍31.2.4 describes some exceptions to this rule: times when it makes sense to write a single polymorphic qualifier in a signature.)

Most frequently, the polymorphic qualifier appears on at least one formal parameter and also on the return type.

It can also be useful to have polymorphic qualifiers on (only) multiple formal parameters, especially if the method side-effects one of its arguments. For example, consider

    void moveBetweenStacks(Stack<@PolyNull Object> s1, Stack<@PolyNull Object> s2) {
      s1.push(s2.pop());
    }

In this particular example, it would be cleaner to rewrite your code to use Java generics, if you can do so:

    <T> void moveBetweenStacks(Stack<T> s1, Stack<T> s2) {
      s1.push(s2.pop());
    }

### 31.2.4 Using a single polymorphic qualifier in a method signature

As explained in Section ‍31.2.3, you will usually use a polymorphic qualifier multiple times in a signature. This section describes situations when it makes sense to write just one polymorphic qualifier in a method signature. Some of these situations can be avoided by writing a generic method, but in legacy code it may not be possible for you to change a method to be generic.

#### Using a single polymorphic qualifier on a return type

It is unusual, but permitted, to write just one polymorphic qualifier, on a return type.

This is just like it is unusual, but permitted, to write just one occurrence of a generic type parameter, on a return type. An example of such a method is Collections.emptyList().

#### Using a single polymorphic qualifier on an element type

It can make sense to use a polymorphic qualifier just once, on an array or generic element type.

For example, consider a routine that returns the index, in an array, of a given element:

      public static int indexOf(@PolyNull Object[] a, @Nullable Object elt) { ... }

If @PolyNull were replaced with either @Nullable or @NonNull, then one of these safe client calls would be rejected:

      @Nullable Object[] a1;
      @NonNull Object[] a2;

      indexOf(a1, someObject);
      indexOf(a2, someObject);

Of course, it would be better style to use a generic method, as in either of these signatures:

     public static <T extends @Nullable Object> int indexOf(T[] a, @Nullable Object elt) { ... }
     public static <T extends @Nullable Object> int indexOf(T[] a, T elt) { ... }

Another example is a method that writes bytes to a file. It accepts an array of signed or unsigned bytes, and it behaves identically for both:

      void write(@PolySigned byte[] b) { ... }

These examples use arrays, but there are similar examples that use collections.

#### Don’t use a single polymorphic qualifier on a formal parameter type

There is no point to writing just one polymorphic qualifier in a method signature, as the main type qualifier on a formal parameter type. Consider this signature:

      void m(@PolyNull Object obj)

which expands to

      void m(@NonNull Object obj)
      void m(@Nullable Object obj)

This is no different (in terms of which calls to the method will type-check) than writing just

      void m(@Nullable Object obj)

However, it does make sense to write a single polymorphic qualifier within a formal parameter type, as in

      void m2a(List<@PolyNull MySubClass> strings)

which is similar to

      void m2b(List<? extends @Nullable MySubClass> strings)

Method m2b() can be called on (for example) List<@Nullable MySubClass> and List<@NonNull MySubClass>, which are not legal arguments to method m2a().

## 31.3 Class qualifier parameters

Class qualifier parameters permit you to supply a type qualifier (only, without a Java basetype) to any class (generic or not).

When a generic class represents a collection, a user can write a type qualifier on the type argument, as in List<@Tainted Character> versus List<@Untainted Character>. When a non-generic class represents a collection with a hard-coded type (as StringBuffer hard-codes Character), you can use a class qualifier parameter to distinguish StringBuffers that contain different types of characters.

To add a qualifier parameter to a class, annotate its declaration with @HasQualifierParameter and write the class of the top qualifier as its element.

    @HasQualifierParameter(Tainted.class)
    class StringBuffer { ... }

A qualifier on a use of StringBuffer is treated as appearing both on the StringBuffer and on its conceptual type argument. That is:
@Tainted StringBuffer ≈ @Tainted Collection<@Tainted Character>
@Untainted StringBuffer ≈ @Untainted Collection<@Untainted Character>

If two types have different qualifier arguments, they have no subtyping relationship. (This is “invariant subtyping”, also used by Java for generic classes.) In particular, @Untainted StringBuffer is not a subtype of @Tainted StringBuffer; an attempt to cast between them, in either direction, will yield an invariant.cast.unsafe error.

@HasQualifierParameter is inherited. If type T has a @HasQualifierParameter annotation, then its subtypes are automatically treated as having the same annotation.

Within a class with a qualifier parameter, the default qualifier for uses of that class is the polymorphic qualifier.

### 31.3.1 Resolving polymorphism when the receiver type has a polymorphic qualifier

Qualifier polymorphism changes the rules for instantiating polymorphic qualifiers (Section ‍31.2). If the receiver type has a qualifier parameter and is annotated with a polymorphic qualifier, then at a call site all polymorphic annotations are instantiated to the same qualifier as the type of the receiver expression of the method call. Otherwise, use the rules of Section ‍31.2

For example, consider

    @HasQualifierParameter(Tainted.class)
    class Buffer {
      void append(@PolyTainted Buffer this, @PolyTainted String s) { ... }
    }

Because @PolyTainted applies to a type (Buffer) with a qualifier parameter, all uses of @PolyTainted are instantiated to the qualifiers on the type of the receiver expression at call sites to append. For example,

    @Untainted Buffer untaintedBuffer = ...;
    @Tainted String taintedString = ...;
    untaintedBuffer.append(taintedString); // error: [argument]

The above append call is illegal because the @PolyTainted is instantiated to @Untainted and the type of the argument is @Tainted which is a supertype of @Untainted. If the type of untaintedBuffer were @Tainted then the call would be legal.

### 31.3.2 Using class qualifier parameters in the type of a field

To express that the type of a field should have the same qualifier as the class qualifier parameter, annotate the field type with the polymorphic qualifier for the type system.

    @HasQualifierParameter(Tainted.class)
    class Buffer {
      @PolyTainted String field;
    }

At a field access where the declared type of the field has a polymorphic qualifier, that polymorphic qualifier is instantiated to the qualifier on the type of the receiver of the field access (or in the case of type variables, the qualifier on the upper bound). That is, the qualifier on myBuffer.field is that same as that on myBuffer.

### 31.3.3 Local variable defaults for types with qualifier parameters

Local variables default to the top type (see Section ‍32.5.3). Type refinement determines if a variable can be treated as a suitable subtype, and annotations on local variables are rarely needed as a result. However, since qualifier parameters add invariant subtyping, type refinement is no longer valid. For example, suppose in the following code that StringBuffer is annotated with @HasQualifierParameter(Tainted.class).

        void method(@Untainted StringBuffer buffer) {
            StringBuffer local = buffer;
            executeSql(local.toString());
        }

        void executeSql(@Untainted String code) {
            // ...
        }

Normally, the framework would determine that local has type @Untainted StringBuffer and the call to executeSql would be valid. However, since by default local has type @Tainted StringBuffer, and @Untainted StringBuffer is not a subtype, no type refinement would be performed, leading to an error. Fixing this would require manually annotating local as an @Untainted StringBuffer, increasing the annotation burden on programmers.

For this reason, local variables with types that have a qualifier parameter use different defaulting rules. When a local variable has an initializer, the type of that initializer is used as the default type of that variable if no other annotations are written. For example, in the above code, the type of local would be @Untainted StringBuffer. This eliminates the need for type refinement.

### 31.3.4 Qualifier parameters by default

If many classes in a project should have @HasQualifierParameter, it’s possible to enable it on all classes in a package by default. Writing @HasQualifierParameter on a package is equivalent to writing @HasQualifierParameter on each class in that package and all subpackages with the same arguments.

For example, writing this annotation enables @HasQualifierParameter for all classes in mypackage.

    @HasQualifierParameter(Tainted.class)
    package mypackage;

When using @HasQualifierParameter on a package, it’s possible to disable it for a specific class using @NoQualifierParameter. Writing this on a class indicates it has no class qualifier parameter and @HasQualifierParameter will not be enabled by default. Like @HasQualifierParameter, it takes one or more top annotations. It is illegal to explicitly write both @HasQualifierParameter and @NoQualifierParameter on the same class for the same hierarchy.

### 31.3.5 Types with qualifier parameters as type arguments

Types with qualifier parameters are only allowed as type arguments to type parameters whose upper bound have a qualifier parameter. If they were allowed for as type arguments for any type parameter, then unsound casts would be permitted. For example:

        @HasQualifierParameter(Tainted.class)
        interface Buffer {
            void append(@PolyTainted String s);
        }

        public class ClassQPTypeVarTest {
            <T> @Tainted T cast(T param) {
                return param;
            }

            void bug(@Untainted Buffer b, @Tainted String s) {
                cast(b).append(s);  // error
            }
        }

# Chapter ‍32 Advanced type system features

This chapter describes features that are automatically supported by every checker written with the Checker Framework. You may wish to skim or skip this chapter on first reading. After you have used a checker for a little while and want to be able to express more sophisticated and useful types, or to understand more about how the Checker Framework works, you can return to it.

## 32.1 Invariant array types

Java’s type system is unsound with respect to arrays. That is, the Java type-checker approves code that is unsafe and will cause a run-time crash. Technically, the problem is that Java has “covariant array types”, such as treating String[] as a subtype of Object[]. Consider the following example:

      String[] strings = new String[] {"hello"};
      Object[] objects = strings;
      objects[0] = new Object();
      String myString = strs[0];

The above code puts an Object in the array strings and thence in myString, even though myString = new Object() should be, and is, rejected by the Java type system. Java prevents corruption of the JVM by doing a costly run-time check at every array assignment; nonetheless, it is undesirable to learn about a type error only via a run-time crash rather than at compile time.

When you pass the -AinvariantArrays command-line option, the Checker Framework is stricter than Java, in the sense that it treats arrays invariantly rather than covariantly. This means that a type system built upon the Checker Framework is sound: you get a compile-time guarantee without the need for any run-time checks. But it also means that the Checker Framework rejects code that is similar to what Java unsoundly accepts. The guarantee and the compile-time checks are about your extended type system. The Checker Framework does not reject the example code above, which contains no type annotations.

Java’s covariant array typing is sound if the array is used in a read-only fashion: that is, if the array’s elements are accessed but the array is not modified. However, facts about read-only usage are not built into any of the type-checkers. Therefore, when using type systems along with -AinvariantArrays, you will need to suppress any warnings that are false positives because the array is treated in a read-only way.

## 32.2 Context-sensitive type inference for array constructors

When you write an expression, the Checker Framework gives it the most precise possible type, depending on the particular expression or value. For example, when using the Regex Checker (Chapter ‍14), the string "hello" is given type @Regex String because it is a legal regular expression (whether it is meant to be used as one or not) and the string "(foo" is given the type @RegexBottom String because it is not a legal regular expression.

Array constructors work differently. When you create an array with the array constructor syntax, such as the right-hand side of this assignment:

    String[] myStrings = {"hello"};

then the expression does not get the most precise possible type, because doing so could cause inconvenience. Rather, its type is determined by the context in which it is used: the left-hand side if it is in an assignment, the declared formal parameter type if it is in a method call, etc.

In particular, if the expression `{"hello"}` were given the type @Regex String[], then the assignment would be illegal! But the Checker Framework gives the type String[] based on the assignment context, so the code type-checks.

If you prefer a specific type for a constructed array, you can indicate that either in the context (change the declaration of myStrings) or in a new construct (change the expression to new @Regex String[] `{"hello"}`).

## 32.3 Upper bound of qualifiers on uses of a given type (annotations on a class declaration)

The examples in this section use the type qualifier hierarchy @A :> @B :> @C.

A qualifier on a use of a certain type must be a subtype or equal to the upper bound for that type. The upper bound of qualifiers used on a given type is specified by annotating the type declaration with some qualifier — that is, by writing an annotation on a class declaration.

      @C class MyClass {}

This means that @B MyClass is an invalid type. (Annotations on class declarations may also specify default annotations for uses of the type; see Section ‍32.5.1)

If it is not possible to annotate the class’s definition (e.g., for primitives and some library classes), the type-system designer can specify an upper bound by using the meta-annotation @UpperBoundFor.

If no annotation is present on a type declaration and if no @UpperBoundFor mentions the type, then the bound is top. This can be changed by overriding AnnotatedTypeFactory#getTypeDeclarationBounds.

There are two exceptions.

- An expression can have a supertype of the upper bound; that is, some expression could have type @B MyClass. This type is not written explicitly, but results from viewpoint adaptation.
- Using usual CLIMB-to-top rules (Section ‍32.5.3), local variables of type MyClass default to @A MyClass. It is legal for @A MyClass to be the type of a local variable. For consistency, users are allowed to write such a type on a local variable declaration.

Due to existing type rules, an expression of type @A MyClass can only be used in limited ways.

- Since every field, formal parameter, and return type of type MyClass (or lower) is annotated as @B (or lower), it cannot be assigned to a field, passed to a method, or returned from a method.
- It can be used in a context that requires @A Object (or whatever the least supertype is of MyClass for which the @A qualifier is permitted). Examples include being tested against null or (for most type systems) being passed to polymorphic routines such as System.out.println or System.identityHashCode.

These operations might refine its type. If a user wishes to annotate a method that does type refinement, its formal parameter must be of illegal type @A MyClass, which requires a warning suppression.

If the framework were to forbid expressions and local variables from having types inconsistent with the class annotation, then important APIs and common coding paradigms would no longer type-check.

Consider the annotation

      @NonNull class Optional { ... }

and the client code

      Map<String, Optional> m;
      String key = ...;
      Optional value = m.get(key);
      if (value != null) {
        ...
      }

The type of m.get(key) is @Nullable Optional, which is an illegal type. However, this is a very common paradigm. Programmers should not need to rewrite the code to test m.containsKey(key) nor suppress a warning in this safe code.

## 32.4 The effective qualifier on a type (defaults and inference)

A checker sometimes treats a type as having a slightly different qualifier than what is written on the type — especially if the programmer wrote no qualifier at all. Most readers can skip this section on first reading, because you will probably find the system simply “does what you mean”, without forcing you to write too many qualifiers in your program. particular, programmers rarely write qualifiers in method bodies (except on type arguments and array component types).

The following steps determine the effective qualifier on a type — the qualifier that the checkers treat as being present.

1. If a type qualifier is present in the source code, that qualifier is used.
2. If there is no explicit qualifier on a type, then a default qualifier is applied; see Section ‍32.5. Defaulted qualifiers are treated by checkers exactly as if the programmer had written them explicitly.
3. The type system may refine a qualified type on a local variable — that is, treat it as a subtype of how it was declared or defaulted. This refinement is always sound and has the effect of eliminating false positive error messages. See Section ‍32.7.

## 32.5 Default qualifier for unannotated types

An unannotated Java type is treated as if it had a default annotation. Both the type system designer and an end-user programmer can control the defaulting. Defaulting never applies to uses of type variables, even if they do not have an explicit type annotation. Most of this section is about defaults for source code that is read by the compiler. When the compiler reads a .class file, different defaulting rules apply. See Section ‍32.5.6 for these rules.

There are several defaulting mechanisms, for convenience and flexibility. When determining the default qualifier for a use of an unannotated type, MyClass, the following rules are used in order, until one applies.

1. The qualifier specified via @DefaultQualifierForUse on the declaration of MyClass. (Section ‍32.5.1)
2. If no @NoDefaultQualifierForUse is written on the declaration of MyClass, the qualifier explicitly written on the declaration of MyClass. (Section ‍32.5.1)
3. The qualifier with a meta-annotation @DefaultFor(types = MyClass.class). (Section ‍32.5.1)
4. The qualifier with a meta-annotation @DefaultFor(typeKinds = KIND), where KIND is the TypeKind of MyClass. (Section ‍32.5.1)
5. The qualifier with a meta-annotation @DefaultFor(names = REGEX), where REGEX matches the name of the variable being defined (if any). For return types, the name of the method is used. (Section ‍32.5.1)
6. The qualifier in the innermost user-written @DefaultQualifier for the location of the use of MyClass. (Section ‍32.5.2)
7. The qualifier in the meta-annotation @DefaultFor for the location of the use of MyClass. These are defaults specified by the type system designer (Section ‍36.5.4); this is usually CLIMB-to-top (Section ‍32.5.3).
8. The qualifier with the meta-annotation @DefaultQualifierInHierarchy.

If the unannotated type is the type of a local variable, then the first 5 rules are skipped and only rules 6 and 7 apply. If rule 6 applies, it makes the type of local variables top so they can be refined.

### 32.5.1 Default for use of a type

The type declaration annotation @DefaultQualifierForUse indicates that the specified qualifier should be added to all unannotated uses of the type.

For example:

    @DefaultQualifierForUse(B.class)
    class MyClass {}

This means any unannotated use of MyClass is treated as @B MyClass by the checker. (Except for locals, which can be refined.)

Similarly, the meta-annotation @DefaultFor can be used to specify defaults for uses of types, using the types element, or type kinds, using the typeKinds elements.

Interaction between qualifier bounds and DefaultQualifierForUse:

- If a type declaration is annotated with a qualifier bound, but not a @DefaultQualifierForUse, then the qualifier bound is added to all unannotated uses of that type (except locals). For example, @C class MyClass is equivalent to

      @DefaultQualifierForUse(C.class)
      @C class MyClass {}

- If the qualifier bound should not be added to all unannotated uses, then @NoDefaultQualifierForUse should be written on the declaration:

      @NoDefaultQualifierForUse
      @C class MyClass {}

  This means that unannotated uses of MyClass are defaulted normally.
- If neither @DefaultQualifierForUse nor a qualifier bound is present on a type declaration, that is equivalent to writing @NoDefaultQualifierForUse.

### 32.5.2 Controlling defaults in source code

The end-user programmer specifies a default qualifier by writing the @DefaultQualifier(ClassName, [locations]) annotation on a package, class, method, or variable declaration. The argument to @DefaultQualifier is the Class name of an annotation. The optional second argument indicates where the default applies. If the second argument is omitted, the specified annotation is the default in all locations. See the Javadoc of DefaultQualifier for details.

For example, using the Nullness type system (Chapter ‍3):

    import org.checkerframework.framework.qual.DefaultQualifier;
    import org.checkerframework.checker.nullness.qual.NonNull;

    @DefaultQualifier(NonNull.class)
    class MyClass {

      public boolean compile(File myFile) { // myFile has type "@NonNull File"
        if (!myFile.exists())          // no warning: myFile is non-null
          ...
        @Nullable File srcPath = ...;  // must annotate to specify "@Nullable File"
        if (srcPath.exists())          // warning: srcPath might be null
          ...
      }

      @DefaultQualifier(Tainted.class)
      public boolean isJavaFile(File myfile) {  // myFile has type "@Tainted File"
        ...
      }
    }

You may write multiple @DefaultQualifier annotations at a single location.

If @DefaultQualifier[s] is placed on a package (via the package-info.java file), then it applies to the given package and all subpackages.

### 32.5.3 Defaulting rules and CLIMB-to-top

Each type system defines a default qualifier (see Section ‍36.5.4). For example, the default qualifier for the Nullness Checker is @NonNull. When a user writes an unqualified type such as Date, the Nullness Checker interprets it as @NonNull Date.

The type system applies that default qualifier to most but not all type uses. In particular, unless otherwise stated, every type system uses the CLIMB-to-top rule. This rule states that the top qualifier in the hierarchy is the default for the CLIMB locations: Casts, Locals, and (some) Implicit Bounds. For example, when the user writes an unqualified type such as Date in such a location, the Nullness Checker interprets it as @Nullable Date (because @Nullable is the top qualifier in the hierarchy, see Figure ‍3.1). (Casts are treated a bit specially; see below.)

The CLIMB-to-top rule is used only for unannotated source code that is being processed by a checker. For unannotated libraries (code read by the compiler in .class or .jar form), see Section ‍32.5.6.

The rest of this section explains the rationale and implementation of CLIMB-to-top.

Here is the rationale for CLIMB-to-top:

- Local variables are defaulted to top because type refinement (Section ‍32.7) is applied to local variables. If a local variable starts as the top type, then the Checker Framework refines it to the best (most specific) possible type based on assignments to it. As a result, a programmer rarely writes an explicit annotation on any of those locations.

  Variables defaulted to top include local variables, resource variables in the try-with-resources construct, variables in for statements, and catch arguments (known as exception parameters in the Java Language Specification).

  Exception parameters default to the top type because they might catch an exception thrown anywhere in the program.

  An alternate design for exception parameters would be to default exception parameters some other type T (instead of the top type); then the Checker Framework would need to issue a warning at every throw statement whose argument might not be a subtype of T. A checker can implement this alternate design by overriding a few methods. The alternative is not appropriate for all type systems. The alternative is unsound for deep type systems because the JDK’s annotations are trusted rather than checked. A deep type system is one where the type of a field can determine the type of its containing instance, such as tainting, Example: a user passes a secret regex to the JDK, and the JDK throws a format exception that includes the regex. This could be caught by a catch clause in the program whose exception parameter is not annotated as secret. As another example, the user passes a secret integer and the JDK throws a DivideByZeroException that reveals the value.

- Cast types are defaulted to the same type as their argument expression. This has the same effect as if they were given the top type and then flow-sensitively refined to the type of their argument. However, note that programmer-written type qualifiers are not refined, so writing the top annotation is not the same as writing no annotation.
- Implicit upper bounds are defaulted to top to allow them to be instantiated in any way.

  Suppose a user declares a class as class C<T> { ... }. The Checker Framework assumes that the user intended to allow any instantiation of the class. The Checker Framework interprets the declaration as class C<T extends @Nullable Object> { ... } rather than as class C<T extends @NonNull Object> { ... }. The latter would forbid instantiations such as C<@Nullable String>, or would require rewriting of code. On the other hand, if a user writes an explicit bound such as class C<T extends D> { ... }, then the user intends some restriction on instantiation and can write a qualifier on the upper bound as desired.

  This rule means that the upper bound of class C<T> is defaulted differently than the upper bound of class C<T extends Object>. This is a bit unfortunate, but it is the least bad option. The more confusing alternative would be for “Object” to be defaulted differently in class C<T extends Object> and in an instantiation C<Object>, and for the upper bounds to be defaulted differently in class C<T extends Object> and class C<T extends Date>.

- Implicit lower bounds are defaulted to the bottom type, again to allow maximal instantiation. Note that Java does not allow a programmer to express both the upper and lower bounds of a type, but the Checker Framework allows the programmer to specify either or both; see Section ‍31.1.2.

A @DefaultQualifier that specifies a CLIMB-to-top location takes precedence over the CLIMB-to-top rule.

Here is how the Nullness Checker overrides part of the CLIMB-to-top rule:

    @DefaultQualifierInHierarchy
    @DefaultFor({ TypeUseLocation.EXCEPTION_PARAMETER })
    public @interface NonNull {}

    public @interface Nullable {}

As mentioned above, the exception parameters are always non-null, so @DefaultFor({ TypeUseLocation.EXCEPTION_PARAMETER }) on @NonNull overrides the CLIMB-to-top rule.

### 32.5.4 Inherited defaults

When overriding a method, programmers must fully specify types in the overriding method, which duplicates information on the overridden method. By contrast, declaration annotations that are meta-annotated with @InheritedAnnotation are inherited by overriding methods.

An example for type annotations is that when defining an equals() method, programmers must write the type annotation @Nullable:

      public boolean equals(@Nullable Object obj) {
          ...
      }

An alternate design would be for every annotation on a superclass member to to be automatically inherited by subclasses that override it.

The alternate design would reduce annotation effort.

The alternate design would reduce program comprehensibility. Currently, a user can determine the annotation on a parameter or return value by looking at a single file. If annotations could be inherited from supertypes, then a user would have to examine all supertypes, and do computations over them, to understand the meaning of an unannotated type in a given file. For declaration annotations, no computation is necessary; that is why they may be inherited. Computation is necessary for type annotations because different annotations might be inherited from a supertype and an interface, or from two interfaces. For return types, the inherited type should be the least upper bound of all annotations on overridden implementations in supertypes. For method parameters, the inherited type should be the greatest lower bound of all annotations on overridden implementations in supertypes. In each case, the Checker Framework would need to issue an error if no such annotations existed.

Because a program is read more often than it is edited/annotated, the Checker Framework does not currently support the alternate design. In the future, this feature may be added.

### 32.5.5 Inherited wildcard annotations

If a wildcard is unbounded and has no annotation (e.g. List<?>), the annotations on the wildcard’s bounds are copied from the type parameter to which the wildcard is an argument.

For example, the two wildcards in the declarations below are equivalent.

    class MyList<@Nullable T extends @Nullable Object> {}

    MyList<?> listOfNullables;
    MyList<@Nullable ? extends @Nullable Object> listOfNullables;

The Checker Framework copies these annotations because wildcards must be within the bounds of their corresponding type parameter. By contrast, if the bounds of a wildcard were defaulted differently from the bounds of its corresponding type parameter, then there would be many false positive type.argument warnings.

Here is another example of two equivalent wildcard declarations:

    class MyList<@Regex(5) T extends @Regex(1) Object> {}

    MyList<?> listOfRegexes;
    MyList<@Regex(5) ? extends @Regex(1) Object> listOfRegexes;

Note, this copying of annotations for a wildcard’s bounds applies only to unbounded wildcards. The two wildcards in the following example are equivalent.

    class MyList<@NonNull T extends @Nullable Object> {}

    MyList<? extends Object> listOfNonNulls;
    MyList<@NonNull ? extends @NonNull Object> listOfNonNulls2;

Note, the upper bound of the wildcard ? ‍extends Object is defaulted to @NonNull using the CLIMB-to-top rule (see Section ‍32.5.3). Also note that the MyList class declaration could have been more succinctly written as: class MyList<T extends @Nullable Object> where the lower bound is implicitly the bottom annotation: @NonNull.

### 32.5.6 Default qualifiers for .class files (library defaults)

The defaulting rules presented so far apply to source code that is read by the compiler. When the compiler reads a .class file, different defaulting rules apply. The rules are the same regardless of whether the .class file is from your own codebase or from a library, including the JDK.

If the .class file was compiled using the checker (that is, if the checker was run during the compiler execution that created the .class file), then there is no need for defaults: the .class file has an explicit qualifier at each type use. (Furthermore, unless warnings were suppressed, those qualifiers are guaranteed to be correct.)

The rest of this section discusses the situation when the .class file was not compiled using the checker (that is, the checker was not run during the compiler execution that created the .class file). The .class file contains only the type qualifiers that the programmer wrote explicitly — possibly none. (Furthermore, there is no guarantee that these qualifiers are correct, since they have not been checked.) Each checker decides what qualifier to use where the programmer did not write an annotation.

- With -AuseConservativeDefaultsForUncheckedCode=-bytecode: (This is also the behavior if -AuseConservativeDefaultsForUncheckedCode is not supplied or is supplied without “bytecode” in its argument.)

  The source code defaulting rules are used.

  For example, an unannotated method

        String concatenate(String p1, String p2)

  in a classfile would be interpreted as

        @Default String concatenate(@Default String p1, @Default String p2)

  for an appropriate value of @Default. (The default might be different for return types and parameter types, depending on how the checker is written.)

  These defaulting rules are unsafe: the default might be different than what the library author intended, so the Checker Framework may not warn about some misuses of the library. (This includes both misuse by passing in a bad value and misuse by using a returned value incorrectly.) However, this unsafe default can be useful because it permits you to focus on errors within the codebase before you have annotated the libraries it uses.

- With -AuseConservativeDefaultsForUncheckedCode=bytecode:
  - For method parameters and lower bounds, use the bottom qualifier (see Section ‍36.5.7).
  - For method return values, fields, and upper bounds, use the top qualifier (see Section ‍36.5.7).

  For example, an unannotated method

        String concatenate(String p1, String p2)

  in a classfile would be interpreted as

        @MyTopQualifier String concatenate(@MyBottomQualifier String p1, @MyBottomQualifier String p2)

  where @MyTopQualifier is the top qualifier in the hierarchy and @MyBottomQualifier is the bottom qualifier in the hierarchy.

  These choices are conservative. They are likely to cause many false-positive type-checking errors, which will help you to know which library methods need annotations. You can then write those library annotations (see Chapter ‍35) or alternately suppress the warnings (see Chapter ‍33).

  There is one case in which these choices are not conservative. Fields default to the top qualifier, so any assignment into a field is permitted. If an unannotated library has a mutable public field (which would be poor code style), the Checker Framework won’t warn about incorrect assignments to the field.

## 32.6 Annotations on constructors

### 32.6.1 Annotations on constructor declarations

An annotation on the “return type” of a constructor declaration indicates what the constructor creates. For example,

    @B class MyClass {
      @C MyClass() {}
    }

means that invoking that constructor creates a @C MyClass.

The Checker Framework cannot verify that the constructor really creates such an object, because the Checker Framework does not know the type-system-specific semantics of the @C annotation. Therefore, if the constructor result type is different than the top annotation in the hierarchy, the Checker Framework will issue a warning. The programmer should check the annotation manually, then suppress the warning.

#### Defaults

If a constructor declaration is unannotated, it defaults to the same type as that of its enclosing class (rather than the default qualifier in the hierarchy). For example, the Tainting Checker (Chapter ‍12) has @Tainted as its default qualifier. Consider the following class:

      @Untainted class MyClass {
        MyClass() {}
      }

The constructor declaration is equivalent to @Untainted MyClass() {}.

The Checker Framework produces the same error messages for explicitly-written and defaulted annotations.

### 32.6.2 Annotations on constructor invocations

The type of a method call expression x.myMethod(y, z) is determined by the return type of the declaration of myMethod. There is no way to write an annotation on the call to change its type. However, it is possible to write a cast: (@Anno SomeType) x.myMethod(y, z). The Checker Framework will issue a warning that it cannot verify that the downcast is correct. The programmer should manually determine that the annotation is correct and then suppress the warning.

A constructor invocation new MyClass() is also a call, so its semantics are similar. The type of the expression is determined by the annotation on the result type of the constructor declaration. It is possible to write a cast (@Anno MyClass) new MyClass(). The syntax new @Anno MyClass() is shorthand for the cast. For either syntax, the Checker Framework will issue a warning that it cannot verify that the cast is correct. The programmer may suppress the warning if the code is correct.

## 32.7 Type refinement (flow-sensitive type qualifier inference)

A checker can sometimes deduce that an expression’s type is more specific than — that is, a subtype of — its declared or defaulted (Section ‍32.5). This is called “flow-sensitive type refinement” or “local type inference”.

Due to local type refinement, a programmer typically does not write any qualifiers on local variables within a method body (except on type arguments and array component types). However, the programmer must write type annotations for method signatures (arguments and return values) and fields, unless the default annotations are correct. Local type refinement does not change the source code to insert the inferred annotations on local variables.

### 32.7.1 Type refinement examples

Here is an example for the Nullness Checker (Chapter ‍3). myVar is declared as @Nullable String, but it is treated as @NonNull String within the body of the if test.

      @Nullable String myVar;
      ...                   // myVar has type @Nullable String here.
      myVar.hashCode();     // warning: possible dereference of null.
      ...
      if (myVar != null) {
        ...                 // myVar has type @NonNull String here.
        myVar.hashCode();   // no warning.
      }

Here is another example. Note that the same expression may yield a warning or not depending on its context (that is, depending on the current type refinement).

      @Nullable String myVar;
      ...                   // myVar has type @Nullable String
      myVar = "hello";
      ...                   // myVar has type @NonNull String
      myVar.hashCode();     // no warning
      ...
      myVar = myMap.get(someKey);
      ...                   // myVar has type @Nullable String
      myVar.hashCode();     // warning: possible dereference of null

Type refinement applies to every checker, including new checkers that you write. Here is an example for the Regex Checker (Chapter ‍14):

      void m2(@Unannotated String s) {
        s = RegexUtil.asRegex(s, 2);  // asRegex throws an exception if its argument is not
                                      // a regex with the given number of capturing groups
        ...   // s now has type "@Regex(2) String"
      }

### 32.7.2 Type refinement behavior

The checker treats a variable or expression as a subtype of its declared type:

- starting at the time that it is assigned a value, a method establishes a postcondition (e.g., as expressed by @EnsuresNonNull or @EnsuresQualifierIf), or a run-time check is performed (e.g., via an assertion or if statement).
- until its value might change (e.g., via an assignment, or because a method call might have a side effect).

The checker never treats a variable as a supertype of its declared type. For example, an expression with declared type @NonNull type is never treated as possibly-null, and such an assignment is always illegal.

The functionality has a variety of names: type refinement, flow-sensitive type qualifier inference, local type inference, and sometimes just “flow”.

### 32.7.3 Which types are refined

You generally do not need to annotate the top-level type of a local variable. You do need to annotate its type arguments or array element types. (Type refinement does not change them, because doing so would not produce a subtype, as explained in see Section ‍31.1.6 and Section ‍32.1.) Type refinement works within a method, so you still need to annotate method signatures (parameter and return type) and field types.

If you find examples where you think a value should be inferred to have (or not have) a given annotation, but the checker does not do so, please submit a bug report (see Section ‍40.2) that includes a small piece of Java code that reproduces the problem.

#### Fields and type refinement

Type refinement infers the type of fields in some restricted cases:

- A final initialized field: Type inference is performed for final fields that are initialized to a compile-time constant at the declaration site; so the type of protocol is @NonNull String in the following declaration:

          public final String protocol = "https";

  Such an inferred type may leak to the public interface of the class. If you wish to override such behavior, you can explicitly insert the desired annotation, e.g.,

          public final @Nullable String protocol = "https";

- Within method bodies: Type inference is performed for fields in the context of method bodies, like local variables or any other expression. Consider the following example, where updatedAt is a nullable field:

      class DBObject {
        @Nullable Date updatedAt;

        void m() {
          // updatedAt is @Nullable, so warning about .getTime()
          ... updatedAt.getTime() ... // warning about possible NullPointerException

          if (updatedAt == null) {
            updatedAt = new Date();
          }

          // updatedAt is now @NonNull, so .getTime() call is OK
          ... updatedAt.getTime() ...
        }
      }

  A method call may invalidate inferences about field types; see Section ‍32.7.5.

### 32.7.4 Run-time tests and type refinement

Some type systems support a run-time test that the Checker Framework can use to refine types within the scope of a conditional such as if, after an assert statement, etc.

Whether a type system supports such a run-time test depends on whether the type system is computing properties of data itself, or properties of provenance (the source of the data). An example of a property about data is whether a string is a regular expression. An example of a property about provenance is units of measure: there is no way to look at the representation of a number and determine whether it is intended to represent kilometers or miles.

Type systems that support a run-time test are:

- Nullness Checker for null pointer errors (see Chapter ‍3)
- Map Key Checker to track which values are keys in a map (see Chapter ‍4)
- Optional Checker for errors in using the Optional type (see Chapter ‍5)
- Lock Checker for concurrency and lock errors (see Chapter ‍10)
- Index Checker for array accesses (see Chapter ‍11)
- Resource Leak Checker for ensuring that resources are disposed of properly (see Chapter ‍8)
- SQL Quotes Checker to mitigate SQL injection attacks (see Chapter ‍13)
- Regex Checker to prevent use of syntactically invalid regular expressions (see Chapter ‍14)
- Format String Checker to ensure that format strings have the right number and type of % directives (see Chapter ‍15)
- Internationalization Format String Checker to ensure that i18n format strings have the right number and type of {} directives (see Chapter ‍16)

Type systems that do not currently support a run-time test, but could do so with some additional implementation work, are

- Interning Checker for errors in equality testing and interning (see Chapter ‍6)
- Property File Checker to ensure that valid keys are used for property files and resource bundles (see Chapter ‍17)
- Internationalization Checker to ensure that code is properly internationalized (see Chapter ‍17.2)
- Signature String Checker to ensure that the string representation of a type is properly used, for example in Class.forName (see Chapter ‍18).
- Constant Value Checker to determine whether an expression’s value can be known at compile time (see Chapter ‍23)

Type systems that cannot support a run-time test are:

- Initialization Checker to ensure all fields are set in the constructor (see Chapter ‍3.8)
- Called Methods Checker for the builder pattern (see Chapter ‍7)
- Fake Enum Checker to allow type-safe fake enum patterns and type aliases or typedefs (see Chapter ‍9)
- Tainting Checker for trust and security errors (see Chapter ‍12)
- GUI Effect Checker to ensure that non-GUI threads do not access the UI, which would crash the application (see Chapter ‍19)
- Units Checker to ensure operations are performed on correct units of measurement (see Chapter ‍20)
- Signedness Checker to ensure unsigned and signed values are not mixed (see Chapter ‍21)
- Purity Checker to identify whether methods have side effects (see Chapter ‍22)
- Initialized Fields Checker to ensure all fields are set in the constructor (see Chapter ‍3.8)
- Aliasing Checker to identify whether expressions have aliases (see Chapter ‍27)
- Must Call Checker to over-approximate the methods that should be called on an object before it is de-allocated (see Chapter ‍28)
- Subtyping Checker for customized checking without writing any code (see Chapter ‍29)

### 32.7.5 Side effects, determinism, purity, and type refinement

Calling a method typically causes the checker to discard its knowledge of the refined type, because the method might assign a field. The @SideEffectFree annotation indicates that the method has no side effects, so calling it does not invalidate any dataflow facts.

Calling a method twice might have different results, so facts known about one call cannot be relied upon at another call. The @Deterministic annotation indicates that the method returns the same result every time it is called on the same arguments.

@Pure means both @SideEffectFree and @Deterministic. The @TerminatesExecution annotation indicates that a given method never returns. This can enable the type refinement to be more precise.

Chapter ‍22 gives more information about these annotations. This section explains how to use them to improve type refinement.

#### Side effects

Consider the following declarations and uses:

      @Nullable Object myField;

      int computeValue() { ... }

      void m() {
        ...
        if (myField != null) {
                            // The type of myField is now "@NonNull Object".
          int result = computeValue();
                            // The type of myField is now "@Nullable Object",
                            // because computeValue might have set myField to null.
          myField.toString(); // Warning: possible null pointer exception.
        }
      }

There are three ways to express that computeValue does not set myField to null, and thus to prevent the Nullness Checker from issuing a warning about the call myField.toString().

1. If computeValue has no side effects, declare the method as @SideEffectFree:

         @SideEffectFree
         int computeValue() { ... }

   The Nullness Checker issues no warnings, because it can reason that the second occurrence of myField has the same (non-null) value as the one in the test.

2. If no method resets myField to null after it has been initialized to a non-null value (even if a method has some other side effect), declare the field as @MonotonicNonNull:

         @MonotonicNonNull Object myField;

3. If computeValue sets myField to a non-null value (or maintains it as a non-null value), declare the method as @EnsuresNonNull:

         @EnsuresNonNull("myField")
         int computeValue() { ... }

   If computeValue maintains myField as a non-null value, even if it might have other side effects and even if other methods might set myField to null, declare it as

         @RequiresNonNull("myField")
         @EnsuresNonNull("myField")
         int computeValue() { ... }

There are two other ways to suppress the warning:

- Put the expression in a local variable before the invocation. A method call never affects the value of a local variable.
- If the warning is about a formal parameter (including the receiver) to a method foo(), then instead of declaring foo() as @SideEffectFree, you can supply the -AinvocationPreservesArgumentNullness command-line argument; see Section ‍3.1.1.

#### Deterministic methods

Consider the following declaration and uses:

      @Nullable Object getField(Object arg) { ... }

      void m() {
        ...
        if (x.getField(y) != null) {
          x.getField(y).toString(); // warning: possible null pointer exception
        }
      }

The Nullness Checker issues a warning regarding the toString() call, because its receiver x.getField(y) might be null, according to the @Nullable return type in the declaration of getField. The Nullness Checker cannot assume that getField returns non-null on the second call, just based on the fact that it returned non-null on the first call.

To indicate that a method returns the same value each time it is called on the same arguments, use the @Deterministic annotation. Actually, it is necessary to use @Pure which means both @Deterministic and @SideEffectFree, because otherwise the first call might change a value that the method depends on.

If you change the declaration of getField to

      @Pure
      @Nullable Object getField(Object arg) { ... }

then the Nullness Checker issues no warnings. Because getField is @SideEffectFree, the values of x and y are the same at both invocations. Because getField is @Deterministic, the two invocations of x.getField(y) have the same value. Therefore, x.getField(y) is non-null within the then branch of the if statement.

### 32.7.6 Assertions

If your code contains an assert statement, then your code could behave in two different ways at run time, depending on whether assertions are enabled or disabled via the -ea, -enableassertions, -da, or -disableassertions command-line options to java.

By default, the Checker Framework outputs warnings about any error that could happen at run time, whether assertions are enabled or disabled.

If you supply the -AassumeAssertionsAreEnabled command-line option, then the Checker Framework assumes assertions (that is, Java assert statements) are enabled, as if Java is run with the -ea or -enableassertions command-line argument. If you supply the -AassumeAssertionsAreDisabled command-line option, then the Checker Framework assumes assertions are disabled, as if Java is run with the -da or -disableassertions command-line argument. You may not supply both command-line options. It is uncommon to supply either one.

These command-line arguments have no effect on processing of assert statements whose message contains the text @AssumeAssertion; see Section ‍33.2.

### 32.7.7 Var Keyword

The initial type of a variable declared with var is exactly the type of the initializer expression at the declaration.

        var list1 = new ArrayList<@Tainted String>(); // type of list1 is ArrayList<@Tainted String>
        var list2 = new ArrayList<@Untainted String>(); // type of list2 is ArrayList<@Untainted String>

Flow-sensitive type refinement applies to these variables as usual. The base type qualifier may change at a later program point due to a subsequent assignment, but the qualifiers on type parameters and array contents remain exactly the same.

        var list1 = new ArrayList<@Tainted String>(); // type of list1 is ArrayList<@Tainted String>
        var list2 = new ArrayList<@Untainted String>(); // type of list2 is ArrayList<@Untainted String>
        var list3 = list1; // type of list3 is ArrayList<@Tainted String>
        list2 = list1; // assignment error

## 32.8 Writing Java expressions as annotation arguments

Sometimes, it is necessary to write a Java expression as the argument to an annotation. The annotations that take a Java expression as an argument include:

- @RequiresQualifier
- @EnsuresQualifier
- @EnsuresQualifierIf
- @RequiresNonNull
- @EnsuresNonNull
- @EnsuresNonNullIf
- @RequiresPresent
- @EnsuresPresent
- @EnsuresPresentIf
- @KeyFor
- @EnsuresKeyFor
- @EnsuresKeyForIf
- @I18nFormatFor
- @EnsuresLockHeld
- @EnsuresLockHeldIf
- @GuardedBy
- @Holding

The set of permitted expressions is a subset of all Java expressions, with a few extensions. The extensions are formal parameters like “#1” and (for some type systems) “<self>”.

- this, the receiver object. You can write this to annotate any variable or declaration where you could write this in code. Notably, it cannot be used in annotations on declarations of static fields or static methods. For a field, this is the field’s receiver (sometimes called its container). For a local variable, it is the method’s receiver.
- super, the receiver object as seen from the superclass. This can be used to refer to fields shadowed in the subclass (although shadowing fields is discouraged in Java).
- <self>, the value of the annotated reference (non-primitive) variable. Currently only defined for the @GuardedBy type system. For example, @GuardedBy("<self>") Object o indicates that the value referenced by o is guarded by the intrinsic (monitor) lock of the value referenced by o.
- a formal parameter, e.g., #2. It is represented as # followed by the one-based parameter index. For example: #1, #3. It is not permitted to write #0 to refer to the receiver object; use “this” instead.

  The formal parameter syntax #1 is less natural in source code than writing the formal parameter name. This syntax is necessary for separate compilation, because no formal parameter name information is available in a .class file. Suppose an annotated method m has already been compiled into a .class file, perhaps by a compilation that did not use the Checker Framework. When a client of m is later compiled, it cannot interpret a formal parameter name, but it can interpret a number.

  Within a method body, you may use the formal parameter name. The formal parameter name never works within a method signature or for a contract (pre- or post-condition) annotation; in those locations, an identifier is interpreted as a field name (not a formal parameter).

- a local variable, e.g., myLocalVar. The variable must be in scope; for example, a method annotation on method m cannot mention a local variable that is declared inside m.
- a static variable, e.g., System.out. Write the class name and the variable.
- a field of any expression. For example: next, this.next, #1.next. You may optionally omit a leading “this.”, just as in Java. Thus, this.next and next are equivalent.
- an array access. For example: this.myArray[i], vals[#1].
- an array creation. For example: new int[10], new String[] "a", "b".
- literals: string, integer, char, long, float, double, null, class literals.
- a method invocation on any expression. This even works for overloaded methods and methods with type parameters. For example: m1(x, y.z, #2), a.m2("hello").

  Currently, the Checker Framework cannot prove all contracts about method calls, so you may need to suppress some warnings.

  One unusual feature of the Checker Framework’s Java expressions is that a method call is allowed to have side effects. Other tools forbid methods with side effects (and doing so is necessary if a specification is going to be checked at run time via assertions). The Checker Framework enables you to state more facts. For example, consider the annotation on java.io.BufferedReader.ready():

        @EnsuresNonNullIf(expression="readLine()", result=true)
        @Pure public boolean ready() throws IOException { ... }

  This states that if readLine() is called immediately after ready() returns true, then readLine() returns a non-null value.

- a binary expression, e.g., x + y or #1 - 1. These are used by the Index Checker, for example.
- a class name expression within another expression, e.g., “String” in String.class or “pkg.MyClass” in pkg.MyClass.staticField. The class name must be fully-qualified unless it can be referenced by its simple name without an import statement at the location where the annotation appears. For example, an annotation in class C can use the simple name of a class in java.lang or in the same package as C.

### 32.8.1 Limitations

If you mention a class in a Java expression, then that class must be available to the compiler. For example, if you write @EnsuresNonNull("package1.package2.MyClass.myField.myOtherField"), then you should pass MyClass.java to the compiler. Otherwise, the compiler does not know which components of the dotted name are packages, classes, and field names. In that case, you might get confusing error messages such as “Invalid ’package1.package2’ because could not find class package1 in package package2”.

It is not possible to write a quantification over all array components (e.g., to express that all array elements are non-null). There is no such Java expression, but it would be useful when writing specifications.

## 32.9 Field invariants

Sometimes a field declared in a superclass has a more precise type in a subclass. To express this fact, write @FieldInvariant on the subclass. It specifies the field’s type in the class on which this annotation is written. The field must be declared in a superclass and must be final.

For example,

    class Person {
      final @Nullable String nickname;
      public Person(@Nullable String nickname) {
        this.nickname = nickname;
      }
    }

    // A rapper always has a nickname.
    @FieldInvariant(qualifier = NonNull.class, field = "nickname")
    class Rapper extends Person {
      public Rapper(String nickname) {
        super(nickname);
      }
      void method() {
        ... nickname.length() ...   // legal, nickname is non-null in this class.
      }
    }

A field invariant annotation can refer to more than one field. For example, @FieldInvariant(qualifier = NonNull.class, field = {fieldA, fieldB}) means that fieldA and fieldB are both non-null in the class upon which the annotation is written. A field invariant annotation can also apply different qualifiers to different fields. For example, @FieldInvariant(qualifier = {NonNull.class, Untainted.class}, field = {fieldA, fieldB}) means that fieldA is non-null and fieldB is untainted.

This annotation is inherited: if a superclass is annotated with @FieldInvariant, its subclasses have the same annotation. If a subclass has its own @FieldInvariant, then it must include the fields in the superclass annotation and those fields’ annotations must be a subtype (or equal) to the annotations for those fields in the superclass @FieldInvariant.

Currently, the @FieldInvariant annotation is trusted rather than checked. In other words, the @FieldInvariant annotation introduces a loophole in the type system, which requires verification by other means such as manual examination.

## 32.10 Unused fields

In an inheritance hierarchy, subclasses often introduce new methods and fields. For example, a Marsupial (and its subclasses such as Kangaroo) might have a variable pouchSize indicating the size of the animal’s pouch. The field does not exist in superclasses such as Mammal and Animal, so Java issues a compile-time error if a program tries to access myMammal.pouchSize.

If you cannot use subtypes in your program, you can enforce similar requirements using type qualifiers. For fields, use the @Unused annotation (Section ‍32.10.1), which enforces that a field or method may only be accessed from a receiver expression with a given annotation (or one of its subtypes). For methods, annotate the receiver parameter this; then a method call type-checks only if the actual receiver is of the specified type.

Also see the discussion of typestate checkers, in Chapter ‍30.30.

### 32.10.1@Unused annotation

A Java subtype can have more fields than its supertype. For example:

    class Animal {}
    class Mammal extends Animal { ... }
    class Marsupial extends Mammal {
      int pouchSize;  // pouch capacity, in cubic centimeters
      ...
    }

You can simulate the same effect for type qualifiers: the @Unused annotation on a field declares that the field may not be accessed via a receiver of the given qualified type (or any supertype). For example:

    class Animal {
      @Unused(when=Mammal.class)
      int pouchSize;  // pouch capacity, in cubic centimeters
      ...
    }
    @interface Mammal {}
    @interface Marsupial {}

    @Marsupial Animal joey = ...;
    ... joey.pouchSize ...    // OK
    @Mammal Animal mae = ...;
    ... mae.pouchSize ...    // compile-time error

The above class declaration is like writing

    class @Mammal-Animal { ... }
    class @Marsupial-Animal {
      int pouchSize;  // pouch capacity, in cubic centimeters
      ...
    }

# Chapter ‍33 Suppressing warnings

When the Checker Framework reports a warning, it’s best to fix the underlying problem, by changing the code or its annotations. For each warning, follow the methodology in Section ‍2.4.5 to correct the underlying problem.

This section describes what to do if the methodology of Section ‍2.4.5 indicates that you need to suppress the warning. You won’t change your code, but you will prevent the Checker Framework from reporting this particular warning to you. (Changing the code to fix a bug is another way to prevent the Checker Framework from issuing a warning, but it is not what this chapter is about.)

You may wish to suppress checker warnings because of unannotated libraries or un-annotated portions of your own code, because of application invariants that are beyond the capabilities of the type system, because of checker limitations, because you are interested in only some of the guarantees provided by a checker, or for other reasons. Suppressing a warning is similar to writing a cast in a Java program: the programmer knows more about the type than the type system does and uses the warning suppression or cast to convey that information to the type system.

You can suppress a warning message in a single variable initializer, method, or class by using the following mechanisms:

- the @SuppressWarnings annotation (Section ‍33.1), or
- the @AssumeAssertion string in an assert message (Section ‍33.2).

You can suppress warnings throughout the codebase by using the following mechanisms:

- the -AsuppressWarnings command-line option (Section ‍33.3),
- the -AskipUses and -AonlyUses command-line options (Section ‍33.4),
- the -AskipDefs and -AonlyDefs command-line options (Section ‍33.5),
- the -AskipFiles and -AonlyFiles command-line options (Section ‍33.6),
- the -AuseConservativeDefaultsForUncheckedCode=source command-line option (Section ‍35.4),
- the -Alint command-line option enables/disables optional checks (Section ‍33.7),
- changing the specification of a method (Section ‍33.8), or
- not running the annotation processor (Section ‍33.9).

Some type checkers can suppress warnings via

- checker-specific mechanisms (Section ‍33.10).

The rest of this chapter explains these mechanisms in turn.

You can use the -AwarnUnneededSuppressions command-line option to issue a warning if a @SuppressWarnings did not suppress any warnings issued by the current checker.

## 33.1@SuppressWarnings annotation

You can suppress specific errors and warnings by use of the @SuppressWarnings annotation, for example @SuppressWarnings("interning") or @SuppressWarnings("nullness"). Section ‍33.1.1 explains the syntax of the argument string.

A @SuppressWarnings annotation may be placed on program declarations such as a local variable declaration, a method, or a class. It suppresses all warnings within that program element. Section ‍33.1.2 discusses where the annotation may be written in source code.

Section ‍33.1.3 gives best practices for writing @SuppressWarnings annotations.

### 33.1.1@SuppressWarnings syntax

The @SuppressWarnings annotation takes a string argument, in one of the following forms: "checkername:messagekey", "checkername", or "messagekey".

The argument checkername is the checker name, without “Checker”. It is lower case by default, though a checker can choose a different casing. For example, if you invoke a checker as javac -processor MyNiftyChecker ..., then you would suppress its error messages with @SuppressWarnings("mynifty"). (An exception is the Subtyping Checker, for which you use the annotation name; see Section ‍29.1.) Sometimes, a checker honors multiple checkername arguments; use the -AshowSuppressWarningsStrings command-line option to see them.

The argument messagekey is the message key for the error. Each warning message from the compiler gives the most specific suppression string that can be used to suppress that warning. An example is “dereference.of.nullable” in

    MyFile.java:107: error: [dereference.of.nullable] dereference of possibly-null reference myList
              myList.add(elt);
              ^

You are allowed to use any substring of a message key, so long as the substring extends at each end to a period or an end of the key. For example, to suppress a warning with message key "lock.expression.possibly.not.final", you could use @SuppressWarnings("lock.expression.possibly.not.final"), @SuppressWarnings("lock.expression"), @SuppressWarnings("not.final"), or other variants. We recommend using the longest possible message key; a short message might suppress more warnings than you expect.

The checkername "allcheckers" means all checkers. Using this is not recommended, except for messages common to all checkers such as purity-related messages when using -AcheckPurityAnnotations. If you use "allcheckers", you run some checker that does not issue any warnings, and you supply the -AwarnUnneededSuppressions command-line argument, then the Checker Framework will issue an unneeded.suppression warning.

The special messagekey “all” means to suppress all warnings.

If the checkername part is omitted, the @SuppressWarnings applies to all checkers. If the messagekey part is omitted, the @SuppressWarnings applies to all messages (it suppresses all warnings from the given checker).

With the -ArequirePrefixInWarningSuppressions command-line option, the Checker Framework only suppresses warnings when the string is in the "checkername" or "checkername:messagekey" format, as in @SuppressWarnings("nullness") or @SuppressWarnings("nullness:assignment"). For example, @SuppressWarnings("assignment") and @SuppressWarnings("all") have no effect (they are ignored) when -ArequirePrefixInWarningSuppressions is used. You can use @SuppressWarnings("allcheckers") to suppress all Checker Framework warnings.

### 33.1.2 Where @SuppressWarnings can be written

@SuppressWarnings is a declaration annotation, so it may be placed on program declarations such as a local variable declaration, a method, or a class.

@SuppressWarnings cannot be used on statements, expressions, or types. To work around this, you can

- extract an expression into a local variable and suppress a warning on its declaration,
- write assert plus @AssumeAssertion between statements; it can affect arbitrary expressions (see Section ‍33.2), or
- use a type-system-specific method call, such as NullnessUtil.castNonNull.

Always write a @SuppressWarnings annotation on the smallest possible scope. To reduce the scope of a @SuppressWarnings annotation, it is sometimes desirable to refactor the code. You might extract an expression into a local variable, so that warnings can be suppressed just for that local variable’s initializer expression. Likewise, you might extract some code into a separate method, so that warnings can be suppressed just for its body. Or, you can use @AssumeAssertion on an assert statement; see Section ‍33.2.

As an example, consider suppressing a warning at an assignment that you know is safe. Here is an example that uses the Tainting Checker (Section ‍12). Assume that expr has compile-time (declared) type @Tainted String, but you know that the run-time value of expr is untainted.

      @SuppressWarnings("tainting:cast.unsafe") // expr is untainted because ... [explanation goes here]
      @Untainted String myvar = expr;

Java does not permit annotations (such as @SuppressWarnings) on assignments (or on other statements or expressions), so it would have been illegal to write

      @Untainted String myvar;
      ...
      @SuppressWarnings("tainting:cast.unsafe") // expr is untainted because ...
      myvar = expr;

### 33.1.3 Good practices when suppressing warnings

#### Suppress warnings in the smallest possible scope

Prefer @SuppressWarnings on a local variable declaration to one on a method, and prefer one on a method to one on a class. @SuppressWarnings on a local variable declaration applies only to the declaration (including its initializer if any), not to all uses of the variable.

You may be able to suppress a warning about a use of an expression by writing @AssumeAssertion for the expression, before the use. See Section ‍33.2.

Another way to reduce the scope of a @SuppressWarnings is to extract the expression into a new local variable and place a @SuppressWarnings annotation on the variable declaration. See Section ‍33.1.2.

#### Use a specific argument to @SuppressWarnings

It is best to use the most specific possible message key to suppress just a specific error that you know to be a false positive. The checker outputs this message key when it issues an error. If you use a broader @SuppressWarnings annotation, then it may mask other errors that you needed to know about.

Any of the following would have suppressed the warning in Section ‍33.1.2:

      @SuppressWarnings("tainting")              // suppresses all tainting-related warnings
      @SuppressWarnings("cast")                  // suppresses warnings from all checkers about casts
      @SuppressWarnings("unsafe")                // suppresses warnings from all checkers about unsafe code
      @SuppressWarnings("cast.unsafe")           // suppresses warnings from all checkers about unsafe casts
      @SuppressWarnings("tainting:cast")         // suppresses tainting warnings about casts
      @SuppressWarnings("tainting:unsafe")       // suppresses tainting warnings about unsafe code
      @SuppressWarnings("tainting:cast.unsafe")  // suppresses tainting warnings about unsafe casts

The last one is the most specific, and therefore is the best style.

#### Justify why the warning is a false positive

A @SuppressWarnings annotation asserts that the programmer knows that the code is actually correct or safe (that is, no undesired behavior will occur), even though the type system is unable to prove that the code is correct or safe.

Whenever you write a @SuppressWarnings annotation, you should also write, typically on the same line, a code comment explaining why the code is actually correct. In some cases you might also justify why the code cannot be rewritten in a simpler way that would be amenable to type-checking. Also make it clear what error is being suppressed. (This is particularly important when the @SuppressWarnings is on a method declaration and the suppressed warning might be anywhere in the method body.)

This documentation will help you and others to understand the reason for the @SuppressWarnings annotation. It will also help you audit your code to verify all the warning suppressions. (The code is correct only if the checker issues no warnings and each @SuppressWarnings is correct.)

A suppression message like “a.f is not null” is not useful. The fact that you are suppressing the warning means that you believe that a.f is not null. The message should explain why you believe that; for example, “a.f was checked above and no subsequent side effect can affect it”.

Here are some terse examples from libraries in plume-lib:

    @SuppressWarnings("cast") // cast is redundant (except when checking nullness)
    @SuppressWarnings("interning") // FbType.FREE is interned but is not annotated
    @SuppressWarnings("interning") // equality testing optimization
    @SuppressWarnings("nullness") // used portion of array is non-null
    @SuppressWarnings("nullness") // oi.factory is a static method, so null first argument is OK
    @SuppressWarnings("purity") // side effect to local state of type BitSet

A particularly good (and concise) justification is to reference an issue in the issue tracker, as in these two from Daikon:

    @SuppressWarnings("flowexpr.parse.error") // https://tinyurl.com/cfissue/862
    @SuppressWarnings("keyfor") // https://tinyurl.com/cfissue/877

Please report false positive warnings, then reference them in your warning suppressions. This permits the Checker Framework maintainers to know about the problem, it helps them with prioritization (by knowing how often in your codebase a particular issue arises), and it enables you to know when an issue has been fixed (though the -AwarnUnneededSuppressions command-line option also serves the latter purpose).

## 33.2@AssumeAssertion string in an assert message

Sometimes, it is too disruptive to refactor your code to create a location where @SuppressWarnings can be written. You can instead suppress a warning by writing an assertion whose message contains the string @AssumeAssertion(checkername).

For example, in this code:

    while (c != Object.class) {
      ...
      c = c.getSuperclass();
      assert c != null
        : "@AssumeAssertion(nullness): c was not Object, so its superclass is not null";
     }

the Nullness Checker assumes that c is non-null from the assert statement forward (including on the next iteration through the loop).

The assert expression must be an expression that would affect flow-sensitive type refinement (Section ‍32.7), if the expression appeared in a conditional test. Each type system has its own rules about what type refinement it performs.

The value in parentheses is a checker name (typically lowercase), exactly as in the @SuppressWarnings annotation (Section ‍33.1.1). Any subcheckers will also assume that the assertion is true (e.g., the Map Key Checker will assume that the assertion in the example above cannot fail, when it runs as a subchecker of the Nullness Checker).

The same good practices apply as for @SuppressWarnings annotations, such as writing a comment justifying why the assumption is safe (Section ‍33.1.3).

The -AassumeAssertionsAreEnabled and -AassumeAssertionsAreDisabled command-line options (Section ‍32.7.6) do not affect processing of assert statements that have @AssumeAssertion in their message. Writing @AssumeAssertion means that the assertion would succeed if it were executed, and the Checker Framework makes use of that information regardless of the -AassumeAssertionsAreEnabled and -AassumeAssertionsAreDisabled command-line options.

### 33.2.1 Suppressing warnings and defensive programming

This section explains the distinction between two different uses for assertions: debugging a program (also known as defensive programming) versus specifying a program. The examples use nullness annotations, but the concepts apply to any checker.

The purpose of assertions is to aid debugging by throwing an exception when a program does not work correctly. Sometimes, programmers use assertions for a different purpose: documenting how the program works. By default, the Checker Framework assumes that each assertion is used for its primary purpose of debugging: the assertion might fail at run time, and the programmer wishes to be informed at compile time about such possible run-time errors.

Suppose that a programmer encounters a failing test, adds an assertion to aid debugging, and fixes the test. The programmer leaves the assertion in the program if the programmer is worried that the program might fail in a similar way in the future. The Checker Framework should not assume that the assertion succeeds — doing so would defeat the very purpose of the Checker Framework, which is to detect errors at compile time and prevent them from occurring at run time.

A non-standard use for annotations is to document facts that a programmer has independently verified to be true. The Checker Framework can leverage these assertions in order to avoid issuing false positive warnings. The programmer marks such assertions with the @AssumeAssertion string in the assert message (see Section ‍33.2). Only do so if you are sure that the assertion always succeeds at run time.

Methods such as Objects.requireNonNull, JUnit’s Assert.assertNotNull, and Guava’s verifyNotNull and checkNotNull are similar to assertions. Just as for assertions, their intended use is as debugging aids, they might fail at run time, and the Checker Framework warns if that might happen. Some programmers may use assert methods as documentation of facts that the programmer has verified in some other manner. If you know that a particular codebase always uses an assertion method not for defensive programming but to indicate facts that are guaranteed to be true (that is, these assertions cannot fail at run time), then there are two approaches to avoid false positive warnings: write specifications or suppress warnings; see below for an explanation of each approach.

The method NullnessUtil.castNonNull is not an assertion method. It is a warning suppression method.

Note that some libraries have an imprecise/incorrect specification of their assertion methods. For example, Guava’s Verify.verifyNotNull is imprecisely/incorrectly specified to have a @Nullable formal parameter. In a correct execution, null never flows there, so its type can and should be annotated as @NonNull. That annotation allows the Nullness Checker to warn about programs that crash due to passing null to verifyNotNull. (A comment in Guava’s Preconditions.java agrees with this reasoning: “the first parameter to checkNotNull should be annotated to require it to be non-null.” The comment goes on to say “I had hoped to take a principled stand on this” (that, is, write the annotation @NonNull), but Guava annotates it as @Nullable to accommodate misuses within the Google codebase.)

By default, the Checker Framework uses Guava’s annotations as written. If you wish to take the principled stand in order to prevent all NullPointerExceptions, you can use a different version of Guava with a small change to your build file. Where the build file refers to Maven Central’s guava artifact, change the group name from “com.google.guava” to “org.checkerframework.annotatedlib”. (The code is identical; the only difference is annotations.)

##### Option 1: Write specifications based on uses of assertion methods

Suppose you are annotating a codebase that already contains precondition checks, such as:

      public String myGet(String key, String def) {
        checkNotNull(key, "key"); // NOI18N
        ...
      }

Because key is non-null in every correct execution, its type should be @NonNull in myGet’s signature. (@NonNull is the default, so in this case there is nothing to write.) The checker will not issue a warning about the checkNotNull call, but will issue a warning at incorrect calls to myGet.

##### Option 2: Suppress warnings at uses of assertion methods

This section explains how to suppress warnings at all uses of an assertion method. As with any warning suppression, you will compromise the checker’s guarantee that your code is correct and will not fail at run time.

- If the method is defined in your source code, annotate its definition just as NullnessUtil.castNonNull is annotated; see its Javadoc or the source code for the Checker Framework. Also, be sure to document the intention in the method’s Javadoc, so that programmers do not accidentally misuse it for defensive programming.
- If the method is defined in an external library, write a stub file that changes the method’s annotations, or use -AskipUses to make the Checker Framework ignore all calls to an entire class.

  As a special case, if you want the Nullness Checker to prevent most null pointer exceptions in your code, but to permit null pointer exceptions at nullness assertion methods, you can pass -Astubs=permit-nullness-assertion-exception.astub.

## 33.3-AsuppressWarnings command-line option

Supplying the -AsuppressWarnings command-line option is equivalent to writing a @SuppressWarnings annotation on every class that the compiler type-checks. The argument to -AsuppressWarnings is a comma-separated list of warning suppression strings, as in -AsuppressWarnings=purity,uninitialized.

When possible, it is better to write a @SuppressWarnings annotation with a smaller scope, rather than using the -AsuppressWarnings command-line option.

## 33.4-AskipUses and -AonlyUses command-line options

You can suppress all errors and warnings at all uses of a given class, or suppress all errors and warnings except those at uses of a given class. (The class itself is still type-checked, unless you also use the -AskipDefs, -AonlyDefs, -AskipFiles, or -AonlyFiles command-line option, see ‍33.5). You can also use these options to affect entire packages or directories/folders.

Set the -AskipUses command-line option to a regular expression that matches fully-qualified class names (not file names) for which warnings and errors should be suppressed. Or, set the -AonlyUses command-line option to a regular expression that matches fully-qualified class names (not file names) for which warnings and errors should be emitted; warnings about uses of all other classes will be suppressed. The regular expressions are unanchored, unless you anchor them yourself with “`^`” and/or “`$`”.

For example, suppose that you use “`-AskipUses=^java\.`” on the command line (with appropriate quoting) when invoking javac. Then the checkers will suppress all warnings related to classes whose fully-qualified name starts with `java.`, such as all warnings relating to invalid arguments and all warnings relating to incorrect use of the return value.

To suppress all errors and warnings related to multiple classes, you can use the regular expression alternative operator “|”, as in “`-AskipUses="java\.lang\.|java\.util\."`” to suppress all warnings related to uses of classes that belong to the java.lang or java.util packages. (Depending on your shell or other tool, you might need to change or remove the quoting.)

You can supply both -AskipUses and -AonlyUses, in which case the -AskipUses argument takes precedence, and -AonlyUses does further filtering but does not add anything that -AskipUses removed.

Warning: Use the -AonlyUses command-line option with care, because it can have unexpected results. For example, if the given regular expression does not match classes in the JDK, then the Checker Framework will suppress every warning that involves a JDK class such as Object or String. The meaning of -AonlyUses may be refined in the future. Oftentimes -AskipUses is more useful.

## 33.5-AskipDefs and -AonlyDefs command-line options

You can suppress all errors and warnings in the definition of a given class, or suppress all errors and warnings except those in the definition of a given class. (Uses of the class are still type-checked, unless you also use the -AskipUses or -AonlyUses command-line option, see ‍33.4.) You can also use these options to affect entire packages or directories/folders.

Set the -AskipDefs command-line option to a regular expression that matches fully-qualified class names (not file names) in whose definition warnings and errors should be suppressed. Or, set the -AonlyDefs command-line option to a regular expression that matches fully-qualified class names (not file names) whose definitions should be type-checked. (This is somewhat similar to NullAway’s -XepOpt:NullAway:AnnotatedPackages command-line argument.) (For file names, use -AskipFiles and -AonlyFiles instead; see Section ‍33.6.) The regular expressions are unanchored, unless you anchor them yourself with “`^`” and/or “`$`”.

For example, if you use “`-AskipDefs=^mypackage\.`” on the command line (with appropriate quoting) when invoking javac, then the definitions of classes whose fully-qualified name starts with `mypackage.` will not be checked.

If you supply both -AskipDefs and -AonlyDefs, then -AskipDefs takes precedence.

Another way not to type-check a file is not to pass it on the compiler command-line: the Checker Framework type-checks only files that are passed to the compiler on the command line, and does not type-check any file that is not passed to the compiler. The -AskipDefs and -AonlyDefs command-line options are intended for situations in which the build system is hard to understand or change. In such a situation, a programmer may find it easier to supply an extra command-line argument, than to change the set of files that is compiled.

A common scenario for using the arguments is when you are starting out by type-checking only part of a legacy codebase. After you have verified the most important parts, you can incrementally check more classes until you are type-checking the whole thing.

## 33.6-AskipFiles and -AonlyFiles command-line options

You can suppress all errors and warnings originating from classes that are located in a given file or directory/folder, or all those that are not in a given file or directory/folder.

Set the -AskipFiles command-line option to a regular expression that matches the path to files or directories/folders containing classes for which errors and warnings should be suppressed. Similarly, -AonlyFiles suppresses warnings everywhere except in the given files. The regular expressions are unanchored, unless you anchor them yourself with “`^`” and/or “`$`”.

For example, if you use “`-AskipFiles=/build/generated/`” on the command line when invoking javac, then the definitions of classes located in any `/build/generated` directory will not be checked.

If you supply both -AskipFiles and -AonlyFiles, then -AskipFiles takes precedence.

A common scenario for using this argument is when you want to exclude certain directories/folders (such as those containing generated or legacy code) from type-checking. Another scenario is when a file contains multiple top-level classes, only one of which has a name that corresponds to the file name.

## 33.7-Alint command-line option

The -Alint option enables or disables optional checks, analogously to javac’s -Xlint option. Each of the distributed checkers supports at least the following lint options (and possibly more, see the checker’s documentation):

- cast:unsafe (default: on) warn about unsafe casts that are not checked at run time, as in ((@NonNull String) myref). Such casts are generally not necessary because of type refinement (Section ‍32.7).
- cast:redundant (default: on) warn about redundant casts that are guaranteed to succeed at run time, as in ((@NonNull String) "m"). Such casts are not necessary, because the target expression of the cast already has the given type qualifier.
- cast Enable or disable all cast-related warnings.
- all Enable or disable all lint warnings, including checker-specific ones if any. Examples include redundantNullComparison for the Nullness Checker (see Section ‍2) and dotequals for the Interning Checker (see Section ‍6.4). This option does not enable/disable the checker’s standard checks, just its optional ones.
- none The inverse of all: disable or enable all lint warnings, including checker-specific ones if any.

To activate a lint option, write -Alint= followed by a comma-delimited list of check names. If the option is preceded by a hyphen (-), the warning is disabled. For example, to disable all lint options except redundant casts, you can pass -Alint=-all,cast:redundant on the command line.

Only the last -Alint option is used; all previous -Alint options are silently ignored. In particular, this means that -Alint=all -Alint=cast:redundant is not equivalent to -Alint=-all,cast:redundant.

## 33.8 Change the specification of a method

To prevent a checker from issuing a warning at calls to a specific method, you can change the annotations on that method by writing a stub file (see Section ‍35.5).

Stub files are usually used to provide correct specifications for unspecified code.

Stub files can also be used to provide incorrect specifications, for the purpose of suppressing warnings. For example, suppose that you are running the Nullness Checker to prevent null pointer exceptions. Further suppose that for some reason you do not care if method Objects.requireNonNull crashes with a NullPointerException. You can supply a stub file containing:

    package java.util;
    class Objects {
      @EnsuresNonNull("#1")
      public static <T> @NonNull T requireNonNull(@Nullable T obj);
    }

This particular stub file already exists. If you want the Nullness Checker to prevent most null pointer exceptions in your code, but to permit null pointer exceptions at nullness assertion methods, you can pass -Astubs=permit-nullness-assertion-exception.astub.

## 33.9 Don’t run the processor

You can compile parts of your code without use of the -processor switch to javac. No checking is done during such compilations, so no warnings are issued related to pluggable type-checking.

You can direct your build system to avoid compiling certain parts of your code. For example, the -Dmaven.test.skip=true command-line argument tells Maven not to compile (or run) the tests.

## 33.10 Checker-specific mechanisms

Finally, some checkers have special rules. For example, the Nullness checker (Chapter ‍3) uses the special castNonNull method to suppress warnings (Section ‍3.4.1). This manual also explains special mechanisms for suppressing warnings issued by the Fenum Checker (Section ‍9.4) and the Units Checker (Section ‍20.5).

# Chapter ‍34 Type inference

This chapter is about tools that infer annotations for your program’s method signatures and fields, before you run a type-checker. To learn about local type inference within a method body, see Section ‍32.7.

A typical workflow (Section ‍2.4) is for a programmer to first write annotations on method signatures and fields, then run a type-checker. Type inference performs the first step automatically for you. This saves time for programmers who would otherwise have to understand the code, then write annotations manually.

Type inference outputs type qualifiers that are consistent with your program’s source code. Your program still might not type-check if your program contains a bug or contains tricky code that is beyond the capabilities of the type checker.

The qualifiers are output into an annotation file. They can be viewed and adjusted by the programmer, can be used by tools such as the type-checker, and can be inserted into the source code or the class file.

Inserting the inferred annotations into the program source code creates documentation in the form of type qualifiers, which can aid programmer understanding and may make type-checking warnings more comprehensible. Storing annotations in side-files is more desirable if the program’s source code cannot be modified for some reason, if the typechecking is "one-off" (typechecking will be done once and its results will be evaluated, but it will not be done repeatedly), or if the set of annotations is extremely voluminous and would clutter the code.

Type inference is most effective when you run it on a program rather than on a library — unless you also run it on an extensive test suite for the library. See Section ‍34.6 for an explanation.

Type inference is costly: it takes several times longer than type-checking does. However, it only needs to be run once, after which you can use and possibly modify the results.

## 34.1 Type inference tools

This section lists tools that take a program and output a set of annotations for it. It first lists tools that work only for a single type system (but may do a more accurate job for that type system) then lists general tools that work for any type system.

For the Nullness Checker:
  Section ‍3.3.7 lists several tools that infer annotations for the Nullness Checker.
For the Purity Checker:
  If you run the Checker Framework with the -AsuggestPureMethods command-line option, it will suggest methods that can be marked as @SideEffectFree, @Deterministic, or @Pure; see Section ‍32.7.5.
WPI, for any type system:
  “Whole program inference”, or WPI, is distributed with the Checker Framework. See Section ‍34.2.
CFI, for any type system:
  “Checker Framework Inference”, or CFI, is a type inference framework built on a variant of the Checker Framework. You need to slightly rewrite your type system to work with CFI. The CFI repository contains rewritten versions of some of the type systems that are distributed with the Checker Framework.
Cascade, for any type system:
  Cascade‍[VPEJ15] is an Eclipse plugin that implements interactive type qualifier inference. Cascade is interactive rather than fully-automated: it makes it easier for a developer to insert annotations. Cascade starts with an unannotated program and runs a type-checker. For each warning it suggests multiple fixes, the developer chooses a fix, and Cascade applies it. Cascade works with any checker built on the Checker Framework. You can find installation instructions and a video tutorial at https://github.com/reprogrammer/cascade. Cascade was last updated in November 2014, so it might or might not work for you.

Except for one of the nullness inference tools, all these type inference tools are static analyses. They analyze your program’s source code, but they do not run your program.

## 34.2 Whole-program inference

Whole-program inference infers types for fields, method parameters, and method return types that do not have a user-written qualifier (for the given type system). The inferred type qualifiers are output into annotation files. The inferred type is the most specific type that is compatible with all the uses in the program. For example, the inferred type for a field is the least upper bound of the types of all the expressions that are assigned into the field.

There are three scripts that you can use to run whole-program inference. Each has advantages and disadvantages, discussed below:

- To run whole-program inference on a single project without modifying its source code, use the wpi.sh script (Section ‍34.3). This script can automatically understand many Ant, Maven, and Gradle build files, so it requires little manual configuration.
- To run whole-program inference on many projects without modifying their source code (say, when running it on projects from GitHub), use the wpi-many.sh script (Section ‍34.4). This script can understand the same build files as wpi.sh.
- If you want to insert the inferred annotations directly into a single project’s source code, use the infer-and-annotate.sh script (Section ‍34.5).

These type inference scripts appear in the checker/bin/ directory. The remainder of this chapter describes them (Sections ‍34.3–34.5), then concludes with discussion that applies to all of them.

## 34.3 Running whole-program inference on a single project

A typical invocation of wpi.sh is

      $CHECKERFRAMEWORK/checker/bin/wpi.sh -- --checker nullness

The result is a set of log files placed in the dljc-out/ folder of the target project. See Section ‍34.3.1 for an explanation of the log files generated by an invocation of wpi.sh. The inferred annotations appear in .ajava files in a temporary directory whose name appears in the dlcj-out/wpi-stdout.log file; you can find their location by examining the -Aajava argument to the last javac command that was run. The annotation files generated in each round of inference appear in directories labeled iteration0, iteration1, etc.

The wpi.sh script is most useful when analyzing projects that follow the standard conventions of their build system for single-module projects. When analyzing a project that requires non-standard build system commands, use the -c and -b options to override the defaults used by wpi.sh. For example, suppose that you wanted to use wpi.sh to infer annotations for the Resource Leak Checker on Apache Zookeeper’s zookeeper-server module only. Without wpi.sh, one might use the following command: mvn -B --projects zookeeper-server --also-make install -DskipTests. To use this command via wpi.sh, use the following: sh wpi.sh -b "-B --projects zookeeper-server --also-make" -c "install -DskipTests" -- --checker resourceleak.

The full syntax for invoking wpi.sh is

      wpi.sh [-d PROJECTDIR] [-t TIMEOUT] [-c COMPILATION_TARGET] [-b EXTRA_BUILD_ARGS] [-g GRADLECACHEDIR] -- [DLJC-ARGS]

Arguments in square brackets are optional. Here is an explanation of the arguments:

-d PROJECTDIR
  The top-level directory of the project. It must contain an Ant, Gradle, or Maven buildfile. The default is the current working directory.
-t TIMEOUT
  The timeout for running the checker, in seconds.
-c COMPILATION_TARGET
  The name(s) of the build system target(s) used to compile the target project. The default is chosen based on the build system (e.g., compileJava for Gradle, compile for Maven, etc.). This argument is passed directly to the build system when compiling the target project, so it can also include command-line arguments that only apply to the compilation targets (and not other targets like clean); for general command-line arguments that apply to all targets, use the -b option instead. When running WPI on a single module of a multi-module project, you might want to use this option (possibly in combination with -b, below), depending on the setup of the target project. The argument may contain spaces and is re-tokenized by the shell.
-b EXTRA_BUILD_ARGS
  Extra arguments to pass to the build script invocation. This argument will be passed to compilation tasks, such as ant compile, gradle compileJava, or mvn compile. The main difference between this option and -c is that the values of the -b option are also passed to other, non-compilation build system commands, such as ant clean, gradle clean, or mvn clean. The argument may contain spaces and is re-tokenized by the shell.
-g GRADLECACHEDIR
  The directory to use for the -g option to Gradle (the Gradle home directory). This option is ignored if the target project does not build with Gradle. The default is .gradle relative to the target project (i.e., each target project has its own Gradle home). This default is motivated by Gradle issue #1319.

DLJC-ARGS
  Arguments that are passed directly to do-like-javac’s dljc program without modification. One argument is required: --checker, which indicates what type-checker(s) to run (in the format described in Section ‍2.2.4).

  The documentation of do-like-javac describes the other commands that its WPI tool supports. Notably, to pass checker-specific arguments to invocations of javac, use the --extraJavacArgs argument to dljc. For example, to use the -AignoreRangeOverflow option for the Constant Value Checker (Chapter ‍23) when running inference, you would add --extraJavacArgs=’-AignoreRangeOverflow’ anywhere after the -- argument to wpi.sh.

You may need to wait a few minutes for the command to complete.

### 34.3.1 Whole-program inference results

The result of invoking wpi.sh are log files stored in the dljc-out/ folder of the target project. The dljc-out/ folder contains:

- typecheck.out: The final error messages produced by javac, comprising the results of type-checking obtained with the latest iteration of WPI (i.e., using the most precise, consistent set of annotations).
- build_output.txt: Logs collected from compiling the target project without the Checker Framework, using the build file found at the top-level directory.
- wpi-stdout.log: The results of type-checking with each candidate set of annotations, concatenated together in the order in which the annotations were inferred. The final results (i.e., those obtained using the most precise, consistent set of annotations) will appear at the end of this file.
- wpi-stdout-*: These files are separate from the logs in wpi-stdout.log. They may be useful in the case where dljc is not working as expected.
- toplevel.log: The log of the command executed at the top-level of the target directory to invoke whole-program inference.
- javac.json: The list of all .java files for which inference was attempted by wpi.sh, and the options passed to javac.
- stats.json: Statistics from the invocation of wpi.sh on the target project, including build time, the number of built .jar files, the number of executable .jar files, the number of javac invocations, and the number of source files.

### 34.3.2 Requirements for whole-program inference scripts

The requirements to run wpi.sh and wpi-many.sh are the same:

- The project on which inference is run must contain an Ant, Gradle, or Maven buildfile that compiles the project.
- At least one of the `JAVA_HOME`, `JAVA17_HOME`, or `JAVA21_HOME` environment variables must be set.
- If set, the `JAVA_HOME` environment variable must point to a Java JDK.
- If set, the `JAVA17_HOME` environment variable must point to a Java 17 JDK.
- If set, the `JAVA21_HOME` environment variable must point to a Java 21 JDK.
- If set, the `JAVA22_HOME` environment variable must point to a Java 22 JDK.
- CHECKERFRAMEWORK environment variable must point to a built copy of the Checker Framework.
- If set, the `DLJC` environment variable must point to a copy of the dljc script from do-like-javac. (If this variable is not set, the WPI scripts will download this dependency automatically.)
- Other dependencies: ant, awk, curl, git, gradle, mvn, python3 (for dljc), wget.

  Python2.7 modules: subprocess32.

## 34.4 Running whole-program inference on many projects

The requirements to run wpi.sh and wpi-many.sh are the same. See Section ‍34.3.2 for the list of requirements.

To run an experiment on many projects:

1. Use query-github.sh to search GitHub for candidate repositories. File docs/examples/wpi-many/securerandom.query is an example query, and file docs/manual/securerandom.list is the standard output created by running query-github.sh securerandom.query 100. If you do not want to use GitHub, construct a file yourself that matches the format of the file securerandom.list.
2. Use wpi-many.sh to run whole-program inference on multiple Ant, Gradle, or Maven projects. You provide it a file in which each line is “GitHub repository URL” “git hash” (with no quotes, and with whitespace between them).
   - If you are using a checker that is distributed with the Checker Framework, use wpi-many.sh directly.
   - If you are using a checker that is not distributed with the Checker Framework (also known as a "custom checker"), file docs/examples/wpi-many/wpi-many-custom-checker-example.sh is a no-arguments script that serves as an example of how to use wpi-many.sh.

   Log files are copied into a results directory. For a failed run, the log file indicates the reason that WPI could not be run to completion on the project. For a successful run, the log file indicates whether the project was verified (i.e., no errors were reported), or whether the checker issued warnings (which might be true positive or false positive warnings).

3. Use wpi-summary.sh to summarize the logs in the output results directory. Use its output to guide your analysis of the results of running wpi-many.sh: you should manually examine the log files for the projects that appear in the "results available" list it produces. This list is the list of every project that the script was able to successfully run WPI on. (This does not mean that the project type-checks without errors afterward, or even typechecks at all — just that the Checker Framework attempted to typecheck the project and some output was produced.)
4. (Optional) Add annotations that WPI does not infer, to eliminate false positive warnings. There are two ways you can add annotations to a target program:
   - Fork the project and add the annotations directly to the project’s source code, and add a dependency on org.checkerframework:checker-qual to the project’s build system. This approach is the most difficult, but has the advantage that the checker will attempt to verify any annotations you add.
   - Add the annotations in a stub file, creating the .astub file as a copy of the .java file. When you run the checker, supply the -AmergeStubsWithSource and -Astubs=... command-line arguments.

A typical invocation is

    wpi-many.sh -o outdir -i /path/to/repo.list -t 7200 -- --checker optional

The wpi-many.sh script takes the following command-line arguments. The -o and -i arguments are mandatory. An invocation should also include -- [DLJC-ARGS] at the end; DLJC-ARGS is documented in Section ‍34.3.

-o outdir
  run the experiment in the outdir directory, and place the results in the outdir-results directory. Both will be created if they do not exist. The directory may be specified as an absolute or relative path.
-i infile
  Read the list of repositories to use from the file infile. The file must be specified as an absolute, not relative, path. Each line should have 2 elements, separated by whitespace:
  1. The URL of the git repository on GitHub. The URL must be of the form https://github.com/username/repository . The script is reliant on the number of slashes, so excluding “https://” is an error.
  2. The commit hash to use.
-t timeout
  The timeout for running the checker on each project, in seconds.
-g GRADLECACHEDIR
  The directory to use for the -g option to Gradle (the Gradle home directory). This option is ignored if the target project does not build with Gradle. The default is .gradle relative to the target project (i.e., each target project has its own Gradle home). This default is motivated by Gradle issue #1319.
-s
  If this flag is present, then projects which are not buildable — for which no supported build file is present or for which running the standard build commands fail — are skipped on future runs but are not deleted immediately (such projects are deleted immediately if this flag is not present). This flag is useful if you intend to run wpi-many.sh several times on the same set of repositories (for example, during checker development), to avoid re-downloading unusable projects.

To obtain the locations of the annotation files generated by an invocation of wpi-many.sh, you can run wpi-annotation-paths.sh on the result directory, e.g., outdir-results.

A typical invocation is

    wpi-annotation-paths.sh outdir-results

Here, the results of wpi-many.sh are located in outdir-results.

## 34.5 Whole-program inference that inserts annotations into source code

To use this version of whole-program inference, make sure that insert-annotations-to-source, from the Annotation File Utilities project, is on your path (for example, its directory is in the $PATH environment variable). Then, run the script checker-framework/checker/bin/infer-and-annotate.sh. Its command-line arguments are:

1. Optional: Command-line arguments to insert-annotations-to-source.
2. Processor’s name.
3. Target program’s classpath. This argument is required; pass "" if it is empty.
4. Optional: Extra processor arguments which will be passed to the checker, if any. You may supply any number of such arguments, or none. Each such argument must start with a hyphen.
5. Optional: Paths to .jaif files used as input in the inference process.
6. Paths to .java files in the program.

For example, to add annotations to the plume-lib project:

    git clone https://github.com/mernst/plume-lib.git
    cd plume-lib
    make jar
    $CHECKERFRAMEWORK/checker/bin/infer-and-annotate.sh \
        "LockChecker,NullnessChecker" java/plume.jar:java/lib/junit-4.12.jar:$JAVA_HOME/lib/tools.jar \
        $(find java/src/plume/ -name "*.java")
    # View the results
    git diff

You may need to wait a few minutes for the command to complete. You can ignore warnings that the command outputs while it tries different annotations in your code.

It is recommended that you run infer-and-annotate.sh on a copy of your code, so that you can see what changes it made and so that it does not change your only copy. One way to do this is to work in a clone of your repository that has no uncommitted changes.

## 34.6 Inference results depend on uses in your program or test suite

Type inference outputs the most specific type qualifiers that are consistent with all the source code it is given. (Section ‍34.6.1 explains when type inference ignores some code.) This may be different than the specification the programmer had in mind when writing tho code. If the program uses a method or field in a limited way, then the inferred annotations will be legal for the program as currently written but may not be as general as possible and may not accommodate future program changes.

Here are some examples:

- Suppose that your program (or test suite) currently calls method m1 only with non-null arguments. The tool will infer that m1’s parameter has @NonNull type. If you had intended the method to be able to take null as an argument and you later add such a call, the type-checker will issue a warning because the inferred @NonNull annotation is inconsistent with the new call.
- If your program (or test suite) passes only null as an argument, the inferred type will be the bottom type, such as @GuardedByBottom.
- Suppose that method m2 has no body, because it is defined in an interface or abstract class. Type inference can still infer types for its signature, based on the overriding implementations. If all the methods that override m2 return a non-null value, type inference will infer that m2’s return type has @NonNull type, even if some other overriding method is allowed to return null.

If the program contains erroneous calls, the inferred annotations may reflect those errors. Suppose you intend method m3 to be called with non-null arguments, but your program contains an error and one of the calls to m3 passes null as the argument. Then the tool will infer that m3’s parameter has @Nullable type.

If you run whole-program inference on a library that contains mutually recursive routines, and there are no non-recursive calls to the routines, then whole-program inference may run a long time and eventually produce incorrect results. In this case, write type annotations on the formal parameters of one of the routines.

Whole-program inference is a “forward analysis”. It determines a method parameter’s type annotation based on what arguments are passed to the method but not on how the parameter is used within the method body. It determines a method’s return type based on code in the method body but not on uses of the method return value in client code.

### 34.6.1 Whole-program inference ignores some code

Whole-program inference ignores code within the scope of a @SuppressWarnings annotation with an appropriate key (Section ‍33.1). In particular, uses within the scope do not contribute to the inferred type, and declarations within the scope are not changed. You should remove @SuppressWarnings annotations from the class declaration of any class you wish to infer types for.

As noted above, whole-program inference generalizes from invocations of methods and assignments to fields. If a field is set via reflection (such as via injection), there are no explicit assignments to it for type inference to generalize from, and type inference will produce an inaccurate result. There are two ways to make whole-program inference ignore such a field. (1) You probably have an annotation such as @Inject or @Option that indicates such fields. Meta-annotate the declaration of the Inject or Option annotation with @IgnoreInWholeProgramInference. (2) Annotate the field to be ignored with @IgnoreInWholeProgramInference.

Whole-program inference, for a type-checker other than the Nullness Checker, ignores assignments and pseudo-assignments where the right-hand-side is the null literal.

### 34.6.2 Manually checking whole-program inference results

With any type inference tool, it is a good idea to manually examine the results. This can help you find bugs in your code or places where type inference inferred an overly-precise result. You can correct the inferred results manually, or you can add tests that pass additional values and then re-run inference.

When arguments or assignments are literals, whole-program inference commonly infers overly precise type annotations, such as @Interned and @Regex annotations when the analyzed code only uses a constant string.

When an annotation is inferred for a use of a type variable, you may wish to move the annotation to the corresponding upper bounds of the type variable declaration.

## 34.7 How whole-program inference works

This section explains how the wpi.sh and infer-and-annotate.sh scripts work. If you merely want to run the scripts and you are not encountering trouble, you can skip this section.

Each script repeatedly runs the checker with an -Ainfer= command-line option to infer types for fields and method signatures. The output of this step is a .jaif (for infer-and-annotate.sh) or .ajava (for wpi.sh) file that records the inferred types. Each script adds the inferred annotation to the next run, so that the checker takes them into account (and checks them). wpi.sh does this by updating the set of .ajava files that are passed to the checker via the -Aajava command-line argument; infer-and-annotate.sh inserts the inferred annotations in the program using the Annotation File Utilities.

On each iteration through the process, there may be new annotations in the .jaif or .ajava files, and some type-checking errors may be eliminated (though others might be introduced). The process halts when there are no more changes to the inference results, that is, the .jaif or .ajava files are unchanged between two runs.

When the type-checker is run on the program with the final annotations inserted, there might still be errors. This may be because the tool did not infer enough annotations, or because your program cannot typecheck (either because contains a defect, or because it contains subtle code that is beyond the capabilities of the type system). However, each of the inferred annotations is sound, and this reduces your manual effort in annotating the program.

The iterative process is required because type-checking is modular: it processes each class and each method only once, independently. Modularity enables you to run type-checking on only part of your program, and it makes type-checking fast. However, it has some disadvantages:

- The first run of the type-checker cannot take advantage of whole-program inference results because whole-program inference is only complete at the end of type-checking, and modular type-checking does not revisit any already-processed classes.
- Revisiting an already-processed class may result in a better estimate.

## 34.8 Type inference compared to other whole-program analyses

There exist monolithic whole-program analyses that run without requiring any annotations in the source code. An advantage of such a tool is that the programmer never needs to write any type annotations.

Running a whole-program inference tool, then running a type-checker, has some benefits:

- The type qualifiers act as machine-checked documentation, which can aid programmer understanding.
- Error messages may be more comprehensible. With a monolithic whole-program analysis, error messages can be obscure, because the analysis has already inferred (possibly incorrect) types for a number of variables.
- Errors are localized. A change to one part of the program does not lead to an error message in a far-removed part of the program.
- Type-checking is modular, which can be faster than re-doing a whole-program analysis every time the program changes.

# Chapter ‍35 Annotating libraries

When your code uses a library that is not currently being compiled, the Checker Framework looks up the library’s annotations in its class files. Section ‍2.2.1 tells you how to find and use a version of a library that contains type annotations.

If your code uses a library that does not contain type annotations, then the type-checker has no way to know the library’s behavior. The type-checker makes conservative assumptions about unannotated bytecode. (See Section ‍32.5.6 for details, an example, and how to override this conservative behavior.) These conservative library annotations invariably lead to checker warnings.

This chapter describes how to eliminate the warnings by adding annotations to the library. (Alternately, the -AskipUses or -AonlyUses command-line option can suppress all warnings related to an unannotated library, or to part of your codebase; see Sections ‍33.4–33.5.)

You can write annotations for a library, and make them known to a checker, in two ways.

1. Write annotations in a copy of the library’s source code (for instance, in a fork of the library’s GitHub project). In addition to writing annotations, adjust the build system to run pluggable-type-checking when compiling (see Chapter ‍38). Now, when you compile the library, the resulting .jar file contains type annotations.

   When checking a client of the library, put the annotated library’s .jar file on the classpath, as explained in Section ‍2.2.1.

   With this compilation approach, the syntax of the library annotations is validated ahead of time. Thus, this compilation approach is less error-prone, and the type-checker runs faster. You get correctness guarantees about the library in addition to your code.

   For instructions, see Section ‍35.2.

2. Write annotations in a “stub file”, if you do not have access to the source code.

   Then, when checking a client of the library, supply the “stub file” textually to the Checker Framework.

   This approach does not require you to compile the library source code. A stub file is applicable to multiple versions of a library, so the stub file does not need to be updated when a new version of the library is released, unless the API has changed (such as defining a new class or method).

   For instructions, see Section ‍35.5.

If you annotate a new library (either in its source code or in a stub file), please inform the Checker Framework developers so that your annotated library can be advertised to users of the Checker Framework. Sharing your annotations is useful even if the library is only partially annotated. However, as noted in Sections ‍2.4.2 and ‍35.1.4, you should annotate an entire class at a time. You may find type inference tools (Chapter ‍34) helpful when getting started, but you should always examine their results.

## 35.1 Tips for annotating a library

Section ‍2.4 gives general tips for writing annotations. This section gives tips that are specific to annotating a third-party library.

### 35.1.1 Don’t change the code

If you annotate a library that you maintain, you can refactor it to improve its design.

When you annotate a library that you do not maintain, you should only add annotations and, when necessary, documentation of those annotations. You can place the documentation in a Java comment (// or /*...*/) or in a @CFComment annotation.

Do not change the library’s code, which will change its behavior and make the annotated version inconsistent with the unannotated version. (Sometimes it is acceptable to make a refactoring, such as extracting an expression into a new local variable in order to annotate its type or suppress a warning. Perform refactoring only when you cannot use @AssumeAssertion.)

Do not change publicly-visible documentation, such as Javadoc comments. That also makes the annotated version inconsistent with the unannotated version.

Do not change formatting and whitespace.

Any of these changes would increase the difference between upstream (the original version) and your annotated version. Unnecessary differences make it harder for others to understand what you have done, and they make it harder to pull changes from upstream into the annotated library.

### 35.1.2 Library annotations should reflect the specification, not the implementation

Publicly-visible annotations (including those on public method formal parameters and return types) should be based on the documentation, typically the method’s Javadoc. In other words, your annotations should re-state facts that are in the Javadoc documentation.

Do not add requirements or guarantees beyond what the library author has already committed to. If a project’s Javadoc says nothing about nullness, then you should not assume that the specification forbids or permits null. (If the project’s Javadoc explicitly mentions null everywhere it is permitted, then you can assume it is forbidden elsewhere, where the author omitted those statements.)

If a fact is not mentioned in the documentation, then it is usually an implementation detail. Clients should not depend on implementation details, which are prone to change without notice. (In some cases, you can infer some facts from the implementation, such as that null is permitted for a method’s parameter or return type if the implementation explicitly passes null to the method or the method implementation returns null.)

If there is a fact that you think should be in the library’s documentation but was unintentionally omitted by its authors, then please submit a bug report asking them to update the documentation to reflect this fact. After they do, you can also express the fact as an annotation.

### 35.1.3 Report bugs upstream

While annotating the library, you may discover bugs or missing/wrong documentation. If you have a documentation improvement or a bug fix, then open a pull request against the upstream version of the library. This will benefit all users of the library. And, once the documentation is updated, you will be able to add annotations that are consistent with the documentation.

### 35.1.4 Fully annotate the library, or indicate which parts you did not

If you do not annotate all the files in the library, then use @AnnotatedFor to indicate what files you have annotated.

Whenever you annotate any part of a file, fully annotate the file! That is, write annotations for all the methods and fields, based on their documentation. Here are reasons for this rule:

- If you annotate just part of the file, then users may be surprised that calls to some methods type-check as expected whereas other methods do not (because they have not been annotated).
- Annotating one method or field at a time may lead to inconsistencies between different parts of the file. Different people may make different assumptions, might write annotations in a way that is locally convenient but globally inconsistent, or might not read all the documentation of the class to understand how it works.
- It is not much more effort to annotate an entire class versus one method or field. In either case it is usually necessary to understand the entire class’s design and implementation. Once you have done that, you might as well annotate the whole thing.
- If you fully annotate the file, it is possible to type-check the library to verify the annotations. (Even if you do not do this right now, it eases the task in the future.)

### 35.1.5 Verify your annotations

Ideally, after you annotate a file, you should type-check the file to verify the correctness and completeness of your annotations.

An alternative is to only annotate method signatures. The alternative is quicker but more error-prone. There is no difference from the point of view of clients, who can only see annotations on public methods and fields. When you compile the library, the type-checker will probably issue warnings; you can supply -Awarns so that the compiler will still produce .class files.

## 35.2 Creating an annotated library

This section describes how to create an annotated library.

1. See the org.checkerframework.annotatedlib group in the Maven Central Repository to find out whether an annotated version of the library already exists.
   - If it exists, but you want to add annotations for a different checker:

     Clone its repository from https://github.com/typetools/, and tweak its buildfile to run an additional checker.

   - If it does not already exist:

     Fork the project. (First, verify that its license permits forking.)

     Add a line in its main README to indicate that this is an annotated version of the library. That line should also indicate how to obtain the corresponding upstream version (typically a git tag corresponding to a release), so that others can see exactly what edits you have made.

     Adjust the library’s build process, such as an Ant, Gradle, or Maven buildfile (for guidance, see Chapter ‍38). Every time the build system runs the compiler, it should:

     - pass the -AuseConservativeDefaultsForUncheckedCode=source,bytecode command-line option and
     - run every pluggable type-checker for which any annotations exist, using -processor TypeSystem1,TypeSystem2,TypeSystem3

     You are not adding new build targets, but modifying existing targets. The reason to run every type-checker is to verify the annotations you wrote, and to use appropriate defaults for all unannotated type uses.

2. Annotate some files.

   When you annotate a file, annotate the whole thing, not just a few of its methods. Add an @AnnotatedFor({"checkername"}) annotation to each class declaration, or augment an existing @AnnotatedFor annotation.

3. Build the library.

   Because of the changes that you made in step 1, this will run pluggable type-checkers. If there are any compiler warnings, fix them and re-compile.

   Now you have a .jar file that you can use while type-checking and at run time.

4. Tell other people about your work so that they can benefit from it.
   - Please inform the Checker Framework developers about your new annotated library by opening a pull request or an issue. This will let us add your annotations to a repository in https://github.com/typetools/ and upload a compiled artifact to the Maven Central Repository.
   - Optionally, encourage the library’s maintainers to accept your annotations into its main version control repository. This suggestion will be most compelling if you have already reported bugs in the library that were revealed by pluggable type-checking. Once the annotations are in the main version control repository, they will be easier to maintain, the library will obtain the correctness guarantees of pluggable type-checking, and there will be no need to distribute an annotated version of the library.

     If the library maintainers do not accept the annotations, then periodically, such as when a new version of the library is released, pull changes from upstream (the library’s main version control system) into your fork, add annotations to any newly-added methods in classes that are annotated with @AnnotatedFor, rebuild to create an updated .jar file, and inform the Checker Framework developers by opening an issue or issuing a pull request.

## 35.3 Creating an annotated JDK

When you create a new checker, you need to also supply annotations for parts of the JDK. You can do so either as stub files or in a copy of the JDK source code, as described in Section ‍36.10. Section ‍40.2.1 tells how to improve JDK annotations for an existing type system.

## 35.4 Compiling partially-annotated libraries

If you completely annotate a library, then you can compile it using a pluggable type-checker. You get a guarantee that the library contains no errors (that is, it is consistent with the specifications you wrote).

The rest of this section tells you how to compile a library if you partially annotate it: that is, you write annotations for some of its classes but not others. (There is another type of partial annotation, which is when you annotate method signatures but do not type-check the bodies. See Section ‍35.1.)

Here are two concerns you may have when partially annotating a library:

- Ignoring type-checking errors in unannotated parts of the library. Use the -AskipDefs, -AonlyDefs, -AskipFiles, or -AonlyFiles command-line arguments; see Section ‍33.5.
- Putting conservative annotations in unannotated parts of the library. The checker needs to use normal defaulting rules (Section ‍32.5.3) for code you have annotated and conservative defaulting rules (Section ‍32.5.6) for code you have not yet annotated. This section describes how to do this. You use @AnnotatedFor to indicate which classes you have annotated.

### 35.4.1 The -AuseConservativeDefaultsForUncheckedCode=source,bytecode command-line argument

When compiling a library that is not fully annotated, use command-line argument -AuseConservativeDefaultsForUncheckedCode=source,bytecode. This has the following effect. For classes with @AnnotatedFor for the current checker, the checker applies defaults normally. For classes without @AnnotatedFor for the current checker, the checker uses conservative defaults (see Section ‍32.5.6) for any type use with no explicit user-written annotation, and the checker issues no warnings.

For example, an unannotated method

      String concatenate(String p1, String p2)

in a class with a relevant @AnnotatedFor is defaulted normally.

In a class without a relevant @AnnotatedFor, it is interpreted as

      @MyTopQualifier String concatenate(@MyBottomQualifier String p1, @MyBottomQualifier String p2)

where @MyTopQualifier is the top qualifier in the hierarchy and @MyBottomQualifier is the bottom qualifier in the hierarchy. These choices are conservative. They are likely to cause many false-positive type-checking errors in client code, which will help you to know which library methods need annotations. You can then write those library annotations.

The @AnnotatedFor annotation, written on a class, indicates that the class has been annotated for certain type systems. You may write @AnnotatedFor on a class that contains no explicit annotations from the given type system, if the defaulting rules already give appropriate qualifiers for every type in the class.

For example, @AnnotatedFor({"nullness", "regex"}) means that the programmer has written annotations for the Nullness and Regular Expression type systems. For any type system other than Nullness or Regex, that @AnnotatedFor annotation has no effect — the checker treats the class as unannotated for that type system. The argument to @AnnotatedFor is not an annotation name, but a checker name.

Whenever you compile a class using the Checker Framework, including when using the -AuseConservativeDefaultsForUncheckedCode=source,bytecode command-line argument, the resulting .class files are fully-annotated; each type use in the .class file has an explicit type qualifier for any checker that is run.

## 35.5 Using stub classes

You use a “stub file” to write annotations for a library, when you cannot edit and recompile the library. A checker uses the annotated signatures at compile time, instead of or in addition to annotations that appear in the library’s .class files.

A stub file cannot override the types of a package-private class, method, or field.

A stub class is Java source code that is allowed to omit certain parts, such as method bodies. Section ‍35.5.4 describes the stub file format.

Section ‍35.3 explains how you should choose between creating stub classes or creating an annotated library. Section ‍35.5.5 describes how to create stub classes. Section ‍35.5.1 describes how to use stub classes. These sections illustrate stub classes via the example of creating a @Interned-annotated version of java.lang.String. You don’t need to repeat these steps to handle java.lang.String for the Interning Checker, but you might do something similar for a different class and/or checker.

There are two types of annotation files, which are files containing annotations that can be read by the Checker Framework. This section describes stub files, which are used by programmers and type system designers. Section ‍35.6 describes ajava files, which are used by tools, such as type inference.

### 35.5.1 Using a stub file

The -Astubs argument causes the Checker Framework to read annotations from annotated stub classes in preference to the unannotated original library classes. For example:

      javac -processor org.checkerframework.checker.interning.InterningChecker \
        -Astubs=path/to/String.astub:stubs MyFile.java MyOtherFile.java ...

Each stub path entry is a stub file (ending with .astub), directory, or .jar file; specifying a directory or .jar file is equivalent to specifying every file in it whose name ends with .astub. The stub path entries are delimited by File.pathSeparator (“:” for Linux and Mac, “;” for Windows).

A checker automatically reads some of its own stub files, even without a -Astubs command-line argument; see Section ‍36.10.

User-supplied stub files override a checker’s built-in stub files and the annotated JDK.

### 35.5.2 Multiple specifications for a method

There are three possible sources of information for a given element: source files, stub files, and bytecode files. Usually source files take precedence over the other two, and stub files take precedence over bytecode. In other words:

- If file A.java is being compiled, then by default any stub for class A is ignored. See below for how to change this behavior and respect both types of annotations.
- An un-annotated type variable in a stub file is used instead of annotations on a type variable in bytecode.
  Use the -AstubWarnIfRedundantWithBytecode command-line option to get a warning whenever a stub file specification is redundant with bytecode annotations.
- If a stub file does not mention a method or field (and no source is available), its annotations are taken from bytecode.

If a method appears in more than one stub file (or twice in the same stub file), then, for each type hierarchy, the last annotation read is used.

The annotated JDK is read as a stub file. You can override JDK annotations by providing your own stub file. If your stub file contains a JDK method m, then no type annotations from the JDK’s m are used. If the JDK’s m contains a declaration annotation @D, it is used unless your stub file contains a different @D annotation for m.

The command-line option -AmergeStubsWithSource tells the checker to use both source files and stub files. The checker permits only values that are permitted by both the source and stub annotations. (This is called the greatest lower bound, or GLB, of the types.)

As an example of GLB, suppose the source annotation says the value is in the range [1..20] and the stub file says the value is in the range [11..30]. Since both sources of information are trusted, the value must be in the range [11..20]. Equivalently, the GLB of @IntRange(from=1, to=20) and @IntRange(from=11, to=30) is @IntRange(from=11, to=20). As another example, the GLB of @KeyFor({"map1", "map2"}) and @KeyFor({"map2", "map3"}) is @KeyFor({"map1", "map2", "map3"}) since the value is known to be a key for all three maps.

### 35.5.3 Stub methods in subclasses of the declaring class

Sometimes, a method’s return type is different in a subclass than in a superclass, even though the subclass inherits (does not define) the method. An example, SecureRandom.nextInt(), is shown below.

To express that method m has a different type in subclass Sub than where m is defined, write a “fake override”: write the method in class Sub in a stub file. The Checker Framework will use that type for m at calls where the receiver’s declared type is Sub or lower.

In a fake override of m, the formal parameter Java types of the method must be identical to those where m is defined, but you can change its annotations. (The return type and the receiver type can be different.) If a fake override has no annotations on a given type use, that type use is defaulted according to the usual defaulting rules.

This feature currently only works for return types. That is, a fake override changes the return type at certain calls, but formal parameter type annotations on fake overrides have no effect. Type annotations on fake overrides of fields also have no effect. Fake overrides will be able to change formal parameter and field type annotations in a future release of the Checker Framework.

##### Example

As an example, consider a type system that tracks whether a value is cryptographically secure. @Secure is a subtype of @Insecure. Although SecureRandom does not override Random.nextInt (it overrides the source of randomness instead), you are allowed to write the following stub file:

    package java.util;
    class Random {
      @Insecure int nextInt();
    }

    package java.security;
    class SecureRandom extends Random {
      @Secure int nextInt();
    }

Client code behaves as follows:

    Random r = ...;
    SecureRandom sr = ...;

    @Secure int i1 = r.nextInt(); // error
    @Secure int i2 = sr.nextInt(); // OK

### 35.5.4 Stub file format

Every Java file is a valid stub file. However, you can omit information that is not relevant to pluggable type-checking, as explained below; this makes the stub file smaller and easier for people to read and write. The stub file’s extension must be .astub, not .java.

As an illustration, a stub file for the Interning type system (Chapter ‍6) could be:

      import org.checkerframework.checker.interning.qual.Interned;
      package java.lang;
      @Interned class Class<T> {}
      class String {
        @Interned String intern();
      }

The stub file format is allowed to differ from Java source code in the following ways:

  Method bodies: The stub class does not require method bodies for classes; any method body may be replaced by a semicolon (;), as in an interface or abstract method declaration.
  Method declarations: You only have to specify the methods that you need to annotate. Any method declaration may be omitted, in which case the checker reads its annotations from library’s .class files. If you are using a stub class, then often the library is unannotated, but you can use stub files to override library annotations.
  Declaration specifiers: Declaration specifiers (e.g., public, final, volatile) may be omitted.
  Return types: The return type of a method does not need to match the real method. In particular, it is valid to use java.lang.Object for every method. This simplifies the creation of stub files.
  Multiple classes and packages: The stub file format permits having multiple classes and packages. The packages are separated by a package statement: package my.package;. Each package declaration may occur only once; in other words, all classes from a package must appear together.
  Import statements: Imports may appear at the beginning of the file or after any package declaration. The only required import statements are the ones to import type annotations. Import statements for types are optional. If you omit an import statement for an annotation, occurrences of that annotation are silently ignored; this is a common error in stub files.

When you create a stub file, it is recommended that you validate it by copying the stub file to a file with the .java extension and running the type-checker. This will ensure that your annotations are syntactically valid, are written in the correct locations, and are properly imported. When you run the type-checker on your annotations, there should not be any stub file that also contains annotations for the class. In particular, if you are type-checking a stub file for a class in the JDK, then you should use the -Aignorejdkastub command-line option.

### 35.5.5 Creating a stub file

#### If you have access to the Java source code

Every Java file is a stub file. If you have access to the Java file, copy file A.java to A.astub. You can add annotations to the signatures, leaving the method bodies unchanged. The stub file parser silently ignores any annotations that it cannot resolve to a type, so don’t forget the import statement.

This copy-and-annotate approach retains the original documentation and source code, making it easier for a programmer to double-check the annotations. It also enables creation of diffs, easing the process of upgrading when a library adds new methods. And, the annotations are in a format that the library maintainers can even incorporate.

The downside of the copy-and-annotate approach is that the stub files are larger. This can slow down the Checker Framework, because it parses the stub files each time it runs.

You can minimize source files to speed up parsing them. Use the JavaStubifier to convert, in-place, all .java files in given directories into minimal stub files.

      mkdir project-stubs
      cp -R project/src project-stubs
      java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar org.checkerframework.framework.stub.JavaStubifier project-stubs
      find project-stubs -type f -name "*.java" -exec rename 's/.java$/.astub/' {} \;

Supply it a list of directories to process. Replacement happens in-place, so make sure to process a copy of your sources.

You can now provide project-stubs as a stub directory using -Astubs=project-stubs as an additional command-line option.

If you wish to use a single stub file, rather than a stub directory, you can concatenate all the minimized .java files into a single .astub file:

    find project-stubs/ -name "*.java" -type f | xargs cat > project.astub

#### If you do not have access to the Java source code

If you do not have access to the library source code, you can create a stub file by hand.

If you have access to the API documentation for the element you wish to annotate, it is often easiest to copy–paste names, types, and signatures from the API documentation. (The API documentation is a .html file that is sometimes colloquially called the “Javadoc”.) This avoids common mistakes when writing stub files by hand, such as typos in the package name or in a method signature.

Here are steps to write a stub file by hand:

1. Create a file whose name ends in .astub, or use an existing one.
2. Write a package declaration, exactly as if you are writing a Java file for the target class. (If the file already contains a declaration for that package, re-use it by doing the rest of your work under it; don’t write a duplicate package declaration in the file.) Typically, it is easiest to copy–paste the package name from API documentation or from an import statement in the code that you are type-checking, to avoid typos.
3. Write a class declaration header: class followed by the class name. The stub file format does not care whether the enclosing type is a class, interface, etc., so you can write “class” regardless of what kind of type it is.

   A common mistake at this stage is that the target method or field may be defined in a superclass of the static type of the class that is actually being used in the code you are trying to analyze. For example, suppose that you are trying to annotate the drawImage(Image,int,int,int,int,ImageObserver) method of an object whose static type is java.awt.Graphics2D. This method is actually defined in the abstract Graphics class (which is the superclass of Graphics2D), so it needs to appear within the class Graphics rather than the class Graphics2D in the stub file.

4. Write the element declaration that you wish to annotate, within the class you just created.
   For a method, this is the method signature, followed by “;”.
   For a field, this is the type, field name, and “;”, without an initializer.
5. Annotate the declaration that you just wrote.
6. Add import statements for any annotations you wrote.

### 35.5.6 Distributing stub files

If you are writing stub files but are not writing a checker, you can place the stub files anywhere that is convenient. However, please consider providing your stub files to the checker author, so that other users can also benefit from the stub files you wrote.

Stub files distributed with a checker appear in the same directory as the checker implementation (the *Checker.java file, see Section ‍36.2). For example, in the Checker Framework they appear in files such as

    checker/src/main/java/org/checkerframework/checker/regex/apache-xerces.astub
    checker/src/main/java/org/checkerframework/checker/signature/javaparser.astub

If you are distributing stub files with a checker implementation, you must configure your build system to copy the stub files into the distributed jar for your checker. Here are instructions if you are using the Checker Framework Gradle plugin:

- If the stub files appear in the same directory as the checker class, which is a subtype of BaseTypeChecker, use the following build configuration:

        sourceSets {
          main {
              resources {
                  srcDirs += ['src/main/java']
              }
          }
        }

- If the stub files appear under the src/main/resources/ directory in a sub-directory matching the package of your checker class, the default Gradle build configuration is sufficient.

In either case, recall that a @StubFiles annotation on the checker class lists stub files that are always used. For stub files whose use is optional (for example, because the behavior is unsound, or unsound except in certain circumstances), users must supply the -Astubs=... command-line option.

If a stub file contains annotations that are used by the framework rather than by any specific checker (such as purity annotations), and you wish to distribute it with the Checker Framework, put the stub file in directory checker/src/main/resources/. You can also do this if the stub file has annotations for multiple checkers. To use a stub file in directory checker/src/main/resources/, users must supply the -Astubs=checker.jar/stub-file-name.astub command-line option.

### 35.5.7 Troubleshooting stub libraries

#### Type-checking does not yield the expected results

By default, the stub parser silently ignores annotations on unknown classes and methods. The stub parser also silently ignores unknown annotations, so don’t forget to import any annotations. Some command-line options make the stub parser issue more warnings:

-AstubWarnIfNotFound
-AstubNoWarnIfNotFound
  The Checker Framework can issue a “Type not found” warning when a stub file contains a nonexistent element, for example when the stub file mentions a method that the class does not contain. By default, the Checker Framework warns about such problems in a stub file provided on the command line, but does not warn about built-in stub files. These command-line options turn the warnings on or off (respectively) for all stub files.
  The @NoAnnotationFileParserWarning annotation on a package or type in a stub file causes no warning to be issued for that package or type, regardless of the command-line options.
-AstubWarnIfNotFoundIgnoresClasses
  Modifies the behavior of -AstubWarnIfNotFound to report only missing methods/fields, but ignore missing classes, even if other classes from the same package are present. Useful if a package spans more than one jar.
-AstubWarnIfRedundantWithBytecode
  Warn if a stub file entry is redundant with bytecode information. The warning means that the stub file’s type is the same as the bytecode type, so that entry in the stub file has no effect. You could remove the entry in the stub file to make it shorter, or you could add an annotation to the stub file to make the entry have an effect.
-AstubWarnNote
  Issue a “note”, not a warning, for the -AstubWarn* command-line arguments. A warning may prevent further compilation (depending on whether the -Werror command-line argument is passed), but a note permits compilation to proceed.
-AstubDebug
  Output debugging messages while parsing stub files, including about unknown classes, methods, and annotations. This overrides the @NoAnnotationFileParserWarning annotation.

#### Problems parsing stub libraries

When using command-line option -AstubWarnIfNotFound, an error is issued if a stub file has a typo or the API method does not exist.

Fix an error of the form

    AnnotationFileParser: Method isLLowerCase(char) not found in type java.lang.Character

by removing the extra “L” in the method name.

Fix an error of the form

    AnnotationFileParser: Method enableForegroundNdefPush(Activity,NdefPushCallback)
          not found in type android.nfc.NfcAdapter

by removing the method enableForgroundNdefPush(...) from the stub file, because it is not defined in class android.nfc.NfcAdapter in the version of the library you are using.

## 35.6 Ajava files

There are two types of annotation files, which are files containing annotations that can be read by the Checker Framework. Section ‍35.5 describes stub files, which are used by programmers and type system designers. This section describes ajava files. Ajava files are primarily read and written by tools, such as whole-program type inference (see Section ‍34.2). (However, programmers are permitted to write .ajava files.) This section about ajava files is primarily of interest to the maintainers of such tools.

An ajava file is simply a valid Java source file. The Checker Framework reads its annotations. (This includes annotations that are written on anonymous and local classes — annotations in such locations are ignored in stub files.)

### 35.6.1 Using an Ajava file

The -Aajava command-line argument works like -Astubs (see Section ‍35.5.1). It takes a colon-separated list of files and directories containing ajava files, which end in the .ajava extension.

For example:

      javac -processor org.checkerframework.checker.interning.InterningChecker \
        -Aajava=path/to/String.ajava:ajavadir MyFile.java MyOtherFile.java ...

If there’s an annotation in both a source file and the corresponding a source file, the Checker Framework uses the greatest lower bound, as with the -AmergeStubsWithSource option.

### 35.6.2 Corresponding source files and ajava files

If the Checker Framework reads both a source file and an ajava file that corresponds to it, then the source file and the ajava file must have a specific relationship to one another, to prevent errors that could occur due to mismatches.

#### Names of ajava files

The ajava file must either (1) be found below one of the arguments to -Aajava, in a subdirectory matching its package name, or (2) be unambiguously specified directly with its full path. A pair of .ajava files are specified ambiguously if one of their fully-qualified names is a substring of the other, and both are specified as .ajava files (rather than being found under directories). For example, if the -Aajava argument was -Aajava= ‍/foo/Bar.ajava: ‍/baz/foo/Bar.ajava, the two .ajava files would be considered ambiguous and a ambiguous.ajava warning would be issued. Files found under a directory are always unambiguous, because their file structure must match their package name; the solution to a ambiguous.ajava warning is to convert the specific files in the -Aajava argument to directories.

The ajava file must be named ClassName-checker.qualified.name.ajava where checker.qualified.name is the fully qualified name of the checker that the ajava file’s annotations belong to.

For example, an ajava file with tainting annotations for a class outerpackage.innerpackage.MyClass would be located in a subdirectory outerpackage/innerpackage and it would be named MyClass-org.checkerframework.checker.tainting.TaintingChecker.ajava.

#### Contents of an ajava file

The ajava file must contain the same source code as the source file with the following exceptions:

- The two may differ in annotations.
- The two may have different import statements.
- The ajava file may have explicit receivers where the source file doesn’t. If a source file has a method declaration void myMethod(), the ajava file might contain void myMethod(MyClass this) or void myMethod(@Interned MyClass this).
- The ajava file may have explicit type bounds that are implicit in the source file. If a source file has a type argument ?, the ajava file might contain ? extends Object or ? extends @Nullable Object.

#### Inserting annotations from ajava files

The InsertAjavaAnnotations.java program inserts annotations from ajava files into Java files. This can be used to insert the annotations inferred by whole program inference into the source code of a program, similarly to the insert-annotations-to-source script from the Annotation File Utilities project.

Its arguments are a directory containing ajava files and a directory containing Java files. The checker.jar file must be in the classpath. Here is an example invocation:

      java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar \
        org.checkerframework.framework.ajava.InsertAjavaAnnoations \
        <path-to-ajava-files> <path-to-java-files>

## 35.7 Troubleshooting/debugging annotated libraries

Sometimes, it may seem that a checker is treating a library as unannotated even though the library has annotations. The compiler has a flag that may help you in determining which library files are read.

  -verbose Outputs info about compile phases — when the compiler reads/parses/attributes/writes any file. Also outputs the classpath and sourcepath paths.

A syntax error in a stub file or the annotated JDK can lead to the file being silently ignored. A typo in an annotation name, or a missing import statement, can lead to annotations being silently ignored.

# Chapter ‍36 How to create a new checker

This chapter describes how to create a checker — a type-checking compiler plugin that detects bugs or verifies their absence. After a programmer annotates a program, the checker verifies that the code is consistent with the annotations. If you only want to use a checker, you do not need to read this chapter. People who wish to edit the Checker Framework source code or make pull requests should read the Checker Framework Developer Manual.

Writing a simple checker is easy! For example, here is a complete, useful type-checker:

    import java.lang.annotation.Documented;
    import java.lang.annotation.Target;
    import java.lang.annotation.ElementType;
    import org.checkerframework.common.subtyping.qual.Unqualified;
    import org.checkerframework.framework.qual.SubtypeOf;

    @Documented
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf(Unqualified.class)
    public @interface Encrypted {}

This checker is so short because it builds on the Subtyping Checker (Chapter ‍29). See Section ‍29.2 for more details about this particular checker. When you wish to create a new checker, it is often easiest to begin by building it declaratively on top of the Subtyping Checker, and then return to this chapter when you need more expressiveness or power than the Subtyping Checker affords.

Three choices for creating your own checker are:

- Customize an existing checker. Checkers that are designed for extension include the Subtyping Checker (Chapter ‍29), the Accumulation Checker (Chapter ‍37), the Fake Enumeration Checker (Chapter ‍9), and the Units Checker (Chapter ‍20).
- Follow the instructions in this chapter to create a checker from scratch. This enables creation of checkers that are more powerful than customizing an existing checker.
- Copy and then modify a different existing checker — whether one distributed with the Checker Framework or a third-party one. A problem with this strategy is that you can get tangled up if you don’t fully understand the subtleties of the existing checker that you are modifying. Usually, it is easier to follow the instructions in this chapter. (If you are going to copy a checker despite the dangers, one good choice to copy and modify is the Regex Checker (Chapter ‍14). A bad choice is the Nullness Checker (Chapter ‍3), which is more sophisticated than anything you want to start out building.)

You do not need all of the details in this chapter, at least at first. In addition to reading this chapter of the manual, you may find it helpful to examine the implementations of the checkers that are distributed with the Checker Framework. The Javadoc documentation of the framework and the checkers is in the distribution and is also available online at https://checkerframework.org/api/.

If you write a new checker and wish to advertise it to the world, let us know so we can mention it in Chapter ‍30 or even include it in the Checker Framework distribution.

## 36.1 How checkers build on the Checker Framework

This table shows the relationship among tools that the Checker Framework builds on or that are built on the Checker Framework. You use the Checker Framework to build pluggable type systems, and the Annotation File Utilities to manipulate .java and .class files.

Subtyping Checker | Nullness Checker | Index Checker | Tainting Checker | … | Your Checker |
Base Checker (enforces subtyping rules) | Type inference | Other tools
Checker Framework (enables creation of pluggable type-checkers) | Annotation File Utilities (.java ↔ .class files)
Java type annotations syntax and classfile format (no built-in semantics)

The Base Checker (more precisely, the BaseTypeChecker) enforces the standard subtyping rules. The Subtyping Checker is a simple use of the Base Checker that supports providing type qualifiers on the command line. You usually want to build your checker on the Base Checker.

## 36.2 The parts of a checker

The Checker Framework provides abstract base classes (default implementations), and a specific checker overrides as little or as much of the default implementations as necessary. To simplify checker implementations, by default the Checker Framework automatically discovers the parts of a checker by looking for specific files. Thus, checker implementations follow a very formulaic structure. To illustrate, a checker for MyProp must be laid out as follows:

    myPackage/
      | qual/                               type qualifiers
      | MyPropChecker.java                  interface to the compiler
      | MyPropVisitor.java                  [optional] type rules
      | MyPropAnnotatedTypeFactory.java     [optional] type introduction and dataflow rules

MyPropChecker.java is occasionally optional, such as if you are building on the Subtyping Checker. If you want to create an artifact containing just the qualifiers (similar to the Checker Framework’s checker-qual artifact), you should put the qual/ directory in a separate Maven module or Gradle subproject.

Sections ‍36.5–36.9 describe the individual components of a type system as written using the Checker Framework:

  36.5 Type qualifiers and hierarchy. You define the annotations for the type system and the subtyping relationships among qualified types (for instance, @NonNull Object is a subtype of @Nullable Object). This is also where you specify the default annotation that applies whenever the programmer wrote no annotation and no other defaulting rule applies.
  36.6 Interface to the compiler. The compiler interface indicates which annotations are part of the type system, which command-line options and @SuppressWarnings annotations the checker recognizes, etc.
  36.7 Type rules. You specify the type system semantics (type rules), violation of which yields a type error. A type system has two types of rules.
  - Subtyping rules related to the type hierarchy, such as that in every assignment, the type of the right-hand-side is a subtype of the type of the left-hand-side. Your checker automatically inherits these subtyping rules from the Base Checker (Chapter ‍29), so there is nothing for you to do.
  - Additional rules that are specific to your particular checker. For example, in the Nullness type system, only references whose type is @NonNull may be dereferenced. You write these additional rules yourself.
  36.8 Type introduction rules. You specify the type of some expressions where the rules differ from the built-in framework rules.
  36.9 Dataflow rules. These optional rules enhance flow-sensitive type qualifier inference (also sometimes called “local variable type inference”).

## 36.3 Compiling and using a custom checker

You can place your checker’s source files wherever you like. One choice is to write your checker in a fork of the Checker Framework repository https://github.com/typetools/checker-framework. Another choice is to write it in a stand-alone repository. Here is a template for a stand-alone repository: https://github.com/typetools/templatefora-checker; at that URL, click the “Use this template” button.

Once your custom checker is written, using it is very similar to using a built-in checker (Section ‍2.2): simply pass the fully-qualified name of your BaseTypeChecker subclass to the -processor command-line option:

      javac -processor mypackage.MyPropChecker SourceFile.java

Note that your custom checker’s .class files must be on the same path (the classpath or processorpath) as the Checker Framework. Invoking a custom checker that builds on the Subtyping Checker is slightly different (Section ‍29.1).

## 36.4 Tips for creating a checker

To make your job easier, we recommend that you build your type-checker incrementally, testing at each phase rather than trying to build the whole thing at once.

Here is a good way to proceed.

1. Write the user manual. Do this before you start coding. The manual explains the type system, what it guarantees, how to use it, etc., from the point of view of a user. Writing the manual will help you flesh out your goals and the concepts, which are easier to understand and change in text than in an implementation. Section ‍36.13 gives a suggested structure for the manual chapter, which will help you avoid omitting any parts. If your checker is in a clone of the Checker Framework, don’t forget to add a LATEX \include directive to manual.tex. Get feedback from someone else at this point to ensure that your manual is comprehensible.

   Once you have designed and documented the parts of your type system, you should “play computer”, manually type-checking some code according to the rules you defined. During manual checking, ask yourself what reasoning you applied, what information you needed, and whether your written-down rules were sufficient. It is more efficient to find problems now rather than after coding up your design.

2. Implement the type qualifiers and hierarchy (Section ‍36.5). Compile your changes.

   Write simple test cases that consist of only assignments, to test your type hierarchy. For instance, if your type hierarchy consists of a supertype @UnknownSign and a subtype @NonNegative, then you could write a test case such as:

       import org.checkerframework.checker.mychecker.qual.UnknownSign;
       import org.checkerframework.checker.mychecker.qual.NonNegative;

       class TestHierarchy {
         void testHierarchy(@UnknownSign int us, @NonNegative int nn) {
           @UnknownSign int a = us;
           @UnknownSign int b = nn;
           // :: error: [assignment]
           @NonNegative int c = us; // expected error on this line
           @NonNegative int d = nn;
         }
       }

   If you are working in a clone of the Checker Framework, put the above test case in file checker/tests/mycheckername/TestHierarchy.java.

   Type-check your test files using the Subtyping Checker (Chapter ‍29), using a command like

       javacheck \
         -processor org.checkerframework.common.subtyping.SubtypingChecker \
         -Aquals=org.checkerframework.checker.mycheckername.qual.MyTopQual,org.checkerframework.checker.mycheckername.qual.MyBottomQual \
         TestHierarchy.java

3. Write the checker class itself (Section ‍36.6).

   Ensure that you can still type-check your test files and that the results are the same. You will not use the Subtyping Checker any more; you will call the checker directly, as in

         javac -processor mypackage.MyChecker File1.java File2.java ...

   If the default qualifier is not the top qualifier, you may see warning: (inconsistent.constructor.type). If the checker should never issue such an error (because a newly-constructed object can be correctly viewed as any type at all), then copy TaintingVisitor.java into your checker, changing “Tainting”.

4. Create test infrastructure. If your checker source code is in a clone of the Checker Framework repository, integrate your checker with the Checker Framework’s Gradle targets for testing (Section ‍36.11). This will make it much more convenient to run tests, and to ensure that they are passing, as your work proceeds.

   If there are errors in the “all-systems” tests, don’t worry about them until after you have annotated the JDK. However, specifying @RelevantJavaTypes (Section ‍36.5.5) may eliminate some of those errors, especially if the default qualifier is not the top qualifier.

5. Annotate parts of the JDK, if relevant (Section ‍36.10). You will clone the Checker Framework JDK, naming it jdk in a sibling directory of checker-framework (that is, they have the same parent). Most changes will consist of adding an import statement, plus uses of qualifiers, in Java files under src/java.base/share/classes/. Also follow the instructions in section “Qualifier definitions” of file README.md.

   Write test cases for a few of the annotated JDK methods to ensure that the annotations are being properly read by your checker.

   If there are errors in the Checker Framework’s “all-systems” tests, you can suppress them if they are expected. See framework/tests/all-systems/README for more information.

6. Implement type rules, if any (Section ‍36.7). Some type systems need JDK annotations but don’t have any additional type rules.

   Before implementing type rules (or any other code in your type-checker), read the Javadoc to familiarize yourself with the utility routines in the org.checkerframework.javacutil package, especially AnnotationBuilder, AnnotationUtils, ElementUtils, TreeUtils, TypeAnnotationUtils, and TypesUtils. You will learn how to access needed information and avoid reimplementing existing functionality.

   Write simple test cases to test the type rules, and ensure that the type-checker behaves as expected on those test files. For example, if your type system forbids indexing an array by a possibly-negative value, then you would write a test case such as:

         void testArrayIndexing(String[] myArray, @UnknownSign int us, @NonNegative int nn) {
           myArray[us];  // expected error on this line
           myArray[nn];
         }

7. Implement type introduction rules, if any (Section ‍36.8). Some type systems need JDK annotations but don’t have any type introduction rules.

   Test your type introduction rules. For example, if your type system sets the qualifier for manifest literal integers and for array lengths, you would write a test case like the following:

         void testTypeIntroduction(String[] myArray) {
           @NonNegative nn1 = -1;  // expected error on this line
           @NonNegative nn2 = 0;
           @NonNegative nn3 = 1;
           @NonNegative nn4 = myArray.length;
         }

8. Optionally, implement dataflow refinement rules (Section ‍36.9). Dataflow refinement rules are only possible for a type system that supports a run-time test (Section ‍32.7.4).

   Test them if you wrote any. For instance, if after an arithmetic comparison, your type system infers which expressions are now known to be non-negative, you could write a test case such as:

         void testDataflow(@UnknownSign int us, @NonNegative int nn) {
           @NonNegative nn2;
           nn2 = us;  // expected error on this line
           if (us > j) {
             nn2 = us;
           }
           if (us >= j) {
             nn2 = us;
           }
           if (j < us) {
             nn2 = us;
           }
           if (j <= us) {
             nn2 = us;
           }
           nn = us;  // expected error on this line
         }

## 36.5 Annotations: Type qualifiers and hierarchy

A type system designer specifies the qualifiers in the type system (Section ‍36.5.1) and the type hierarchy that relates them. The type hierarchy — the subtyping relationships among the qualifiers — can be defined either declaratively via meta-annotations (Section ‍36.5.2), or procedurally through subclassing QualifierHierarchy or TypeHierarchy (Section ‍36.5.3).

### 36.5.1 Defining the type qualifiers

Type qualifiers are defined as Java annotations. In Java, an annotation is defined using the Java @interface keyword. Here is how to define a two-qualifier hierarchy:

    package mypackage.qual;
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
    import org.checkerframework.framework.qual.SubtypeOf;
    /**
     * The run-time value of the integer is unknown.
     *
     * @checker_framework.manual #nonnegative-checker Non-Negative Checker
     */
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf({})
    @DefaultQualifierInHierarchy
    public @interface UnknownSign {}

    package mypackage.qual;
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    import org.checkerframework.framework.qual.LiteralKind;
    import org.checkerframework.framework.qual.SubtypeOf;
    /**
     * Indicates that the value is greater than or equal to zero.
     *
     * @checker_framework.manual #nonnegative-checker Non-Negative Checker
     */
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf({UnknownSign.class})
    public @interface NonNegative {}

The @SubtypeOf meta-annotation indicates the parent in the type hierarchy.

The @Target meta-annotation indicates where the annotation may be written. All type qualifiers that users can write in source code should have the value ElementType.TYPE_USE and optionally with the additional value of ElementType.TYPE_PARAMETER, but no other ElementType values.

The qualifiers should be in a qual subpackage of the type checker package — by default, the checker reflectively determines the qualifiers and the only thing that matters for that is the package name. As mentioned in creating-parts-of-a-checker, the files for that qual package can be either in a subdirectory or in a separate Gradle module, depending on whether they should be distributable separately. The Checker Framework automatically treats any annotation that is declared in the qual package as a type qualifier. (See Section 36.6.1 for more details.) For example, the Nullness Checker’s source file is located at .../nullness/NullnessChecker.java. The @NonNull qualifier is defined in file .../nullness/qual/NonNull.java.

Your type system should include a top qualifier and a bottom qualifier (Section ‍36.5.7). The top qualifier is conventionally named CheckerName Unknown. Most type systems should also include a polymorphic qualifier @Poly MyTypeSystem (Section ‍36.5.2).

One of the annotations must be meta-annotated with @DefaultQualifierInHierarchy or @DefaultFor(TypeUseLocation.OTHERWISE). It is usually easiest to make the top type be the default; this may or may not be the correct default for the type system design.

Choose good names for the qualifiers, because users will write these in their source code. The Javadoc of every type qualifier should include a precise English description and an example use of the qualifier.

In source code, declaration qualifiers are written on their own line, and type qualifiers are written on the same line as the type they apply to. Section ‍39.6.10 explains how to configure tools to format type annotations properly.

### 36.5.2 Declaratively defining the qualifier hierarchy

Declaratively, the type system designer uses two meta-annotations (written on the declaration of qualifier annotations) to specify the qualifier hierarchy.

- @SubtypeOf denotes that a qualifier is a subtype of another qualifier or qualifiers, specified as an array of class literals. For example, for any type T, @NonNull T is a subtype of @Nullable T:

          @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
          @SubtypeOf( { Nullable.class } )
          public @interface NonNull {}

  @SubtypeOf accepts multiple annotation classes as an argument, permitting the type hierarchy to be an arbitrary DAG.

  All type qualifiers, except for polymorphic qualifiers (see below and also Section ‍31.2), need to be properly annotated with SubtypeOf.

  The top qualifier is annotated with @SubtypeOf( { } ). The top qualifier is the qualifier that is a supertype of all other qualifiers. For example, @Nullable is the top qualifier of the Nullness type system, hence is defined as:

          @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
          @SubtypeOf( {} )
          public @interface Nullable {}

  If the top qualifier of the hierarchy is the generic unqualified type (this is not recommended!), then its children will use @SubtypeOf(Unqualified.class), but no @SubtypeOf({}) annotation on the top qualifier Unqualified is necessary. For an example, see the Encrypted type system of Section ‍29.2.

- @PolymorphicQualifier denotes that a qualifier is a polymorphic qualifier. For example:

        import java.lang.annotation.ElementType;
        import java.lang.annotation.Target;
        import org.checkerframework.framework.qual.PolymorphicQualifier;

        @PolymorphicQualifier(Nullable.class)
        @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
        public @interface PolyNull {}

  A polymorphic qualifier must not have a @SubtypeOf meta-annotation nor be mentioned in any other @SubtypeOf meta-annotation.

  For how to restrict instantiation, see Section ‍36.5.2. For a general description of polymorphic qualifiers, see Section ‍31.2.

The declarative and procedural mechanisms for specifying the hierarchy can be used together. If any of the annotations representing type qualifiers have elements, then the relationships between those qualifiers must be defined procedurally.

#### Restricting instantiation

Sometimes, you wish to restrict how a polymorphic qualifier can be instantiated. An example is the Signedness Checker (Chapter ‍21). Its two main qualifiers are @Signed and @Unsigned, and the top qualifier (which should be rarely used) is @UnknownSignedness.

As described so far, there is no way for a programmer to require that two method arguments have the same signedness type. Writing

      void foo(@PolySigned Object a, @PolySigned Object b) { ... }

permits every invocation, because one of its instantiations is

      void foo(@UnknownSignedness Object a, @UnknownSignedness Object b) { ... }

which is consistent with any two arguments, regardless of their signedness type.

It is possible to permit only method calls whose arguments have exactly the same type qualifier. This is enabled for an entire type system. To do so, create a subclass of DefaultQualifierPolymorphism, override its combine method, and make method createQualifierPolymorphism in the type factory return an instance of the new subclass. The Signedness Checker gives an example.

Changing the qualifier polymorphism allows a loophole: upcasts to top. For example, the type system permits a call like

      Objects.equals((@UnknownSignedness) mySigned, (@UnknownSignedness) myUnsigned)

A type system can forbid upcasts to top (including in assignments, not just explicit casts) to close this loophole.

If you change the qualifier polymorphism, your type system may need class qualifier polymorphism (see Section ‍31.3). And, you may need to suppress false positive warnings within the body of methods whose formal parameter types are annotated with the polymorphic qualifier (e.g., @PolySigned).

### 36.5.3 Procedurally defining the qualifier hierarchy

The declarative syntax suffices for most cases. More complex type hierarchies can be expressed by overriding, in your subclass of BaseAnnotatedTypeFactory, either createQualifierHierarchy or createTypeHierarchy (typically only one of these needs to be overridden).

For more details, see the Javadoc of those methods and of the classes QualifierHierarchy and TypeHierarchy.

The QualifierHierarchy class represents the qualifier hierarchy (not the type hierarchy). A type-system designer may subclass QualifierHierarchy to express customized qualifier relationships (e.g., relationships based on annotation arguments).

The TypeHierarchy class represents the type hierarchy — that is, relationships between annotated types, rather than merely type qualifiers, e.g., @NonNull Date is a subtype of @Nullable Date. The default TypeHierarchy uses QualifierHierarchy to determine all subtyping relationships. The default TypeHierarchy handles generic type arguments, array components, type variables, and wildcards in a similar manner to the Java standard subtype relationship but with taking qualifiers into consideration. Some type systems may need to override that behavior. For instance, the Java Language Specification specifies that two generic types are subtypes only if their type arguments are identical: for example, List<Date> is not a subtype of List<Object>, or of any other generic List. (In the technical jargon, the generic arguments are “invariant” or “novariant”.)

### 36.5.4 Defining the default annotation

A type system applies the default qualifier where the user has not written a qualifier (and no other default qualifier is applicable), as explained in Section ‍32.5.

The type system designer must specify the default annotation. The designer can specify the default annotation declaratively, using the @DefaultQualifierInHierarchy meta-annotation. Note that the default will apply to any source code that the checker reads, including stub libraries, but will not apply to compiled .class files that the checker reads.

Alternately, the type system designer may specify a default procedurally, by overriding the GenericAnnotatedTypeFactory.addCheckedCodeDefaults method. You may do this even if you have declaratively defined the qualifier hierarchy.

If the default qualifier in the type hierarchy requires a value, there are ways for the type system designer to specify a default value both declaratively and procedurally, as well. To do so declaratively, append the string default value where value is the actual value you want to be the default, after the declaration of the value in the qualifier file. For instance, int value() default 0; would make value default to zero. Alternatively, the procedural method described above can be used.

The default qualifier applies to most, but not all, unannotated types. Section ‍32.5.3 describes other defaulting rules that are automatically added to every checker. Also, Section ‍32.5 describes other meta-annotations used to specify default annotations.

### 36.5.5 Relevant Java types

Sometimes, a checker is only relevant to certain Java types. The @RelevantJavaTypes annotation on the checker class indicates that its qualifiers may only be written on those types, their subtypes, and their supertypes.

As an example of relevant types, the Format String Checker is relevant only to CharSequence and its subtypes such as String. You cannot write @Format on a List or a Number. As another example, the Index Checker and the Signedness Checker are only relevant to integral numeric types. You cannot write @Positive on a List or a float.

If a user writes an annotation on an irrelevant type, the Checker Framework issues an anno.on.irrelevant error.

Irrelevant types are ordinarily defaulted to the top annotation. A checker designer may change the default, and it may differ from type to type. For example, the Signedness Checker treats all types that are not numeric integral types as @Signed, but treats the char and Character types as @Unsigned.

#### Relevant types and supertypes

If a type is relevant, so are all its supertypes including Object. This prevents qualifiers from being lost via an assignment to a supertype. For example, in this code:

    @SomeAnn MyClass x = ...;
    Object y = x;
    MyClass z = (MyClass) y;

variables x and z have the same type qualifiers, without them being lost via the assignment to Object.

### 36.5.6 Do not re-use type qualifiers

Every annotation should belong to only one type system. No annotation should be used by multiple type systems. This is true even of annotations that are internal to the type system and are not intended to be written by the programmer.

Suppose that you have two type systems that both use the same type qualifier @Bottom. In a client program, a use of type T may require type qualifier @Bottom for one type system but a different qualifier for the other type system. There is no annotation that a programmer can write to make the program type-check under both type systems.

This also applies to type qualifiers that a programmer does not write, because the compiler outputs .class files that contain an explicit type qualifier on every type — a defaulted or inferred type qualifier if the programmer didn’t write a type qualifier explicitly.

### 36.5.7 Completeness of the qualifier hierarchy

When you define a type system, its qualifier hierarchy must be a lattice: every set of qualifiers has a unique least upper bound and a unique greatest lower bound. This implies that there must be a top qualifier that is a supertype of all other qualifiers, and there must be a bottom qualifier that is a subtype of all other qualifiers. Furthermore, the top qualifier and bottom qualifier should be defined specifically for the type system. Don’t reuse an existing qualifier from the Checker Framework such as @Unqualified.

It is possible that a single type-checker checks multiple qualifier hierarchies. An example is the Nullness Checker, which has three separate qualifier hierarchies, one each for nullness, initialization, and map keys. In this case, each qualifier hierarchy would have its own top qualifier and its own bottom qualifier; they don’t all have to share a single top qualifier or a single bottom qualifier.

##### Bottom qualifier

Your qualifier hierarchy must have a bottom qualifier — a qualifier that is a (direct or indirect) subtype of every other qualifier.

null has the bottom type. Because the only value with type Void is null, uses of the type Void are also bottom. (The only exception is if the type system has special treatment for null values, as the Nullness Checker does. In that case, add the meta-annotation @QualifierForLiterals(LiteralKind.NULL) to the correct qualifier.) This legal code will not type-check unless null has the bottom type:

    <T> T f() {
        return null;
    }

Some type systems have a special bottom type that is used only for the null value, and for dead code and other erroneous situations. In this case, users should only write the bottom qualifier on explicit bounds. In this case, the definition of the bottom qualifier should be meta-annotated with:

    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @TargetLocations({TypeUseLocation.EXPLICIT_LOWER_BOUND, TypeUseLocation.EXPLICIT_UPPER_BOUND})

Furthermore, by convention the name of such a qualifier ends with “Bottom”.

It might seem that the hierarchy shown in Figure ‍3.5 lacks a bottom qualifier. The actual implementation does contain a bottom qualifier, @FBCBottom, that users rarely write.

##### Top qualifier

Your qualifier hierarchy must have a single top qualifier — a qualifier that is a (direct or indirect) supertype of every other qualifier. There are many reasons this is essential. One reason is that the default type for local variables is the top qualifier, and there must be an unambiguous choice for the default type.

### 36.5.8 Annotations whose argument is a Java expression (dependent type annotations)

Sometimes, an annotation needs to refer to a Java expression. Section ‍32.8 gives examples of such annotations and also explains what Java expressions can and cannot be referred to.

This section explains how to implement a dependent type annotation.

A “dependent type annotation” must have one attribute, value, that is an array of strings. The Checker Framework verifies that the annotation’s arguments are valid expressions according to the rules of Section ‍32.8. If the expression is not valid, an error is issued and the string in the annotation is changed to indicate that the expression is not valid.

The Checker Framework standardizes the expression strings. For example, a field f can be referred to as either “f” or “this.f”. If the programmer writes “f”, the Checker Framework treats it as if the programmer had written “this.f”. An advantage of this canonicalization is that comparisons, such as isSubtype, can be implemented as string comparisons.

The Checker Framework viewpoint-adapts type annotations on method, constructor, and field declarations at uses for those methods. For example, given the following class

    class MyClass {
       Object field = ...;
       @Anno("this.field") Object field2 = ...;
    }

and assuming the variable myClass is of type MyClass, then the type of myClass.field is viewpoint-adapted to @Anno("myClass.field").

To use this built-in functionality, add a @JavaExpression annotation to any annotation element that should be interpreted as a Java expression. The type of the element must be an array of Strings. If your checker requires special handling of Java expressions, your checker implementation should override GenericAnnotatedTypeFactory.createDependentTypesHelper to return a subclass of DependentTypesHelper.

Given a specific expression in the program (of type Tree or Node), a checker may need to obtain its canonical string representation. This enables the checker to create an dependent type annotation that refers to it, or to compare to the string expression of an existing expression annotation. To obtain the string, first create a JavaExpression object by calling fromTree(ExpressionTree) or fromNode(Node). Then, call toString() on the JavaExpression object.

### 36.5.9 Repeatable annotations

Some pre- and post-condition annotations that have multiple elements (that is, annotations that take multiple arguments) should be repeatable, so that programmers can specify them more than once. An example is @EnsuresNonNullIf; it could not be defined with each of its elements being a list, as (for example) @KeyFor is.

Make an annotation A repeatable by defining a nested annotation (within A’s definition) named List, and writing @Repeatable(A.List.class) on A’s definition.

## 36.6 The checker class: Compiler interface

A checker’s entry point is a subclass of SourceChecker, and is usually a direct subclass of either BaseTypeChecker or AggregateChecker. This entry point, which we call the checker class, serves two roles: an interface to the compiler and a factory for constructing type-system classes.

Because the Checker Framework provides reasonable defaults, oftentimes the checker class has no work to do. Here are the complete definitions of the checker classes for the Interning Checker and the Nullness Checker:

      package my.package;
      import org.checkerframework.common.basetype.BaseTypeChecker;
      @SupportedLintOptions({"dotequals"})
      public final class InterningChecker extends BaseTypeChecker {}

      package my.package;
      import org.checkerframework.common.basetype.BaseTypeChecker;
      @SupportedLintOptions({"flow", "cast", "cast:redundant"})
      public class NullnessChecker extends BaseTypeChecker {}

(The @SupportedLintOptions annotation is optional, and many checker classes do not have one.)

The checker class bridges between the Java compiler and the checker. It invokes the type-rule check visitor on every Java source file being compiled. The checker uses SourceChecker.reportError and SourceChecker.reportWarning to issue errors.

Also, the checker class follows the factory method pattern to construct the concrete classes (e.g., visitor, factory) and annotation hierarchy representation. It is a convention that, for a type system named Foo, the compiler interface (checker), the visitor, and the annotated type factory are named as FooChecker, FooVisitor, and FooAnnotatedTypeFactory. BaseTypeChecker uses the convention to reflectively construct the components. Otherwise, the checker writer must specify the component classes for construction.

A checker can customize the default error messages through a Properties-loadable text file named messages.properties that appears in the same directory as the checker class. The property file keys are the strings passed to reportError and reportWarning (like "type.incompatible") and the values are the strings to be printed ("cannot assign ..."). The messages.properties file only need to mention the new messages that the checker defines. It is also allowed to override messages defined in superclasses, but this is rarely needed. Section ‍33.1.3 discusses best practices when using a message key in a @SuppressWarnings annotation.

### 36.6.1 Indicating supported annotations

A checker must indicate the annotations that it supports (that make up its type hierarchy).

By default, a checker supports all type annotations located in a subdirectory called qual that’s located in the same directory as the checker (unless you are creating multiple distribution artifacts, one for your checker and one for its qualifiers). A type annotation is meta-annotated with either @Target(ElementType.TYPE_USE) or @Target(ElementType.TYPE_USE, ElementType.TYPE_PARAMETER).

To indicate support for annotations that are located outside of the qual subdirectory, annotations that have other ElementType values, checker writers can override the createSupportedTypeQualifiers method (see its Javadoc for details). It is required to define createSupportedTypeQualifiers if you are mixing qualifiers from multiple directories (including when extending an existing checker that has its own qualifiers) and when using the Buck build tool, whose class loader cannot find the qualifier directory.

An aggregate checker (which extends AggregateChecker) does not need to specify its type qualifiers, but each of its component checkers should do so.

### 36.6.2 Bundling multiple checkers

Sometimes, multiple checkers work together and should always be run together. There are two different ways to bundle multiple checkers together, by creating either an “aggregate checker” or a “compound checker”.

1. An aggregate checker runs multiple independent, unrelated checkers. There is no communication or cooperation among them.

   The effect is the same as if a user passes multiple processors to the -processor command-line option, except the error messages are interleaved such that all errors about a line of code appear together.

   For example, instead of a user having to run

         javac -processor DistanceUnitChecker,VelocityUnitChecker,MassUnitChecker MyFile.java

   the user can write

         javac -processor MyUnitCheckers MyFile.java

   if you define an aggregate checker class. Extend AggregateChecker and override the getSupportedCheckers method, like the following:

         public class MyUnitCheckers extends AggregateChecker {
           @Override
           protected Collection<Class<? extends SourceChecker>> getSupportedCheckers() {
             Collection<Class<? extends SourceChecker>> checkers = new ArrayList<>(3);
             Collections.addAll(
               checkers,
               DistanceUnitChecker.class, VelocityUnitChecker.class, MassUnitChecker.class);
             return checkers;
           }
         }

   An example of an aggregate checker is I18nChecker (see Chapter ‍17.2), which consists of I18nSubchecker and LocalizableKeyChecker.

2. Use a compound checker to express dependencies among checkers. Suppose it only makes sense to run MyChecker if MyHelperChecker has already been run; that might be the case if MyHelperChecker computes some information that MyChecker needs to use.

   Override MyChecker.getImmediateSubcheckerClasses to return a list of the checkers that MyChecker depends on. Every one of them will be run before MyChecker is run. One of MyChecker’s subcheckers may itself be a compound checker, and multiple checkers may declare a dependence on the same subchecker. The Checker Framework will run each checker once, and in an order consistent with all the dependences.

   A checker obtains information from its subcheckers (those that ran before it) by querying their AnnotatedTypeFactory to determine the types of variables. Obtain the AnnotatedTypeFactory by calling getTypeFactoryOfSubcheckerOrNull.

   An example of a compound checker is SignednessChecker (see Chapter ‍21) for which ValueChecker is a subchecker.

### 36.6.3 Providing command-line options

A checker can provide two kinds of command-line options: boolean flags and named string values (the standard annotation processor options).

#### Boolean flags

To specify a simple boolean flag, add:

      @SupportedLintOptions({"myflag"})

to your checker subclass. The value of the flag can be queried using

      checker.getLintOption("myflag", false)

The second argument sets the default value that should be returned.

To pass a flag on the command line, call javac as follows:

      javac -processor mypackage.MyChecker -Alint=myflag

#### Named string values

For more complicated options, one can use the standard @SupportedOptions annotation on the checker, as in:

      @SupportedOptions({"myoption"})

The value of the option can be queried using

      checker.getOption("myoption")

To pass an option on the command line, call javac as follows:

      javac -processor mypackage.MyChecker -Amyoption=p1,p2

The value is returned as a single string and you have to perform the required parsing of the option.

#### Default Options

The default options passed to a custom checker may be customized by overriding the getOptions method in the custom checker class.

For example, to ignore the introduce.eliminate warnings raised by the Optional Checker (Chapter ‍5), override getOptions as follows:

      @Override
      public Map<String, String> getOptions() {
        Map<String, String> options = new HashMap<>(super.getOptions());
        options.put("suppressWarnings", "optional:introduce.eliminate");
        return options;
      }

in the OptionalChecker class.

## 36.7 Visitor: Type rules

A type system’s rules define which operations on values of a particular type are forbidden. These rules must be defined procedurally, not declaratively. Put them in a file MyChecker Visitor.java that extends BaseTypeVisitor.

BaseTypeVisitor performs type-checking at each node of a source file’s AST. It uses the visitor design pattern to traverse Java syntax trees as provided by Oracle’s jdk.compiler API, and it issues a warning (by calling reportError or reportWarning) whenever the type system is violated.

Most type-checkers override only a few methods in BaseTypeVisitor. A checker’s visitor overrides one method in the base visitor for each special rule in the type qualifier system. The last line of the overridden version is “return super.visit TreeType(node, p);”. If the method didn’t raise any error, the superclass implementation can perform standard checks.

By default, BaseTypeVisitor performs subtyping checks that are similar to Java subtype rules, but taking the type qualifiers into account. BaseTypeVisitor issues these errors:

- invalid assignment (type.incompatible) for an assignment from an expression type to an incompatible type. The assignment may be a simple assignment, or pseudo-assignment like return expressions or argument passing in a method invocation

  In particular, in every assignment and pseudo-assignment, the left-hand side of the assignment is a supertype of (or the same type as) the right-hand side. For example, this assignment is not permitted:

          @Nullable Object myObject;
          @NonNull Object myNonNullObject;
          ...
          myNonNullObject = myObject;  // invalid assignment

- invalid generic argument (type.argument) when a type is bound to an incompatible generic type variable
- invalid method invocation (method.invocation) when a method is invoked on an object whose type is incompatible with the method receiver type
- invalid overriding parameter type (override.param) when a parameter in a method declaration is incompatible with that parameter in the overridden method’s declaration
- invalid overriding return type (override.return) when the return type in a method declaration is incompatible with the return type in the overridden method’s declaration
- invalid overriding receiver type (override.receiver) when a receiver in a method declaration is incompatible with that receiver in the overridden method’s declaration

### 36.7.1 AST traversal

The Checker Framework needs to do its own traversal of the AST even though it operates as an ordinary annotation processor ‍[Dar06]. Java provides a visitor for Java code that is intended to be used by annotation processors, but that visitor only visits the public elements of Java code, such as classes, fields, methods, and method arguments — it does not visit code bodies or various other locations. The Checker Framework hardly uses the built-in visitor — as soon as the built-in visitor starts to visit a class, then the Checker Framework’s visitor takes over and visits all of the class’s source code.

Because there is no standard API for the AST of Java code1, the Checker Framework uses the javac implementation. This is why the Checker Framework is not deeply integrated with Eclipse or IntelliJ IDEA, but runs as an external tool (see Section ‍38.8).

### 36.7.2 Avoid hardcoding

If a method’s contract is expressible in the type system’s annotation syntax, then you should write annotations, in a stub file or annotated JDK (Chapter ‍35).

If the contract is not expressible, you can create a declaration annotation, and your checker can treat methods with that annotation specially. An example is the @FormatMethod annotation; see Section ‍15.5.

A final alternative is to write a type-checking rule for method invocation, where your rule checks the (class and) name of the method being called and then treats the method in a special way.

## 36.8 Type factory: Type introduction rules

The annotated type of expressions and types are defined via type introduction rules in the type factory. For most expressions and types, these rules are the same regardless of the type system. For example, the type of a method invocation expression is the return type of the invoked method, viewpoint-adapted for the call site. The framework implements these rules so that all type systems automatically use them. For other expressions, such as string literals, their (annotated) types depend on the type system, so the framework provides way to specify what qualifiers should apply to these expressions.

Defaulting rules are type introduction rules for computing the annotated type for an unannotated type; these rules are explained in Section ‍36.5.4. The meta-annotation @QualifierForLiterals can be written on an annotation declaration to specify that the annotation should be applied to the type of literals listed in the meta-annotation.

### 36.8.1 Procedurally specifying type introduction rules

If the meta-annotations are not sufficiently expressive, then you can write your own type introduction rules. There are three ways to do so. Each makes changes to an AnnotatedTypeMirror, which is the Checker Framework’s representation of an annotated type.

1. Define a subclass of TreeAnnotator, typically as a private inner class of your AnnotatedTypeFactory. There is a method of TreeAnnotator for every AST node, and the visitor has access to both the tree (the AST node) and its type.

   A TreeAnnotator is best suited to changing the type of an expression. Don’t try to use a TreeAnnotator to change the type of a variable or method declaration, because that change will not generally be seen at uses of the variable or method.

   After defining your TreeAnnotator, override createTreeAnnotator (in your subclass of AnnotatedTypeFactory) to return a ListTreeAnnotator containing your new TreeAnnotator, as in:

         @Override
         protected TreeAnnotator createTreeAnnotator() {
             return new ListTreeAnnotator(super.createTreeAnnotator(), new MyTreeAnnotator(this));
         }

   In code like the above, you might choose to put your TreeAnnotator first. Several tree annotators are run by default, and among them, PropagationTreeAnnotator adds annotations to AnnotatedTypeMirrors that do not have an annotation, but has no effect on those that have an annotation.

2. Define a subclass of a TypeAnnotator, typically as a private inner class of your AnnotatedTypeFactory. There is a method of TypeAnnotator for every kind of type, and the visitor has access to only the type (including base type and qualifiers). In your subclass of AnnotatedTypeFactory, override createTypeAnnotator to return a ListTypeAnnotator containing that annotator, as in

         @Override
         protected TypeAnnotator createTypeAnnotator() {
             return new ListTypeAnnotator(new MyTypeAnnotator(this), super.createTypeAnnotator());
         }

   (or put your TypeAnnotator last, depending on the behavior you want).

3. Create a subclass of AnnotatedTypeFactory and override two addComputedTypeAnnotations methods: addComputedTypeAnnotations(Tree,AnnotatedTypeMirror) (or addComputedTypeAnnotations(Tree,AnnotatedTypeMirror,boolean) if extending GenericAnnotatedTypeFactory) and addComputedTypeAnnotations(Element,AnnotatedTypeMirror). The methods can make arbitrary changes to the annotations on a type.

   Recall that AnnotatedTypeFactory, when given a program expression, returns the expression’s type. This should include not only the qualifiers that the programmer explicitly wrote in the source code, but also default annotations and type refinement (see Section ‍32.4 for explanations of these concepts).

   The approach of overriding addComputedTypeAnnotations is a last resort, if your logic cannot be implemented using a TreeAnnotator or a TypeAnnotator. The implementation of addComputedTypeAnnotations in GenericAnnotatedTypeFactory calls the tree annotator and the type annotator (in that order), but by overriding the method you can cause your logic to be run even earlier or even later.

### 36.8.2 Recursion in TreeAnnotators

A subclass of TreeAnnotator should not, in general, call getAnnotatedType() more than once. This is primarily a concern for visitBinary() and visitCompoundAssignment(). For example, the implementation of visitBinary() should not start with

        AnnotatedTypeMirror lExpr = getAnnotatedType(tree.getLeftOperand());
        AnnotatedTypeMirror rExpr = getAnnotatedType(tree.getRightOperand());

The reason is that PropagationTreeAnnotator already does such recursion. If two TreeAnnotators do so, then expressions at the base of a large binary tree will be visited exponentially many times, leading to slowdowns or the appearance of an infinite loop.

One approach is to not call getAnnotatedType() in visitBinary(); this is what UpperBoundAnnotatedTypeFactory does.

Another approach is to disable PropagationTreeAnnotator, doing all work in a different TreeAnnotator. Create a subclass of PropagationTreeAnnotator whose visitBinary() method does no work, and use that subclass in place of PropagationTreeAnnotator itself. This is what RegexAnnotatedTypeFactory and UnitsAnnotatedTypeFactory do.

## 36.9 Dataflow: enhancing flow-sensitive type refinement

By default, every checker performs flow-sensitive type refinement, as described in Section ‍32.7.

This section of the manual explains how to enhance the Checker Framework’s built-in type refinement. Most commonly, you will inform the Checker Framework about a run-time test that gives information about the type qualifiers in your type system. Section ‍32.7.4 gives examples of type systems with and without run-time tests.

The steps to customizing type refinement are:

1. §36.9.1 Determine which expressions will be refined.
2. §36.9.2 Create required class and configure its use.
3. §36.9.3 Override methods that handle Nodes of interest.
4. §36.9.4 Implement the refinement.

The Regex Checker’s dataflow customization for the RegexUtil.asRegex run-time check is used as a running example.

If needed, you can find more details about the implementation of type refinement, and the control flow graph (CFG) data structure that it uses, in the Dataflow Manual.

### 36.9.1 Determine expressions to refine the types of

A run-time check or run-time operation involves multiple expressions (arguments, results). Determine which expression the customization will refine. This is usually specific to the type system and run-time test. There is no code to write in this step; you are merely determining the design of your type refinement.

For the program operation op(a,b), you can refine the types in either or both of the following ways:

1. Change the result type of the entire expression op(a,b).

   As an example (and as the running example of implementing a dataflow refinement), the RegexUtil.asRegex method is declared as:

         @Regex(0) String asRegex(String s, int groups) { ... }

   This annotation is sound and conservative: it says that an expression such as RegexUtil.asRegex(myString, myInt) has type @Regex(0) String. However, this annotation is imprecise. When the group argument is known at compile time, a better estimate can be given. For example, RegexUtil.asRegex(myString, 2) has type @Regex(2) String.

2. Change the type of some other expression, such as a or b.

   As an example, consider an equality test in the Nullness type system:

         @Nullable String s;
           if (s != null) {
             ...
           } else {
             ...
           }

   The type of s != null is always boolean. However, in the true branch, the type of s can be refined to @NonNull String.

If you are refining the types of arguments or the result of a method call, then you may be able to implement your flow-sensitive refinement rules by just writing @EnsuresQualifier and/or @EnsuresQualifierIf annotations. When this is possible, it is the best approach.

Sections ‍36.9.2–36.9.4 explain how to create a transfer class when the @EnsuresQualifier and @EnsuresQualifierIf annotations are insufficient.

### 36.9.2 Create required class

In the same directory as MyChecker Checker.java, create a class named MyChecker Transfer that extends CFTransfer.

Leave the class body empty for now, except for a constructor:

      public MyCheckerTransfer(CFAnalysis analysis) {
        super(analysis);
      }

Your class will add functionality by overriding methods of CFTransfer, which performs the default Checker Framework type refinement.

As an example, the Regex Checker’s extended CFTransfer is RegexTransfer.

(If you disregard the instructions above and choose a different name or a different directory for your MyChecker Transfer class, you will also need to override the createFlowTransferFunction method in your type factory to return a new instance of the class.)

(As a reminder, use of @EnsuresQualifier and @EnsuresQualifierIf may obviate the need for a transfer class.)

### 36.9.3 Override methods that handle Nodes of interest

Decide what source code syntax is relevant to the run-time checks or run-time operations you are trying to support. The CFG (control flow graph) represents source code as Node, a node in the abstract syntax tree of the program being checked (see Section ‍36.9.3).

In your extended CFTransfer override the visitor method that handles the Nodes relevant to your run-time check or run-time operation. Leave the body of the overriding method empty for now.

For example, the Regex Checker refines the type based on a call to the RegexUtil.isRegex method. A method call is represented by a MethodInvocationNode. Therefore, RegexTransfer overrides the visitMethodInvocation method:

      public TransferResult<CFValue, CFStore> visitMethodInvocation(
        MethodInvocationNode n, TransferInput<CFValue, CFStore> in)  { ... }

#### Program representation

The Node subclasses can be found in the org.checkerframework.dataflow.cfg.node package. Some examples are EqualToNode, LeftShiftNode, VariableDeclarationNode.

A Node is basically equivalent to a javac compiler Tree.

See Section ‍36.14 for more information about Trees. As an example, the statement String a = ""; is represented as this abstract syntax tree:

    VariableTree:
      name: "a"
      type:
        IdentifierTree
          name: String
      initializer:
        LiteralTree
          value: ""

### 36.9.4 Implement the refinement

Each visitor method in CFAbstractTransfer returns a TransferResult. A TransferResult represents the refined information that is known after an operation. It has two components: the result type for the Node being evaluated, and a map from expressions in scope to estimates of their types (a Store). Each of these components is relevant to one of the two cases in Section ‍36.9.1:

1. Changing the TransferResult’s result type changes the type that is returned by the AnnotatedTypeFactory for the tree corresponding to the Node that was visited. (Remember that BaseTypeVisitor uses the AnnotatedTypeFactory to look up the type of a Tree, and then performs checks on types of one or more Trees.)

   For example, When RegexTransfer evaluates a RegexUtils.asRegex invocation, it updates the TransferResult’s result type. This changes the type of the RegexUtils.asRegex invocation when its Tree is looked up by the AnnotatedTypeFactory. See below for details.

2. Updating the Store treats an expression as having a refined type for the remainder of the method or conditional block. For example, when the Nullness Checker’s dataflow evaluates myvar != null, it updates the Store to specify that the variable myvar should be treated as having type @NonNull for the rest of the then conditional block. Not all kinds of expressions can be refined; currently method return values, local variables, fields, and array values can be stored in the Store. Other kinds of expressions, like binary expressions or casts, cannot be stored in the Store.

The rest of this section details implementing the visitor method RegexTransfer.visitMethodInvocation for the RegexUtil.asRegex run-time test. You can find other examples of visitor methods in LockTransfer and FormatterTransfer.

1. Determine if the visited Node is of interest

   A visitor method is invoked for all instances of a given Node kind in the program. The visitor must inspect the Node to determine if it is an instance of the desired run-time test or operation. For example, visitMethodInvocation is called when dataflow processes any method invocation, but the RegexTransfer should only refine the result of RegexUtils.asRegex invocations:

         @Override
         public TransferResult<CFValue, CFStore> visitMethodInvocation(...)
           ...
           MethodAccessNode target = n.getTarget();
           ExecutableElement method = target.getMethod();
           Node receiver = target.getReceiver();
           if (receiver instanceof ClassNameNode) {
             String receiverName = ((ClassNameNode) receiver).getElement().toString();

             // Is this a call to static method isRegex(s, groups) in a class named RegexUtil?
             if (receiverName.equals("RegexUtil")
                 && ElementUtils.matchesElement(method,
                        "isRegex", String.class, int.class)) {
                   ...

2. Determine the refined type

   Sometimes the refined type is dependent on the parts of the operation, such as arguments passed to it.

   For example, the refined type of RegexUtils.asRegex is dependent on the integer argument to the method call. The RegexTransfer uses this argument to build the resulting type @Regex(i), where i is the value of the integer argument. For simplicity the below code only uses the value of the integer argument if the argument was an integer literal. It could be extended to use the value of the argument if it was any compile-time constant or was inferred at compile time by another analysis, such as the Constant Value Checker (Chapter ‍23).

         AnnotationMirror regexAnnotation;
         Node count = n.getArgument(1);
         if (count instanceof IntegerLiteralNode) {
           // argument is a literal integer
           IntegerLiteralNode iln = (IntegerLiteralNode) count;
           Integer groupCount = iln.getValue();
           regexAnnotation = factory.createRegexAnnotation(groupCount);
         } else {
           // argument is not a literal integer; fall back to @Regex(), which is the same as @Regex(0)
           regexAnnotation = AnnotationBuilder.fromClass(factory.getElementUtils(), Regex.class);
         }

3. Return a TransferResult with the refined types

   Recall that the type of an expression is refined by modifying the TransferResult returned by a visitor method. Since the RegexTransfer is updating the type of the run-time test itself, it will update the result type and not the Store.

   A CFValue is created to hold the type inferred. CFValue is a wrapper class for values being inferred by dataflow:

         CFValue newResultValue = analysis.createSingleAnnotationValue(regexAnnotation,
             result.getResultValue().getType().getUnderlyingType());

   Then, RegexTransfer’s visitMethodInvocation creates and returns a TransferResult using newResultValue as the result type.

         return new RegularTransferResult<>(newResultValue, result.getRegularStore());

   As a result of this code, when the Regex Checker encounters a RegexUtils.asRegex method call, the checker will refine the return type of the method if it can determine the value of the integer parameter at compile time.

### 36.9.5 Further customizing flow-sensitive inference

By default, the Checker Framework assumes that modifications to an object’s fields do not change its type qualifiers. This is true for all provenance-based type systems (see Section ‍32.7.4), for all value-based type systems over immutable values, and for some other type systems. To indicate that side effects can change a value’s type qualifiers, write sideEffectsUnrefineAliases = true; in the type factory constructor, before the call to this.postInit();. Note that this setting may lead to large numbers of false positive warnings. You can reduce the number of false positive warnings by writing more precise side effect annotations.

In the uncommon case that you wish to disable the Checker Framework’s built-in flow inference in your checker (this is different than choosing not to extend it as described in Section ‍36.9), put the following two lines at the beginning of the constructor for your subtype of BaseAnnotatedTypeFactory:

            // disable flow inference
            super(checker, /*useFlow=*/ false);

## 36.10 Annotated JDK and other annotated libraries

You will need to supply annotations for relevant parts of the JDK; otherwise, your type-checker may produce spurious warnings for code that uses the JDK. You have two options:

- Write JDK annotations in a fork of https://github.com/typetools/jdk.

  If your checker is written in a fork of https://github.com/typetools/jdk, then the checker and the JDK must use the same fork name (GitHub organization) and branch name; this is necessary so that the CI jobs use the right annotated JDK.

  Clone the JDK and the Checker Framework in the same place; that is, your working copy of the JDK (a jdk directory) should be a sibling of your working copy of the Checker Framework (a checker-framework directory).

  Here are some tips:

  - Add an @AnnotatedFor annotation to each file you annotate.
  - Whenever you add a file, fully annotate it, as described in Section ‍35.1.
  - If you are only annotating fields and method signatures (but not ensuring that method bodies type-check), then you don’t need to suppress warnings, because the JDK is not type-checked.
  - Double-check your work. If you forget to import an annotation, or you misspell it in the import statement or at some use, it will be silently ignored, which is frustrating.
- Write JDK annotations as stub files (partial Java source files).

  Create a file jdk.astub in the checker’s main source directory. You can also create jdk N.astub files that contain methods or classes that only exist in certain JDK versions. The JDK stub files will be automatically used by the checker, unless the user supplies the command-line option -Aignorejdkastub.

  You can also supply .astub files in that directory for other libraries. You should list those other libraries in a @StubFiles annotation on the checker’s main class, so that they will also be automatically used.

  When a stub file should be used by multiple checkers (for example, if it contains purity annotations that are needed by multiple distinct checkers), the stub file should appear in directory checker/src/main/resources/. In the distribution, it will appear at the top level of the checker.jar file. It will not be used automatically; a user must pass the -Astubs=checker.jar/stubfilename.astub command-line argument (see Section ‍2.2.1)

  While creating a stub file, you may find the debugging options described in Section ‍35.5.7 useful.

## 36.11 Testing framework

The Checker Framework provides a convenient way to write tests for your checker. Each test case is a Java file, with inline indications of what errors and warnings (if any) a checker should emit. An example is

    class MyNullnessTest {
      void method() {
        Object nullable = null;
        // :: error: [dereference.of.nullable]
        nullable.toString();
      }
    }

When the Nullness Checker is run on the above code, it should produce exactly one error, whose message key is dereference.of.nullable, on the line following the “// ::” comment.

The testing infrastructure is extensively documented in file checker-framework/checker/tests/README.md.

If your checker’s source code is within a fork of the Checker Framework repository, then you can copy the testing infrastructure used by some existing type system.

## 36.12 Debugging options

The Checker Framework provides debugging options that can be helpful when implementing a checker. These are provided via the standard javac “-A” switch, which is used to pass options to an annotation processor.

### 36.12.1 Amount of detail in messages

- -AprintAllQualifiers: print all type qualifiers, including qualifiers meta-annotated with @InvisibleQualifier, which are usually not shown.
- -AprintVerboseGenerics: print more information about type parameters and wildcards when they appear in warning messages. Supplying this also implies -AprintAllQualifiers.
- -Anomsgtext: use message keys (such as “type.invalid”) rather than full message text when reporting errors or warnings. This is used by the Checker Framework’s own tests, so they do not need to be changed if the English message is updated.
- -Aonelinemsg: print each error message on a single line, with logical lines separated by “/”. This is useful when using a tool that only shows the first line of the error.
- -AnoPrintErrorStack: don’t print a stack trace when an internal Checker Framework error occurs. Setting this option is rare. You should only do it if you have discovered a bug in a checker, you have already reported the bug, and you want to continue using the checker on a large codebase without being inundated in stack traces.
- -AnoWarnMemoryConstraints: when memory constraints are impeding performance, issue a note rather than a warning. This prevents the message from being elevated to an error by -Werror.
- -AdumpOnErrors: Outputs a stack trace when reporting errors or warnings.
- -AexceptionLineSeparator: Controls what line separator is used when printing exceptions. This is useful when tools, such as Maven, only display the first line of exception messages. For example, -AexceptionLineSeparator=" ".

### 36.12.2 Format of output

- -Adetailedmsgtext: Output error/warning messages in a stylized format that is easy for tools to parse. This is useful for tools that run the Checker Framework and parse its output, such as IDE plugins. See the source code of SourceChecker.java for details about the format.

The javac-diagnostic-wrapper tool can transform javac’s textual output into other formats, such as JSON in LSP (Language Server Protocol) format.

### 36.12.3 Stub and JDK libraries

- -Aignorejdkastub: ignore the jdk.astub and jdk N.astub files in the checker directory. Files passed through the -Astubs option are still processed. This is useful when experimenting with an alternative stub file.
- -ApermitMissingJdk: don’t issue an error if no annotated JDK can be found.
- -AparseAllJdk: parse all JDK files at startup rather than as needed.
- -AstubDebug: Print debugging messages while processing stub files. Section ‍35.5.7 describes more diagnostic command-line arguments.

### 36.12.4 Progress tracing

- -Afilenames: print the name of each file before type-checking it. This can be useful for determining that a long compilation job is making progress.

  This option can also help to keep a Travis CI job alive (since Travis CI terminates any job that does not produce output for 10 minutes). This does not work if you are using Maven, because it does not print the full output of the Java compiler (see https://github.com/codehaus-plexus/plexus-compiler/issues/149). (Also, maven-compiler-plugin is buggy: it sometimes doesn’t print any output if it cannot parse it.)

- -Ashowchecks: print debugging information for each pseudo-assignment check (commonAssignmentCheck) performed by BaseTypeVisitor; see Section ‍36.7.
- -AshowWpiFailedInferences: print debugging information about failed inference steps during whole-program inference. Must be used with -Ainfer; see Section ‍34.2.

If the Checker Framework is in an infinite loop, you can use -Afilenames and -Ashowchecks (which is very verbose) to determine what source code triggers the infinite loop.

While the Checker Framework is in its infinite loop (no more progress is being made), get a traceback (stack trace) from from each Java thread, using any of the following four techniques:

- From a different shell than the Checker Framework is running in (running just jcmd gives a list of pids or process ids):
  - jstack pid
  - jcmd pid Thread.print
  - kill -QUIT pid
- From the same shell that the Checker Framework is running in:
  - Press ctrl-\

### 36.12.5 Saving the command-line arguments to a file

- -AoutputArgsToFile: This saves the final command-line parameters as passed to the compiler in a file. The file can be used as a script to re-execute the same compilation command. (The script file must be marked as executable on Unix, or must have a .bat extension on Windows.) Example usage: -AoutputArgsToFile=$HOME/scriptfile

  The -AoutputArgsToFile command-line argument is processed by CheckerMain, not by the annotation processor. That means that it can be supplied only when you use the Checker Framework compiler (the “Checker Framework javac wrapper”), and it cannot be written in a file containing command-line arguments passed to the compiler using the @argfile syntax.

### 36.12.6 Visualizing the dataflow graph

To understand control flow in your program and the resulting type refinement, you can create a graphical representation of the CFG.

Typical use is:

    javacheck -processor mypackage.MyProcessor -Aflowdotdir=. MyClass.java
    for dotfile in *.dot; do dot -Tpdf -o "$dotfile.pdf" "$dotfile"; done

or (for more verbose output)

    javacheck -processor mypackage.MyProcessor \
      -Acfgviz=org.checkerframework.dataflow.cfg.visualize.DOTCFGVisualizer,outdir=.,verbose \
      MyClass.java
    for dotfile in *.dot; do dot -Tpdf -o "$dotfile.pdf" "$dotfile"; done

where the first command creates file MyClass.dot that represents the CFG, and the second command writes the CFG to a PDF file. The dot program is part of Graphviz.

In the output, conditional basic blocks are represented as octagons with two successors. Special basic blocks are represented as ovals (e.g., the entry and exit point of the method).

#### Creating a CFG while running a type-checker

To create a CFG while running a type-checker, use the following command-line options.
(See above for typical usage.)

- -Aflowdotdir=somedir: Specify directory for .dot files visualizing the CFG. Shorthand for
  -Acfgviz=org.checkerframework.dataflow.cfg.visualize.DOTCFGVisualizer,outdir=somedir. The directory must already exist.
- -Averbosecfg: Enable additional output in the CFG visualization. Equivalent to passing verbose to cfgviz, e.g. as in -Acfgviz=MyVisualizer,verbose
- -Acfgviz=VizClassName[,opts,...]: Mechanism to visualize the control flow graph (CFG) of all the methods and code fragments analyzed by the dataflow analysis (Section ‍36.9). The graph also contains information about flow-sensitively refined types of various expressions at many program points.

  The argument is a comma-separated sequence. The first element is the fully-qualified name of the org.checkerframework.dataflow.cfg.visualize.CFGVisualizer implementation that should be used. Currently, the possibilities are:

  - org.checkerframework.dataflow.cfg.visualize.DOTCFGVisualizer
  - org.checkerframework.dataflow.cfg.visualize.StringCFGVisualizer

  The remaining keys or key–value pairs are passed to CFGVisualizer.init. Supported keys include

  - verbose outputs the store after each basic block (in addition to before); shows transfer result values, unique IDs, and the processing order
  - outdir directory into which to write files
  - checkerName

#### Creating a CFG by running CFGVisualizeLauncher

You can also use CFGVisualizeLauncher to generate a DOT or String representation of the control flow graph of a given method in a given class. The CFG is generated and output, but no dataflow analysis is performed.

    java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar \
      org.checkerframework.dataflow.cfg.visualize.CFGVisualizeLauncher \
      MyClass.java --class MyClass --method test --pdf

The above command will generate the corresponding .dot and .pdf files for the method test in the class MyClass in the project directory. To generate a string representation of the graph to standard output, remove --pdf but add --string. For example:

    java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar \
      org.checkerframework.dataflow.cfg.visualize.CFGVisualizeLauncher \
      MyClass.java --class MyClass --method test --string

For more details about invoking CFGVisualizeLauncher, run it with no arguments.

### 36.12.7 Miscellaneous debugging options

- -AresourceStats: Whether to output resource statistics at JVM shutdown.
- -AatfDoNotCache: If provided, the Checker Framework will not cache results but will recompute them. This makes the Checker Framework run slower. If the Checker Framework behaves differently with and without this flag, then there is a bug in its caching code. Please report that bug.
- -AatfCacheSize: The size of the Checker Framework’s internal caches. Ignored if -AatfDoNotCache is provided. Most users have no need to set this.

### 36.12.8 Examples

The following example demonstrates how these options are used:

    $ javac -processor org.checkerframework.checker.interning.InterningChecker \
        docs/examples/InternedExampleWithWarnings.java -Ashowchecks -Anomsgtext -Afilenames

    [InterningChecker] InterningExampleWithWarnings.java
     success (line  18): STRING_LITERAL "foo"
         actual: DECLARED @org.checkerframework.checker.interning.qual.Interned java.lang.String
       expected: DECLARED @org.checkerframework.checker.interning.qual.Interned java.lang.String
     success (line  19): NEW_CLASS new String("bar")
         actual: DECLARED java.lang.String
       expected: DECLARED java.lang.String
    docs/examples/InterningExampleWithWarnings.java:21: (not.interned)
        if (foo == bar) {
                ^
     success (line  22): STRING_LITERAL "foo == bar"
         actual: DECLARED @org.checkerframework.checker.interning.qual.Interned java.lang.String
       expected: DECLARED java.lang.String
    1 error

### 36.12.9 Using an external debugger

You can use any standard debugger to observe the execution of your checker.

You can also set up remote (or local) debugging using the following command as a template:

    java -jar "$CHECKERFRAMEWORK/checker/dist/checker.jar" \
        -J-Xdebug -J-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005 \
        -processor org.checkerframework.checker.nullness.NullnessChecker \
        src/sandbox/FileToCheck.java

## 36.13 Documenting the checker

This section describes how to write a chapter for this manual that describes a new type-checker. This is a prerequisite to having your type-checker distributed with the Checker Framework, which is the best way for users to find it and for it to be kept up to date with Checker Framework changes. Even if you do not want your checker distributed with the Checker Framework, these guidelines may help you write better documentation.

When writing a chapter about a new type-checker, see the existing chapters for inspiration. (But recognize that the existing chapters aren’t perfect: maybe they can be improved too.)

A chapter in the Checker Framework manual should generally have the following sections:

Chapter: Belly Rub Checker
  The text before the first section in the chapter should state the guarantee that the checker provides and why it is important. It should give an overview of the concepts. It should state how to run the checker.
Section: Belly Rub annotations
  This section includes descriptions of the annotations with links to the Javadoc. Separate type annotations from declaration annotations, and put any type annotations that a programmer may not write (they are only used internally by the implementation) last within variety of annotation.

  Draw a diagram of the type hierarchy. A textual description of the hierarchy is not sufficient; the diagram really helps readers to understand the system. The diagram will appear in directory docs/manual/figures/; see its README file for tips.

  The Javadoc for the annotations deserves the same care as the manual chapter. Each annotation’s Javadoc comment should use the @checker_framework.manual Javadoc taglet to refer to the chapter that describes the checker, and the polymorphic qualifier’s Javadoc should also refer to the qualifier-polymorphism section. For example, in PolyPresent.java:

       * @checker_framework.manual #optional-checker Optional Checker
       * @checker_framework.manual #qualifier-polymorphism Qualifier polymorphism

Section: What the Belly Rub Checker checks
  This section gives more details about when an error is issued, with examples. This section may be omitted if the checker does not contain special type-checking rules — that is, if the checker only enforces the usual Java subtyping rules.
Section: Examples
  Code examples.

Sometimes you can omit some of the above sections. Sometimes there are additional sections, such as tips on suppressing warnings, comparisons to other tools, and run-time support.

You will create a new belly-rub-checker.tex file, then `\input` it at a logical place in manual.tex (not necessarily as the last checker-related chapter). Also add two references to the checker’s chapter: one at the beginning of chapter ‍1, and identical text in the appropriate part of Section ‍32.7.4. Add the new file to docs/manual/Makefile. Keep the lists in the same order as the manual chapters, to help us notice if anything is missing.

For a chapter or (sub)*section, use `\sectionAndLabel{Section title}{section-label}`. Section labels should start with the checker name (as in `bellyrub-examples`) and not with “sec:”. Figure labels should start with “fig-checkername” and not with “fig:”. These conventions are for the benefit of the Hevea program that produces the HTML version of the manual. Use `\begin{figure}` for all figures, including those whose content is a table, in order to have a single consistent numbering for all figures.

Don’t forget to write Javadoc for any annotations that the checker uses. That is part of the documentation and is the first thing that many users may see. The documentation for any annotation should include an example use of the annotation. Also ensure that the Javadoc links back to the manual, using the @checker_framework.manual custom Javadoc tag.

## 36.14 javac implementation survival guide

Since this section of the manual was written, the useful “The Hitchhiker’s Guide to javac” has become available at https://openjdk.org/groups/compiler/doc/hhgtjavac/index.html. See it first, and then refer to this section. (This section of the manual should be revised, or parts eliminated, in light of that document.)

A checker built using the Checker Framework makes use of a few interfaces from the underlying compiler (Oracle’s OpenJDK javac). This section describes those interfaces.

### 36.14.1 Checker access to compiler information

The compiler uses and exposes three hierarchies to model the Java source code and classfiles.

#### Types — Java Language Model API

A TypeMirror represents a Java type.

There is a TypeMirror interface to represent each type kind, e.g., PrimitiveType for primitive types, ExecutableType for method types, and NullType for the type of the null literal.

TypeMirror does not represent annotated types though. A checker should use the Checker Framework types API, AnnotatedTypeMirror, instead. AnnotatedTypeMirror parallels the TypeMirror API, but also presents the type annotations associated with the type.

The Checker Framework and the checkers use the types API extensively.

#### Elements — Java Language Model API

An Element represents a potentially-public declaration that can be accessed from elsewhere: classes, interfaces, methods, constructors, and fields. Element represents elements found in both source code and bytecode.

There is an Element interface to represent each construct, e.g., TypeElement for classes/interfaces, ExecutableElement for methods/constructors, and VariableElement for local variables and method parameters.

If you need to operate on the declaration level, always use elements rather than trees (see below). This allows the code to work on both source and bytecode elements.

Example: retrieve declaration annotations, check variable modifiers (e.g., strictfp, synchronized)

#### Trees — Compiler Tree API

A Tree represents a syntactic unit in the source code, such as a method declaration, statement, block, for loop, etc. Trees only represent source code to be compiled (or found in -sourcepath); no tree is available for classes read from bytecode.

There is a Tree interface for each Java source structure, e.g., ClassTree for class declaration, MethodInvocationTree for a method invocation, and ForEachTree for an enhanced for statement (also known as a foreach loop).

You should limit your use of trees. A checker uses Trees mainly to traverse the source code and retrieve the types/elements corresponding to them. Then, the checker performs any needed checks on the types/elements instead.

To obtain the position (line number and column number) of a Tree, cast the Tree to a com.sun.tools.javac.tree.JCTree and then use one of its methods, such as pos() which returns a JCDiagnostic.DiagnosticPosition. com.sun.source.tree.CompilationUnitTree has a LineMap that lets you convert positions to line numbers.

#### Using the APIs

The three APIs use some common idioms and conventions; knowing them will help you to create your checker.

Type-checking: Do not use instanceof Subinterface to determine which kind of TypeMirror or Element an expression is, because some of the classes that implement the TypeMirror and Element subinterfaces implement multiple subinterfaces. For example, type instanceof DeclaredType and type instanceof UnionType both return true if type is an com.sun.tools.javac.code.Type.UnionClassType object.

Instead, use the TypeMirror.getKind() or Element.getKind() method. For example, if type is an com.sun.tools.javac.code.Type.UnionClassType object, then expr.getKind() == TypeKind.DECLARED is false and expr.getKind() == TypeKind.UNION is true.

For Trees, you can use either Tree.getKind() or instanceof.

Visitors and Scanners: The compiler and the Checker Framework use the visitor pattern extensively. For example, visitors are used to traverse the source tree (BaseTypeVisitor extends TreePathScanner) and for type checking (TreeAnnotator implements TreeVisitor).

Utility classes: Some useful methods appear in a utility class. The OpenJDK convention is that the utility class for a Foo hierarchy is Foos (e.g., Types, Elements, and Trees). The Checker Framework uses a common Utils suffix to distinguish the class names (e.g., TypesUtils, TreeUtils, ElementUtils), with one notable exception: AnnotatedTypes.

#### Equality for annotations

AnnotationMirror is an interface that is implemented both by javac and the Checker Framework. The documentation of AnnotationMirror says, “Annotations should be compared using the equals method. There is no guarantee that any particular annotation will always be represented by the same object.” The second sentence is true, but the first sentence is wrong. You should never compare AnnotationMirrors using equals(), which (for some implementations) is reference equality. AnnotationUtils has various methods that should be used instead. Also, AnnotationMirrorMap and AnnotationMirrorSet can be used.

### 36.14.2 How a checker fits in the compiler as an annotation processor

The Checker Framework builds on the Annotation Processing API introduced in Java 6. A type-checking annotation processor is one that extends AbstractTypeProcessor; it gets run on each class source file after the compiler confirms that the class is valid Java code.

The most important methods of AbstractTypeProcessor are typeProcess and getSupportedSourceVersion. The former class is where you would insert any sort of method call to walk the AST, and the latter just returns a constant indicating that we are targeting version 8 of the compiler. Implementing these two methods should be enough for a basic plugin; see the Javadoc for the class for other methods that you may find useful later on.

The Checker Framework uses Oracle’s Tree API to access a program’s AST. The Tree API is specific to the Oracle OpenJDK, so the Checker Framework only works with the OpenJDK javac, not with Eclipse’s compiler ecj. This also limits the tightness of the integration of the Checker Framework into other IDEs such as IntelliJ IDEA. An implementation-neutral API would be preferable. In the future, the Checker Framework can be migrated to use the Java Model AST of JSR 198 (Extension API for Integrated Development Environments) ‍[Cro06], which gives access to the source code of a method. But, at present no tools implement JSR ‍198. Also see Section ‍36.7.1.

#### Learning more about javac

Sun’s javac compiler interfaces can be daunting to a newcomer, and its documentation is a bit sparse. The Checker Framework aims to abstract a lot of these complexities. You do not have to understand the implementation of javac to build powerful and useful checkers. Beyond this document, other useful resources include the Java Infrastructure Developer’s guide at https://netbeans.apache.org/wiki/main/javahowto/Java_DevelopersGuide/ and the compiler mailing list archives at https://mail.openjdk.org/pipermail/compiler-dev/ (subscribe at https://mail.openjdk.org/mailman/listinfo/compiler-dev).

## 36.15 Integrating a checker with the Checker Framework

To integrate a new checker with the Checker Framework release, perform the following:

- Make sure check-compilermsgs and check-purity run without warnings or errors.

1
  Actually, there is a standard API for Java ASTs — JSR 198 (Extension API for Integrated Development Environments) ‍[Cro06]. If tools were to implement it (which would just require writing wrappers or adapters), then the Checker Framework and similar tools could be portable among different compilers and IDEs.

# Chapter ‍37 Building an accumulation checker

This chapter describes how to build a checker for an accumulation analysis. If you want to use an existing checker, you do not need to read this chapter.

An accumulation analysis is a program analysis where the analysis abstraction is a monotonically increasing set — that is, the analysis learns new facts, and facts are never retracted. Typically, some operation in code is legal only when the set is large enough — that is, the estimate has accumulated sufficiently many facts.

The Called Methods Checker (Chapter ‍7) is an accumulation analysis. The Test Accumulation Checker is a simplified version of the Called Methods Checker that you can also use as a model.

Accumulation analysis is a special case of typestate analysis in which (1) the order in which operations are performed does not affect what is subsequently legal, and (2) the accumulation does not add restrictions; that is, as more operations are performed, more operations become legal. Unlike a traditional typestate analysis, an accumulation analysis does not require an alias analysis for soundness. It can therefore be implemented as a flow-sensitive type system.

The rest of this chapter assumes you have read how to create a checker (Chapter ‍36).

##### Defining type qualifiers

Define 2 or 3 type qualifiers.

- The “accumulator” type qualifier has a single argument: a String[] named value that defaults to the an empty array. Note that the Checker Framework’s support for accumulation analysis requires you to accumulate a string representation of whatever you are accumulating. For example, when accumulating which methods have been called, you might choose to accumulate method names.

  The accumulator qualifier should have no supertypes (@SubtypeOf()) and should be the default qualifier in the hierarchy (@DefaultQualifierInHierarchy).

  An example of such a qualifier can be found in the Checker Framework’s tests: TestAccumulation.java.

- Define a bottom type, analogous to TestAccumulationBottom.java. It should take no arguments, and should be a subtype of the accumulator type you defined earlier.
- Optionally, define a predicate annotation, analogous to TestAccumulationPredicate.java. It must have a single argument named value of type String. The predicate syntax supports
  - || disjunctions
  - && conjunctions
  - ! logical complement. "!x" means “it is not true that x was definitely accumulated” or, equivalently, “there is no path on which x was accumulated”. Note that this does not mean “x was not accumulated” — it is not a violation of the specification "!x" if x is accumulated on some paths, but not others.
  - (...) parentheses for precedence

##### Setting up the checker

Define a new class that extends AccumulationChecker. It does not need any content.

Define a new class that extends AccumulationAnnotatedTypeFactory. You must create a new constructor whose only argument is a BaseTypeChecker. Your constructor should call one of the super constructors defined in AccumulationAnnotatedTypeFactory (which one depends on whether or not you defined a predicate annotation).

##### Adding accumulation logic

Define a class that extends AccumulationTransfer. To update the estimate of what has been accumulated, override some method in CFAbstractTransfer to call accumulate.

For example, to accumulate the names of methods called, a checker would override CFAbstractTransfer.visitMethodInvocation to: call super to get a TransferResult, compute the method name from the MethodInvocationNode, and then call accumulate.

##### Enforcing program properties

At this point, your checker ensures that all annotations are consistent with one another and the source code, and it flow-sensitively refines the annotations. To enforce properties in the code being type-checked, write type rules (Section ‍36.7) that are specific to your type system.

## 37.1 Publications

The paper “Accumulation Analysis”‍[KSSE22] describes theoretical properties of accumulation analysis. The papers “Verifying Object Construction”‍[KRS+20] and “Lightweight and modular resource leak verification”‍[KSSE21] describe specific accumulation analyses.

# Chapter ‍38 Integration with external tools

This chapter discusses how to run a checker from the command line, from a build system, or from an IDE. You can skip to the appropriate section:

- Android (Section ‍38.1)
- Android Gradle Plugin (Section ‍38.2)
- Ant (Section ‍38.3)
- Bazel (Section ‍38.4)
- Buck (Section ‍38.5)
- Command line, via Checker Framework javac wrapper (Section ‍38.6)
- Command line, via JDK javac (Section ‍38.7)
- Eclipse (Section ‍38.8)
- Gradle (Section ‍38.9)
- IntelliJ IDEA (Section ‍38.10)
- javac Diagnostics Wrapper (Section ‍38.11)
- Lombok (Section ‍38.12)
- Maven (Section ‍38.13)
- NetBeans (Section ‍38.14)
- sbt (Section ‍38.15)
- tIDE (Section ‍38.16)

If your build system or IDE is not listed above, you should customize how it runs the javac command on your behalf. See your build system or IDE documentation to learn how to customize it, adapting the instructions for javac in Section ‍38.7. If you make another tool support running a checker, please inform us via the mailing list or issue tracker so we can add it to this manual.

All examples in this chapter are in the public domain, with no copyright nor licensing restrictions.

## 38.1 Android

When creating an Android app, you may wish to use checker-qual-android whenever this document mentions checker-qual. This can lead to smaller dex files (smaller distributed apps).

The checker-qual-android artifact is identical to the checker-qual artifact, except that in checker-qual-android annotations have classfile retention. The default Android Gradle plugin retains types annotated with runtime-retention annotations in the main dex, but strips out class-retention annotations.

## 38.2 Android Studio and the Android Gradle Plugin

Android Studio 3.0 and later, and Android Gradle Plugin 3.0.0 and later, support type annotations. (See https://developer.android.com/studio/write/java8-support for more details.) This section explains how to configure your Android project to use the Checker Framework. All the changes should be made to the module’s build.gradle file — not the project’s build.gradle file.

1. Add a build variant for running checkers:

        android {
           ...
             buildTypes {
             ...
               checkTypes {
                   javaCompileOptions.annotationProcessorOptions.
                           classNames.add("org.checkerframework.checker.nullness.NullnessChecker")
                   // You can pass options like so:
                   // javaCompileOptions.annotationProcessorOptions.arguments.put("warns", "")
               }
           }
       }

2. Declare the Checker Framework dependencies:

       dependencies {
           ... existing dependencies...
           ext.checkerFrameworkVersion = "4.1.0"
           implementation "org.checkerframework:checker-qual-android:${checkerFrameworkVersion}"
           // or if you use no annotations in source code the above line could be
           // compileOnly "org.checkerframework:checker-qual-android:${checkerFrameworkVersion}"
           annotationProcessor "org.checkerframework:checker:${checkerFrameworkVersion}"
       }

3. To run the checkers, build using the checkTypes variant:

       gradlew assembleCheckTypes

## 38.3 Ant task

If you use the Ant build tool to compile your software, then you can add an Ant task that runs a checker. We assume that your Ant file already contains a compilation target that uses the javac task, and that the CHECKERFRAMEWORK environment variable is set.

1. Set the cfJavac property:

         <property environment="env"/>
         <property name="checkerframework" value="${env.CHECKERFRAMEWORK}" />
         <condition property="cfJavac" value="javac.bat" else="javac">
           <os family="windows" />
         </condition>
         <presetdef name="cf.javac">
           <javac fork="yes" executable="${checkerframework}/checker/bin/${cfJavac}" >
             <compilerarg value="-version"/>
             <compilerarg value="-implicit:class"/>
           </javac>
         </presetdef>

2. Duplicate the compilation target, then modify it slightly as indicated in this example:

         <target name="check-nullness"
                 description="Check for null pointer dereferences"
                 depends="clean,...">
           <!-- use cf.javac instead of javac -->
           <cf.javac ... >
             <compilerarg line="-processor org.checkerframework.checker.nullness.NullnessChecker"/>
             <!-- optional, to not check uses of library methods:
               <compilerarg value="-AskipUses=^(java\.awt\.|javax\.swing\.)"/>
             -->
             <compilerarg line="-Xmaxerrs 10000"/>
             ...
           </cf.javac>
         </target>

   Fill in each ellipsis (…) from the original compilation target. However, do not copy any fork= setting from the original <javac> task invocation.

   If your original compilation target set the bootclasspath, then you cannot use the javac wrapper script as the above instructions do. You should edit your Ant buildfile to make invocations similar to those described in Section ‍38.7, but accommodating your bootclasspath.

   In the example, the target is named check-nullness, but you can name it whatever you like.

### 38.3.1 Explanation

This section explains each part of the Ant task.

1. Definition of cf.javac:

   The fork field of the javac task ensures that an external javac program is called. Otherwise, Ant will run javac via a Java method call, and there is no guarantee that it will get correct version of javac.

   The -version compiler argument is just for debugging; you may omit it.

   The -implicit:class compiler argument causes annotation processing to be performed on implicitly compiled files. (An implicitly compiled file is one that was not specified on the command line, but for which the source code is newer than the .class file.) This is the default, but supplying the argument explicitly suppresses a compiler warning.

2. The check-nullness target:

   The target assumes the existence of a clean target that removes all .class files. That is necessary because Ant’s javac target doesn’t re-compile .java files for which a .class file already exists.

   The -processor ... compiler argument indicates which checker to run. You can supply additional arguments to the checker as well.

## 38.4 Bazel

Bazel is a build system maintained by Google.

To run the Checker Framework, see Stack Overflow discussion 48555320 (https://stackoverflow.com/questions/48555320/).

## 38.5 Buck

Buck is a build system maintained by Meta.

Buck has support for annotation processors, but that support is undocumented because the Buck maintainers may change the syntax in the future and they don’t wish to ever change anything that is documented. You can learn more about Buck and annotation processors at these URLs: https://stackoverflow.com/questions/32915721/documentation-for-annotation-processors-buck, https://github.com/facebook/buck/issues/85.

Here is an example BUCK build file showing how to enable the Checker Framework:

    prebuilt_jar(
        name = 'checker-framework',
        binary_jar = 'checker-4.1.0.jar',
        visibility = [ 'PUBLIC' ]
    )

    prebuilt_jar(
        name = 'checker-qual',
        binary_jar = 'checker-qual-4.1.0.jar',
        visibility = [ 'PUBLIC' ]
    )

    java_library (
        name = 'hello',
        srcs = glob(['src/main/java/**/*.java']),
        java_version = '17',
        provided_deps = [ ':checker-framework', ':checker-qual' ],
    # To add annotation processing
        annotation_processors = [ 'org.checkerframework.checker.units.UnitsChecker' ],
        annotation_processor_deps = [ ':checker-framework', ':checker-qual' ],
    )

### 38.5.1 Troubleshooting

Use --verbose 8 to see diagnostic output including a command line that ought to be equivalent to Buck’s use of an in-process Java compiler. For example, you might run

    ./buck clean
    ./buck fetch //...
    ./buck build --verbose 8 //app:main

However, the command lines that buck prints with -v 8 is not what Buck actually executes. Buck’s invocation of the Checker Framework may produce errors that cannot be reproduced from the command line. This is because Buck uses in-process compilation, potentially multi-threaded, and because Buck may use multiple classloaders that are cached.

You can force Buck to run an external javac, thus behaving exactly like the command line that its diagnostics print. Use the configuration flag -c tools.javac=path-to-javac, for example:

    ./buck clean && ./buck fetch //... && ./buck build //app:main -c tools.javac=$(which javac)

## 38.6 Command line, via Checker Framework javac wrapper

To perform pluggable type-checking from the command line, run the javac command that ships with the Checker Framework. This is called the “Checker Framework compiler” or the “Checker Framework javac wrapper”. It is exactly the same as the OpenJDK javac compiler, with one small difference: it includes the Checker Framework jar file on its classpath.

You cannot use the javac wrapper if you wish to set the bootclasspath.

There are three ways to use the Checker Framework compiler from the command line. You can use any one of them. However, if you are using the Windows command shell, you must use the last one. Adjust the pathnames if you have installed the Checker Framework somewhere other than $HOME/checker-framework-4.1.0/.

- Option 1: Add directory .../checker-framework-4.1.0/checker/bin to your path, before any other directory that contains a javac executable.

  If you are using the bash shell, a way to do this is to add the following to your `~/.profile` (or alternately `~/.bash_profile` or `~/.bashrc`) file:

        export CHECKERFRAMEWORK=${HOME}/checker-framework-4.1.0
        export PATH=${CHECKERFRAMEWORK}/checker/bin:${PATH}

  After editing the file, log out and back in to ensure that the environment variable setting takes effect.

- Option 2: Whenever this document tells you to run javac, instead run $CHECKERFRAMEWORK/checker/bin/javac.

  You can simplify this by introducing an alias javacheck. Then, whenever this document tells you to run javac, instead run javacheck. Here is the syntax for your `~/.bashrc`, `~/.profile`, or `~/.bash_profile` file:

        export CHECKERFRAMEWORK=${HOME}/checker-framework-4.1.0
        alias javacheck='$CHECKERFRAMEWORK/checker/bin/javac'

  After editing the file, log out and back in to ensure that the environment variable setting and alias take effect.

- Option 3: Whenever this document tells you to run javac, instead run checker.jar via java (not javac) as in:

        java -jar "$CHECKERFRAMEWORK/checker/dist/checker.jar" -cp "myclasspath" -processor nullness MyFile.java

  You can simplify the above command by introducing an alias javacheck. Then, whenever this document tells you to run javac, instead run javacheck. For example:

        # Unix
        export CHECKERFRAMEWORK=${HOME}/checker-framework-4.1.0
        alias javacheck='java -jar "$CHECKERFRAMEWORK/checker/dist/checker.jar"'

        # Windows
        set CHECKERFRAMEWORK = C:\Program Files\checker-framework-4.1.0\
        doskey javacheck=java -jar "%CHECKERFRAMEWORK%\checker\dist\checker.jar" $*

  (Explanation for advanced users: More generally, anywhere that you would use javac.jar, you can substitute $CHECKERFRAMEWORK/checker/dist/checker.jar; the result is to use the Checker Framework compiler instead of the regular javac.)

## 38.7 Command line, via JDK javac

This section explains how to use the Checker Framework with the OpenJDK or OracleJDK javac, rather than with the javac wrapper script described in Section ‍38.6.

This section assumes you have downloaded the Checker Framework release zip and set the environment variable CHECKERFRAMEWORK to point to the unzipped directory. Alternately, you can get all of the jars mentioned in this section from Maven Central:

- checker-qual.jar: https://repo1.maven.org/maven2/org/checkerframework/checker-qual/4.1.0/checker-qual-4.1.0.jar
- checker-util.jar: https://repo1.maven.org/maven2/org/checkerframework/checker-util/4.1.0/checker-util-4.1.0.jar
- checker.jar: https://repo1.maven.org/maven2/org/checkerframework/checker/4.1.0/checker-4.1.0-all.jar

These instructions should work on JDK 17 or later, but we only test with supported versions of Java: JDK 17, JDK 21, and JDK 25.

#### Non-modularized code

To compile non-modularized code:

    javac \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
    -J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED \
    -J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED \
    -processorpath $CHECKERFRAMEWORK/checker/dist/checker.jar \
    -cp $CHECKERFRAMEWORK/checker/dist/checker-qual.jar \
    -processor org.checkerframework.checker.nullness.NullnessChecker

The arguments are explained above, except for two of them:

1. -J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED which opens the jdk.compiler/com.sun.tools.java.comp package. This is required because the Checker Framework reflectively accesses private members of this package.
2. -J--add-exports=...=ALL-UNNAMED which exports the listed packages. This is required because the Checker Framework accesses non-exported members of these packages.

If this option is missing, javac may issue one of the below warnings:

    WARNING: An illegal reflective access operation has occurred
    WARNING: Illegal reflective access by org.checkerframework.javacutil.Resolver
      (file:$CHECKERFRAMEWORK/checker/dist/checker.jar) to method com.sun.tools.javac.comp.Resolve.findMethod(...)
    WARNING: Please consider reporting this to the maintainers of org.checkerframework.javacutil.Resolver
    WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
    WARNING: All illegal access operations will be denied in a future release

    java.lang.IllegalAccessError: class org.checkerframework.javacutil.AbstractTypeProcessor
      (in unnamed module ...) cannot access class com.sun.tools.javac.processing.JavacProcessingEnvironment
      (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.processing
      to unnamed module ...

#### Modularized code

To compile a module, first add requires org.checkerframework.checker.qual; to your module-info.java. The Checker Framework inserts inferred annotations into bytecode even if none appear in source code, so you must do this even if you write no annotations in your code.

    javac \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
      -J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED \
      -J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED \
      -processorpath $CHECKERFRAMEWORK/checker/dist/checker.jar \
      --module-path $CHECKERFRAMEWORK/checker/dist/checker-qual.jar \
      -processor org.checkerframework.checker.nullness.NullnessChecker

checker-qual.jar must be on the module path rather than the class path, but do not put checker.jar on the processor module path as it is not modularized.

## 38.8 Eclipse

You need to run the Checker Framework via a build tool (Ant, Gradle, Maven, etc.), rather than by supplying the -processor command-line option to the ejc compiler, which is also known as eclipsec. The reason is that the Checker Framework is built upon javac, and ejc represents the Java program differently. (If both javac and ejc implemented JSR 198 ‍[Cro06], then it would be possible to build an annotation processor that works with both compilers.)

There is no dedicated Eclipse plug-in for running the Checker Framework, but it’s still easy to run the Checker Framework. First, create a target/task in your build system to run the Checker Framework. Then, run the target/task from Eclipse. Section ‍38.8.1 gives details for Ant, but other build systems are similar.

### 38.8.1 Using an Ant task

Add an Ant target as described in Section ‍38.3. You can run the Ant target by executing the following steps (instructions copied from https://help.eclipse.org/luna/index.jsp?topic=%2Forg.eclipse.platform.doc.user%2FgettingStarted%2Fqs-84_run_ant.htm):

1. Select build.xml in one of the navigation views and choose Run As > Ant Build... from its context menu.
2. A launch configuration dialog is opened on a launch configuration for this Ant buildfile.
3. In the Targets tab, select the new ant task (e.g., check-interning).
4. Click Run.
5. The Ant buildfile is run, and the output is sent to the Console view.

### 38.8.2 Troubleshooting Eclipse

Eclipse issues an “Unhandled Token in @SuppressWarnings” warning if you write a @SuppressWarnings annotation containing a string that Eclipse does not know about. Unfortunately, Eclipse hard-codes this list.

To eliminate the warnings: disable all “Unhandled Token in @SuppressWarnings” warnings in Eclipse. Look under the menu headings “Java → Compiler → Errors/Warnings → Annotations → Unhandled Token in ’@SuppressWarnings’,” and set it to ignore.

## 38.9 Gradle

To run a checker on a project that uses the Gradle build system, use the Checker Framework Gradle plugin. Its documentation explains how to use it.

## 38.10 IntelliJ IDEA

This section tells you how to make IntelliJ IDEA automatically run a checker on every compile for Java projects and/or modules.

If your project uses a build tool (Ant, Gradle, Maven, etc.), do not use the instructions in this section. Follow the instructions for that build tool instead (they are in a different section of this chapter). To compile your project, run the build tool (possibly from within IntelliJ IDEA, possibly not).

### 38.10.1 Running a checker on every IntelliJ compilation

If your project does not use a build tool, use the following instructions to run a checker within IntelliJ IDEA on every compile:

1. Change the project SDK to 17 or later, as explained at https://www.jetbrains.com/help/idea/sdk.html#change-project-sdk.
2. Make sure the language level for your project is 8 or higher, as explained at https://www.jetbrains.com/help/idea/project-page.html.
3. Create and configure an annotation profile, following the instructions at https://www.jetbrains.com/help/idea/annotation-processors-support.html. When configuring the profile:
   1. Add .../checker-framework/checker/dist/checker.jar to the processor path.
   2. Add checkers to be run during compilation by writing the fully-qualified name of the checker in the “Processor FQ Name” section. An example of a fully-qualified checker name is “org.checkerframework.checker.nullness.NullnessChecker”.
4. Add .../checker-framework/checker/dist/checker-qual.jar, as a dependency to all modules you wish to type check. (They should all have been associated with the annotation profile above.) Instructions appear at https://www.jetbrains.com/help/idea/creating-and-managing-projects.html.

Now, when you compile your code, the checker will be run.

It is necessary to manually inform the IDE via a plugin if an annotation system adds any dependencies beyond those that normally exist in Java. For information about the extension points, see https://youtrack.jetbrains.com/issue/IDEA-159286.

### 38.10.2 Running a checker on every IntelliJ change or save

To make IntelliJ compile on every change or save, follow the instructions at https://www.jetbrains.com/help/idea/compiling-applications.html#auto-build.

You can also configure IntelliJ to automatically save (and thus automatically compile) your work periodically. Instructions appear at https://www.jetbrains.com/help/idea/system-settings.html#sync.

## 38.11 javac diagnostics wrapper

The javac diagnostics wrapper project can post-process the javac diagnostics output into other formats, such as the LSP (Language Server Protocol) JSON style.

## 38.12 Lombok

Project Lombok (https://projectlombok.org/) is a library that generates getter, setter, and builder methods, among other features. For example, if you declare a field:

      @Getter @Setter
      private @Regex String role;

then Lombok will generate getter and setter methods:

      public @Regex String getRole() { return role; }
      public void setRole(@Regex String role) { this.role = role; }

### 38.12.1 Annotations on generated code

As illustrated in the example above, Lombok copies type annotations from fields to generated methods, when the user writes Lombok’s @Getter, @Setter, and @Builder annotations. Lombok does so only for certain type annotations (including all annotations in the Checker Framework distribution); see variable BASE_COPYABLE_ANNOTATIONS in file HandlerUtil.java.

To make Lombok copy other type annotations from fields to generated code, add those type annotations to the lombok.copyableAnnotations configuration key in your lombok.config file. For example:

      lombok.copyableAnnotations += my.checker.qual.MyTypeAnnotation

Directory docs/examples/lombok contains an example Gradle project that augments the configuration key.

### 38.12.2 Type-checking code with Lombok annotations

If you run the Checker Framework and Lombok in the same javac invocation, the Checker Framework cannot type-check a class that contains Lombok annotations. The way that Lombok changes the class prevents the Checker Framework from seeing any of the class. (The Checker Framework works fine on classes that do not contain Lombok annotations, including if they call Lombok-generated code.)

Therefore, you must run the Checker Framework in a postpass after the javac that runs Lombok has completed. Use the Delombok tool (distributed with Lombok) to generate Java source code, then run the Checker Framework on that. The Checker Framework Gradle plugin does this for you automatically.

### 38.12.3 Maven Projects using Lombok

Lombok Maven Plugin can be used to delombok source code for Maven projects. The Lombok Maven Plugin FAQ discusses how to configure the plugin when code using Lombok is mixed with standard Java code. Once the Lombok Maven plugin is configured correctly, the Checker Framework Maven configuration can be added independently.

## 38.13 Maven

If you use the Maven tool, then you can enable Checker Framework checkers by following the instructions below. These instructions have been tested with Maven 3.9.3, and they should work with any 3.x version of Maven.

See the directory docs/examples/MavenExample/ for examples of the use of Maven build files. This example can be used to verify that Maven is correctly downloading the Checker Framework from the Maven Central Repository and executing it.

Please note that the -AoutputArgsToFile command-line option (see Section ‍36.12.5) and shorthands for built-in checkers (see Section ‍2.2.4) are not available when following these instructions. Both these features are available only when a checker is launched via checker.jar such as when $CHECKERFRAMEWORK/checker/bin/javac is run. The instructions in this section bypass checker.jar and cause the compiler to run a checker as an annotation processor directly.

Debugging your Maven configuration can be tricky because of a bug in maven-compiler-plugin versions before 3.10.1, and still affecting some users with 3.11.0: Maven does not report annotation processor exceptions, even when the -X command-line argument is passed.

1. Declare a dependency on the Checker Framework artifacts from Maven Central. Find the existing <dependencies> section (not within <dependencyManagement>, but somewhere else) and add the following new <dependency> items:

         <dependencies>
           ... existing <dependency> items ...

           <!-- Annotations from the Checker Framework: nullness, interning, locking, ... -->
           <dependency>
             <groupId>org.checkerframework</groupId>
             <artifactId>checker-qual</artifactId>
             <version>4.1.0</version>
           </dependency>
         </dependencies>

   Periodically update to the most recent version, to obtain the latest bug fixes and new features:

         mvn versions:use-latest-versions -Dincludes="org.checkerframework:*"

2. Direct the Maven compiler plugin to use the desired checkers by creating a new profile as shown below (the example uses the Nullness Checker).

   If your POM file does not already use the maven-compiler-plugin plugin, you can use the text below as is. If your POM file already uses the plugin, copy and edit the first profile to incorporate the existing configuration. If the existing configuration uses proc:none, you need to remove that or comment it out.

         <profiles>
           <profile>
             <id>checkerframework</id>
             <!-- If you omit the activation block, run mvn with "-P checkerframework" to run checkers. -->
             <activation>
               <jdk>[17,)</jdk>
             </activation>
             <build>
               <plugins>
                 <plugin>
                   <artifactId>maven-compiler-plugin</artifactId>
                   <version>3.11.0</version>
                   <configuration>
                     <fork>true</fork> <!-- Must fork or else JVM arguments are ignored. -->
                     <annotationProcessorPaths>
                       <path>
                         <groupId>org.checkerframework</groupId>
                         <artifactId>checker</artifactId>
                         <version>4.1.0</version>
                       </path>
                     </annotationProcessorPaths>
                     <annotationProcessors>
                       <!-- Add all the checkers you want to enable here -->
                       <annotationProcessor>org.checkerframework.checker.nullness.NullnessChecker</annotationProcessor>
                     </annotationProcessors>
                     <compilerArgs>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED</arg>
                       <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED</arg>
                       <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED</arg>
                       <arg>-Xmaxerrs</arg>
                       <arg>10000</arg>
                       <arg>-Xmaxwarns</arg>
                       <arg>10000</arg>
                       <!-- <arg>-Awarns</arg> --> <!-- -Awarns turns type-checking errors into warnings. -->
                     </compilerArgs>
                   </configuration>
                 </plugin>
               </plugins>
             </build>
           </profile>
         </profiles>

   If you wish to run checkers while compiling your source code but not your tests, wrap the <configuration>...</configuration> within

       <executions>
         <execution>
           <id>default-compile</id>
           ...
         </execution>
       </executions>

   To compile without using the Checker Framework, pass -P ’!checkerframework’ on the Maven command line.

   Warning: adding

       <compilerArgs>
         ...
         <arg>-verbose</arg>

   may suppress warnings from the stub parser.

If you use animal-sniffer-maven-plugin, you must upgrade to version 1.20 or later. Version 1.19 and earlier are broken. If necessary, you can disable animal sniffer by modifying your POM file’s properties:

      <properties>
        ... existing <property> items ...

        <!-- Needed for animal-sniffer-maven-plugin version 1.19 which is broken (version 1.20 is fixed). -->
        <animal.sniffer.skip>true</animal.sniffer.skip>
      </properties>

### 38.13.1 Maven, with a locally-built version of the Checker Framework

To use a locally-built version of the Checker Framework, first run:

    ./gradlew publishToMavenLocal

Then use the Maven instructions, but modify the version number for the Checker Framework artifacts. Instead of 4.1.0, use the version number that is output when you run ./gradlew version.

### 38.13.2 Maven troubleshooting

If you get an error like “module jdk.compiler does not export com.sun.tools.javac.processing to unnamed module” (discussed in Section ‍38.7) even though your pom.xml contains lines such as

      <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED</arg>

then perhaps you have omitted this line from your pom.xml:

              <fork>true</fork> <!-- Must fork or else JVM arguments are ignored. -->

## 38.14 NetBeans

There are two approaches to running a checker in NetBeans: via modifying the project properties, or via a custom ant target.

Note: The “compile and save” action in NetBeans 8.1 automatically runs the Checker Framework, but this functionality has not yet been incorporated into NetBeans 8.2. Additionally, JDK annotations are currently unavailable on NetBeans 8.1 and 8.2.

### 38.14.1 Adding a checker via the Project Properties window

1. Add the Checker Framework libraries to your project’s library. First, right click on the project in the Projects panel, and select “Properties” in the drop-down menu. Then, in the “Project Properties” window, navigate to the “Libraries” tab.
2. Add checker-qual.jar to the compile-time libraries. To do so, select the “Compile” tab, click the “Add JAR/Folder” button on the right and browse to add $CHECKERFRAMEWORK/checker/dist/checker-qual.jar.
3. Add checker.jar to the processor-path libraries. To do so, select the “Processor” tab, click the “Add JAR/Folder” button on the right and browse to add $CHECKERFRAMEWORK/checker/dist/checker.jar.
4. Enable annotation processor underlining in the editor. Go to “Build>Compiling” and check the box “Enable Annotation Processing,” and under that, “Enable Annotation Processing in Editor.”
5. Add the checker to run, by clicking “Add” next to the box labeled “Annotation Processors” and enter the fully qualified name of the checker (for example, org.checkerframework.checker.nullness.NullnessChecker) and click “OK” to add.

The selected checker should be run on the project either on a save (if Compile on Save is enabled), or when the project is built, and annotation processor output will appear in the editor.

### 38.14.2 Adding a checker via an ant target

1. Set the cfJavac property:

         <property environment="env"/>
         <property name="checkerframework" value="${env.CHECKERFRAMEWORK}" />
         <condition property="cfJavac" value="javac.bat" else="javac">
           <os family="windows" />
         </condition>
         <presetdef name="cf.javac">
           <javac fork="yes" executable="${checkerframework}/checker/bin/${cfJavac}" >
             <compilerarg value="-version"/>
             <compilerarg value="-implicit:class"/>
           </javac>
         </presetdef>

2. Override the -init-macrodef-javac-with-processors target to use cf.javac instead of javac and to run the checker. In this example, a nullness checker is run:

         <target depends="-init-ap-cmdline-properties" if="ap.supported.internal"
               name="-init-macrodef-javac-with-processors">
           <echo message = "${checkerframework}"/>
           <macrodef name="javac" uri="http://www.netbeans.org/ns/j2se-project/3">
               <attribute default="${src.dir}" name="srcdir"/>
               <attribute default="${build.classes.dir}" name="destdir"/>
               <attribute default="${javac.classpath}" name="classpath"/>
               <attribute default="${javac.processorpath}" name="processorpath"/>
               <attribute default="${build.generated.sources.dir}/ap-source-output" name="apgeneratedsrcdir"/>
               <attribute default="${includes}" name="includes"/>
               <attribute default="${excludes}" name="excludes"/>
               <attribute default="${javac.debug}" name="debug"/>
               <attribute default="${empty.dir}" name="sourcepath"/>
               <attribute default="${empty.dir}" name="gensrcdir"/>
               <element name="customize" optional="true"/>
               <sequential>
                   <property location="${build.dir}/empty" name="empty.dir"/>
                   <mkdir dir="${empty.dir}"/>
                   <mkdir dir="@{apgeneratedsrcdir}"/>
                   <cf.javac debug="@{debug}" deprecation="${javac.deprecation}"
                           destdir="@{destdir}" encoding="${source.encoding}"
                           excludes="@{excludes}" fork="${javac.fork}"
                           includeantruntime="false" includes="@{includes}"
                           source="${javac.source}" sourcepath="@{sourcepath}"
                           srcdir="@{srcdir}" target="${javac.target}"
                           tempdir="${java.io.tmpdir}">
                       <src>
                           <dirset dir="@{gensrcdir}" erroronmissingdir="false">
                               <include name="*"/>
                           </dirset>
                       </src>
                       <classpath>
                           <path path="@{classpath}"/>
                       </classpath>
                       <compilerarg line="${endorsed.classpath.cmd.line.arg}"/>
                       <compilerarg line="${javac.profile.cmd.line.arg}"/>
                       <compilerarg line="${javac.compilerargs}"/>
                       <compilerarg value="-processorpath"/>
                       <compilerarg path="@{processorpath}:${empty.dir}"/>
                       <compilerarg line="${ap.processors.internal}"/>
                       <compilerarg line="${annotation.processing.processor.options}"/>
                       <compilerarg value="-s"/>
                       <compilerarg path="@{apgeneratedsrcdir}"/>
                       <compilerarg line="${ap.proc.none.internal}"/>
                       <compilerarg line="-processor org.checkerframework.checker.nullness.NullnessChecker"/>
                       <compilerarg line="-Xmaxerrs 10000"/>
                       <compilerarg line="-Xmaxwarns 10000"/>
                       <customize/>
                   </cf.javac>
               </sequential>
           </macrodef>
         </target>
         <target name="-post-jar">
         </target>

   When Build and Clean Project is used, the output of the checker will now appear in the build console. However, annotation processor output will not appear in the editor.

## 38.15 sbt

sbt is a build tool for Scala, Java, and more.

Adjust the -processor command-line argument for the processor(s) you wish to run (see Section ‍2.2).

    javacOptions ++= Seq(
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED",
        "-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED",
        "-processorpath $CHECKERFRAMEWORK/checker/dist/checker.jar",
        "-cp $CHECKERFRAMEWORK/checker/dist/checker-qual.jar",
        "-processor org.checkerframework.checker.nullness.NullnessChecker"
      )

### 38.15.1 For modularized code

    For compiling modularized code:
    javacOptions ++= Seq(
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
        "-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED",
        "-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED",
        "-processorpath $CHECKERFRAMEWORK/checker/dist/checker.jar",
        "--module-path $CHECKERFRAMEWORK/checker/dist/checker-qual.jar",
        "-processor org.checkerframework.checker.nullness.NullnessChecker"
      )

## 38.16 tIDE

tIDE, an open-source Java IDE, supports the Checker Framework. You can download it from https://sourceforge.net/projects/tide/.

## 38.17 Type inference tools

A type inference tool infers type annotations for a program’s method signatures and fields, so that the programmer does not need to manually annotate the program’s source code. Section ‍34.1 lists type inference tools.

# Chapter ‍39 Frequently Asked Questions (FAQs)

These are some common questions about the Checker Framework and about pluggable type-checking in general. Feel free to suggest improvements to the answers, or other questions to include here.

Contents:

39.1: Motivation for pluggable type-checking
39.1.1: I don’t make type errors, so would pluggable type-checking help me?
39.1.2: Should I use pluggable types (type qualifiers), or should I use Java subtypes?

39.2: Getting started
39.2.1: How do I get started annotating an existing program?
39.2.2: Which checker should I start with?
39.2.3: How can I join the checker-framework-dev mailing list?

39.3: Usability of pluggable type-checking
39.3.1: Are type annotations easy to read and write?
39.3.2: Will my code become cluttered with type annotations?
39.3.3: Will using the Checker Framework slow down my program? Will it slow down the compiler?
39.3.4: How do I shorten the command line when invoking a checker?
39.3.5: Method pre-condition contracts, including formal parameter annotations, make no sense for public methods.

39.4: How to handle warnings
39.4.1: What should I do if a checker issues a warning about my code?
39.4.2: Why does a checker issue a warning even though the code checks a property?
39.4.3: What does a certain Checker Framework warning message mean?
39.4.4: What do square brackets mean in a Checker Framework warning message?
39.4.5: Can a pluggable type-checker guarantee that my code is correct?
39.4.6: What guarantee does the Checker Framework give for concurrent code?
39.4.7: How do I make compilation succeed even if a checker issues errors?
39.4.8: Why does the checker always say there are 100 errors or warnings?
39.4.9: Why does the Checker Framework report an error regarding a type I have not written in my program?
39.4.10: Why does the Checker Framework accept code on one line but reject it on the next?
39.4.11: How can I do run-time monitoring of properties that were not statically checked?

39.5: False positive warnings
39.5.1: What is a “false positive” warning?
39.5.2: How can I improve the Checker Framework to eliminate a false positive warning?
39.5.3: Why doesn’t the Checker Framework infer types for fields and method return types?
39.5.4: Why are some operations legal in constructors but not in methods?
39.5.5: Why doesn’t the Checker Framework track relationships between variables?
39.5.6: Why isn’t the Checker Framework path-sensitive?

39.6: Syntax of type annotations
39.6.1: What is a “receiver”?
39.6.2: What is the meaning of an annotation after a type, such as @NonNull Object @Nullable?
39.6.3: What is the meaning of array annotations such as @NonNull Object @Nullable []?
39.6.4: What is the meaning of varargs annotations such as @English String @NonEmpty ‍...?
39.6.5: What is the meaning of a type qualifier at a class declaration?
39.6.6: How are type qualifiers written on upper and lower bounds?
39.6.7: Why shouldn’t a qualifier apply to both types and declarations?
39.6.8: How do I annotate a fully-qualified type name?
39.6.9: What is the difference between type annotations and declaration annotations?
39.6.10: How should type annotations be formatted in source code? Where should I write type annotations?
39.6.11: How does the Checker Framework handle obsolete declaration annotations?
39.6.12: How do I convert from other tools’ declaration annotations to type annotations?

39.7: Semantics of type annotations
39.7.1: How can I handle typestate, or phases of my program with different data properties?
39.7.2: Why are explicit and implicit bounds defaulted differently?
39.7.3: How should I annotate code that uses generics?
39.7.4: Why are type annotations declared with @Retention(RetentionPolicy.RUNTIME)?

39.8: Creating a new checker
39.8.1: How do I create a new checker?
39.8.2: What properties can and cannot be handled by type-checking?
39.8.3: Why is there no declarative syntax for writing type rules?

39.9: Tool questions
39.9.1: How does pluggable type-checking work?
39.9.2: What classpath is needed to use an annotated library?
39.9.3: Why do .class files contain more annotations than the source code?
39.9.4: Is there a type-checker for managing checked and unchecked exceptions?
39.9.5: The Checker Framework runs too slowly
39.9.6: What does the Checker Framework version number mean?

39.10: Relationship to other tools
39.10.1: Why not just use a bug detector (like SpotBugs or Error Prone)?
39.10.2: How does the Checker Framework compare with Eclipse’s Null Analysis?
39.10.3: How does the Checker Framework compare with NullAway?
39.10.4: How does the Checker Framework compare with JSpecify?
39.10.5: How does the Checker Framework compare with the EISOP Checker Framework?
39.10.6: How does the Checker Framework compare with the JDK’s Optional type?
39.10.7: How does pluggable type-checking compare with JML?
39.10.8: Is the Checker Framework an official part of Java?
39.10.9: What is the relationship between the Checker Framework and JSR 305?
39.10.10: What is the relationship between the Checker Framework and JSR 308?

## 39.1 Motivation for pluggable type-checking

### 39.1.1 I don’t make type errors, so would pluggable type-checking help me?

Occasionally, a developer says that he makes no errors that type-checking could catch, or that any such errors are unimportant because they have low impact and are easy to fix. When I investigate the claim, I invariably find that the developer is mistaken.

Very frequently, the developer has underestimated what type-checking can discover. Not every type error leads to an exception being thrown; and even if an exception is thrown, it may not seem related to classical types. Remember that a type system can discover null pointer dereferences, incorrect side effects, security errors such as information leakage or SQL injection, partially-initialized data, wrong units of measurement, and many other errors. Every programmer makes errors sometimes and works with other people who do. Even where type-checking does not discover a problem directly, it can indicate code with bad smells, thus revealing problems, improving documentation, and making future maintenance easier.

There are other ways to discover errors, including extensive testing and debugging. You should continue to use these. But type-checking is a good complement to these. Type-checking is more effective for some problems, and less effective for other problems. It can reduce (but not eliminate) the time and effort that you spend on other approaches. There are many important errors that type-checking and other automated approaches cannot find; pluggable type-checking gives you more time to focus on those.

### 39.1.2 Should I use pluggable types (type qualifiers), or should I use Java subtypes?

In brief, use subtypes when you can, and use type qualifiers when you cannot use subtypes.

For some programming tasks, you can use either a Java subtype (interfaces or subclasses) or a type qualifier. As an example, suppose that your code currently uses String to represent an address. You could use Java subclasses by creating a new Address class and refactor your code to use it, or you could use type qualifiers by creating an @Address annotation and applying it to some uses of String in your code. As another example, suppose that your code currently uses MyClass in two different ways that should not interact with one another. You could use Java subclasses by changing MyClass into an interface or abstract class, defining two subclasses, and ensuring that neither subclass ever refers to the other subclass nor to the parent class.

If Java subclasses solve your problem, then that is probably better. We do not encourage you to use type qualifiers as a poor substitute for classes. An advantage of using classes is that the Java type-checker runs every time you compile your code; by contrast, it is possible to forget to run the pluggable type-checker. However, sometimes type qualifiers are a better choice; here are some reasons:

Backward compatibility
  Using a new class may make your code incompatible with existing libraries or clients. You may need to write a conversion at every call to and return from a library call; these conversions add clutter, reduce performance, and (by creating new objects) break object equality properties. Brian Goetz also notes some problems in an article on the pseudo-typedef antipattern ‍[Goe06].

  It is possible to add annotations to code you do not maintain or cannot change; for example, you can write library annotations (see Chapter ‍35).

Composability
  A Java object has just one class at run time. You care about multiple correctness properties, then using the Java type system requires a combinatorial number of classes (all possible combinations). By contrast, arbitrary type qualifiers can be used at the same time.
No new bugs
  A code change may introduce bugs, whereas adding annotations does not change the run-time behavior.
Broader applicability
  Type annotations can be applied to primitives and to final classes such as String, which cannot be subclassed.
Richer semantics and new supertypes
  Type qualifiers permit you to remove operations, with a compile-time guarantee. More generally, type qualifiers permit creating a new supertype, not just a subtype, of an existing Java type.
More precise type-checking
  The Checker Framework is able to verify the correctness of code that the Java type-checker would reject. Here are a few examples.
  - It uses a dataflow analysis to determine a more precise type for variables after conditional tests or assignments.
  - It treats certain Java constructs more precisely, such as reflection (see Chapter ‍25).
  - It includes special-case logic for type-checking specific methods, such as the Nullness Checker’s treatment of Map.get.
Partial annotation
  You can add type annotations to just part of your code, and suppress warnings at entry points to the annotated portion. By contrast, using a new Java type requires using it everywhere or writing conversion routines at every entry point.
Efficiency
  Type qualifiers have no run-time representation. Therefore, there is no space overhead for separate classes or for wrapper classes for primitives. There is no run-time overhead for due to extra dereferences or dynamic dispatch for methods that could otherwise be statically dispatched.
Less code clutter
  The programmer does not have to convert primitive types to wrappers, which would make the code both uglier and slower. Thanks to defaults and type refinement (Section ‍32.5), you may be able to write and think in terms of the original Java type, rather than having to explicitly write one of the subtypes in all locations.

For more details, see Section ‍39.1.2.

## 39.2 Getting started

### 39.2.1 How do I get started annotating an existing program?

See Section ‍2.4.2.

### 39.2.2 Which checker should I start with?

You should start with a property that matters to you. Think about what aspects of your code cause the most errors, or cost the most time during maintenance, or are the most common to be incorrectly-documented. Focusing on what you care about will give you the best benefits.

When you first start out with the Checker Framework, it’s usually best to get experience with an existing type-checker before you write your own new checker.

Many users are tempted to start with the Nullness Checker (see Chapter ‍3), since null pointer errors are common and familiar. The Nullness Checker works very well, but be warned of three facts that make the absence of null pointer exceptions challenging to verify.

1. Dereferences happen throughout your codebase, so there are a lot of potential problems. By contrast, fewer lines of code are related to locking, regular expressions, etc., so those properties are easier to check.
2. Programmers use null for many different purposes. More seriously, programmers write run-time tests against null, and those are difficult for any static analysis to capture.
3. The Nullness Checker interacts with initialization and map keys.

If null pointer exceptions are most important to you, then by all means use the Nullness Checker. But if you just want to try some type-checker, there are others that are easier to use.

We do not recommend indiscriminately running all the checkers on your code. The reason is that each one has a cost — not just at compile time, but also in terms of code clutter and human time to maintain the annotations. If the property is important to you, is difficult for people to reason about, or has caused problems in the past, then you should run that checker. For other properties, the benefits may not repay the effort to use it. You will be the best judge of this for your own code, of course.

Some of the third-party checkers (see Chapter ‍30) have known bugs that limit their usability. (Report the ones that affect you, so the developers will prioritize fixing them.)

### 39.2.3 How can I join the checker-framework-dev mailing list?

The checker-framework-dev@googlegroups.com mailing list is for Checker Framework developers. Anyone is welcome to join checker-framework-dev, after they have had several pull requests accepted.

Anyone is welcome to send mail to the checker-framework-dev@googlegroups.com mailing list — for implementation details it is generally a better place for discussions than the general checker-framework-discuss@googlegroups.com mailing list.

Anyone is welcome to join checker-framework-discuss@googlegroups.com and send mail to it. This list is for user-focused discussions. It is not for submitting bug reports, which should use the issue tracker (Section ‍40.2).

## 39.3 Usability of pluggable type-checking

### 39.3.1 Are type annotations easy to read and write?

The papers “Practical pluggable types for Java”‍[PAC+08] and “Building and using pluggable type-checkers”‍[DDE+11] discuss case studies in which programmers found type annotations to be natural to read and write. The code continued to feel like Java, and the type-checking errors were easy to comprehend and often led to real bugs.

You don’t have to take our word for it, though. You can try the Checker Framework for yourself.

The difficulty of adding and verifying annotations depends on your program. If your program is well-designed and -documented, then skimming the existing documentation and writing type annotations is extremely easy. Otherwise, you may find yourself spending a lot of time trying to understand, reverse-engineer, or fix bugs in your program, and then just a moment writing a type annotation that describes what you discovered. This process inevitably improves your code. You must decide whether it is a good use of your time. For code that is not causing trouble now and is unlikely to do so in the future (the code is bug-free, and you do not anticipate changing it or using it in new contexts), then the effort of writing type annotations for it may not be justified.

### 39.3.2 Will my code become cluttered with type annotations?

In summary: annotations do not clutter code; they are used much less frequently than generic types, which Java programmers find acceptable; and they reduce the overall volume of documentation that a codebase needs.

As with any language feature, it is possible to write ugly code that over-uses annotations. However, in normal use, very few annotations need to be written. Figure 1 of the paper Practical pluggable types for Java‍[PAC+08] reports data for over 350,000 lines of type-annotated code:

- 1 annotation per 62 lines for nullness annotations (@NonNull, @Nullable, etc.)
- 1 annotation per 1736 lines for interning annotations (@Interned)

These numbers are for annotating existing code. New code that is written with the type annotation system in mind is cleaner and more correct, so it requires even fewer annotations.

Each annotation that a programmer writes replaces a sentence or phrase of English descriptive text that would otherwise have been written in the Javadoc. So, use of annotations actually reduces the overall size of the documentation, at the same time as making it machine-processable and less ambiguous.

### 39.3.3 Will using the Checker Framework slow down my program? Will it slow down the compiler?

Using the Checker Framework has no impact on the execution of your program: the javac compiler emits identical bytecodes whether or not the Checker Framework is used, so there is no run-time effect. Because there is no run-time representation of type qualifiers, there is no way to use reflection to query the qualifier on a given object, though you can use reflection to examine a class/method/field declaration.

Using the Checker Framework does increase compilation time. It can increase the compilation time by 2–10 times — or more, if you run many pluggable type-checkers at once. For workarounds, see Section ‍39.9.5.

### 39.3.4 How do I shorten the command line when invoking a checker?

The compile options to javac can be long to type; for example, javac -processor org.checkerframework.checker.nullness.NullnessChecker .... You can use shorthand for built-in checkers, such as javac -processor nullness ... (see Section ‍2.2.4), or you can use auto-discovery to eliminate the need for the -processor command-line option (see Section ‍2.2.3).

### 39.3.5 Method pre-condition contracts, including formal parameter annotations, make no sense for public methods

Some people go further and say that pre-condition contracts make no sense for any method. This objection is sometimes stated as, "A method parameter should never be annotated as @NonNull. A client could pass any value at all, so the method implementation cannot depend on the value being non-null. Furthermore, if a client passes an illegal value, it is the method’s responsibility to immediately tell the client about the illegal value."

Here is an example that invalidates this general argument. Consider a binary search routine. Its specification requires that clients pass in a sorted array.

      /** Return index of the search key, if it is contained it the sorted array a; otherwise ... */
      int binarySearch(Object @Sorted [] a, Object key)

The binarySearch routine is fast — it runs in O(log n) time where n is the length of the array. If the routine had to validate that its input array is sorted, then it would run in O(n) time, negating all benefit of binary search. In other words, binarySearch should not validate its input!

The nature of a contract is that if the caller violates its responsibilities by passing bad values, then the callee is absolved of its responsibilities. It is polite for the callee to try to provide a useful diagnostic to the misbehaving caller (for example, by raising a particular exception quickly), but it is not required. In such a situation, the callee has the flexibility to do anything that is convenient.

In some cases a routine has a complete specification: the contract permits the caller to pass any value, and the callee is required to throw particular exceptions for particular inputs. This approach is common for public methods, but it is not required and is not always the right thing. As explained in section ‍2.4.3, even when a method has a complete specification, the annotations should indicate normal behavior: behavior that will avoid exceptions.

## 39.4 How to handle warnings and errors

### 39.4.1 What should I do if a checker issues a warning about my code?

For a discussion of this issue, see Section ‍2.4.5.

### 39.4.2 Why does a checker issue a warning even though the code checks a property?

Suppose a checker issues a warning about a line of code. One way to eliminate a warning is to perform a run-time test (Section ‍32.7.4), such as adding if (x != null) before a dereference of x to avoid a warning about a potential null pointer dereference. However, sometimes the checker issues a warning even though the code contains a run-time test. Two common reasons are non-determinism in the expression being tested and or side effects to the expression after the run-time test. For details and examples, see Section ‍32.7.5.

### 39.4.3 What does a certain Checker Framework warning message mean?

Read the error message first; sometimes that is enough to clarify it.

Search through this manual for the text of the warning message or for words that appear in it.

If nothing else explains it, then open an issue (Section ‍40.2). Be sure to say what you think it means or what specific part does not make sense to you, and what you have already done to try to understand it.

### 39.4.4 What do square brackets mean in a Checker Framework warning message?

In a message like this:

      found   : ? extends T extends @UnknownKeyFor Object
      required: [extends @UnknownKeyFor Object super @UnknownKeyFor null]

the square brackets enclose the upper and lower bounds that the type parameter needs to be within.

### 39.4.5 Can a pluggable type-checker guarantee that my code is correct?

Each checker looks for certain errors. You can use multiple checkers to detect more errors in your code, but you will never have a guarantee that your code is completely bug-free.

If the type-checker issues no warning, then you have a guarantee that your code is free of some particular error. There are some limitations to the guarantee.

Most importantly, if you run a pluggable checker on only part of a program, then you only get a guarantee that those parts of the program are error-free. For example, if your code uses a library that has not been type-checked, then the library might be incorrect, or your code might misuse the library. As another example, suppose you have type-checked a framework that clients are intended to extend. You should recommend that clients run the pluggable checker. There is no way to force users to do so, so you may want to retain dynamic checks or use other mechanisms to detect errors.

Section ‍2.3 states other limitations to a checker’s guarantee, such as regarding concurrency. Java’s type system is also unsound in certain situations, such as for arrays and casts (however, the Checker Framework is sound for arrays and casts). Java uses dynamic checks is some places it is unsound, so that errors are thrown at run time. The pluggable type-checkers do not currently have built-in dynamic checkers to check for the places they are unsound. Writing dynamic checkers would be an interesting and valuable project.

Other types of dynamism in a Java application do not jeopardize the guarantee, because the type-checker is conservative. For example, at a method call, dynamic dispatch chooses some implementation of the method, but it is impossible to know at compile time which one it will be. The type-checker gives a guarantee no matter what implementation of the method is invoked.

Even if a pluggable checker cannot give an ironclad guarantee of correctness, it is still useful. It can find errors, exclude certain types of possible problems (e.g., restricting the possible class of problems), improve documentation, and increase confidence in your software.

### 39.4.6 What guarantee does the Checker Framework give for concurrent code?

The Lock Checker (see Chapter ‍10) offers a way to detect and prevent certain concurrency errors.

By default, the Checker Framework assumes that the code that it is checking is sequential: that is, there are no concurrent accesses from another thread. This means that the Checker Framework is unsound for concurrent code, in the sense that it may fail to issue a warning about errors that occur only when the code is running in a concurrent setting. For example, the Nullness Checker issues no warning for this code:

      if (myobject.myfield != null) {
        myobject.myfield.toString();
      }

This code is safe when run on its own. However, in the presence of multithreading, the call to toString may fail because another thread may set myobject.myfield to null after the nullness check in the if condition, but before the if body is executed.

If you supply the -AconcurrentSemantics command-line option, then the Checker Framework assumes that any field can be changed at any time. This limits the amount of type refinement (Section ‍32.7) that the Checker Framework can do.

### 39.4.7 How do I make compilation succeed even if a checker issues errors?

Section ‍2.2 describes the -Awarns command-line option that turns checker errors into warnings, so type-checking errors will not cause javac to exit with a failure status.

### 39.4.8 Why does the checker always say there are 100 errors or warnings?

By default, javac only reports the first 100 errors or warnings. Furthermore, once javac encounters an error, it doesn’t try compiling any more files (but does complete compilation of all the ones that it has started so far).

To see more than 100 errors or warnings, use the javac options -Xmaxerrs and -Xmaxwarns. To convert Checker Framework errors into warnings so that javac will process all your source files, use the option -Awarns. See Section ‍2.2 for more details.

### 39.4.9 Why does the Checker Framework report an error regarding a type I have not written in my program?

Sometimes, a Checker Framework warning message will mention a type you have not written in your program. This is typically because a default has been applied where you did not write a type; see Section ‍32.5. In other cases, this is because type refinement has given an expression a more specific type than you wrote or than was defaulted; see Section ‍32.7. Note that an invocation of an impure method may cause the loss of all information that was determined via type refinement; see Section ‍32.7.5.

### 39.4.10 Why does the Checker Framework accept code on one line but reject it on the next?

Sometimes, the Checker Framework permits code on one line, but rejects the same code a few lines later:

      if (myField != null) {
        myField.hashCode();   // no warning
        someMethod();
        myField.hashCode();   // warning about potential NullPointerException
      }

The reason is explained in the section on type refinement (Section ‍32.7), which also tells how to specify the side effects of someMethod (Section ‍32.7.5).

### 39.4.11 How can I do run-time monitoring of properties that were not statically checked?

Some properties are not checked statically (see Chapter ‍33 for reasons that code might not be statically checked). In such cases, it would be desirable to check the property dynamically, at run time. Currently, the Checker Framework has no support for adding code to perform run-time checking.

Adding such support would be an interesting and valuable project. An example would be an option that causes the Checker Framework to automatically insert a run-time check anywhere that static checking is suppressed. If you are able to add run-time verification functionality, we would gladly welcome it as a contribution to the Checker Framework.

Some checkers have library methods that you can explicitly insert in your source code. Examples include the Nullness Checker’s NullnessUtil.castNonNull method (see Section ‍3.4.1) and the Regex Checker’s RegexUtil class (see Section ‍14.2.4). But, it would be better to have more general support that does not require the user to explicitly insert method calls.

## 39.5 False positive warnings

### 39.5.1 What is a “false positive” warning?

A “false positive” is when the tool reports a potential problem, but the code is actually correct and will never violate the given property at run time.

The Checker Framework aims to be sound; that is, if the Checker Framework does not report any possible errors, then your code is correct.

Every sound tool suffers false positive errors. Wherever the Checker Framework issues an error, you can think of it as saying, “I can’t prove this code is safe,” but the code might be safe for some complex, tricky reason that is beyond the capabilities of its analysis.

If you are sure that the warning is a false positive, you have several options. Perhaps you just need to write annotations, especially on method signatures but perhaps within method bodies as well. Sometimes you can rewrite the code in a clearer way that the Checker Framework can verify, and that might be easier for people to understand, too. If these don’t work, then you can suppress the warning (Section ‍2.4.5). You also might want to report the false positive in the Checker Framework issue tracker (Section ‍40.2), if it appears in real-world, well-written code. Finally, you could improve the Checker Framework to make it more precise, so that it does not suffer that false positive (see Section ‍39.5.2).

### 39.5.2 How can I improve the Checker Framework to eliminate a false positive warning?

As noted in Section ‍39.5.1, every sound analysis tool suffers false positives.

For any given false positive warning, it is theoretically possible to improve the Checker Framework to eliminate it. (But, it’s theoretically impossible to eliminate all false positives. That is, there will always exist some programs that don’t go wrong at run time but for which the Checker Framework issues a warning.)

Some improvements affect the implementation of the type system; they do not add any new types. Such an improvement is invisible to users, except that the users suffer fewer false positive warnings. This type of improvement to the type checker’s implementation is often worthwhile.

Other improvements change the type system or add a new type system. Defining new types is a powerful way to improve precision, but it is costly too. A simpler type system is easier for users to understand, less likely to contain bugs, and more efficient.

By design, each type system in the Checker Framework has limited expressiveness. Our goal is to implement enough functionality to handle common, well-written real-world code, not to cover every possible situation.

When reporting bugs, please focus on realistic scenarios. We are sure that you can make up artificial code that stymies the type-checker! Those bugs aren’t a good use of your time to report nor the maintainers’ time to evaluate and fix. When reporting a bug, it’s very helpful to minimize it to give a tiny example that is easy to evaluate and fix, but please also indicate how it arises in real-world, well-written code.

### 39.5.3 Why doesn’t the Checker Framework infer types for fields and method return types?

Consider the following code. A programmer can tell that all three invocations of format are safe — they never suffer an IllegalFormatException exception at run time:

    class MyClass {

      final String field = "%d";

      String method() {
        return "%d";
      }

      void method m() {
        String local = "%d";
        String.format(local, 5);    // Format String Checker verifies the call is safe
        String.format(field, 5);    // Format String Checker warns the call might not be safe
        String.format(method(), 5); // Format String Checker warns the call might not be safe
      }
    }

However, the Format String Checker can only verify the first call. It issues a false positive warning about the second and third calls.

The Checker Framework can verify all three calls, with no false positive warnings, if you annotate the type of field and the return type of method as @Format(INT).

By default, the Checker Framework infers types for local variables (Section ‍32.7), but not for fields and method return types. (The Checker Framework includes a whole-program type inference tool that infers field and method return types; see Section ‍34.2.) There are several reasons for this design choice.

Separation of specification from implementation
  The designer of an API makes certain promises to clients; these are codified in the API’s specification or contract. The implementation might return a more specific type today, but the designer does not want clients to depend on that. For example, a string might happen to be a regular expression because it contains no characters with special meaning in regexes, but that is not guaranteed to always be true. It’s better for the programmer to explicitly write the intended specification.
Separate compilation
  To infer types for a non-final method, it is necessary to examine every overriding implementation, so that the method’s return type annotation is compatible with all values that are returned by any overriding implementation. In general, examining all implementations is impossible, because clients may override the method. When possible, it is inconvenient to provide all that source code, and it would slow down the type-checker.

  A related issue is that determining which values can be returned by a method m requires analyzing m’s body, which requires analyzing all the methods called by m, and so forth. This quickly devolves to analyzing the whole program. Determining all possible values assigned to a field is equally hard.

  Type-checking is modular — it works on one procedure at a time, examining that procedure and the specifications (not implementations) of methods that it calls. Therefore, type-checking is fast and can work on partial programs. The Checker Framework performs modular type-checking, not a whole-program analysis.

Order of compilation
  When the compiler is called on class Client and class Library, the programmer has no control over which class is analyzed first. When the first class is compiled, it has access only to the signature of the other class. Therefore, a programmer would see inconsistent results depending on whether Client was compiled first and had access only to the declared types of Library, or the Library was compiled first and the compiler refined the types of its methods and fields before Client looked them up.
Consistent behavior with or without pluggable type-checking
  The .class files produced with or without pluggable type-checking should specify the same types for all public fields and methods. If pluggable type-checking changed the those types, then users would be confused. Depending on how a library was compiled, pluggable type-checking of a client program would give different results.

### 39.5.4 Why are some operations legal in constructors but not in methods?

Java permits you to initialize an instance field either in the constructor or in a field initializer. The Checker Framework behaves as if the field initializer assignments are performed at the beginning of the constructor (after superclass construction), because that is how Java executes the program.

Here is an example:

    class MyClass1 {
      @Nullable Object field;
      MyClass1() {
        field = new Object();
        field.toString(); // no possible NullPointerException
      }
    }

    class MyClass2 {
      @Nullable Object field = new Object(); // Same initialization as for MyClass1
      MyClass2() {
        field.toString(); // no possible NullPointerException
      }
    }

    class MyClass3 {
      @Nullable Object field;
      void myMethod() {
        field.toString(); // Nullness Checker warns: possible NullPointerException
      }
    }

For consistency, the Checker Framework treats the MyClass1 and MyClass2 constructors the same.

### 39.5.5 Why doesn’t the Checker Framework track relationships between variables?

The Checker Framework estimates the possible run-time value of each variable, as expressed in a type system. In general, the Checker Framework does estimate relationships between two variables, except for specific relationships listed in Section ‍32.8.

For example, the Checker Framework does not track which variables are equal to one another. The Nullness Checker issues a warning, “dereference of possibly-null reference y”, for expression y.toString():

    void nullnessExample1(@Nullable Object x) {
        Object y = x;
        if (x != null) {
          System.out.println(y.toString());
        }
      }

Code that checks one variable and then uses a different variable is confusing and is often considered poor style.

The Nullness Checker is able to verify the correctness of a small variant of the program, thanks to type refinement (Section ‍32.7):

      void nullnessExample12(@Nullable Object x) {
        if (x != null) {
          Object y = x;
          System.out.println(y.toString());
        }
      }

The difference is that in the first example, nothing was known about x at the time y was set to it, and so the Nullness Checker recorded no facts about y. In the second example, the Nullness Checker knew that x was non-null when y was assigned to it.

In the future, the Checker Framework could be enriched by tracking which variables are equal to one another, a technique called “copy propagation”.

This would handle the above examples, but wouldn’t handle other examples. For example, the following code is safe:

      void requiresPositive(@Positive int arg) {}

      void intExample1(int x) {
        int y = x*x;
        if (x > 0) {
          requiresPositive(y);
        }
      }

      void intExample2(int x) {
        int y = x*x;
        if (y > 0) {
          requiresPositive(x);
        }
      }

However, the Index Checker (Chapter ‍11), which defines the @Positive type qualifier, issues warnings saying that it cannot prove that the arguments are @Positive.

A slight variant of intExample1 can be verified:

      void intExample1a(int x) {
        if (x > 0) {
          int y = x*x;
          requiresPositive(y);
        }
      }

No variant of intExample2 can be verified. It is not worthwhile to make the Checker Framework more complex and slow by tracking rich properties such as arbitrary arithmetic.

As another example of a false positive warning due to arbitrary arithmetic properties, consider the following code:

      void falsePositive2(int arg) {
        Object o;
        if (arg * arg >= arg) { // always true!
          o = new Object();
        }
        o.toString();  // Nullness Checker issues a false positive warning
      }

### 39.5.6 Why isn’t the Checker Framework path-sensitive?

The Checker Framework is not path-sensitive. That is, it maintains one estimate for each variable, and it assumes at every branch (such as if statement) that every choice could be taken.

In the following code, there are two if statements.

      void falsePositive1(boolean b) {
        Object o;
        if (b) {
          o = new Object();
        }
        if (b) {
          o.toString();  // Nullness Checker issues a false positive warning
        }
      }

In general, if code has two if statements in succession, then there are 4 possible paths through the code: [true, true], [true, false], [false, true], and [false, false]. However, for this code only two of those paths are feasible: namely, [true, true] and [false, false].

The Checker Framework is not path-sensitive, so it issues a warning.

The lack of path-sensitivity can be viewed as special case of the fact that the Checker Framework maintains a single estimate for each variable value, rather than tracking relationships between multiple variables (Section ‍39.5.5).

Making the Checker Framework path-sensitive would make it more powerful, but also much more complex and much slower. We have not yet found this necessary.

## 39.6 Syntax of type annotations

There is also a separate FAQ for the type annotations syntax (https://checkerframework.org/jsr308/jsr308-faq.html).

### 39.6.1 What is a “receiver”?

The receiver of a method is the this formal parameter, sometimes also called the “current object”. Within the method declaration, this is used to refer to the receiver formal parameter. At a method call, the receiver actual argument is written before a period and the method name.

The method compareTo takes two formal parameters. At a call site like x.compareTo(y), the two arguments are x and y. It is desirable to be able to annotate the types of both of the formal parameters, and doing so is supported by both Java’s type annotations syntax and by the Checker Framework.

A type annotation on the receiver is treated exactly like a type annotation on any other formal parameter. At each call site, the type of the argument must be a consistent with (a subtype of or equal to) the declaration of the corresponding formal parameter. If not, the type-checker issues a warning.

Here is an example. Suppose that @A Object is a supertype of @B Object in the following declaration:

      class MyClass {
        void requiresA(@A MyClass this) { ... }
        void requiresB(@B MyClass this) { ... }
      }

Then the behavior of four different invocations is as follows:

      @A MyClass myA = ...;
      @B MyClass myB = ...;

      myA.requiresA()    // OK
      myA.requiresB()    // compile-time error
      myB.requiresA()    // OK
      myB.requiresB()    // OK

The invocation myA.requiresB() does not type-check because the actual argument’s type is not a subtype of the formal parameter’s type.

A top-level constructor does not have a receiver. An inner class constructor does have a receiver, whose type is the same as the containing outer class. The receiver is distinct from the object being constructed. In a method of a top-level class, the receiver is named this. In a constructor of an inner class, the receiver is named Outer.this and the result is named this.

A method in an anonymous class has a receiver, but it is not possible to write the receiver in the formal parameter list because there is no name for the type of this.

### 39.6.2 What is the meaning of an annotation after a type, such as @NonNull Object @Nullable?

In a type such as @NonNull Object @Nullable [], it may appear that the @Nullable annotation is written after the type Object. In fact, @Nullable modifies []. See the next FAQ, about array annotations (Section ‍39.6.3).

### 39.6.3 What is the meaning of array annotations such as @NonNull Object @Nullable []?

You should parse this as: (@NonNull Object) (@Nullable []). Each annotation precedes the component of the type that it qualifies.

Thus, @NonNull Object @Nullable [] is a possibly-null array of non-null objects. Note that the first token in the type, “@NonNull”, applies to the element type Object, not to the array type as a whole. The annotation @Nullable applies to the array ([]).

Similarly, @Nullable Object @NonNull [] is a non-null array of possibly-null objects.

Some older tools have inconsistent semantics for annotations on array and varargs elements. Their semantics is unfortunate and confusing; developers should convert their code to use type annotations instead. Section ‍39.6.10 explains how the Checker Framework handles declaration annotations in the meanwhile.

### 39.6.4 What is the meaning of varargs annotations such as @English String @NonEmpty ‍...?

Varargs annotations are treated similarly to array annotations. (A way to remember this is that when you write a varargs formal parameter such as void method(String... x) {}, the Java compiler generates a method that takes an array of strings; whenever your source code calls the method with multiple arguments, the Java compiler packages them up into an array before calling the method.)

Either of these annotations

      void method(String @NonEmpty [] x) {}
      void method(String @NonEmpty ... x) {}

applies to the array: the method takes a non-empty array of strings, or the varargs list must not be empty.

Either of these annotations

      void method(@English String [] x) {}
      void method(@English String ... x) {}

applies to the element type. The annotation documents that the method takes an array of English strings.

### 39.6.5 What is the meaning of a type qualifier at a class declaration?

See Section ‍32.3.

### 39.6.6 How are type qualifiers written on upper and lower bounds?

See Section ‍31.1.2.

### 39.6.7 Why shouldn’t a qualifier apply to both types and declarations?

It is bad style for an annotation to apply to both types and declarations. In other words, every annotation should have a @Target meta-annotation, and the @Target meta-annotation should list either only declaration locations or only type annotations. (It’s OK for an annotation to target both ElementType.TYPE_PARAMETER and ElementType.TYPE_USE, but no other declaration location along with ElementType.TYPE_USE.)

Sometimes, it may seem tempting for an annotation to apply to both type uses and (say) method declarations. Here is a hypothetical example:

> “Each Widget type may have a @Version annotation. I wish to prove that versions of widgets don’t get assigned to incompatible variables, and that older code does not call newer code (to avoid problems when backporting).

> A @Version annotation could be written like so:

>       @Version("2.0") Widget createWidget(String value) { ... }

> @Version("2.0") on the method could mean that the createWidget method only appears in the 2.0 version. @Version("2.0") on the return type could mean that the returned Widget should only be used by code that uses the 2.0 API of Widget. It should be possible to specify these independently, such as a 2.0 method that returns a value that allows the 1.0 API method invocations.”

Both of these are type properties and should be specified with type annotations. No method annotation is necessary or desirable. The best way to require that the receiver has a certain property is to use a type annotation on the receiver of the method. (Slightly more formally, the property being checked is compatibility between the annotation on the type of the formal parameter receiver and the annotation on the type of the actual receiver.) If you do not know what “receiver” means, see Section ‍39.6.1.

Another example of a type-and-declaration annotation that represents poor design is JCIP’s @GuardedBy annotation ‍[GPB+06]. As discussed in Section ‍10.6.1, it means two different things when applied to a field or a method. To reduce confusion and increase expressiveness, the Lock Checker (see Chapter ‍10) uses the @Holding annotation for one of these meanings, rather than overloading @GuardedBy with two distinct meanings.

A final example of a type-and-declaration annotation is some @Nullable or @NonNull annotations that are intended to work both with modern tools that process type annotations and with old tools that were written before Java had type annotations. Such type-and-declaration annotations were a temporary measure, intended to be used until the tool supported Java 8 (which was released in March 2014), and should not be necessary any longer.

### 39.6.8 How do I annotate a fully-qualified type name?

If you write a fully-qualified type name in your program, then the Java language requires you to write a type annotation on the simple name part, such as

      entity.hibernate. @Nullable User x;

If you try to write the type annotation before the entire fully-qualified name, such as

      @Nullable entity.hibernate.User x;  // illegal Java syntax

then you will get an error like one of the following:

    error: scoping construct for static nested type cannot be annotated
    error: scoping construct cannot be annotated with type-use annotation

### 39.6.9 What is the difference between type annotations and declaration annotations?

Java has two distinct varieties of annotation: type annotations and declaration annotations.

A type annotation can be written on any use of a type. It conceptually creates a new, more specific type. That is, it describes what values the type represents.

As an example, the int type contains these values: ..., -2, -1, 0, 1, 2, ...
The @Positive int type contains these values: 1, 2, ...
Therefore, @Positive int is a subtype of int.

A declaration annotation can be written on any declaration (a class, method, or variable). It describes the thing being declared, but does not describe run-time values. Here are examples of declaration annotations:

        @Deprecated    // programmers should not use this class
        class MyClass { ... }

        @Override      // this method overrides a method in a supertype
        void myMethod() { ... }

        @SuppressWarnings(...) // compiler should not warn about the initialization expr
        int myField = INITIALIZATION-EXPRESSION;

Here are examples that use both a declaration annotation and a type annotation:

        @Override
        @Regex String getPattern() { ... }

        @GuardedBy("myLock")
        @NonNull String myField;

Note that the type annotation describes the value, and the declaration annotation says something about the method or use of the field.

As a matter of style, declaration annotations are written on their own line, and type annotations are written directly before the type, on the same line.

### 39.6.10 How should type annotations be formatted in source code? Where should I write type annotations?

The Java Language Specification, section 9.7.4 says: “It is customary, though not required, to write declaration annotations before all other modifiers, and type annotations immediately before the type to which they apply.”

A type annotation should be written immediately before the type it qualifies, on the same line. There should not be any modifiers (such as public) between them. This emphasizes that the type qualifier plus the Java basetype is a logical unit, which aids people reading the code.

      // Wrong formatting
      @Positive
      public int numItems;

      // Correct formatting
      public @Positive int numItems;

      // Wrong formatting
      @Nullable
      public Object getThing() {
        ...
      }

      // Correct formatting
      public @Nullable Object getThing() {
        ...
      }

By contrast, declaration annotations are conventionally written on their own line.

      // Wrong formatting
      @Deprecated URL toURL() {
        ...
      }

      // Correct formatting
      @Deprecated
      URL toURL() {
        ...
      }

The preferred order of modifiers is

1. declaration annotations
2. keyword modifiers such as public and strictfp
3. type annotations
4. type

Here is an example of a declaration annotation and a type annotation on a class declaration:

      @Deprecated
      public @Interned class Level {
        ...
      }

The popular google-java-format tool formats type annotations incorrectly, with type annotations on their own line instead of on the same line with the type they modify. (Actually, you can see from method JavaInputAstVisitor.typeAnnotations() that it handles two of the many @Nullable @NonNull annotations correctly, but all other type annotations incorrectly! This would be fixed by merging pull request #802.) Until this bug in google-java-format is fixed, you have these options:

- Run google-java-format via Spotless and use the formatAnnotations step to correct the formatting of type annotations.
- Run google-java-format via run-google-java-format

If you define a new type qualifier — for example, because you are using the Subtyping Checker (Chapter ‍29) or Fake Enum Checker (Chapter ‍9), or because you have created a new checker (Chapter ‍36) — then the above tools will format your new type qualifiers incorrectly, like declaration qualifiers. Here is how to fix the formatting.

- For Spotless, change your configuration to indicate the new type qualifiers. If you use the Spotless plugin for Gradle, edit your build.gradle file to change formatAnnotations() to (for example) formatAnnotations().addTypeAnnotation("Growable").addTypeAnnotation("Modifiable")....
- For run-google-java-format, make a pull request to change variable type_annotations in file fixup-google-java-format.py.

### 39.6.11 How does the Checker Framework handle obsolete declaration annotations?

When a declaration annotation is an alias for a type annotation, the Checker Framework may move the annotation before replacing it by the canonical version. (If the declaration annotation is in an org.checkerframework package, it is not moved.)

For example,

      import android.support.annotation.NonNull;
      ...
      @NonNull Object [] returnsArray();

is treated as if the programmer had written

      import org.checkerframework.checker.nullness.qual.NonNull;
      ...
      Object @NonNull [] returnsArray();

because Android’s @NonNull annotation is a declaration annotation, which is understood to apply to the top-level return type of the annotated method.

When possible, you should use type annotations rather than declaration annotations, and write them in the correct location (Section ‍39.6.10).

Users who are using old Java 5–7 declaration annotations (for instance, from the FindBugs tool, which has not been maintained since 2015, and from its successor SpotBugs, which has not yet adopted type annotations, even though type annotations were added to Java in 2014) can use annotations in package org.checkerframework.checker.nullness.compatqual to avoid name conflicts. These are available in package checker-compat-qual on Maven Central. Once the users are ready to upgrade to Java 8+ type annotations, those compatibility annotations are no longer necessary.

### 39.6.12 How do I convert from other tools’ declaration annotations to type annotations?

In order to convert from another tool’s declaration annotations to the Checker Framework’s type annotations, you need to change imports, rename annotations, move annotations written on arrays (see Section ‍39.6.11), and move annotations to simple type names.

For example, if a user writes

    import android.support.annotation.Nullable;
    ...
    @Nullable
    public Map.Entry<Object, Object> intersects() { ... }

and then changes the import to org.checkerframework.checker.nullness.qual.Nullable, the code will not compile. (See Section ‍40.1.3 for more details.) The user needs to also move the @Nullable annotation to a location where Java permits type annotations:

    import org.checkerframework.checker.nullness.qual.Nullable;
    ...
    public Map.@Nullable Entry<Object, Object> intersects() { ... }

## 39.7 Semantics of type annotations

### 39.7.1 How can I handle typestate, or phases of my program with different data properties?

Sometimes, your program works in phases that have different behavior. For example, you might have a field that starts out null and becomes non-null at some point during execution, such as after a method is called. You can express this property as follows:

1. Annotate the field type as @MonotonicNonNull.
2. Annotate the method that sets the field as @EnsuresNonNull("myFieldName"). (If method m1 calls method m2, which actually sets the field, then you would probably write this annotation on both m1 and m2.)
3. Annotate any method that depends on the field being non-null as @RequiresNonNull("myFieldName"). The type-checker will verify that such a method is only called when the field isn’t null — that is, the method is only called after the setting method.

You can also use a typestate checker (see Chapter ‍30.30), but they have not been as extensively tested.

### 39.7.2 Why are explicit and implicit bounds defaulted differently?

The following two bits of code have the same semantics under Java, but are treated differently by the Checker Framework’s CLIMB-to-top defaulting rules (Section ‍32.5.3):

    class MyClass<T> { ... }
    class MyClass<T extends Object> { ... }

The difference is the annotation on the upper bound of the type argument T. They are treated in the following way.

    class MyClass<T>                 ==  class MyClass<T extends @TOPTYPEANNO Object>
    class MyClass<T extends Object>  ==  class MyClass<T extends @DEFAULTANNO Object>

@TOPTYPEANNO is the top annotation in the type qualifier hierarchy. For example, for the nullness type system, the top type annotation is @Nullable, as shown in Figure ‍3.1. @DEFAULTANNO is the default annotation for the type system. For example, for the nullness type system, the default type annotation is @NonNull.

In some type systems, the top qualifier and the default are the same. For such type systems, the two code snippets shown above are treated the same. An example is the regular expression type system; see Figure ‍14.1.

The CLIMB-to-top rule reduces the code edits required to annotate an existing program, and it treats types written in the program consistently.

When a user writes no upper bound, as in class C<T> { ... }, then Java permits the class to be instantiated with any type parameter. The Checker Framework behaves exactly the same, no matter what the default is for a particular type system — and no matter whether the user has changed the default locally.

When a user writes an upper bound, as in class C<T extends OtherClass> { ... }, then the Checker Framework treats this occurrence of OtherClass exactly like any other occurrence, and applies the usual defaulting rules. Use of Object is treated consistently with all other types in this location and all other occurrences of Object in the program.

Here are some style guidelines:

- Omit the extends clause when possible. To indicate no constraints on type qualifiers, write class MyClass<T> rather than class MyClass<T extends @TOPTYPEANNO Object>.
- When you write an Object upper bound, give an explicit type annotation. That is, write class C<T extends @DEFAULTANNO Object> { ... } even though it is equivalent to writing class C<T extends Object> { ... }.

  If you write just extends Object, then someone who is reading the code might think that it is irrelevant (which it is in plain Java). Also, some IDEs will suggest removing it.

### 39.7.3 How should I annotate code that uses generics?

Suppose unannotated code contains a declaration of a type parameter <T>. How should you annotate that declaration?

This is really a question about Java’s generic types, so if you understand Java generics, this question is moot. However, Java generics can be hard to understand, so here is a brief explanation of some concrete annotations, using nullness annotations for concreteness.

<T>
  Any type argument may be supplied for T.

  It is equivalent to <T extends @Nullable Object>, because @Nullable Object is the top type in the type hierarchy.

<T extends @Nullable Object>
  Any type argument may be supplied for T.

  It is equivalent to <T>, as noted above.

<T extends Object>
  T can be instantiated by any type whose qualifier is @NonNull.

  The default annotation is @NonNull, and annotation defaults apply to type uses such as Object but not to type variables such as T. Therefore, <T extends Object> is equivalent to <T extends @NonNull Object>. It permits any type argument that is a subtype of @NonNull Object, which is any type argument whose qualifier is @NonNull since @NonNull is the bottom type qualifier in the type qualifier hierarchy.

<T extends @NonNull Object>
  T can be instantiated by any type whose qualifier is @NonNull.

  It is equivalent to <T extends Object>, as noted above.

<@Nullable T>
  T can be instantiated by any type whose qualifier is @Nullable.

  The annotation @Nullable before T applies to T’s implicit lower bound. There is no explicit upper bound (that is, no extends), so the upper bound is the top type, @Nullable Object, just as for <T>, which was discussed above. Therefore, <@Nullable T> is the same as <T super @Nullable void extends @Nullable Object>, except that the latter is not legal Java.

<T super @Nullable String>
  T can be instantiated by any supertype of @Nullable String, which is any supertype of String (Object, Serializable, CharSequence, etc.) so long as its type qualifier is @Nullable.
<T super @NonNull String>
  T can be instantiated by any supertype of @NonNull String. Since @NonNull is the bottom type qualifier, the instantiating type can have any type qualifier.

For more details about how the Checker Framework supports generics and polymorphism, see Chapter ‍31.

### 39.7.4 Why are type annotations declared with @Retention(RetentionPolicy.RUNTIME)?

Annotations such as @NonNull are declared with @Retention(RetentionPolicy.RUNTIME). In other words, these type annotations are available to tools at run time. Such run-time tools could check the annotations (like an assert statement), type-check dynamically-loaded code, check casts and instanceof operations, resolve reflection more precisely, or other tasks that we have not yet thought of. Not many such tools exist today, but the annotation designers wanted to accommodate them in the future.

RUNTIME retention has negligible costs (no run-time dependency, minimal increase in heap size). To avoid these costs, a project can use the checker-qual-android dependency; as explained in Section ‍38.1, it is identical to checker-qual, except that in checker-qual-android annotations have CLASS retention.

For the purpose of static checking at compile time, CLASS retention is sufficient. Note that SOURCE retention would not be sufficient, because of separate compilation: when type-checking a class, the compiler needs to read the annotations on libraries that it uses, and separately-compiled libraries are available to the compiler only as class files.

## 39.8 Creating a new checker

### 39.8.1 How do I create a new checker?

In addition to using the checkers that are distributed with the Checker Framework, you can write your own checker to check specific properties that you care about. Thus, you can find and prevent the bugs that are most important to you.

Chapter ‍36 gives complete details regarding how to write a checker. It also suggests places to look for more help, such as the Checker Framework API documentation (Javadoc) and the source code of the distributed checkers.

To whet your interest and demonstrate how easy it is to get started, here is an example of a complete, useful type-checker.

      @SubtypeOf(Unqualified.class)
      @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
      public @interface Encrypted {}

Section ‍29.2 explains this checker and tells you how to run it.

### 39.8.2 What properties can and cannot be handled by type-checking?

In theory, any property about a program can be expressed and checked within a type system. In practice, types are a good choice for some properties and a bad choice for others.

A type expresses the set of possible values for an expression. Therefore, types are a good choice for any property that is about variable values or provenance.

Types are a poor choice for expressing properties about timing, such as that action B will happen within 10 milliseconds of action A. Types are not good for verifying the results of calculations; for example, they could ensure that code always call an encrypt routine in the appropriate places, but not that the encrypt routine is correctly implemented. Types are not a good solution for preventing infinite loops, except perhaps in special cases.

### 39.8.3 Why is there no declarative syntax for writing type rules?

A type system implementer can declaratively specify the type qualifier hierarchy (Section ‍36.5.2) and the type introduction rules (Section ‍36.8). However, the Checker Framework uses a procedural syntax for specifying type-checking rules (Section ‍36.7). A declarative syntax might be more concise, more readable, and more verifiable than a procedural syntax.

We have not found the procedural syntax to be the most important impediment to writing a checker.

Previous attempts to devise a declarative syntax for realistic type systems have failed; see a technical paper ‍[PAC+08] for a discussion. When an adequate syntax exists, then the Checker Framework can be extended to support it.

## 39.9 Tool questions

### 39.9.1 How does pluggable type-checking work?

The Checker Framework enables you to define a new type system. It finds errors, or guarantees their absence, by performing type-checking that is similar to that already performed by the Java compiler.

Type-checking examines each statement of your program in turn, one at a time.

- Expressions are processed bottom-up. Given types for each sub-expression, the type-checker determines whether the types are legal for the expression’s operator and determines the type of the expression.
- An assignment is legal if the type of the right-hand side is a subtype of the declared type of the left-hand side.
- At a method call, the arguments are legal if they can be assigned to the formal parameters (this is called a “pseudo-assignment” and it follows the normal rules for assignment). The type of the method call is the declared type of the return type, where the method is declared. If the method declaration is not annotated, then a default annotation is used.
- Suppose that method Sub.m overrides method Super.m. The return type of Sub.m must be equal to or a subtype of the return type of Super.m (this is called “covariance”). The type of formal parameter i of Sub.m must be equal to or a supertype of the type of formal parameter i in Super.m (this is called “contravariance”).

### 39.9.2 What classpath is needed to use an annotated library?

Suppose that you distribute a library, which contains Checker Framework annotations such as @Nullable. This enables clients of the library to use the Checker Framework to type-check their programs. To do so, they must have the Checker Framework annotations on their classpath, for instance by using the Checker Framework Compiler.

Clients who do not wish to perform pluggable type-checking do not need to have the Checker Framework annotations (checker-qual.jar) in their classpath, either when compiling or running their programs.

The JVM does not issue a link error if an annotation is not found when a class is loaded. (By contrast, the JVM does issue a link error if a superclass, or a parameter/return/field type, is not found.) Likewise, there is no problem compiling against a library even if the library’s annotations are not on the classpath. These are properties of Java, and are not specific to the Checker Framework’s annotations.

### 39.9.3 Why do .class files contain more annotations than the source code?

A .class file contains an annotation on every type, as computed by defaulting rules; see Section ‍32.4.

When an overridden method has a side effect annotation, the overriding method must have one too. However, if the side effect annotation is declared with the @InheritedAnnotation meta-annotation, the Checker Framework automatically adds the missing annotation. This is the case for most side effect annotations — these annotations are propagated from annotated libraries, such as the JDK, to your code.

### 39.9.4 Is there a type-checker for managing checked and unchecked exceptions?

It is possible to annotate exception types, and any type-checker built on the Checker Framework enforces that type annotations are consistent between throw statements and catch clauses that might catch them.

The Java compiler already enforces that all checked exceptions are caught or are declared to be passed through, so you would use annotations to express other properties about exceptions.

Checked exceptions are an example of a “type and effect” system, which is like a type system but also accounts for actions/behaviors such as side effects. The GUI Effect Checker (Chapter ‍19) is a type-and-effect system that is distributed with the Checker Framework.

### 39.9.5 The Checker Framework runs too slowly

Using a pluggable type-checker increases compile times by a factor of 2–10. Slow compilation speed is probably the worst thing about the Checker Framework.

To improve performance:

- Ensure that the Checker Framework has enough memory. The Checker Framework uses more memory than javac does, and Java’s default heap limit is so small that the Checker Framework might spend most of its time in garbage collection. (If this is the case, the Checker Framework will issue a warning “Garbage collection consumed over 25% of CPU during the past minute.") To permit the Checker Framework to use up to 4GB of memory, pass an argument like -Xmx4g or set an environment variable like export _JAVA_OPTIONS=-Xmx4g. Also consider upgrading to a more recent release of Java, because older versions of Java have worse memory management.
- Set your build system to perform incremental compilation. When compiling just a few source files (the size of a typical edit or commit), you won’t notice the slowdown even if the Checker Framework is very slow. If you compile all files in a large project, you will definitely notice a slowdown. You should structure your build system to make compiling all files rare, by declaring dependencies and using caching. (Note: Maven lacks dependency-driven build and caching. If your project uses Maven, consider switching to a more capable build system such as Gradle.)
- Write generic type arguments. Often, generic type inference is the slowest part of type-checking. You can significantly speed up type-checking by explicitly writing a few generic type arguments. To determine where to write them, temporarily set -AslowTypecheckingSeconds to a small value, such as 1. Write type arguments where slow.typechecking warnings are issued.

If the Checker Framework is still too slow for you to run on every compilation, you can run it periodically, such as in a Git commit hook or in continuous integration.

The Checker Framework team does not currently have the resources to fix performance problems, but we welcome community contributions.

Here are some reasons that the Checker Framework is slow.

- It has to do all the same work as the compiler does, such as resolving overloading and overriding, inferring generics, and type-checking.
- Its analysis is general; it interprets user-defined type systems whereas javac hard-codes one type system and integrates it with other processing.
- Its analysis are much richer. javac can take certain shortcuts that are not correct for all possible type systems.
- It builds a control flow graph and performs a fixpoint analysis on it, which javac does not do.
- It manipulates multiple representations of data, including javac’s internal representation, source code, control flow graph, and its own internal representation. These transformations and lookups take time.
- It switches between dataflow analysis and type analysis, and this sometimes causes it to redo work.
- When running a compound or aggregate checker, it computes the control flow graph multiple times, and it makes multiple passes over the program rather than just one.

### 39.9.6 What does the Checker Framework version number mean?

As explained in Section ‍1.3, a change in the middle number of the Checker Framework version string indicates a possible incompatibility.

Another policy is “semantic versioning” as defined at https://semver.org/. This version number policy, unfortunately, does not define key terms, such as “backwards compatible bug fixes”. It includes an escape hatch, “Semantic Versioning is all about conveying meaning by how the version number changes.” It has not been updated in over a decade (since January 2013), has only ever been updated once (from 1.0.0 to 2.0.0), and has about 100 open issues. It is a proposal, not a standard nor an official definition. Projects that claim to follow it often do so loosely.

If the Checker Framework strictly used semver.org’s definition, every release would be a new major version, and the current version string would be past version 200.0.0. Always incrementing the major version number would not convey meaning to its users — for example, it would not indicate major new functionality or important behavior differences.

## 39.10 Relationship to other tools

### 39.10.1 Why not just use a bug detector (like SpotBugs or Error Prone)?

A pluggable type-checker is a verification tool that prevents or detects all errors of a given variety. If it issues no warnings, your code has no errors of a given variety (for details about the guarantee, see Section ‍2.3).

An alternate approach is to use a bug detector such as Error Prone, FindBugs‍[HP04, HSP05], SpotBugs, Jlint‍[Art01], PMD‍[Cop05], or the tools built into Eclipse (see Section ‍39.10.2) and IntelliJ. The NullAway and Eradicate tools are more like sound type-checking than bug detection, but all of those tools accept unsoundness — that is, false negatives or missed warnings — in exchange for analysis speed.

A pluggable type-checker or verifier differs from a bug detector in several ways:

- A type-checker reports all errors in your code. If a type-checker reports no warnings, then you have a guarantee (a proof) of correctness (Section ‍39.4.5).

  A bug detector aims to find some of the most obvious errors. Even if a bug detector reports no warnings, there may still be errors in your code.

- A type-checker requires you to annotate your code with type qualifiers, or to run an inference tool that does so for you. That is, it requires you to write a specification, then it verifies that your code meets its specification.

  Some bug detectors do not require annotations. This means that it may be easier to get started running a bug detector. The tool makes guesses about the intended behavior of your code, leading to false alarms or missed alarms.

- A verification tool may issue more warnings for a programmer to investigate. Some bug detectors internally generate many warnings, then use heuristics to discard some of them. The cost is missed alarms, when the tool’s heuristics classified the warnings as likely false positives and discarded them.
- A type-checker uses a more sophisticated and precise analysis. For example, it can take advantage of method annotations and annotations on generic type parameters, such as List<@NonNull String>. An example specific to the Nullness Checker (Section ‍3, no other tool correctly handles map keys or initialization.

  A bug detector does a more lightweight analysis. This means that a bug detector usually runs faster, giving feedback to the programmer more rapidly and avoiding slowdowns. Its analysis is often narrow, avoiding properties that are tricky to reason about or that might lead to false alarms. The cost is missed alarms, when analysis is too weak to find the errors.

- Neither type checking nor bug detection subsumes the other. A type-checker finds problems that a bug detector cannot. A bug detector finds problems that a type-checker does not: there is no need for the type-checker to address style rules, when a bug detector is adequate.

  If your code is important to you, it is advantageous to run both types of tools.

For a case study that compared the nullness analysis of FindBugs (equivalent to SpotBugs), Jlint, PMD, and the Checker Framework, see section 6 of the paper “Practical pluggable types for Java”‍[PAC+08]. The case study was on a well-tested program in daily use. The Checker Framework tool found 8 nullness errors (that is, null pointer dereferences). None of the other tools found any errors. A follow-up 10 years later found that Eclipse’s nullness analysis found 0 of the errors, and IntelliJ’s nullness analyses found 3 of the errors, that the Nullness Checker found.

The JSR 308‍[Ern08b] documentation also contains a discussion of related work.

### 39.10.2 How does the Checker Framework compare with Eclipse’s null analysis?

Eclipse comes with a null analysis that can detect potential null pointer errors in your code. Eclipse’s built-in analysis differs from the Checker Framework in several respects.

The Checker Framework’s Nullness Checker (see ‍Chapter ‍3) is more precise: it does a deeper semantic analysis, so it issues fewer false positives than Eclipse. Eclipse’s nullness analysis is missing many features that the Checker Framework supports, such as handling of map keys, partially-initialized objects, method pre- and post-conditions, polymorphism, and a powerful dataflow analysis. These are essential for practical verification of real-world code without poor precision. Furthermore, Eclipse by default ignores unannotated code (even unannotated parameters within a method that contains other annotations). As a result, Eclipse is more useful for bug-finding than for verification, and that is what the Eclipse documentation recommends.

Eclipse assumes by default that all code is multi-threaded, which cripples its local type inference. (This default can be turned off, however.) The Checker Framework allows the user to specify whether code will be run concurrently or not via the -AconcurrentSemantics command-line option (see Section ‍39.4.6).

The Checker Framework builds on javac, so it is easier to run in integration scripts or in environments where not all developers have installed Eclipse.

Eclipse handles only nullness properties and is not extensible, whereas the Checker Framework comes with over 20 type-checkers (for a list, see ‍Chapter ‍1) and is extensible to more properties.

There are also some benefits to Eclipse’s null analysis. It is faster than the Checker Framework, in part because it is less featureful. It is built into Eclipse, so you do not have to add it to your build scripts. Its IDE integration is tighter and slicker.

In a case study, the Nullness Checker found 9 errors in a program, and Eclipse’s analysis found 0.

### 39.10.3 How does the Checker Framework compare with NullAway?

NullAway is a lightweight, unsound type-checker whose aim is similar to that of the Nullness Checker (Chapter ‍3). For both tools, the user writes nullness annotations, and then the tool checks them.

NullAway is faster than the Nullness Checker and requires fewer annotations.

NullAway is unsound: even if NullAway issues no warnings, your code might crash with a null pointer exception. Two differences are that NullAway makes unchecked assumptions about getter methods and NullAway assumes all objects are always fully initialized. NullAway forces all generic arguments to be non-null, which is not an unsoundness but is less flexible than the Nullness Checker.

### 39.10.4 How does the Checker Framework compare with JSpecify?

JSpecify defines the two annotations @NonNull and @Nullable. This is one more possibility in addition to the other definitions of those annotations listed in Section ‍3.7. JSpecify is not an official standard.

You can use the JSpecify annotations or the Checker Framework ones, whichever you prefer. All nullness checkers we know of (e.g., Eclipse, EISOP, IntelliJ, Nullaway, the Nullness Checker, SpotBugs) recognize the JSpecify and Nullness Checker versions of those two type annotations, so there is no tooling reason to prefer one over the other. JSpecify has only nullness annotations, and none for the dozens of other type-checkers that the Checker Framework supports.

The JSpecify specification language is weaker than what the Nullness Checker supports. For example, JSpecify lacks a polymorphic qualifier such as@PolyNull, and it lacks method specifications (Section ‍3.2.2). It lacks annotations for initialization (Section ‍3.8, but you could consider that a benefit, if you always suppress all initialization warnings, opting for convenience and simplicity over sound checking). Even if your code does not use all the specification power of the Nullness Checker, you will benefit from expressive specifications on libraries that your code uses.

If you want precise, sound type-checking, you will need to use some Nullness Checker annotations. You can mix two sets of annotations (those of JSpecify and of the Nullness Checker), or you can use just those of the Nullness Checker. Both approaches work.

JSpecify contains a notion of “unspecified nullness”. It is typically used for warning suppression in unannotated code, but doing so has two problems. First, it makes your specification tool-dependent, because a JSpecify-compliant tool may have any behavior (including issuing warnings or not issuing warnings) for code that uses “unspecified nullness” or use a library that uses “unspecified nullness”. Second, it mixes notions of nullness specification and warning suppression, which are separate concepts. The Checker Framework has well-defined behavior for all specifications, and it has rich facilities for warning suppression (Chapter ‍33). For example, the -Askipuses command-line option (Section ‍33.4) suppresses warnings relating to uses of unannotated libraries.

The Checker Framework does not need JSpecify’s @NullMarked, which makes a checker treat unannotated references as @NonNull. The Nullness Checker by default treats unannotated references as @NonNull. The Checker Framework does not need JSpecify’s @NullUnmarked, which sets the default to “unspecified nullness”. The Nullness Checker supports explicit warning suppressions instead.

### 39.10.5 How does the Checker Framework compare with the EISOP Checker Framework?

The EISOP Checker Framework (also called the EISOP Framework) is a fork of the Checker Framework. The main difference is that the EISOP Nullness Checker supports one interpretation of the full JSpecify “standard” (see Section ‍39.10.4), whereas the Checker Framework’s Nullness Checker only supports the non-ambiguous parts of JSpecify.

The EISOP Checker Framework is an “unfriendly fork” in that it incorporates improvements from the Checker Framework, but its developers do not contribute back to the Checker Framework: they only incorporate their improvements and bug fixes in their own version. The EISOP Checker Framework has introduced some bugs that do not exist in the Checker Framework, and it has fixed some bugs that exist in the Checker Framework, so neither one is strictly more correct than the other. If you use the EISOP Checker Framework, please encourage its developers to be good open-source citizens and contribute back to the Checker Framework. We wish the EISOP project well, because any project that gets more people to verify their code (by writing nullness specifications and running a checker) is a net positive for the programming community.

### 39.10.6 How does the Checker Framework compare with the JDK’s Optional type?

JDK 8 introduced the Optional class, a container that is either empty or contains a non-null value. The Optional Checker (see Chapter ‍5) guarantees that programmers use Optional correctly.

Section ‍3.7.3 explains the relationship between nullness and Optional and the benefits of each.

### 39.10.7 How does pluggable type-checking compare with JML?

JML, the Java Modeling Language ‍[LBR06], is a language for writing formal specifications.

JML aims to be more expressive than pluggable type-checking. A programmer can write a JML specification that describes arbitrary facts about program behavior. Then, the programmer can use formal reasoning or a theorem-proving tool to verify that the code meets the specification. Run-time checking is also possible. By contrast, pluggable type-checking can express a more limited set of properties about your program. Pluggable type-checking annotations are more concise and easier to understand.

JML is not as practical as pluggable type-checking. The JML toolset is less mature. For instance, if your code uses generics or other features of Java 5, then you cannot use JML. However, JML has a run-time checker, which the Checker Framework currently lacks.

### 39.10.8 Is the Checker Framework an official part of Java?

The Checker Framework is not an official part of Java. The Checker Framework relies on type annotations, which became part of Java with Java 8 (released in March 2014). For more about type annotations, see the Type Annotations (JSR 308) FAQ for more details.

### 39.10.9 What is the relationship between the Checker Framework and JSR 305?

JSR 305 aimed to define official Java names for some annotations, such as @NonNull and @Nullable. However, it did not aim to precisely define the semantics of those annotations nor to provide a reference implementation of an annotation processor that validated their use; as a result, JSR 305 was of limited utility as a specification. JSR 305 has been abandoned; there has been no activity by its expert group since 2009.

By contrast, the Checker Framework precisely defines the meaning of a set of annotations and provides powerful type-checkers that validate them. However, the Checker Framework is not an official part of the Java language; it chooses one set of names, but another tool might choose other names.

In the future, the Java Community Process might revitalize JSR 305 or create a replacement JSR to standardize the names and meanings of specific annotations, after there is more experience with their use in practice.

The Checker Framework defines annotations @NonNull and @Nullable that are compatible with annotations defined by JSR 305, SpotBugs, IntelliJ, and other tools; see Section ‍3.7.

### 39.10.10 What is the relationship between the Checker Framework and JSR 308?

JSR 308, also known as the Type Annotations specification, dictates the syntax of type annotations in Java SE 8: how they are expressed in the Java language.

JSR 308 does not define any type annotations such as @NonNull, and it does not specify the semantics of any annotations. Those tasks are left to third-party tools. The Checker Framework is one such tool.

# Chapter ‍40 Troubleshooting, getting help, and contributing

The manual might already answer your question, so first please look for your answer in the manual, including this chapter and the FAQ (Chapter ‍39). If you think you have found a bug in the Checker Framework, please report it (see Section ‍40.2). For discussions of broad interest, you can use the mailing list, checker-framework-discuss@googlegroups.com. For archives and to subscribe, see https://groups.google.com/forum/#!forum/checker-framework-discuss. If you want to help out, you can give feedback (including on the documentation), choose a bug and fix it (start with those labeled ), or select a project from the ideas list at https://checkerframework.org/manual/new-contributor-projects.html.

## 40.1 Common problems and solutions

- To verify that you are using the compiler you think you are, you can add -version to the command line. For instance, instead of running javac -g MyFile.java, you can run javac -version -g MyFile.java. Then, javac will print out its version number in addition to doing its normal processing.

### 40.1.1 Unable to compile the Checker Framework

If you get the following error while compiling the Checker Framework itself:

    checker-framework/stubparser/dist/stubparser.jar(org/checkerframework/stubparser/ast/CompilationUnit.class):
    warning: [classfile] Signature attribute introduced in version 49.0 class files is ignored in version 46.0 class files

and you have used Eclipse to compile the Checker Framework, then probably you are using a very old version of Eclipse. (If you install Eclipse from the Ubuntu 16.04 repository, you get Eclipse version 3.8. Ubuntu 16.04 was released in April 2016, and Eclipse 3.8 was released in June 2012, with subsequent major releases in June 2013, June 2014, and June 2015.) Install the latest version of Eclipse and use it instead.

### 40.1.2 Unable to run the checker, or checker crashes

If you are unable to run the checker, or if the checker or the compiler terminates with an error, then the problem may be a problem with your environment. (If the checker or the compiler crashes, that is a bug in the Checker Framework; please report it. See Section ‍40.2.) This section describes some possible problems and solutions.

- An error that includes java.lang.NoSuchFieldError: RELEASE" means that you are not using the correct compiler (you are not using a Java 9+ compiler).
- If you get the error

      com.sun.tools.javac.code.Symbol$CompletionFailure: class file for com.sun.source.tree.Tree not found

  then you are using the source installation and file tools.jar is not on your classpath. See the installation instructions (Section ‍1.3).

- If you get an error like one of the following,

      ...\build.xml:59: Error running ${env.CHECKERFRAMEWORK}\checker\bin\javac.bat compiler

      .../bin/javac: Command not found

      error: Annotation processor 'org.checkerframework.checker.signedness.SignednessChecker' not found

  then the problem may be that you have not set the CHECKERFRAMEWORK environment variable, as described in Section ‍38.6. Or, maybe you made it a user variable instead of a system variable.

- If you get one of these errors:

      The hierarchy of the type ClassName is inconsistent

      The type com.sun.source.util.AbstractTypeProcessor cannot be resolved.
        It is indirectly referenced from required .class files

  then you are likely not using the Checker Framework compiler. Use either $CHECKERFRAMEWORK/checker/bin/javac or one of the alternatives described in Section ‍38.6.

- If you get the error

        java.lang.ArrayStoreException: sun.reflect.annotation.TypeNotPresentExceptionProxy

  then an annotation is not present at run time that was present at compile time. For example, maybe when you compiled the code, the @Nullable annotation was available, but it was not available at run time.

- The Checker Framework sometimes runs out of memory when processing very large Java files, or files with very large methods.

  If you get an error such as

      error: SourceChecker.typeProcess: unexpected Throwable (OutOfMemoryError) while processing ...
        ; message: GC overhead limit exceeded

  (all on one line), then either give the JVM more memory when running the Checker Framework, or split your files and methods into smaller ones, or both.

- Versions of Error Prone before 2.4.0 are incompatible with the Checker Framework. (More precisely, each version of Error Prone before 2.4.0 is compatible with only one specific version of the Checker Framework.) Those versions of Error Prone use outdated versions of the Checker Framework’s dataflow analysis library, each of which is compatible with only one version of the Checker Framework.

  If you wish to use both Error Prone and the Checker Framework, then use Error Prone version 2.4.0 or later. If you cannot upgrade to Error Prone 2.4.0 or later, there is a way to still use both tools. The Gradle plugin documentation shows how to do so if you use the Gradle build system.

- Versions 0.9.0 and 0.9.1 of NullAway are compatible only with Checker Framework version 3.6.0 through 3.8.0. Those versions of NullAway use version 3.6.0 of the Checker Framework’s dataflow analysis library without shading it. As of version 0.9.2, NullAway uses a shaded version of the library, so this conflict no longer exists. If you want to use a version of the Checker Framework newer than 3.8.0 with NullAway in the same build, then use NullAway 0.9.2 or later.

### 40.1.3 Unexpected warnings not related to type-checking

This section gives solutions for some warning messages that are not related to type errors in your code.

- If you get an error like the following

      error: scoping construct for static nested type cannot be annotated
      error: scoping construct cannot be annotated with type-use annotation

  then you have probably written something like one of the following:

  @Nullable java.util.List
    The correct Java syntax to write an annotation on a fully-qualified type name is to put the annotation on the simple name part, as in java.util.@Nullable List. But, it’s usually better to add import java.util.List to your source file, so that you can just write @Nullable List.
  @Nullable Map.Entry
    You must write Outer.@Nullable StaticNestedClass rather than @Nullable Outer.StaticNestedClass. Since a static nested class does not depend on its outer class, the annotation on the outer class would have no effect and is forbidden.

  Java 8 requires that a type qualifier be written directly on the type that it qualifies, rather than on a scoping mechanism that assists in resolving the name. Examples of scoping mechanisms are package names and outer classes of static nested classes.

  The reason for the Java 8 syntax is to avoid syntactic irregularity. When writing a member nested class (also known as an inner class), it is possible to write annotations on both the outer and the inner class: @A1 Outer. @A2 Inner. Therefore, when writing a static nested class, the annotations should go on the same place: Outer. @A3 StaticNested (rather than @ConfusingAnnotation Outer. Nested where @ConfusingAnnotation applies to Outer if Nested is a member class and applies to Nested if Nested is a static class). It’s not legal to write an annotation on the outer class of a static nested class, because neither annotations nor instantiations of the outer class affect the static nested class.

  Similar arguments apply when annotating pakkage.Outer.Nested.

- An “error: package ... does not exist” or “error: cannot find symbol” about classes in your own project or its dependencies means that you have left your own project or its dependencies off the classpath when you invoked the compiler.

### 40.1.4 Unexpected type-checking results

This section describes possible problems that can lead the type-checker to give unexpected results.

- If the Checker Framework is unable to verify a property that you know is true, then you should formulate a proof about why the property is true. Your proof depends on some set of facts about the program and its operation. State them completely; for example, don’t write “the field f cannot be null when execution reaches line 22”, but justify why the field cannot be null.

  Once you have written down your proof, translate each fact into a Java annotation.

  If you are unable to express some aspect of your proof as an annotation, then the type system is not capable of reproducing your proof. You might need to find a different proof, or extend the type system to be more expressive, or suppress the warning.

  If you are able to express your entire proof as annotations, then look at the Checker Framework error messages.

  - Perhaps your proof was incomplete or incorrect. Your proof might depend on some fact you didn’t write down, such as “method m has no side effects.” In this case, you should revise your proof and add more annotations to your Java code.
  - Perhaps your proof is incorrect, and the errors indicate where.
  - Perhaps there is a bug in the type-checker. In this case, you should report it to the maintainers, giving your proof and explaining why the type-checker should be able to make the same inferences that you did.

  Recall that the Checker Framework does modular verification, one procedure at a time; it observes the specifications, but not the implementations, of other methods.

  Also see Section ‍2.4.5, which explains this same methodology in different words.

- If a checker seems to be ignoring the annotation on a method, then it is possible that the checker is reading the method’s signature from its .class file, but the .class file was not created by a Java 8 or later compiler. You can check whether the annotations actually appear in the .class file by using the javap tool.

  If the annotations do not appear in the .class file, here are two ways to solve the problem:

  - Re-compile the method’s class with the Checker Framework compiler. This will ensure that the type annotations are written to the class file, even if no type-checking happens during that execution.
  - Pass the method’s file explicitly on the command line when type-checking, so that the compiler reads its source code instead of its .class file.
- If a checker issues a warning about a property that it accepted (or that was checked) on a previous line, then probably there was a side-effecting method call in between that could invalidate the property. For example, in this code:

      if (currentOutgoing != null && !message.isCompleted()) {
          currentOutgoing.continueBuffering(message);
      }

  the Nullness Checker will issue a warning on the second line:

      warning: [dereference.of.nullable] dereference of possibly-null reference currentOutgoing
          currentOutgoing.continueBuffering(message);
          ^

  If currentOutgoing is a field rather than a local variable, and isCompleted() is not a pure method, then a null pointer dereference can occur at the given location, because isCompleted() might set the field currentOutgoing to null.

  If you want to communicate that isCompleted() does not set the field currentOutgoing to null, you can use @Pure, @SideEffectFree, or @EnsuresNonNull on the declaration of isCompleted(); see Sections ‍32.7.5 and ‍3.2.2.

- If a checker issues a type-checking error for a call that the library’s documentation states is correct, then maybe that library method has not yet been annotated, so default annotations are being used.

  To solve the problem, add the missing annotations to the library (see Chapter ‍35). The annotations might appear in stub files (which appear in an .astub file together with the checker’s source code) or in the form of annotated libraries (which appear in https://github.com/typetools/).

- If the compiler reports that it cannot find a method from the JDK or another external library, then maybe the stub file for that class is incomplete.

  To solve the problem, add the missing annotations to the library, as described in the previous item.

  The error might take one of these forms:

      method sleep in class Thread cannot be applied to given types
      cannot find symbol: constructor StringBuffer(StringBuffer)

- If you get an error related to a bounded type parameter and a literal such as null, the problem may be missing defaulting. Here is an example:

      mypackage/MyClass.java:2044: warning: incompatible types in assignment.
            T retval = null;
                       ^
        found   : null
        required: T extends @MyQualifier Object

  A value that can be assigned to a variable of type T extends @MyQualifier Object only if that value is of the bottom type, since the bottom type is the only one that is a subtype of every subtype of T extends @MyQualifier Object. The value null satisfies this for the Java type system, and it must be made to satisfy it for the pluggable type system as well. The typical way to address this is to call addStandardLiteralQualifiers on the LiteralTreeAnnotator.

- An error such as

      MyFile.java:123: error: incompatible types in argument.
                              myModel.addElement("Scanning directories...");
                                                 ^
        found   : String
        required: ? extends Object

  may stem from use of raw types. (“String” might be a different type and might have type annotations.) If your declaration was

        DefaultListModel myModel;

  then it should be

        DefaultListModel<String> myModel;

  Running the regular Java compiler with the -Xlint:unchecked command-line option will help you to find and fix problems such as raw types.

- The error

      error: annotation type not applicable to this kind of declaration
          ... List<@NonNull String> ...

  indicates that you are using a definition of @NonNull that is a declaration annotation, which cannot be used in that syntactic location. For example, many legacy annotations such as those listed in Figures ‍3.2 and 3.3 are declaration annotations. You can fix the problem by instead using a definition of @NonNull that is a type annotation, such as the Checker Framework’s annotations; often this only requires changing an import statement.

- If Eclipse gives the warning

      The annotation @NonNull is disallowed for this location

  then you have the wrong version of the org.eclipse.jdt.annotation classes. Eclipse includes two incompatible versions of these annotations. You want the one with a name like org.eclipse.jdt.annotation_2.0.0.....jar, which you can find in the plugins subdirectory under the Eclipse installation directory. Add this .jar file to your build path.

- When one formal parameter’s annotation references another formal parameter’s name, as in this constructor:

      public String(char value[], @IndexFor("value") int offset, @IndexOrHigh("value") int count) { ... }

  you will get an error such as

      [expression.unparsable] Expression in dependent type annotation invalid:
      Use "#1" rather than "value"

  Section ‍32.8 explains that you need to use a different syntax to refer to a formal parameter:

      public String(char value[], @IndexFor("#1") int offset, @IndexOrHigh("#1") int count) { ... }

- The Checker Framework can issue false positive warnings in dead code. An example is

        void m() {
          String t = "";
          t.toString();
          try {
          } catch (Throwable e) {
              t.toString(); // Nullness Checker issues spurious (dereference.of.nullable)
          }

- If the Checker Framework issues an error or warning that you cannot reproduce using a smaller test case, then try running the Checker Framework with the -AatfDoNotCache command-line argument. If the Checker Framework behaves differently with and without that command-line argument, then there is a bug related to its internal caches. Please report that bug at https://github.com/typetools/checker-framework/issues.

### 40.1.5 Unexpected compilation output when running javac without a pluggable type-checker

A message of the form

      error: annotation values must be of the form 'name=value'
            @LTLengthOf("firstName", "lastName") int index;
                        ^

is caused by incorrect Java syntax. When you supply a set of multiple values as an annotation argument, you need to put curly braces around them:

            @LTLengthOf({"firstName", "lastName"}) int index;

## 40.2 How to report problems (bug reporting)

If you have a problem with any checker, or with the Checker Framework, please file a bug at https://github.com/typetools/checker-framework/issues. (First, check whether there is an existing bug report for that issue.) If the problem is with an incorrect or missing annotation on a library, including the JDK, see Section ‍40.2.1. You can also use the issue tracker to make suggestions and feature requests. We also welcome pull requests with annotated libraries, bug fixes, new features, new checkers, and other improvements.

Please ensure that your bug report is clear and that it is complete. Otherwise, we may be unable to understand it or to reproduce it, either of which would prevent us from helping you. Your bug report should include at least the following 4 parts: commands, inputs, outputs, and expectation.

Commands
  Provide one or more commands that can be pasted into a command shell to reproduce the problem.

  In the simplest case, you will provide one command of the form javac -processor ... MyFile.java. In more complex cases, it might be a set of commands clone a repository, check out a branch, and run a build command. Include commands to install software and set environment variables if necessary.

  If you encountered the problem in an IDE, reproduce it from the command line; if you cannot, report the bug to the IDE integration. If you encountered the problem when using a build system, it is most useful to the Checker Framework developers if you provide a single javac command rather than a build file.

  It can be helpful to add
  -version -Aversion -AprintGitProperties -verbose -AprintVerboseGenerics
  to the javac options. This causes the compiler to output debugging information, including its version number.

Inputs
  Include all files that are necessary to reproduce the problem. This includes every file that is used by any of the commands you reported, and possibly other files as well. This is not necessary if the commands you provided obtain the code that is type-checked.

  If you cannot share your code, create a new, small test case. (A small test case is helpful even if your code is not secret!) If the Checker Framework crashed, the progress tracing options (Section ‍36.12.4) can be helpful in determining which file the Checker Framework was processing when it crashed.

  Minimization: If your command invokes a build system such as Gradle or Maven, it can be helpful to the maintainers if you are able to reduce it to a single javac invocation on a single file. This also rules out the possibility that the problem is with the build system integration rather than the checker itself. If you use an invasive annotation processor such as Lombok, then try to reproduce the problem without it — this will indicate whether the problem is in the Checker Framework proper or in its interaction with Lombok.

Output
  Indicate exactly what the result was by attaching a file or using cut-and-paste from your command shell. Don’t merely describe it in words, don’t use a screenshot, and don’t provide just part of the output.
Expectation
  If the problem is not a crash, then indicate what you expected the result to be, since a bug is a difference between desired and actual outcomes. Also, please indicate why you expected that result — explaining your reasoning can reveal how your reasoning is different than the checker’s and which one is wrong. Remember that the checker reasons modularly and intraprocedurally: it examines one method at a time, using only the method signatures of other methods.

  It can also be helpful to indicate what you have already done to try to understand the problem. Did you do any additional experiments? What parts of the manual did you read, and what else did you search for in the manual? Without this information, the maintainers may give you redundant suggestions or may waste time re-doing work you have already done.

When reporting bugs, please focus on realistic scenarios and well-written code. We are sure that you can make up artificial code that stymies the type-checker! Those aren’t a good use of your time to report nor the maintainers’ time to evaluate and fix.

### 40.2.1 Problems with annotated libraries

If a checker reports a warning because a library contains incorrect annotations, then please open a pull request that adds the annotations. If the library’s maintainers have added annotations, make a pull request to them. If the Checker Framework maintainers have added annotations, you can find the annotated source in https://github.com/typetools/.

If a checker reports a warning because a library is not annotated, please do not open a GitHub issue. Instead, we appreciate pull requests to help us improve the annotations. Alternatively or in the interim, you can use stub files as described in Section ‍35.5.

For either approach, please see Chapter ‍35 for tips about writing library annotations.

Thanks for your contribution to the annotated libraries!

## 40.3 Building from source

The Checker Framework release (Section ‍1.3) contains everything that most users need, both to use the distributed checkers and to write your own checkers. This section describes how to compile its binaries from source. You will be using the latest development version of the Checker Framework, rather than an official release.

### 40.3.1 Install prerequisites

You need to install several packages in order to build the Checker Framework. Follow the instructions for your operating system. If your OS is not listed, adapt the instructions for an existing OS; for example, other Linux distributions will be similar to Ubuntu.

Put the setting of the JAVA_HOME environment variable in a startup file such as .bash_profile or .bashrc, if JAVA_HOME is not already set appropriately. It must be the location of your JDK installation (not the JRE installation). You may need to log out and log back in for the setting to take effect.

Ubuntu
  Run the following commands, which should be the same as those in checker/bin-devel/Dockerfile-ubuntu-jdk XX-plus, where XX is a JDK version such as 17, 21, 25, etc.:

  (This list should be the same as in checker/bin-devel/Dockerfile-ubuntu-jdk25-plus; if your build fails because it is missing a package, try the commands there and report a bug in this manual.)

      sudo apt -qqy update
      # Dependencies for building and running the Checker Framework.
      sudo apt -qqy install \
        ant \
        cpp \
        git \
        jq \
        jtreg7 \
        libcurl3-gnutls \
        make \
        maven \
        python3-requests \
        python3-setuptools \
        unzip \
        wget
      # Dependencies for developing the Checker Framework.
      apt -qqy install \
        autoconf \
        devscripts \
        dia \
        graphviz \
        hevea \
        imagemagick \
        junit \
        latexmk \
        librsvg2-bin \
        libasound2-dev libcups2-dev libfontconfig1-dev \
        libx11-dev libxext-dev libxrender-dev libxrandr-dev libxtst-dev libxt-dev \
        pdf2svg \
        rsync \
        shellcheck \
        shfmt \
        texlive-font-utils \
        texlive-fonts-recommended \
        texlive-latex-base \
        texlive-latex-extra \
        texlive-latex-recommended

      # Install uv (manages Python dependencies).
      curl -LsSf https://astral.sh/uv/install.sh | sh
      export PATH=$USER/.local/bin:$PATH

      # Install markdownlint.
      sudo apt -y install \
        npm
      sudo npm install markdownlint-cli2 --global

      # Install the JDK.
      sudo apt -qqy install \
        openjdk-25-jdk \
      && update-java-alternatives --set java-1.25.0-openjdk-amd64
      export JAVA25_HOME=/usr/lib/jvm/java-25-openjdk-amd64

  You may have to answer questions about which time zone your computer is in.

  In a startup file, write:

        export JAVA_HOME=${JAVA_HOME:-$(dirname "$(dirname "$(readlink -f "$(which javac)")")")}

macOS
  If you employ homebrew to install packages, run the following commands:

      brew update
      brew install git graphviz ant hevea maven librsvg unzip make
      brew install --cask temurin17
      brew install --cask mactex

  Note: Running brew install --cask temurin17 is only necessary if you do not already have a JDK that is compatible with the Checker Framework installed.

  Make a latex directory and copy hevea.sty file into it. (https://hevea.inria.fr/ gives the current version number.)

      mkdir -p $HOME/Library/texmf/tex/latex
      cp -p /opt/homebrew/Cellar/hevea/2.38/lib/hevea/hevea.sty $HOME/Library/texmf/tex/latex/

  Note: If copy permission is denied, try sudo.

  If echo $JAVA_HOME outputs nothing (that is, if the JAVA_HOME environment variable is not set), then write the following in a startup file:

      export JAVA_HOME=$(/usr/libexec/java_home -v XX)

  or

      export JAVA_HOME=/Library/Java/JavaVirtualMachines/adoptopenjdk-XX.jdk/Contents/Home/

  In the command, XX is a JDK version such as 17, 21, 25, etc.

Windows
  To build on Windows 10 or later, run bash to obtain a version of Ubuntu (provided by the Windows Subsystem for Linux) and follow the Ubuntu instructions.

### 40.3.2 Obtain the source

Obtain the latest source code from the version control repository:

    git clone https://github.com/typetools/checker-framework.git checker-framework
    export CHECKERFRAMEWORK=$(pwd)/checker-framework

You might want to add an export CHECKERFRAMEWORK=... line to your .bashrc file.

### 40.3.3 Build the Checker Framework

1. As with any Gradle project, run ./gradlew assemble to build the Checker Framework:

         cd $CHECKERFRAMEWORK
         ./gradlew assemble

   Your antivirus program (e.g., Windows Security Virus & threat protection) might make the build run very slowly, or might make it seem to stall. Just give it time.

   If compilation fails, but you have not made any changes, then perhaps one of the supporting repositories is in an inconsistent state; see Section ‍40.3.7.

2. Once it is built, you may wish to put the Checker Framework’s javac even earlier in your PATH:

       export PATH=$CHECKERFRAMEWORK/checker/bin:${PATH}

   The Checker Framework’s javac ensures that all required libraries are on your classpath and boot classpath, but is otherwise identical to the javac Java compiler.

   Putting the Checker Framework’s javac earlier in your PATH will ensure that the Checker Framework’s version is used.

3. If you wish to use Maven or Gradle, publish the Checker Framework artifacts to a local Maven repository, run ./gradlew publishToMavenLocal

         cd $CHECKERFRAMEWORK
         ./gradlew publishToMavenLocal

   Then use the Maven or Gradle instructions, but modify the version number for the Checker Framework artifacts. Use the version number that is output when you run ./gradlew version.
4. Test that everything works:
   - Run ./gradlew allTests:

           cd $CHECKERFRAMEWORK
           ./gradlew allTests

   - Run the Nullness Checker examples (see Section ‍3.5).

### 40.3.4 Build the Checker Framework Manual (this document)

1. Install needed packages; see Section ‍40.3.1 for instructions.
2. Run make in the docs/manual directory to build both the PDF and HTML versions of the manual.

### 40.3.5 Code style, IDE configuration, pull requests, etc.

Please see the Checker Framework Developer Manual.

### 40.3.6 Enable continuous integration builds

We strongly recommend that you enable continuous integration (CI) builds on your fork of the projects, so that you learn quickly about errors. See the CI instructions in the Checker Framework Developer Manual.

#### Debugging continuous integration builds

If a continuous integration job is failing, you can reproduce the problem locally. The CI jobs all run within Docker containers. Install Docker on your local computer, then perform commands like the following to get a shell within Docker:

    docker pull mdernst/cf-ubuntu-jdk21
    docker run -it mdernst/cf-ubuntu-jdk21 /bin/bash

Then, you can run arbitrary commands, including those that appear in the CI configuration files. For example:

    git clone --filter=blob:none -b docker-ubuntu-24-10 https://github.com/mernst/checker-framework.git
    cd checker-framework
    export CHECKERFRAMEWORK=$(pwd)
    alias javacheck='$CHECKERFRAMEWORK/checker/bin/javac'
    export NO_WRITE_VERIFICATION_METADATA=true
    checker/bin-devel/test-cftests-junit.sh

### 40.3.7 Related repositories

Building the Checker Framework depends on other repositories, such as Annotated JDK. You need to have compatible versions of each.

After you git pull in one of the repositories, you should do so in the others as well. The script checker/bin-devel/clone-related.sh pulls all the related repositories, so to build the latest development version of the Checker Framework you can do

    cd $CHECKERFRAMEWORK
    git pull
    checker/bin-devel/clone-related.sh

## 40.4 Contributing

We welcome contributions and pull requests. Section ‍40.3 tells you how to set up your development environment to compile the Checker Framework, and the Checker Framework Developer Manual gives more information.

One good way to contribute is to give feedback about your use of the tool. There is a list of potential projects for new contributors at https://checkerframework.org/manual/new-contributor-projects.html. Or, you can fix a bug that you have encountered during your use of the Checker Framework.

### 40.4.1 Contributing fixes (creating a pull request)

Please see the pull requests section of the Checker Framework Developer Manual. Thanks in advance for your contributions!

The easiest bugs to fix for a newcomer are labeled as “good first issue”.

Please do not spam the issue tracker with requests to assign an issue to you. If you want to contribute by fixing an issue, just open a pull request that fixes it. You can do that even if the issue is already assigned to someone.

Please do not spam the issue tracker asking how to get started fixing a bug. If you have concrete questions, please ask them and we will be happy to assist you. If you don’t know how to get started fixing a particular bug, then you should contribute to the project in other ways. Thanks!

## 40.5 Credits and changelog

Differences from previous versions of the checkers and framework can be found in the docs/CHANGELOG.md file. This file is included in the Checker Framework distribution and is also available on the web at https://checkerframework.org/CHANGELOG.md.

Developers who have contributed code to the Checker Framework include Abhijit Kulkarni, Abraham Lin, Adian Qian, Aditya Singh, Akash Srivastava, Alex Liu, Alvin Abdagic, Anant Jain, Anatoly Kupriyanov, Andy Turner, Ankit Yadav, Aosen Xiong, Arie van Deursen, Artem Pyanykh, Arthur Baars, Ashish Rana, Asumu Takikawa, Atul Dada, Ayush Agarwal, Baorui Zhou, Basil Peace, Benno Stein, Bohdan Sharipov, Brian Corcoran, Calvin Loncaric, Charles Chen, Charlie Garrett, Chris Povirk, Chris Toxiadis, Christopher Mackie, Colin S. Gordon, Craig Day, Damika Anupama, Dan Brotherston, Dan Brown, Daniel Gao, Daniel Zhu, David Lazar, David McArthur, Di Wang, Dilraj Singh, Dmitriy Shepelev, Eric Spishak, Ethan Koenig, Farzan Mirshekari, Felipe R. Monteiro, Florian Lanzinger, Gagan Dhakrey, Gautam Korlam, Google Inc. (via @wmdietlGC), Haaris Ahmed, Haifeng Shi, Hamed Taghani, Heath Borders, Ivory Wang, Jakub Vrána, James Yoo, Jason Waataja, Javier Thaine, Jeff Luo, Jenny Xiang, Jeroen Meijer, Jianchu Li, Jiangqi Zhang, Jiasen (Jason) Xu, Joe Schafer, John Krah, John Vandenberg, JohnnyJayJay, Jonathan Burke, Jonathan Nieder, Joshua Peterson, Jugal Mistry, Junhao Hu, Kanak Das, Kartikeya Goswami, Kenneth Knowles, Kivanc Muslu, Konstantin Weitz, Lazaro Clapp, Leo Liu, Liam Miller-Cushon, Lian Sun, Luqman Aden, Mahmood Ali, Manu Sridharan, Mark Roberts, Marko Bekhta, Markus Frohme, Martin Kellogg, Matt Mullen, Maximilian Gama, Michael Bayne, Michael Coblenz, Michael Ernst, Michael Hixson, Michael Sloan, Michal Stehlik, Mier Ta, Mrigank Arora, Muyeed Ahmed, Narges Shadab, Neil Brown, Nhat Dinh, Nhat Nguyen, Nicholas Breen, Nikhil Shinde, Nima Karimipour, Nitin Kumar Das, Oleg Shchelykalnov, Olek Wojnar, Pascal Wittmann, Patrick Meiring, Paul Vines, Paulo Barros, Philip Lai, Piyush Jha, Pratik Bhusal, Prionti Nasir, Priti Chattopadhyay, Rashmi Mudduluru, Ravi Roshan, Renato Athaydes, René Just, René Kraneis, Rob Bygrave, Rui Fernandes, Ruturaj Mohanty, Ryan Oblak, Sadaf Tajik, Sagar Tewari, Sanjay Malakar, Sascha Kehrli, Sean C. Sullivan, Sean McLaughlin, Sebastian Schuberth, Shinya Yoshida, Shubham Kapoor, Shubham Raj, Sidney Monteiro, Simon Gerst, Stefan Heule, Steph Dietzel, Stephan Schroevers, Stuart Pernsteiner, Suzanne Millstein, Thomas Schweizer, Thomas Weißschuh, Tony Wang, Trask Stalnaker, Travis Haagen, Utsav Oza, Vatsal Sura, Vitalii Ananiev, Vivek Singh Solanki, Vladimir Sitnikov, Vlastimil Dort, Weitian Xing, Werner Dietl, Yuki Yang, Zhiping Cai.

In addition, too many users to list have provided valuable feedback, which has improved the toolset’s design and implementation. Thanks for your help!

## 40.6 License

Two different licenses apply to different parts of the Checker Framework.

- The Checker Framework itself is licensed under the GNU General Public License (GPL), version 2, with the classpath exception. This is the same license that OpenJDK is licensed under. Just as compiling your code with javac does not infect your code with the GPL, type-checking your code with the Checker Framework does not infect your code with the GPL. Running the Checker Framework during development has no effect on your intellectual property or licensing.

  If you want to ship the Checker Framework as part of your product, then your product may need to be licensed under the GPL. Because of the classpath exception, you can include parts of the Checker Framework in your product, even if your product uses a different license. For example, Google’s Error Prone and Uber’s NullAway both use the Checker Framework internally, but neither one is licensed under the GPL.

- The more permissive MIT License applies to code that you might want to include in your own program, such as the annotations and run-time utility classes.

For details, see file LICENSE.txt.

## 40.7 Publications

Here are some papers that use the Checker Framework; for instance, they build a type-checker on top of the Checker Framework, or their methodology depends on using the Checker Framework. (It is not papers that merely cite the Checker Framework.) This list is incomplete; if you know of a paper that is missing, please let us know so that we can add it. Most educational use of the Checker Framework is never published, and most commercial use of the Checker Framework is never discussed publicly. Most of the papers are available for free online; do a web search on the title.

- Practical pluggable types for Java (ISSTA 2008) ‍[PAC+08]
- Building and using pluggable type systems with the Checker Framework (ECOOP 2008) ‍[Ern08a]
- Type-Based Object Immutability with Flexible Initialization (ECOOP 2009) ‍[HP09]
- The nullness analyser of Julia (LPAR 2010) ‍[Spo10]
- Static checking of safety critical Java annotations (JTRES 2010) ‍[TPV10]
- Ownership and immutability in generic Java (OOPSLA 2010) ‍[ZPL+10]
- Rethinking the economics of software engineering (FOSER 2010) ‍[SE10]
- Applying uniqueness to the Java language (Master’s Thesis, 2010) ‍[Har10]
- TIFI+: A Type Checker for Object Immutability with Flexible Initialization (Diploma thesis, 2010) ‍[Noa10]
- Building and using pluggable type-checkers (ICSE 2011) ‍[DDE+11]
- Inference of field initialization (ICSE 2011) ‍[SE11]
- EnerJ: approximate data types for safe and general low-power computation (PLDI 2011) ‍[SDF+11]
- Tunable static inference for generic universe types (ECOOP 2011) ‍[DEM11]
- Separating ownership topology and encapsulation with generic universe types (TOPLAS 2011) ‍[DDM11]
- The design, implementation and evaluation of a pluggable type checker for thread-locality in Java (Master’s Thesis, 2011) ‍[She11]
- Towards Effective Inference and Checking of Ownership Types (IWACO 2011) ‍[HM11]
- Static Dominance Inference (TOOLS 2011) ‍[MV11]
- A Static Memory Safety Annotation System for Safety Critical Java (RTSS 2011) ‍[TPNV11]
- Inference and checking of object ownership (ECOOP 2012) ‍[HDME12]
- A type system for regular expressions (FTfJP 2012) ‍[SDE12]
- Verification games: making verification fun (FTfJF 2012) ‍[DDE+12]
- Reim & ReImInfer: checking and inference of reference immutability and method purity (OOPSLA 2012) ‍[HMDE12]
- ReImInfer: method purity inference for Java (FSE 2012) ‍[HM12]
- Inference and checking of context-sensitive pluggable types (FSE 2012) ‍[MH12]
- Documenting Java database access with type annotations (WorldComp 2012) ‍[Ber12]
- Immutability (book chapter, 2013) ‍[PÖZE13]
- JavaUI: Effects for controlling UI object access (ECOOP 2013) ‍[GDEG13]
- Reified type parameters using Java annotations (GPCE 2013) ‍[GBS13]
- Evaluating the Accuracy of Annotations in the Loci 3.0 Pluggable Type Checker (Master’s Thesis, 2013) ‍[Zaz13]
- Composing polymorphic information flow systems with reference immutability (FTfJP 2013) ‍[MH13]
- Type-based taint analysis for Java web applications (FASE 2014) ‍[HDM14]
- A type system for format strings (ISSTA 2014) ‍[WKSE14]
- Collaborative verification of information flow for a high-assurance app store (CCS 2014) ‍[EJM+14]
- Cascade: A universal type qualifier inference tool (ICSE 2015) ‍[VPEJ15]
- An inference and checking framework for context-sensitive pluggable types (PhD thesis, 2014) ‍[Hua14]
- CFL-reachability and context-sensitive integrity types (PPPJ 2014) ‍[MHD14]
- Static analysis of implicit control flow: resolving Java reflection and Android intents (ASE 2015) ‍[BJM+15]
- Scalable and Precise Taint Analysis for Android (ISSTA 2015) ‍[HDMD15]
- A context-sensitive security type system for Java (Master’s thesis, 2015) ‍[Kai15]
- Locking discipline inference and checking (ICSE 2016) ‍[ELM+16]
- Semantics for locking specifications (NFM 2016) ‍[EMMS16]
- JCrypt: Towards computation over encrypted data (PPPJ 2016) ‍[DMD16a]
- Preventing signedness errors in numerical computations in Java (FSE 2016) ‍[Mac16]
- Enforcing correct array indexes with a type system (FSE 2016) ‍[San16]
- Gradual Pluggable Typing in Java (Master’s Thesis, 2016) ‍[Bro16]
- Security analysis of the IRMA app using SPARTA and fuzzing (Bachelor Thesis, 2016) ‍[Bri16]
- Static Analysis and Program Transformation for Secure Computation on the Cloud (ISSTA 2016 Doctoral Symposium) ‍[DMD16b]
- Towards using concurrent Java API correctly (ICECCS 2016) ‍[LBSD16]
- Reflection-aware static analysis of Android apps (ASE 2016) ‍[LBOK16]
- A crowdsourcing game for formal software verification (Bachelor’s thesis) ‍[Iri16]
- Exploring Language Support for Immutability (ICSE 2016) ‍[CSA+16]
- Granullar: Gradual nullable types for Java (CC 2017) ‍[BDL17]
- Type checking for reliable APIs (WAPI 2017) ‍[KS17]
- Glacier: transitive class immutability for Java (ICSE 2017) ‍[CNA+17]
- A General Pluggable Type Inference Framework and its use for Data-flow Analysis (Master’s Thesis, 2017) ‍[Li17]
- Null safety benchmarks for object initialization (ISP RAS, 2017) ‍[Kog17]
- Static analysis and program transformation for secure computation on the cloud (PhD Thesis, 2017) ‍[Don17]
- Spartan Jester: end-to-end information flow control for hybrid Android applications (SPW 2017) ‍[SCN17]
- Lightweight verification of array indexing (ISSTA 2018) ‍[KDME18]
- Safe Stream-based Programming with Refinement Types (ASE 2018) ‍[SCSC18]
- Don’t miss the end: Preventing unsafe end-of-file comparisons (NFM 2018) ‍[CD18]
- Context sensitive typechecking and inference: ownership and immutability (Master’s Thesis, 2018) ‍[Ta18]
- Usable and Sound Static Analysis through its Integration into Automated and Interactive Workflows (Master’s Thesis, 2018) ‍[Val18]
- Pluggable properties for program understanding: Ontic type checking and inference (Master’s Thesis, 2018) ‍[Che18]
- Definite reference mutability (ECOOP 2018) ‍[Mil18]
- Googletest to CUTE converter (Technical report, 2018) ‍[GV18]
- NullAway: Practical type-based null safety for Java (ESEC/FSE 2019) ‍[BCS19]
- Compile-time detection of machine image sniping (ASE 2019) ‍[Kel19]
- Type-Directed Bounding of Collections in Reactive Programs (VMCAI 2019) ‍[LČCT19]
- Scala with Explicit Nulls (Master’s Thesis, 2019) ‍[NR19]
- A machine-checked proof of security for AWS Key Management Service (CCS, 2019) ‍[ABB+19]
- Synthesis of assurance cases for software certification (ICSE NIER 2020) ‍[BKM20]
- Verifying object construction (ICSE 2020) ‍[KRS+20]
- Precise inference of expressive units of measurement types (OOPSLA 2020) ‍[XLD20]
- Rethinking safe consistency in distributed object-oriented programming (OOPSLA 2020) ‍[KEW+20]
- Continuous compliance (ASE 2020) ‍[KSTE20]
- Light-weight verification of cryptographic API usage (Master’s Thesis, 2020) ‍[Xin20]
- Type checking and whole-program inference for value range analysis (Master’s Thesis, 2020) ‍[Xia20]
- User-Centered Design of Principled Programming Languages (PhD thesis, 2020) ‍[Cob20]
- Behavioural types for memory and method safety in a core object-oriented language (APLAS 2020) ‍[BFG+20]
- Scala with Explicit Nulls (ECOOP 2020) ‍[NZL+20]
- Property Types in Java: Combining Type Systems and Deductive Verification (Master’s Thesis, 2021) ‍[Lan21]
- Interval Type Inference: Improvements and Evaluations (Master’s Thesis, 2021) ‍[Wan21]
- Coping with the reality: adding crucial features to a typestate-oriented language (Master’s Thesis, 2021) ‍[dLM21]
- Java Typestate Checker (Coordination 2021) ‍[MGR21]
- Ensuring correct cryptographic algorithm and provider usage at compile time (FTFjP 2021) ‍[XCD21]
- Scalability and Precision by Combining Expressive Type Systems and Deductive Verification (OOPSLA 2021) ‍[LWUD21]
- Automatic Annotation of Confidential Data in Java Code (FPS 2021) ‍[BBRS21]
- Accumulation Analysis (ECOOP 2022) ‍[KSSE22]
- Lightweight Verification via Specialized Typecheckers (PhD thesis, 2022) ‍[Kel22]

# References

[ABB+19]
  José ‍Bacelar Almeida, Manuel Barbosa, Gilles Barthe, Matthew Campagna, Ernie Cohen, Benjamin Gregoire, Vitor Pereira, Bernardo Portela, Pierre-Yves Strub, and Serdar Tasiran. A machine-checked proof of security for AWS Key Management Service. In CCS 2019: Proceedings of the 21st ACM Conference on Computer and Communications Security, page 63–78, London, UK, November 2019.
[AQKE09]
  Shay Artzi, Jaime Quinonez, Adam Kieżun, and Michael ‍D. Ernst. Parameter reference immutability: Formal definition, inference tool, and comparison. Automated Software Engineering, 16(1):145–192, March 2009.
[Art01]
  Cyrille Artho. Finding faults in multi-threaded programs. Master’s thesis, Swiss Federal Institute of Technology, March ‍15, 2001.
[BBRS21]
  Iulia Bastys, Pauline Bolignano, Franco Raimondi, and Daniel Schoepe. Automatic annotation of confidential data in Java code. In FPS 2021: 14th International Symposium on Foundations & Practice of Security, Paris, France, December 2021.
[BCS19]
  Subarno Banerjee, Lazaro Clapp, and Manu Sridharan. NullAway: Practical type-based null safety for Java. In ESEC/FSE 2019: The ACM 27th joint European Software Engineering Conference and Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 740–750, Tallinn, Estonia, August 2019.
[BDL17]
  Dan Brotherston, Werner Dietl, and Ondřej Lhoták. Granullar: Gradual nullable types for Java. In CC 2017: 26th International Conference on Compiler Construction, pages 87–97, Austin, TX, USA, February 2017.
[Ber12]
  Paul ‍L. Bergstein. Documenting Java database access with type annotations. In WorldComp 2012: Proceedings of The 2012 World Congress in Computer Science, Computer Engineering, and Applied Computing, 2012.
[BFG+20]
  Mario Bravetti, Adrian Francalanza, Iaroslav Golovanov, Hans Hüttel, Mathias ‍S. Jakobsen, Mikkel ‍K. Kettunen, and António Ravara. Behavioural types for memory and method safety in a core object-oriented language. In APLAS 2020: 18th Asian Symposium on Programming Languages and Systems, page 105–124, Fukuoka, Japan, November 2020.
[BJM+15]
  Paulo Barros, René Just, Suzanne Millstein, Paul Vines, Werner Dietl, Marcelo d’Amorim, and Michael ‍D. Ernst. Static analysis of implicit control flow: Resolving Java reflection and Android intents. In ASE 2015: Proceedings of the 30th Annual International Conference on Automated Software Engineering, pages 669–679, Lincoln, NE, USA, November 2015.
[BKM20]
  Hamid Bagheri, Eunsuk Kang, and Niloofar Mansoor. Synthesis of assurance cases for software certification. In ICSE NIER, Proceedings of the 42nd International Conference on Software Engineering, New Ideas and Emerging Results Track, pages 61–64, Seoul, Korea, May 2020.
[Bri16]
  Laurens Brinker. Security analysis of the IRMA app using SPARTA and fuzzing. Bachelor thesis, Radboud University, Nijmegen, Netherlands, 2016.
[Bro16]
  Daniel Brotherston. Gradual pluggable typing in Java. Master’s thesis, U. of Waterloo, Waterloo, Ontario, Canada, 2016.
[CD18]
  Charles ‍Zhuo Chen and Werner Dietl. Don’t miss the end: Preventing unsafe end-of-file comparisons. In NFM 2018: 10th NASA Formal Methods Symposium, pages 87–94, Newport News, VA, USA, April 2018.
[Che18]
  Zhuo Chen. Pluggable properties for program understanding: Ontic type checking and inference. Master’s thesis, U. of Waterloo, Waterloo, Ontario, Canada, 2018.
[CNA+17]
  Michael Coblenz, Whitney Nelson, Jonathan Aldrich, Brad Myers, and Joshua Sunshine. Glacier: Transitive class immutability for Java. In ICSE 2017, Proceedings of the 39th International Conference on Software Engineering, pages 496–506, Buenos Aires, Argentina, May 2017.
[Cob20]
  Michael Coblenz. User-Centered Design of Principled Programming Languages. PhD thesis, Carnegie Mellon University, aug 2020. TR CMU-CS-20-127.
[Cop05]
  Tom Copeland. PMD Applied. Centennial Books, November 2005.
[Cro06]
  Jose Cronembold. JSR 198: A standard extension API for Integrated Development Environments. https://jcp.org/en/jsr/detail?id=198, May ‍8, 2006.
[CSA+16]
  Michael Coblenz, Joshua Sunshine, Jonathan Aldrich, Brad Myers, Sam Weber, and Forrest Shull. Exploring language support for immutability. In ICSE 2016, Proceedings of the 38th International Conference on Software Engineering, pages 736–747, Austin, TX, USA, May 2016.
[Dar06]
  Joe Darcy. JSR 269: Pluggable annotation processing API. https://jcp.org/en/jsr/detail?id=269, May ‍17, 2006. Public review version.
[DDE+11]
  Werner Dietl, Stephanie Dietzel, Michael ‍D. Ernst, Kıvanç Muşlu, and Todd Schiller. Building and using pluggable type-checkers. In ICSE 2011, Proceedings of the 33rd International Conference on Software Engineering, pages 681–690, Waikiki, Hawaii, USA, May 2011.
[DDE+12]
  Werner Dietl, Stephanie Dietzel, Michael ‍D. Ernst, Nathaniel Mote, Brian Walker, Seth Cooper, Timothy Pavlik, and Zoran Popović. Verification games: Making verification fun. In FTfJP: 14th Workshop on Formal Techniques for Java-like Programs, pages 42–49, Beijing, China, June 2012.
[DDM11]
  Werner Dietl, Sophia Drossopoulou, and Peter Müller. Separating ownership topology and encapsulation with Generic Universe Types. ACM Transactions on Programming Languages and Systems, 33(6):20:1–62, December 2011.
[DEM11]
  Werner Dietl, Michael ‍D. Ernst, and Peter Müller. Tunable static inference for Generic Universe Types. In ECOOP 2011 — Object-Oriented Programming, 25th European Conference, pages 333–357, Lancaster, UK, July 2011.
[dLM21]
  João ‍Daniel da ‍Luz ‍Mota. Coping with the reality: adding crucial features to a typestate-oriented language. Master’s thesis, Universidade Nova de Lisboa, Faculdade de Ciências e Tocnologia, February 2021.
[DMD16a]
  Yao Dong, Ana Milanova, and Julian Dolby. JCrypt: Towards computation over encrypted data. In PPPJ 2016: Proceedings of the 13th International Conference on Principles and Practice of Programming in Java: Virtual Machines, Languages, and Tools, Lugano, Switzerland, July 2016.
[DMD16b]
  Yao Dong, Ana Milanova, and Julian Dolby. Static analysis and program transformation for secure computation on the cloud. In ISSTA 2016 Doctoral Symposium, Saarbrücken, Genmany, July 2016.
[Don17]
  Yao Dong. Static analysis and program transformation for secure computation on the cloud. PhD thesis, Rensselaer Polytechnic Institute, December 2017.
[EJM+14]
  Michael ‍D. Ernst, René Just, Suzanne Millstein, Werner Dietl, Stuart Pernsteiner, Franziska Roesner, Karl Koscher, Paulo Barros, Ravi Bhoraskar, Seungyeop Han, Paul Vines, and Edward ‍X. Wu. Collaborative verification of information flow for a high-assurance app store. In CCS 2014: Proceedings of the 21st ACM Conference on Computer and Communications Security, pages 1092–1104, Scottsdale, AZ, USA, November 2014.
[ELM+16]
  Michael ‍D. Ernst, Alberto Lovato, Damiano Macedonio, Fausto Spoto, and Javier Thaine. Locking discipline inference and checking. In ICSE 2016, Proceedings of the 38th International Conference on Software Engineering, pages 1133–1144, Austin, TX, USA, May 2016.
[EMMS16]
  Michael ‍D. Ernst, Damiano Macedonio, Massimo Merro, and Fausto Spoto. Semantics for locking specifications. In NFM 2016: 8th NASA Formal Methods Symposium, pages 355–372, Minneapolis, MN, USA, June 2016.
[Ern08a]
  Michael ‍D. Ernst. Building and using pluggable type systems with the Checker Framework. In ECOOP 2008 — Object-Oriented Programming, 22nd European Conference, Paphos, Cyprus, July 2008. Tool demo.
[Ern08b]
  Michael ‍D. Ernst. Type Annotations specification (JSR 308). https://checkerframework.org/jsr308/, September ‍12, 2008.
[GBS13]
  Prodromos Gerakios, Aggelos Biboudis, and Yannis Smaragdakis. Reified type parameters using java annotations. In GPCE 2013: Proceedings of the 12th International Conference on Generative Programming and Component Engineering, pages 61–64, Indianapolis, IN, USA, October 2013.
[GDEG13]
  Colin ‍S. Gordon, Werner Dietl, Michael ‍D. Ernst, and Dan Grossman. JavaUI: Effects for controlling UI object access. In ECOOP 2013 — Object-Oriented Programming, 27th European Conference, pages 179–204, Montpellier, France, July 2013.
[Goe06]
  Brian Goetz. The pseudo-typedef antipattern: Extension is not type definition. https://web.archive.org/web/20171025205847/https://www.ibm.com/developerworks/java/library/j-jtp02216/, February ‍21, 2006.
[GPB+06]
  Brian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, and Doug Lea. Java Concurrency in Practice. Addison-Wesley, 2006.
[GV18]
  Sascha Gschwind and Renato Venzin. Googletest to CUTE converter. Technical report, University of Applied Sciences Rapperswil, Rapperswil, Switzerland, 2018.
[Har10]
  Artemus Harper. Applying uniqueness to the Java language. Master’s thesis, Washington State University, 2010.
[HDM14]
  Wei Huang, Yao Dong, and Ana Milanova. Type-based taint analysis for Java web applications. In FASE 2014: Fundamental Approaches to Software Engineering, pages 140–154, Grenoble, France, April 2014.
[HDMD15]
  Wei Huang, Yao Dong, Ana Milanova, and Julian Dolby. Scalable and precise taint analysis for Android. In ISSTA 2015, Proceedings of the 2015 International Symposium on Software Testing and Analysis, pages 106–117, Baltimore, MD, USA, July 2015.
[HDME12]
  Wei Huang, Werner Dietl, Ana Milanova, and Michael ‍D. Ernst. Inference and checking of object ownership. In ECOOP 2012 — Object-Oriented Programming, 26th European Conference, pages 181–206, Beijing, China, June 2012.
[HM11]
  Wei Huang and Ana Milanova. Towards effective inference and checking of ownership types. In IWACO 2011: International Workshop on Aliasing, Confinement and Ownership in object-oriented programming, Lancaster, UK, July 2011.
[HM12]
  Wei Huang and Ana Milanova. ReImInfer: Method purity inference for Java. In FSE 2012: Proceedings of the ACM SIGSOFT 20th Symposium on the Foundations of Software Engineering, pages 1–4, Cary, NC, USA, November 2012.
[HMDE12]
  Wei Huang, Ana Milanova, Werner Dietl, and Michael ‍D. Ernst. ReIm & ReImInfer: Checking and inference of reference immutability and method purity. In OOPSLA 2012, Object-Oriented Programming Systems, Languages, and Applications, pages 879–896, Tucson, AZ, USA, October 2012.
[HP04]
  David Hovemeyer and William Pugh. Finding bugs is easy. In OOPSLA Companion: Companion to Object-Oriented Programming Systems, Languages, and Applications, pages 132–136, Vancouver, BC, Canada, October 2004.
[HP09]
  Christian Haack and Erik Poll. Type-based object immutability with flexible initialization. In ECOOP 2009 — Object-Oriented Programming, 23rd European Conference, pages 520–545, Genova, Italy, July 2009.
[HSP05]
  David Hovemeyer, Jaime Spacco, and William Pugh. Evaluating and tuning a static analysis to find null pointer bugs. In PASTE 2005: ACM SIGPLAN/SIGSOFT Workshop on Program Analysis for Software Tools and Engineering (PASTE 2005), pages 13–19, Lisbon, Portugal, September 2005.
[Hua14]
  Wei Huang. An inference and checking framework for context-sensitive pluggable types. PhD thesis, Rennselaer Polytechnic Institute, 2014.
[Iri16]
  Karamitrou Irini. A crowdsourcing game for formal software verification. Bachelor’s thesis, University of Thessaly Polytechnic School Department of Electrical and Computer Engineering, Thessaly, Greece, 2016.
[Kai15]
  Benjamin Kaiser. A context-sensitive security type system for Java. Master’s thesis, Rennselaer Polytechnic Institute, Troy, NY, USA, April 2015.
[KDME18]
  Martin Kellogg, Vlastimil Dort, Suzanne Millstein, and Michael ‍D. Ernst. Lightweight verification of array indexing. In ISSTA 2018, Proceedings of the 2018 International Symposium on Software Testing and Analysis, pages 3–14, Amsterdam, Netherlands, July 2018.
[Kel19]
  Martin Kellogg. Compile-time detection of machine image sniping. In ASE 2019: Proceedings of the 34th Annual International Conference on Automated Software Engineering, pages 1256–1258, San Diego, CA, USA, September 2019.
[Kel22]
  Martin Kellogg. Lightweight Verification via Specialized Typecheckers. PhD thesis, University of Washington Paul G. Allen School of Computer Science and Engineering, Seattle, WA, USA, June 2022.
[KEW+20]
  Mirko Köhler, Nafise Eskandani, Pascal Weisenburger, Alessandro Margara, and Guido Salvaneschi. Rethinking safe consistency in distributed object-oriented programming. In OOPSLA 2020, Object-Oriented Programming Systems, Languages, and Applications, Chicago, IL, USA, November 2020.
[Kog17]
  Alexander ‍V. Kogtenkov. Null safety benchmarks for object initialization. Proceedings of the Institute for System Programming of RAS, 29(6):135–150, 2017.
[KRS+20]
  Martin Kellogg, Manli Ran, Manu Sridharan, Martin Schäf, and Michael ‍D. Ernst. Verifying object construction. In ICSE 2020, Proceedings of the 42nd International Conference on Software Engineering, pages 1447–1458, Seoul, Korea, May 2020.
[KS17]
  Maria Kechagia and Diomidis Spinellis. Type checking for reliable APIs. In WAPI ’17: Proceedings of the 1st International Workshop on API Usage and Evolution, pages 15–18, Buenos Aires, Argentina, May 2017.
[KSSE21]
  Martin Kellogg, Narges Shadab, Manu Sridharan, and Michael ‍D. Ernst. Lightweight and modular resource leak verification. In ESEC/FSE 2021: The ACM 29th joint European Software Engineering Conference and Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 181–192, Athens, Greece, August 2021.
[KSSE22]
  Martin Kellogg, Narges Shadab, Manu Sridharan, and Michael ‍D. Ernst. Accumulation analysis. In ECOOP 2022 — Object-Oriented Programming, 33rd European Conference, pages 10:1–10:31, Berlin, Germany, June 2022.
[KSTE20]
  Martin Kellogg, Martin Schäf, Serdar Tasiran, and Michael ‍D. Ernst. Continuous compliance. In ASE 2020: Proceedings of the 35th Annual International Conference on Automated Software Engineering, pages 511–523, Melbourne, Australia, September 2020.
[Lan21]
  Florian Lanzinger. Property types in Java: Combining type systems and deductive verification. Master’s thesis, Karlsruher Institut für Technologie, feb 2021.
[LBOK16]
  Li ‍Li, Tegawendé ‍F. Bissyandé, Damien Octeau, and Jacques Klein. Reflection-aware static analysis of Android apps. In ASE 2016: Proceedings of the 31st Annual International Conference on Automated Software Engineering, pages 756–761, Singapore, Singapore, September 2016.
[LBR06]
  Gary ‍T. Leavens, Albert ‍L. Baker, and Clyde Ruby. Preliminary design of JML: A behavioral interface specification language for Java. ACM SIGSOFT Software Engineering Notes, 31(3), March 2006.
[LBSD16]
  Shuang Liu, Guangdong Bai, Jun Sun, and J. ‍Dong. Towards using concurrent Java API correctly. In ICECCS 2016: 21st International Conference on Engineering of Complex Computer Systems, pages 219–222, Dubai, UAE, June 2016.
[LČCT19]
  Tianhan Lu, Pavol Černý, Bor-Yuh ‍Evan Chang, and Ashutosh Trivedi. Type-directed bounding of collections in reactive programs. In VMCAI 2019: 20th International Conference on Verification, Model Checking and Abstract Interpretation, pages 275–296, Cascais, Portugal, January 2019.
[Li17]
  Jianchu Li. A general pluggable type inference framework and its use for data-flow analysis. Master’s thesis, U. of Waterloo, Waterloo, Ontario, Canada, 2017.
[LWUD21]
  Florian Lanzinger, Alexander Weigl, Mattias Ulbrich, and Werner Dietl. Scalability and precision by combining expressive type systems and deductive verification. Proceedings of the ACM on Programming Languages, 5(OOPSLA), October 2021.
[Mac16]
  Christopher ‍A. Mackie. Preventing signedness errors in numerical computations in Java. In FSE 2016: Proceedings of the ACM SIGSOFT 24th Symposium on the Foundations of Software Engineering, pages 1148–1150, Seattle, WA, USA, November 2016.
[MGR21]
  João Mota, Marco Giunti, and António Ravara. Java typestate checker. In Coordination 2021: 23rd IFIP WG 6.1 International Conference on Coordination Models and Languages, page 121–133, Valletta, Malta, June 2021.
[MH12]
  Ana Milanova and Wei Huang. Inference and checking of context-sensitive pluggable types. In FSE 2012: Proceedings of the ACM SIGSOFT 20th Symposium on the Foundations of Software Engineering, pages 1–4, Cary, NC, USA, November 2012.
[MH13]
  Ana Milanova and Wei Huang. Composing polymorphic information flow systems with reference immutability. In FTfJP: 14th Workshop on Formal Techniques for Java-like Programs, pages 5:1–5:7, Montpellier, France, July 2013.
[MHD14]
  Ana Milanova, Wei Huang, and Yao Dong. CFL-reachability and context-sensitive integrity types. In PPPJ 2014: Proceedings of the 2014 International Conference on Principles and Practice of Programming in Java: Virtual Machines, Languages, and Tools, page 99–109, Cracow, Poland, September 2014.
[Mil18]
  Ana Milanova. Definite reference mutability. In ECOOP 2018 — Object-Oriented Programming, 32nd European Conference, pages 25:1–25:30, Amsterdam, Netherlands, July 2018.
[MV11]
  Ana Milanova and Jan Vitek. Static dominance inference. In TOOLS 2011: Objects, Models, Components, Patterns, 49th International Conference, pages 211–227, Zurich, Switzerland, June 2011.
[MWME21]
  Rashmi Mudduluru, Jason Waataja, Suzanne Millstein, and Michael ‍D. Ernst. Verifying determinism in sequential programs. In ICSE 2021, Proceedings of the 43rd International Conference on Software Engineering, pages 37–49, Madrid, Spain, May 2021.
[Noa10]
  Gunther Noack. TIFI+: A type checker for object immutability with flexible initialization. Diploma thesis, University of Kaiserslautern, March 2010.
[NR19]
  Abel Nieto ‍Rodriguez. Scala with explicit nulls. Master’s thesis, University of Waterloo Department of Computer Science, Waterloo, Ontario, Canada, December 2019.
[NZL+20]
  Abel Nieto, Yaoyu Zhao, Ondřej Lhoták, Angela Chang, and Justin Pu. Scala with explicit nulls. In ECOOP 2020 — Object-Oriented Programming, 33rd European Conference, pages 25:1–25:26, online, November 2020.
[PAC+08]
  Matthew ‍M. Papi, Mahmood Ali, Telmo ‍Luis Correa ‍Jr., Jeff ‍H. Perkins, and Michael ‍D. Ernst. Practical pluggable types for Java. In ISSTA 2008, Proceedings of the 2008 International Symposium on Software Testing and Analysis, pages 201–212, Seattle, WA, USA, July 2008.
[PÖZE13]
  Alex Potanin, Johan Östlund, Yoav Zibin, and Michael ‍D. Ernst. Immutability. In Aliasing in Object-Oriented Programming, volume 7850 of LNCS, pages 233–269. Springer-Verlag, April 2013.
[QTE08]
  Jaime Quinonez, Matthew ‍S. Tschantz, and Michael ‍D. Ernst. Inference of reference immutability. In ECOOP 2008 — Object-Oriented Programming, 22nd European Conference, pages 616–641, Paphos, Cyprus, July 2008.
[San16]
  Joseph Santino. Enforcing correct array indexes with a type system. In FSE 2016: Proceedings of the ACM SIGSOFT 24th Symposium on the Foundations of Software Engineering, pages 1142–1144, Seattle, WA, USA, November 2016.
[SCN17]
  Julian Sexton, Andrey Chudnov, and David ‍A. Naumann. Spartan Jester: end-to-end information flow control for hybrid Android applications. In SPW 2017: Symposium Security and Privacy Workshops, pages 157–162, San Jose, CA, USA, May 2017.
[SCSC18]
  Benno Stein, Lazaro Clapp, Manu Sridharan, and Bor-Yuh ‍Evan Chang. Safe stream-based programming with refinement types. In ASE 2018: Proceedings of the 33rd Annual International Conference on Automated Software Engineering, pages 565–576, Montpellier, France, September 2018.
[SDE12]
  Eric Spishak, Werner Dietl, and Michael ‍D. Ernst. A type system for regular expressions. In FTfJP: 14th Workshop on Formal Techniques for Java-like Programs, pages 20–26, Beijing, China, June 2012.
[SDF+11]
  Adrian Sampson, Werner Dietl, Emily Fortuna, Danushen Gnanapragasam, Luis Ceze, and Dan Grossman. EnerJ: Approximate data types for safe and general low-power computation. In PLDI 2011: Proceedings of the ACM SIGPLAN 2011 Conference on Programming Language Design and Implementation, pages 164–174, San Jose, CA, USA, June 2011.
[SE10]
  Todd ‍W. Schiller and Michael ‍D. Ernst. Rethinking the economics of software engineering. In FoSER: Workshop on the Future of Software Engineering Research, pages 325–330, Santa Fe, NM, USA, November 2010.
[SE11]
  Fausto Spoto and Michael ‍D. Ernst. Inference of field initialization. In ICSE 2011, Proceedings of the 33rd International Conference on Software Engineering, pages 231–240, Waikiki, Hawaii, USA, May 2011.
[SGT+23]
  Narges Shadab, Pritam Gharat, Shrey Tiwari, Michael ‍D. Ernst, Martin Kellogg, Shuvendu Lahiri, Akash Lal, and Manu Sridharan. Inference of resource management specifications. Proceedings of the ACM on Programming Languages, 7(OOPSLA2, article #282):1705–1728, October 2023.
[She11]
  Amanj Sherwany. The design, implementation and evaluation of a pluggable type checker for thread-locality in Java. Master’s thesis, Uppsala University, Department of Information Technology, Uppsala, Sweden, 2011.
[SM11]
  Alexander ‍J. Summers and Peter Müller. Freedom before commitment: A lightweight type system for object initialisation. In OOPSLA 2011, Object-Oriented Programming Systems, Languages, and Applications, pages 1013–1032, Portland, OR, USA, October 2011.
[Spo10]
  F. ‍Spoto. The nullness analyser of Julia. In LPAR 2010: Proceedings of the 16th International Conference on Logic for Programming, Artificial Intelligence, and Reasoning, pages 405–424, Dakar, Senegal, April 2010.
[Sun21]
  Lian Sun. An immutability type system for classes and objects: Improvements, experiments, and comparisons. Master’s thesis, University of Waterloo Department of Electrical and Computer Engineering, Waterloo, Ontario, Canada, April 2021.
[Ta18]
  Mier Ta. Context sensitive typechecking and inference: Ownership and immutability. Master’s thesis, University of Waterloo Department of Electrical and Computer Engineering, Waterloo, Ontario, Canada, 2018.
[TE05]
  Matthew ‍S. Tschantz and Michael ‍D. Ernst. Javari: Adding reference immutability to Java. In OOPSLA 2005, Object-Oriented Programming Systems, Languages, and Applications, pages 211–230, San Diego, CA, USA, October 2005.
[TPNV11]
  Daniel Tang, Ales Plsek, Kelvin Nilsen, and Jan Vitek. A static memory safety annotation system for Safety Critical Java. In RTSS 2011: The 32nd IEEE Real-Time Systems Symposium, Vienna, Austria, November 2011.
[TPV10]
  Daniel Tang, Ales Plsek, and Jan Vitek. Static checking of safety critical Java annotations. In JTRES 2010: 8th International Workshop on Java Technologies for Real-time and Embedded Systems, pages 148–154, Prague, Czech Republic, August 2010.
[Val18]
  Lembit Valgma. Usable and sound static analysis through its integration into automated and interactive workflows. Master’s thesis, U. of Tartu, Tartu, Estonia, 2018.
[VPEJ15]
  Mohsen Vakilian, Amarin Phaosawasdi, Michael ‍D. Ernst, and Ralph ‍E. Johnson. Cascade: A universal programmer-assisted type qualifier inference tool. In ICSE 2015, Proceedings of the 37th International Conference on Software Engineering, pages 234–245, Florence, Italy, May 2015.
[Wan21]
  Di ‍Wang. Interval type inference: Improvements and evaluations. Master’s thesis, University of Waterloo Department of Electrical and Computer Engineering, Waterloo, Ontario, Canada, December 2021.
[WKSE14]
  Konstantin Weitz, Gene Kim, Siwakorn Srisakaokul, and Michael ‍D. Ernst. A type system for format strings. In ISSTA 2014, Proceedings of the 2014 International Symposium on Software Testing and Analysis, pages 127–137, San Jose, CA, USA, July 2014.
[WPM+09]
  Tobias Wrigstad, Filip Pizlo, Fadi Meawad, Lei Zhao, and Jan Vitek. Loci: Simple thread-locality for Java. In ECOOP 2009 — Object-Oriented Programming, 23rd European Conference, pages 445–469, Genova, Italy, July 2009.
[XCD21]
  Weitian Xing, Yuanhui Cheng, and Werner Dietl. Ensuring correct cryptographic algorithm and provider usage at compile time. In FTfJP: 239d Workshop on Formal Techniques for Java-like Programs, page 43–50, Online, July 2021.
[Xia20]
  Tongtong Xiang. Type checking and whole-program inference for value range analysis. Master’s thesis, University of Waterloo Department of Electrical and Computer Engineering, Waterloo, Ontario, Canada, October 2020.
[Xin20]
  Weitian Xing. Light-weight verification of cryptographic API usage. Master’s thesis, U. of Waterloo, Waterloo, Ontario, Canada, 2020.
[XLD20]
  Tongtong Xiang, Jeff ‍Y. Luo, and Werner Dietl. Precise inference of expressive units of measurement types. In OOPSLA 2020, Object-Oriented Programming Systems, Languages, and Applications, Chicago, IL, USA, November 2020.
[Zaz13]
  Nosheen Zaza. Evaluating the accuracy of annotations in the Loci 3.0 pluggable type checker. Master’s thesis, Uppsala University, Uppsala, Sweden, 2013.
[ZPA+07]
  Yoav Zibin, Alex Potanin, Mahmood Ali, Shay Artzi, Adam Kieżun, and Michael ‍D. Ernst. Object and reference immutability using Java generics. In ESEC/FSE 2007: Proceedings of the 11th European Software Engineering Conference and the 15th ACM SIGSOFT Symposium on the Foundations of Software Engineering, pages 75–84, Dubrovnik, Croatia, September 2007.
[ZPL+10]
  Yoav Zibin, Alex Potanin, Paley Li, Mahmood Ali, and Michael ‍D. Ernst. Ownership and immutability in generic Java. In OOPSLA 2010, Object-Oriented Programming Systems, Languages, and Applications, pages 598–617, Revo, NV, USA, October 2010.