The Checker Framework Manual:
Custom pluggable types for Java

https://checkerframework.org/

Version 2.11.1 (1 Oct 2019)

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 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 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. Lock Checker for concurrency and lock errors (see Chapter 7)
  7. Index Checker for array accesses (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 10)
  10. Regex Checker to prevent use of syntactically invalid regular expressions (see Chapter 11)
  11. Format String Checker to ensure that format strings have the right number and type of % directives (see Chapter 12)
  12. Internationalization Format String Checker to ensure that i18n format strings have the right number and type of {} directives (see Chapter 13)
  13. Property File Checker to ensure that valid keys are used for property files and resource bundles (see Chapter 14)
  14. Internationalization Checker to ensure that code is properly internationalized (see Chapter 14.2)
  15. Signature String Checker to ensure that the string representation of a type is properly used, for example in Class.forName (see Chapter 15)
  16. GUI Effect Checker to ensure that non-GUI threads do not access the UI, which would crash the application (see Chapter 16)
  17. Units Checker to ensure operations are performed on correct units of measurement (see Chapter 17)
  18. Signedness Checker to ensure unsigned and signed values are not mixed (see Chapter 18)
  19. Aliasing Checker to identify whether expressions have aliases (see Chapter 19)
  20. Purity Checker to identify whether methods have side effects (see Chapter 20)
  21. Constant Value Checker to determine whether an expression’s value can be known at compile time (see Chapter 21)
  22. Reflection Checker to determine whether an expression’s value (of type Method or Class) can be known at compile time (see Chapter 22)
  23. Subtyping Checker for customized checking without writing any code (see Chapter 23)
  24. Third-party checkers that are distributed separately from the Checker Framework (see Chapter 24)

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 23 and 31.

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:

1.2  How it works: Pluggable types 🔗

The Checker Framework supports adding pluggable type systems to the Java language in a backward-compatible way. 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 run an additional type-checker 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 only part of your program. Type inference tools exist to help you annotate your code; see Chapter 29.2.

A type system designer uses the Checker Framework to define type qualifiers and their semantics, and a compiler plug-in (a “checker”) enforces the semantics. Programmers can write the type qualifiers in their programs and use the plug-in to detect or prevent errors. 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”, “checker plugin”, “type-checking compiler plugin”, and “annotation processor” as synonyms.

1.3  Installation 🔗

This section describes how to install the Checker Framework.

Requirement: You must have JDK 8 installed. You can get the JDK from Oracle or elsewhere.

The installation process is simple! It has two required steps and one optional step.

  1. Download the Checker Framework distribution: https://checkerframework.org/checker-framework-2.11.1.zip
  2. Unzip it to create a checker-framework directory.
  3. Configure your IDE, build system, or command shell to include the Checker Framework on the classpath. Choose the appropriate section of Chapter 32.

That’s all there is to it! Now you are ready to start using the checkers.

We recommend that you work through the Checker Framework tutorial, which walks you through how to use the Checker Framework on the command line (Nullness, Regex, and Tainting Checkers). There is also a Nullness Checker tutorial by David Bürgin; the setup instructions are out of date, but you can read through the steps.

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

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 gives more extensive instructions for using the Checker Framework on the command line, and a Nullness Checker tutorial by David Bürgin.

  1. 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();
        }
    }
    
  2. Run the Nullness Checker on the class. You can do that from the command line or from an IDE:
    1. From the command line, run this command:
        javac -processor org.checkerframework.checker.nullness.NullnessChecker GetStarted.java
      

      where javac is set as in Section 32.8.

    2. To compile within your IDE, you must have customized it to use the Checker Framework compiler and to pass the extra arguments (see Chapter 32).

    The compilation should complete without any errors.

  3. Let’s introduce an error now. Modify ref’s assignment to:
      @NonNull Object ref = null;
    
  4. Run the Nullness Checker again, just as before. This run should emit the following error:
    GetStarted.java:5: incompatible types.
    found   : @Nullable <nulltype>
    required: @NonNull Object
            @NonNull Object ref = null;
                                  ^
    1 error
    

The type qualifiers (e.g., @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 plugin 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 insert annotations in his code: see Section 3.3.7.) It is possible to annotate only part of your code: see Section 28.1.
  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:

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

Finally, there is a tutorial that walks you through using the Checker Framework on the command line, and a separate Nullness Checker tutorial by David Bürgin.

2.1  Writing annotations 🔗

The syntax of type annotations in Java is specified by the Java Language Specification (Java SE 8 edition).

Java 5 defines declaration annotations such as @Deprecated, which apply to a class, method, or field, but do not apply to the method’s return type or the field’s type. They are typically written on their own line in the source code.

Java 8 defines type annotations, which you write 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 annotations on method signatures, fields, and some type arguments. Most annotations within method bodies are inferred for you; for more details, see Section 26.7.

2.2  Running a checker 🔗

To run a checker plugin, run the compiler javac as usual, but pass the -processor plugin_class command-line option. A concrete example (using the Nullness Checker) is:

  javac -processor nullness MyFile.java

where javac is as specified in Section 32.8.

You can also run a checker from within your favorite IDE or build system. See Chapter 32 for details about build tools such as Ant (Section 32.3), Buck (Section 32.4), Gradle (Section 32.6), and Maven (Section 32.10); IDEs such as IntelliJ IDEA (Section 32.7), Eclipse (Section 32.5), NetBeans (Section 32.11), and tIDE (Section 32.12); 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 (or created by another annotation processor). It may also include other of your Java files (but not if a more recent .class file exists). 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.

You can always compile the code without the -processor command-line option, but in that case 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, it is strongly recommended that whenever you are creating .class files that will be distributed or compiled against, you run the type-checkers for all the annotations that your 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.

Some projects are already distributed with type annotations by their maintainers, so you do not need to do anything special. Over time, this should become more common.

For some other libraries, the Checker Framework developers have provided an annotated version of the library. The annotated libraries appear in the org.checkerframework.annotatedlib group in the Central Repository. The annotated library has identical behavior to the upstream, unannotated version; the source code is identical other than the annotations are different, so the only difference is with respect to pluggable type-checking of clients.

(Some of the annotated libraries are bcel, commons-csv, commons-io, guava, and java-getopt. If the library you are interested in does not appear in the Central Repository, you can contribute by annotating it, which will help you and all other Checker Framework users; see Chapter 30.)

To use an annotated library:

There are two special cases.

2.2.2  Distributing your annotated project 🔗

The distributed .jar files can be used for pluggable type-checking of client code. The .jar files are only compatible with a Java 8 JVM. Developers perform pluggable type-checking in-house to detect errors and verify their absence. When you create .class files, run each relevant type system. Create the distributed .jar files from those .class files.

2.2.3  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.

Unsound checking: ignore some errors

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

Type-checking modes: enable/disable functionality

Partially-annotated libraries

Debugging

Some checkers support additional options, which are described in that checker’s manual section. For example, -Aquals tells the Subtyping Checker (see Chapter 23) 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 the word “processor”, because in javac jargon, a checker is an “annotation processor”.

The Checker Framework does not support -source 1.7 or earlier. You must supply -source 1.8 or later, or no -source command-line argument, when running javac.

2.2.4  Checker auto-discovery 🔗

“Auto-discovery” makes the javac compiler always run a checker plugin, even if you do not explicitly pass the -processor command-line option. This can make your command line shorter, and ensures that your code is checked even if you forget the command-line option.

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 checker plugins 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.

2.2.5  Shorthand for built-in checkers 🔗

Ordinarily, javac’s -processor flag requires fully-qualified class names. When running a built-in checker, 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 can guarantee that a particular property holds throughout the code. For example, the Nullness Checker (Chapter 3) guarantees that every expression whose type is a @NonNull type never evaluates to null. The Interning Checker (Chapter 6) guarantees that every expression whose type is an @Interned type evaluates to an interned value. The checker makes its guarantee by examining every part of your program and verifying that no part of the program violates the guarantee.

There are some limitations to the guarantee.

A checker can be useful in finding bugs or in verifying part of a program, even if the checker is unable to verify the correctness of an entire program.

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

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

In this case, 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. (More often, at least in our experience to date, 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 34.2).

2.4  Tips about writing annotations 🔗

Section 30.1 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; you will find that you spend most of your time understanding the code, and very little time actually writing annotations or running the checker.

When annotating, be systematic; we recommend annotating an entire class 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 means you determined that the default is desirable, or it means you didn’t yet examine that type. You may find it helpful to start annotating the leaves of the call tree — that is, start with methods/classes/packages that have few dependencies on other code or, equivalently, start with code that a lot of your other code depends on. For example, within a package annotate supertypes before you annotated 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.

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 33.1.2).

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. For now, just annotate signatures and fields; there is no need to annotate 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.

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.

You may wonder about the effect of adding a given annotation (that is, of changing the specification for a given method or class): how many other specification changes (added annotations) will it require, and will it conflict with other code? It’s best to reason about the desired design, but you can also do an experiment. Suppose you are considering adding an annotation to a method parameter. One approach is to manually examine all callees. A more automated approach is to save the checker output before adding the annotation, and to compare it to the checker output after adding the annotation. This helps you to focus on the specific consequences of your change.

Chapter 30 tells you how to annotate libraries that your code uses. Section 2.4.5 and Chapter 27 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 clean enough for the type-checker to verify, then it will also be easier for programmers to understand.

Your code should compile cleanly under the regular Java compiler. If you are not willing to write code that type-checks in Java, then there is little point in using an even more powerful, restrictive type system. As a specific example, your code should not use raw types like List; use parameterized types like List<String> instead (Section 25.1.1).

Do not write unnecessary annotations.

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.

As an example, the goal of the Nullness Checker is to guarantee that your program does not crash due to a null value. In a method like this:

  /** @throws IllegalArgumentException if arg is null */
  void m(Object arg) {
    if (arg == null) {
      throw new IllegalArgumentException();
    }
    ...
  }

the program crashes if null is passed. Therefore, the type of arg should be @NonNull Object (which you can write as just Object due to defaulting). The Nullness Checker (Chapter 3) will warn whenever a client passes a value that might cause m to crash. If you wrote the type of the formal parameter as @Nullable Object, the Nullness Checker would permit clients to make calls that lead to a crash.

Many methods are guaranteed to throw an exception if they are passed null as an argument. Examples include

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

@Nullable (see Section 3.2) might seem like a reasonable annotation for the parameter, for two reasons. First, null is a legal argument with a well-defined semantics: throw an exception. Second, @Nullable describes a possible program execution: it might be possible for null to flow there, if your program has a bug.

However, it is never useful for a programmer to pass null. It is the programmer’s intention that null never flows there. If null does flow there, the program will not continue normally (whether or not it throws a NullPointerException).

Therefore, you should specify such parameters as @NonNull, indicating the intended use of the method. When you specify the parameter as the @NonNull annotation, the checker is able to issue compile-time warnings about possible run-time exceptions, which is its purpose. Specifying the parameter as @Nullable would suppress such warnings, which is undesirable. (Since @NonNull is the default, you don’t have to write anything in the source code to specify the parameter as non-null. You are allowed to write a redundant @NonNull annotation, but it is discouraged.)

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.

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 a warning if the client calls orElseThrow on a @MaybePresent value.

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! There are three general causes for the warnings:

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.

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

    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).

  2. Translate the proof into annotations. Here are some examples.

    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 30, Annotating Libraries.

    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 27); be sure to write a comment explaining your reasoning for suppressing the warning. You may wish to submit a feature request (Section 34.2) 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.

  3. 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 27). The comment is an excerpt from your English proof, and the proof guides you to the best place to suppress the warning. Finally, please submit a bug report so that the checker can be improved in the future (Section 34.2).

If you have trouble understanding a Checker Framework warning message, you can search for its text in this manual. Also see Section 34.1.4 and Chapter 34, Troubleshooting. In particular, Section 34.1.4 explains this same methodology in different words.


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. This guarantee enables a programmer to prevent errors from occurring when a program is run. 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 checks 🔗

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 27.6 for more details about the -Alint command-line option.)

3.1.1  Nullness Checker optional warnings 🔗

  1. Options that control soundness:
  2. Options that warn about poor code style:

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 type Boolean is nullable: a variable of type Boolean always has one of the values TRUE, FALSE, or null.
@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 @PolyNull, see Section 25.2.
@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. When the variable is read, its type is treated as @Nullable, but when the variable is assigned, its type is treated as @NonNull.

Because the Nullness Checker works intraprocedurally (it analyzes one method at a time), when a MonotonicNonNull field is first read within a method, the field cannot be assumed to be non-null. The benefit of MonotonicNonNull over Nullable is its different interaction with type refinement (Section 26.7). 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.

It is permitted to initialize a MonotonicNonNull field 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.

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
@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 3.3 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.

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 26.4). 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. The standard defaulting rule is CLIMB-to-top, described in Section 26.5.3. Its effect is to default all types to @NonNull, except that @Nullable is used for casts, locals, instanceof, and implicit bounds. 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 26.8. More examples of the use of these annotations appear in the Javadoc for @EnsuresNonNullIf.

3.3.4  Nullness and arrays 🔗

Suppose that you declare an array to contain non-null elements. Currently, the Nullness Checker does not verify that all the elements of the array are initialized to a non-null value. This is an unsoundness in the checker.

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

When the -Alint=forbidnonnullarraycomponents option is supplied, the following is not allowed:

  Object [] oa = new Object[10]; // error

(recall that Object means the same thing as @NonNull Object).

Instead, your code needs 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  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 26.7.

3.3.6  Additional details 🔗

The Nullness Checker does some special checks in certain circumstances, in order to soundly reduce the number of warnings that it produces.

For example, a call to System.getProperty(String) can return null in general, but it will not return null if the argument is one of the built-in-keys listed in the documentation of System.getProperties(). The Nullness Checker is aware of this fact, so you do not have to suppress a warning for a call like System.getProperty("line.separator"). The warning is still issued for code like this:

  final String s = "line.separator";
  nonNullvar = System.getProperty(s);

though that case could be handled as well, if desired. (Suppression of the warning is, strictly speaking, not sound, because a library that your code calls, or your code itself, could perversely change the system properties; the Nullness Checker assumes this bizarre coding pattern does not happen.)

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 26.7), 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.

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 27). 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 key, 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. In such cases, you can use the @AssumeAssertion string in an assert message (see Section 27.2).

If you need to suppress a warning within an expression, then sometimes writing an assertion is not convenient. In such a case, you can suppress warnings by writing a call to the NullnessUtil.castNonNull method. The rest of this section discusses the castNonNull method.

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 27.2.1.

To use the castNonNull method, the checker-qual.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 27.2.1 for a discussion of the distinction between warning suppression and defensive programming.

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):

  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:

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.

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:

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:

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 33.10.1 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 Figure 3.2. If the other annotation is a declaration annotation, it may be moved; see Section 28.1.1.


 android.annotation.NonNull 
 android.support.annotation.NonNull 
 androidx.annotation.NonNull 
 androidx.annotation.RecentlyNonNull 
 com.sun.istack.internal.NotNull 
 edu.umd.cs.findbugs.annotations.NonNull 
 io.reactivex.annotations.NonNull 
 javax.annotation.Nonnull 
 javax.validation.constraints.NotNull 
 lombok.NonNull 
 org.checkerframework.checker.nullness.compatqual.NonNullDecl 
 org.checkerframework.checker.nullness.compatqual.NonNullType 
 org.eclipse.jdt.annotation.NonNull 
 org.eclipse.jgit.annotations.NonNull 
 org.jetbrains.annotations.NotNull 
 org.jmlspecs.annotation.NonNull 
 org.netbeans.api.annotations.common.NonNull 
 org.springframework.lang.NonNull 
⇒  org.checkerframework.checker.nullness.qual.NonNull 
 
 android.annotation.Nullable 
 android.support.annotation.Nullable 
 androidx.annotation.Nullable 
 androidx.annotation.RecentlyNullable 
 com.sun.istack.internal.Nullable 
 edu.umd.cs.findbugs.annotations.Nullable 
 edu.umd.cs.findbugs.annotations.CheckForNull 
 edu.umd.cs.findbugs.annotations.PossiblyNull 
 edu.umd.cs.findbugs.annotations.UnknownNullness 
 io.reactivex.annotations.Nullable 
 javax.annotation.Nullable 
 javax.annotation.CheckForNull 
 org.checkerframework.checker.nullness.compatqual.NullableDecl 
 org.checkerframework.checker.nullness.compatqual.NullableType 
 org.eclipse.jdt.annotation.Nullable 
 org.eclipse.jgit.annotations.Nullable 
 org.jetbrains.annotations.Nullable 
 org.jmlspecs.annotation.Nullable 
 org.netbeans.api.annotations.common.NullAllowed 
 org.netbeans.api.annotations.common.CheckForNull 
 org.netbeans.api.annotations.common.NullUnknown 
 org.springframework.lang.Nullable 
⇒  org.checkerframework.checker.nullness.qual.Nullable 
Figure 3.2: Correspondence between other nullness annotations and the Checker Framework’s annotations.

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 FindBugs’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. Here is a brief comparison with FindBugs, but similar points apply to other tools. (For example, the NullAway and Eradicate tools are more like sound type-checking than FindBugs, but all of those tools accept unsoundness — that is, false negatives or missed warnings — in exchange for analysis speed.)

The Checker Framework has a more powerful nullness analysis; FindBugs misses some real errors. FindBugs requires you to annotate your code, but usually not as thoroughly as the Checker Framework does. Depending on the importance of your code, you may desire: no nullness checking, the cursory checking of FindBugs, or the thorough checking of the Checker Framework. You might even want to ensure that both tools run, for example if your coworkers or some other organization are still using FindBugs. If you know that you will eventually want to use the Checker Framework, there is no point using FindBugs first; it is easier to go straight to using the Checker Framework.

FindBugs can find other errors in addition to nullness errors; here we focus on its nullness checks. Even if you use FindBugs for its other features, you may want to use the Checker Framework for analyses that can be expressed as pluggable type-checking, such as detecting nullness errors.

Regardless of whether you wish to use the FindBugs nullness analysis, you may continue running all of the other FindBugs analyses at the same time as the Checker Framework; there are no interactions among them.

If FindBugs (or any other tool) discovers a nullness error that the Checker Framework does not, please report it to us (see Section 34.2) so that we can enhance the Checker Framework.

3.7.2  Incompatibility note about FindBugs @Nullable 🔗

FindBugs has a non-standard definition of @Nullable. FindBugs’s 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, FindBugs’s @Nullable is detrimental rather than useful as documentation. In practice, your best bet is to not rely on FindBugs for nullness analysis, even if you find FindBugs useful for other purposes.

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

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

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

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

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

With FindBugs, 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 FindBugs supports only one and gives the programmer less flexibility.

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

(FindBugs 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 choices make the FindBugs annotations less helpful to people, and much less useful for other tools. As a separate issue, the FindBugs analysis is also very imprecise. For type-related analyses, it is best to stay away from the FindBugs nullness annotations, and use a more capable tool like the Checker Framework.)

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”.)

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.

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.

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, pass the command-line argument -AsuppressWarnings=uninitialized when running the Nullness Checker. 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.

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 Nullness 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 3.3: 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 3.3. 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 3.4 contains some examples.


DeclarationsExpressionExpression’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.ferror: deref of nullable
 c.gerror: deref of nullable
@Nullable @UnderInitialization C d;d@Nullable
 d.ferror: deref of nullable
 d.gerror: deref of nullable
Figure 3.4: 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. When treated as an A (e.g., if only the A fields are accessed), the object is initialized, but when treated as a B, the object is still non-initialized.
  4. B’s constructor completes. The object is initialized when treated as an A or a B. (And, the object is fully initialized if B’s constructor was invoked via a new B(). But the type system cannot assume that — 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 MyClass and all subclasses.

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

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.invalid] 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 3.5. Essentially, nothing is known about the initialization status of the value returned unless the receiver was @Initialized.


x.ff is @NonNullf is @Nullable
x is @Initialized@Initialized @NonNull@Initialized @Nullable
x is @UnderInitialization@UnknownInitialization @Nullable@UnknownInitialization @Nullable
x is @UnknownInitialization@UnknownInitialization @Nullable@UnknownInitialization @Nullable
Figure 3.5: Initialization rules for reading a @NotOnlyInitialized field f.

Similarly, Figure 3.6 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 = yy is @Initializedy is @UnderInitializationy is @UnknownInitialization
x is @Initializedyesnono
x is @UnderInitializationyesyesyes
x is @UnknownInitializationyesnono
Figure 3.6: 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.

“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 33.6.1) 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  More details about initialization checking 🔗

Suppressing warnings 🔗

You can suppress warnings related to partially-initialized objects with @SuppressWarnings("initialization"). This can be placed on a single field; on a constructor; or on a class to suppress all initialization warnings for all constructors.

To disable initialization checking, supply the command-line argument -AsuppressWarnings=uninitialized. Do not use -AsuppressWarnings=initialization, because doing so will disable all nullness checking as well as all initialization 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.)

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 known as “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).


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.

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

You can suppress warnings related to map keys with @SuppressWarnings("keyfor"); see Chapter 27.

4.1  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.

4.2  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).

4.2.1  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).

4.2.2  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.type.incompatible] 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 25.1.2):

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

4.3  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

4.4  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 26.7.

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 🔗

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.

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.)

Use of the Optional Checker guarantees that your program will not suffer a NullPointerException nor a NoSuchElementException when calling methods on an expression of Optional type.

5.1  How to run the Optional Checker 🔗

javac -processor optional 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.
@Present
The annotated Optional container definitely contains a (non-null) value.
@PolyPresent
indicates qualifier polymorphism (see Section 25.2).

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.

5.3  What the Optional Checker guarantees 🔗

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

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).


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 errors in your code. First, == should be used only on interned values; using == on non-interned values can result in subtle bugs. For example:

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

The Interning Checker helps programmers to prevent such bugs. Second, the Interning Checker also 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. (Not every value of the given type is necessarily interned; 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. For example, suppose that an array contains no duplicates. Then searching for the index of an element that is known to be in the array can use ==.

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 a type such that each value is not equals() to any other Java value. 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 (see Section 25.2).

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. In other words, neither this class nor any of its superclasses overrides the equals method. 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.
@InternMethod
is a method declaration annotation that indicates that this method returns an interned object and maybe be invoked on an uninterned object. See Section 6.3.1 for more details.

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)) {
    @SuppressWarning("interning:assignment.type.incompatible")
    @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.invalid")
  @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 26.4).

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:

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 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.

You can also restrict which types the checker should examine and type-check, 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  Limitations 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 (http://plse.cs.washington.edu/daikon/) is also annotated with @Interned. From directory java/, run make check-interning.

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 28.1.1.


 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  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 33.4.4).

7.1  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.

7.2  Lock annotations 🔗

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

7.2.1  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 26.8 explains which expressions the Lock Checker is able to analyze as lock expressions. These include <self>, i.e. 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 26.5.3). (Section 7.5.4 discusses this choice.) It is also the conservative default type qualifier for method parameters in unannotated libraries (see Chapter 30).

@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 30).

@GuardedByBottom
If a variable x has type @GuardedByBottom, then the value referred to by x is null and can never be dereferenced.

Figure 7.1: 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 26.5.3).

Figure 7.1 shows the type hierarchy of these qualifiers. All @GuardedBy annotations are incomparable: if exprSet1exprSet2, 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 7.4.2 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 25.2), 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 7.4.6 for an example of a use of @GuardSatisfied.

7.2.2  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 7.4.4 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 30).

7.3  Type-checking rules 🔗

In addition to the standard subtyping rules enforcing the subtyping relationship described in Figure 7.1, the Lock Checker enforces the following additional rules.

7.3.1  Polymorphic qualifiers 🔗

@GuardSatisfied

The overall rules for polymorphic qualifiers are given in Section 25.2.

Here are additional constraints for (pseudo-)assignments:

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.

7.3.2  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.

7.3.3  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({}).

7.3.4  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

7.3.5  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 26.7.5.

7.4  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.

7.4.1  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.
    }
  }
}

7.4.2  @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 exprSet1exprSet2, 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.

7.4.3  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
    }
  }

7.4.4  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();
    }

    ...
}

7.4.5  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
  }
}

7.4.6  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);

7.5  More locking details 🔗

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

7.5.1  Two types of locking: monitor locks and explicit locks 🔗

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

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.

7.5.2  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 26.8 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.

7.5.3  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 26.7.

7.5.4  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.

7.5.5  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:

7.6  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 7.2. If the other annotation is a declaration annotation, it may be moved; see Section 28.1.1.


 net.jcip.annotations.GuardedBy 
 javax.annotation.concurrent.GuardedBy 
⇒  org.checkerframework.checker.lock.qual.GuardedBy (for fields) or …Holding (for methods) 
Figure 7.2: Correspondence between other lock annotations and the Checker Framework’s annotations.

7.6.1  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:

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.)

7.7  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 8  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 8.9).

To run the Index Checker, run the command

  javac -processor index 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) []

8.1  Index Checker structure and annotations 🔗

Internally, the Index Checker computes information about integers that might be indices:

and about sequence lengths:

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") 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 25.2.
@PolyLength
is a special polymorphic qualifier that combines @PolySameLen and @PolyValue from the Constant Value Checker (see Chapter 21). @PolyLength exists as a shorthand for these two annotations, since they often appear together.

8.2  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 8.1) 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 25.2.
@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 8.1: 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. "myArray" in the Upper Bound qualifiers is an example of an array’s name.

8.3  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 8.1) 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”.)
@PolyUpperBound
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 25.2.
@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”.)

@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 startIndex;
  int endIndex;
  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.

8.4  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 21). 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 the Chapter 21) 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 8.2: 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.

8.5  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 8.2):

@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 25.2.
@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.

8.6  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 8.3: 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 8.3) 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.

8.7  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 8.4: 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 8.4) 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.

8.7.1  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):

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 8.2) and upper-bound (Section 8.3) 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.

8.8  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.

8.9  Annotating fixed-size data structures 🔗

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);
        }
    }

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 23), 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 31.4.1.

  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.

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.type.incompatible") // 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.type.incompatible")   // 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.

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 🔗


Chapter 10  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 command-line option to javac.

10.1  Tainting annotations 🔗

The Tainting type system uses the following annotations:

10.2  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 26.5 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 30 describes how to annotate external libraries):

  public boolean execute(@Untainted String sql) throws SQLException;
  public boolean executeUpdate(@Untainted String sql) throws SQLException;

10.3  @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 23). This lack of hard-coded behavior has two consequences. The first consequence is that the annotations can serve many different purposes, such as:

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 31 for more details.

10.4  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).

(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 11  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. At run time, the string is “compiled” into an efficient internal form (Pattern) that is used for text-matching. Regular expression in Java also have capturing groups, which are delimited by parentheses and allow for extraction from text.

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). Ordinarily, the programmer does not learn of these errors until run time. The Regex Checker warns about these problems at compile time.

For further details, including case studies, see a paper about the Regex Checker [SDE12].

To run the Regex Checker, supply the -processor org.checkerframework.checker.regex.RegexChecker command-line option to javac.

11.1  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 (see Section 25.2).

The subtyping hierarchy of the Regex Checker’s qualifiers is shown in Figure 11.1.


Figure 11.1: 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.

11.2  Annotating your code with @Regex 🔗

11.2.1  Implicit qualifiers 🔗

The Regex Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section 26.4). 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.

11.2.2  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 26.7), but you may need to add @Regex annotations with a capturing group count to Pattern and Matcher fields and parameters.

11.2.3  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 11.2: 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 11.2.

11.2.4  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 11.3 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-qual.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 11.3: Example use of RegexUtil methods.

11.2.5  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 11.3 should support most cases so the asRegex method should be used rarely.


Chapter 12  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 12.3 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.

12.1  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.

12.2  Format String Checker annotations 🔗

The @Format qualifier on a string type indicates a valid format string. The JDK documentation for the Formatter class explains the requirements for 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 12.2.1. 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 12.1: 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 12.2.2.

Figure 12.1 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 12.5.

12.2.1  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 12.3.2.) 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.

Further, all conversion categories accept 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:

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. The format string "%1$f %1$c", for example requires that the first format argument be null. Passing a value such as 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 (CHARINT).
INT_AND_TIME is used if a format argument is restricted by an INT and a TIME conversion category (INTTIME).

All other intersections lead to already existing conversion categories. For example, GENERALCHAR = CHAR and UNUSEDGENERAL = GENERAL.

Figure 12.2 summarizes the subset relationship among all conversion categories.


Figure 12.2: The subset relationship among conversion categories.

12.2.2  Subtyping rules for @Format 🔗

Here are the subtyping rules among different @Format qualifiers. It is legal to:

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);

12.3  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)

12.3.1  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.

12.3.2  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)

12.4  Implicit qualifiers 🔗

The Format String Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section 26.4). The checker implicitly adds the @Format qualifier with the appropriate conversion categories to any String literal that is a valid format string.

12.5  @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.

12.6  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-qual.jar file must be on the classpath at run time.


Chapter 13  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 13.6.

The Internationalization Checker or I18n Checker (Chapter 14.2) 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.

13.1  Internationalization Format String Checker annotations 🔗


Figure 13.1: The Internationalization Format String Checker type qualifier hierarchy. The type qualifiers are applicable te CharSequence and its subtypes. The figure does not show the subtyping rules among different @I18nFormat(...) qualifiers; see Section 13.2. All @I18nFormatFor annotations are unrelated by subtyping. 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 13.1 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 13.2.
@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 26.8. 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.

13.2  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 correspoond 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 13.2 summarizes the subset relationship among all conversion categories.


Figure 13.2: The subset relationship among i18n conversion categories.

13.3  Subtyping rules for @I18nFormat 🔗

Here are the subtyping rules among different @I18nFormat qualifiers. It is legal to:

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.

13.4  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 27).

    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}";

13.5  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 14.

13.6  Running the Internationalization Format Checker 🔗

The checker can be invoked by running one of the following commands (with the whole command on one line).

13.7  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 13.8.

13.8  Examples of using the Internationalization Format Checker 🔗


Chapter 14  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 14.1) 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 14.2) verifies that code is properly internationalized. A Compiler Message Key Checker (Section 14.3) 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.

14.1  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 colons ’:’.

14.2  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 13). 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.

14.2.1  Internationalization annotations 🔗

The Internationalization Checker supports two annotations:

  1. @Localized: indicates that the qualified CharSequence is a message that has been localized and/or formatted with respect to the used locale.
  2. @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.

You may need to add the @Localized annotation to more methods in the JDK or other libraries, or in your own code.

14.2.2  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.

14.3  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 error 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 15  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 15.1), 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.

15.1  Signature annotations 🔗

Java defines six formats for the string representation of a type. There is an annotation for each of these representations. Figure 15.1 shows how they are related; examples appear in a table below.


Figure 15.1: 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 only need to write the boldfaced qualifiers; other qualifiers are included to improve the internal handling of String literals.

@FullyQualifiedName
A fully qualified name (JLS §6.7), such as package.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 Lpackage/Outer$Inner;, is used in a .class file’s constant pool, for example to refer to other types. It abbreviates primitive types and array types. It uses internal form (binary names, but with / instead of .; see JVMS §4.2) for class names. See examples below.
@BinaryName
A binary name (JLS §13.1), such as package.Outer$Inner, is the conceptual name of a type in its own .class file.
@InternalForm
The internal form (JVMS §4.2), such as package/Outer$Inner, is how a class name is actually represented in its own .class file. It is also known as the “syntax of binary names that appear in class file structures”. It is the same as the binary name, but with periods (.) replaced by slashes (/). Programmers more often use the binary name, leaving the internal form as a JVM implementation detail.
@ClassGetSimpleName
The type representation returned by the Class.getSimpleName() method. This format is not required by any method in the JDK, so you will rarely write it in source code. The string can be empty. This is not the same as the “simple name” defined in (JLS §6.2), which is the same as @Identifier.
@FqBinaryName
Like @FullyQualifiedName, but using “$” instead of “.” to separate nested classes from their containing classes. For example, "pkg.Outer$Inner" or "pkg.Outer$Inner[][]" or "int[]".

Other type qualifiers are the intersection of two or more qualifiers listed above; for example, a @BinaryNameInUnnamedPackage is a string that is a valid internal form 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: qualified names (JLS §6.2) and canonical names (JLS §6.7). The Signature Checker does not include annotations for these.

Here are examples of the supported formats:

fully qualified nameClass.getNamefield descriptorbinary nameinternal formClass.getSimpleName
intintIn/a for primitive typen/a for primitive typeint
int[][][[I[[In/a for array typen/a for array typeint[][]
MyClassMyClassLMyClass;MyClassMyClassMyClass
MyClass[][LMyClass;[LMyClass;n/a for array typen/a for array typeMyClass[]
n/a for anonymous classMyClass$22LMyClass$22;MyClass$22MyClass$22(empty string)
n/a for array of anon. class[LMyClass$22;[LMyClass$22;n/a for array typen/a for array type[]
java.lang.Integerjava.lang.IntegerLjava/lang/Integer;java.lang.Integerjava/lang/IntegerInteger
java.lang.Integer[][Ljava.lang.Integer;[Ljava/lang/Integer;n/a for array typen/a for array typeInteger[]
pkg.Outer.Innerpkg.Outer$InnerLpkg/Outer$Inner;pkg.Outer$Innerpkg/Outer$InnerInner
pkg.Outer.Inner[][Lpkg.Outer$Inner;[Lpkg/Outer$Inner;n/a for array typen/a for array typeInner[]
n/a for anonymous classpkg.Outer$22Lpkg/Outer$22;pkg.Outer$22pkg/Outer$22(empty string)
n/a for array of anon. class[Lpkg.Outer$22;[Lpkg/Outer$22;n/a for array typen/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;

15.2  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 16  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:

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:

16.1  GUI effect annotations 🔗

There are two primary GUI effect annotations:

@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

16.2  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:

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).

16.3  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 ...

16.4  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:

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.

16.5  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;.

16.5.1  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.

16.5.2  Using an effect-polymorphic type 🔗

To instantiate an effect-polymorphic type, write one of these three type qualifiers before a use of the type:

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.

16.5.3  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.)

16.5.4  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 25.2).

@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 25.1.6), 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.

16.6  References 🔗

The ECOOP 2013 paper “JavaUI: Effects for Controlling UI Object Access” 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. You can obtain the paper at:
https://homes.cs.washington.edu/~mernst/pubs/gui-thread-ecoop2013-abstract.html


Chapter 17  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. Utility class UnitsTools provides constants with which unqualified integer are multiplied to get values of the corresponding unit. The assignment of an unqualified value to meters, as in meters = 99, will be flagged as an error by the Units Checker.

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.

17.1  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
@Length
@Luminance
@Mass
@Speed
@Substance
@Temperature
@Time

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 @Length: Meters @m and the derived units millimeters @mm and kilometers @km
  6. For @Luminance: Candela @cd
  7. For @Mass: kilograms @kg and the derived unit grams @g
  8. For @Speed: meters per second @mPERs and kilometers per hour @kmPERh
  9. For @Substance: Mole @mol
  10. For @Temperature: Kelvin @K and the derived unit Celsius @C
  11. For @Time: seconds @s and the derived units minutes @min and hours @h

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.

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 25.2. For an example of its use, see the @PolyUnit Javadoc.

17.2  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)
@SubtypeOf({Time.class})
@UnitsMultiple(quantity=s.class, prefix=Prefix.nano)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
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 31.4.1.

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.

17.3  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.)

17.4  Running the Units Checker 🔗

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

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

17.5  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.

17.6  References 🔗


Chapter 18  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 integral types: bype, char, short, int, and long. Floating-point types (float and double) do not have operations that interpret the bits as unsigned.

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 positive, but just because a variable’s value is positive 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 8) or the @IntRange annotation of the Constant Value Checker (see Chapter 21).

18.1  Annotations 🔗

The Signedness Checker uses type annotations to indicate the signedness that the programmer intends an expression to have.


Figure 18.1: 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 positive signed range, so it has the same interpretation as signed or unsigned and may be used with either interpretation. 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. This annotation should almost never be written by the programmer.
@PolySigned
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 25.2.
@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.

18.1.1  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:

18.2  Prohibited operations 🔗

The Signedness Checker prohibits the following uses of operators:

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.

18.3  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.

18.4  Utility routines for manipulating unsigned values 🔗

Class SignednessUtil provides static utility methods for working with unsigned values. Some of these re-implement functionality in JDK 8, making it available in earlier versions of Java. Others provide new functionality. All of them are properly annotated with @Unsigned where appropriate, so using them may reduce the number of annotations that you need to write.

Class SignednessUtilExtra contains more utility methods that reference packages not included in Android. This class is not included in checker-qual.jar, so you may want to copy the methods to your code.

18.5  Local type refinement 🔗

Local type refinment/inference (Section 26.7) may be surprising for the Signedness type system. Ordinarily, an expression with unsigned type may not participate in a division, as shown in Sections 18.2 and 18.3. 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. For example, it can participate in division. Since the result of the division is signed, you cannot accidentally assign the division result to an @Unsigned variable.

    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
    }

Chapter 19  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 24.10) 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.

19.1  Aliasing annotations 🔗


Figure 19.1: 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 creates 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.

19.2  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 19.4.)
  @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.
}

19.3  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>.

19.4  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 26.7.

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:

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 19.2 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 19.2: Example of the Aliasing Checker’s type refinement rules.


Chapter 20  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 26.7).

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.util.PurityChecker.

20.1  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.

If you supply the command-line option -AsuggestPureMethods, then the Checker Framework will suggest methods that can be marked as @SideEffectFree, @Deterministic, or @Pure.

20.2  Purity annotations are trusted 🔗

Currently, 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.

20.3  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.invalid.overriding"), or you can use the -AsuppressWarnings=purity.invalid.overriding command-line argument to suppress all such warnings. In the future, the Checker Framework will support inheriting annotations from superclass definitions.

20.4  Suppressing warnings 🔗

The command-line option -AassumeSideEffectFree makes the Checker Framework unsoundly assume that every called method is side-effect-free. 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 you are starting out annotating a project, or if you are using the Checker Framework to find some bugs but not to give a guarantee that no more errors exist of the given type.


Chapter 21  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.

21.1  Annotations 🔗

The Constant Value Checker uses type annotations to indicate the value of an expression (Section 21.1.1), and it uses method annotations to indicate methods that the Constant Value Checker can execute at compile time (Section 21.2.2).

21.1.1  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, and @StringVal. Additional type annotations for arrays and strings are @ArrayLen, @ArrayLenRange, and @MinLen. A polymorphic qualifier (@PolyValue) is also supported (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, @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 annotation may be applied to a char array. Although byte arrays are often converted to/from strings, the @StringVal annotation may not be applied to them. This is because the conversion depends on the platform’s character set.

@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 21.1 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 21.1: 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, 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 21.2 illustrates how the Constant Value Checker infers type annotations (using flow-sensitive type qualifier refinement, Section 26.7).


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 21.2: The Constant Value Checker infers different types for a variable on different lines of the program.

21.2  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 21.3. If the other annotation is a declaration annotation, it may be moved; see Section 28.1.1.


 android.support.annotation.IntRange 
⇒  org.checkerframework.checker.common.value.qual.IntRange 
Figure 21.3: Correspondence between other constant value and range annotations and the Checker Framework’s annotations.

21.2.1  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 operators that do not throw exceptions (e.g., +, -, <<, !=).

21.2.2  @StaticallyExecutable methods and the classpath 🔗

The Constant Value Checker statically executes methods annotated with @StaticallyExecutable, if the method has already been compiled and is on the classpath.


@StaticallyExecutable @Pure
public 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 21.4: The @StaticallyExecutable annotation enables constant propagation through method calls.

A @StaticallyExecutable method must be @Pure (side-effect-free and deterministic).

Additionally, a @StaticallyExecutable method and any method it calls must be on the classpath for the compiler, because they are reflectively called at compile-time to perform the constant value analysis. 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. For example, the command-line arguments to the Checker Framework might include:

  -processor org.checkerframework.common.value.ValueChecker
  -Astubs=statically-executable.astub
  -classpath $CLASSPATH:MY_PROJECT/build/

21.3  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:

There are some other situations in which the Constant Value Checker produces a warning message:

21.4  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 21.5 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 21.5: 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.


Chapter 22  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 22.1 gives an example of reflection resolution. Then, Section 22.2 describes the MethodVal and ClassVal Checkers, which reflection resolution uses internally.

22.1  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.

22.2  MethodVal and ClassVal Checkers 🔗

The implementation of reflection resolution internally uses the ClassVal Checker (Section 22.2.1) and the MethodVal Checker (Section 22.2.2). They are similar to the Constant Value Checker (Section 21) 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 22.2.3).

22.2.1  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.

The arguments are binary names (JLS §13.1).

@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.

The arguments are binary names (JLS §13.1).

@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 22.1: 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 22.1 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.

22.2.2  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 22.2: 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 22.2 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

∀ indexes i ∃ an index j:  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.

22.2.3  MethodVal and ClassVal inference 🔗

The developer rarely has to write @ClassVal or @MethodVal annotations, because the Checker Framework infers them according to Figure 22.3. Most readers can skip this section, which explains the inference rules.


Figure 22.3: 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:

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 23  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 31). This includes type introduction rules via the @QualifierForLiterals meta-annotation , and other features such as type refinement (Section 26.7) and qualifier polymorphism (Section 25.2).

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 24.17. See Section 33.7.1 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 31.

23.1  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:

The annotations listed in -Aquals or -AqualDirs must be accessible to the compiler during compilation in the classpath. In other words, they must already be compiled (and, typically, be on the javac classpath) before you run the Subtyping Checker with javac. It is not sufficient to supply their source files on the command line.


To suppress a warning issued by the Subtyping Checker, use a @SuppressWarnings annotation, with the argument being the unqualified, uncapitalized name of any of the annotations passed to -Aquals. This will suppress all warnings, regardless of which of the annotations is involved in the warning. (As a matter of style, you should choose one of the annotations as your @SuppressWarnings key and stick with it.)

23.2  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.

  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 31.4.1.

    Don’t forget to compile these classes:

    $ javac myPackage/qual/Encrypted.java myPackage/qual/PossiblyUnencrypted.java
    

    The resulting .class files should either be on your classpath, or on the processor path (set via the -processorpath command-line option to javac).

  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.

23.3  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 17), 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 17) 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 23.1.


Figure 23.1: 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?

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 25.1.2), 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 24  Third-party checkers 🔗

The Checker Framework has been used to build other checkers that are not distributed together with the framework. This chapter mentions just a few of them. They are listed in reverse chronological order; newer ones appear first and older ones appear last.

They are externally-maintained, so if you have problems or questions, you should contact their maintainers rather than the Checker Framework maintainers.

If you want this chapter to reference your checker, please send us a link and a short description.

24.1  Object Construction Checker 🔗

The Object Construction Checker verifies that your code correctly uses the builder pattern, never omitting a required argument. The checker has built-in support for Lombok and AutoValue. Programmers can extend it to other builders by writing method specifications.

24.2  AWS crypto policy compliance checker 🔗

The AWS crypto policy compliance checker checks that no weak cipher algorithms are used with the Java crypto API.

24.3  AWS KMS compliance checker 🔗

The AWS KMS compliance checker extends the Constant Value Checker (see Chapter 21) 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 group: 'software.amazon.checkerframework', name: 'aws-kms-compliance-checker', version: '1.0.2'

Other build systems are similar.

24.4  UI Thread Checker for ReactiveX 🔗

The Rx Thread & Effect Checker [SCSC18] enforces UI Thread safety properties for stream-based Android applications and is available at https://github.com/uber-research/RxThreadEffectChecker.

24.5  Glacier: Class immutability 🔗

Glacier [CNA+17] enforces transitive class immutability in Java. According to its webpage:

24.6  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/2016/05/09/jsr-308-and-the-checker-framework-add-even-more-typesafety-to-jooq-3-9/.

24.7  Read Checker for CERT FIO08-J 🔗

CERT rule FIO08-J describes a rule for the correct handling of characters/bytes read from a stream.

The Read Checker enforces this rule. It is available from https://github.com/opprop/ReadChecker.

24.8  Nullness Rawness Checker 🔗

The Nullness Rawness Checker is a 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 them, install Checker Framework version 2.9.0.

24.9  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.

24.10  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 SPARTA toolset is available from https://checkerframework.org/sparta/. The paper “Collaborative verification of information flow for a high-assurance app store” appeared in CCS 2014.

24.11  CheckLT taint checker 🔗

CheckLT uses taint tracking to detect illegal information flows, such as unsanitized data that could result in a SQL injection attack. CheckLT is available from http://checklt.github.io/.

24.12  EnerJ checker 🔗

A checker for EnerJ [SDF+11], 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, is available from http://sampa.cs.washington.edu/research/approximation/enerj.html.

24.13  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/.

24.14  Safety-Critical Java checker 🔗

A checker for Safety-Critical Java (SCJ, JSR 302) [TPV10] is available at https://www.cs.purdue.edu/sss/projects/oscj/checker/checker.html. Developer resources are available at the project page https://code.google.com/archive/p/scj-jsr302/.

24.15  Thread locality checker 🔗

Loci [WPM+09], a checker for thread locality, is available at http://www.it.uu.se/research/upmarc/loci/.

24.16  Units and dimensions checker 🔗

A checker for units and dimensions is available at https://www.lexspoon.org/expannots/.

Unlike the Units Checker that is distributed with the Checker Framework (see Section 17), 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.

24.17  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.

Just as you can extend the Subtyping Checker to create a type-checker, you can extend a typestate checker to create a type-checker that supports typestate analysis. An extensible typestate analysis by Adam Warski that builds on the Checker Framework is available at http://www.warski.org/typestate.html.

24.17.1  Comparison to flow-sensitive type refinement 🔗

The Checker Framework’s flow-sensitive type refinement (Section 26.7) 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 26.10) 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 25  Generics and polymorphism 🔗

This chapter describes support for Java generics (also known as “parametric polymorphism”) and polymorphism over type qualifiers.

Section 25.2 describes support for polymorphism over type qualifiers.

25.1  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 25.1.3); rather, the instantiation of the type parameter is restricted (see Section 25.1.2).

25.1.1  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 parameters and the type qualifiers. If it infers imprecise types that lead to type-checking warnings elsewhere, then you have two options. You can convert the raw types such as List to parameterized types such as List<String>, or you can supply the -AignoreRawTypeArguments command-line option. That option causes the Checker Framework to ignore all subtype tests for type arguments that were inferred for a raw type.

25.1.2  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 25.1.4.

For a concrete example, recall the type system of the Regex Checker (see Figure 11.1) 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 supertype 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:

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:

Because of these rules, the recommended style is:

For further discussion, see Section 33.7.2.

25.1.3  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 25.1.2).

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.

25.1.4  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 26.5.5).

25.1.5  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.)

 MyList1MyList2MyList3
t = null;OKerrorerror
t = nble;OKerrorerror
nble = null;OKOKOK
nn = null;errorerrorerror
t = this.get(0);OKOKOK
nble = this.get(0);OKOKOK
nn = this.get(0);errorOKerror
this.add(t);OKOKOK
this.add(nble);OKerrorerror
this.add(nn);OKOKOK

The differences are more significant when the qualifier hierarchy is more complicated than just @Nullable and @NonNull.

25.1.6  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 values of that type are read but are never assigned — 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. Its elements can be read but not written, 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 performs 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.

25.1.7  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.

25.1.8  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) { ... }

25.2  Qualifier polymorphism 🔗

The Checker Framework supports type qualifier polymorphism for methods, which permits a single method to have multiple different qualified type signatures. This is similar to Java’s generics, but is used in situations where you cannot use Java generics. If you can use generics, you typically do not need to use a polymorphic qualifier such as @PolyNull.

To use a polymorphic qualifier, just write it on a type. For example, you can write @PolyNull anywhere in a method that you would write @NonNull or @Nullable. A polymorphic qualifier can be used in a method signature or body. It may not be used on a class or field.

A method written using a polymorphic qualifier 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. See the examples below in Section 25.2.1.

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.

To define a polymorphic qualifier, mark the definition with @PolymorphicQualifier. For example, @PolyNull is a polymorphic type qualifier for the Nullness type system:

  import java.lang.annotation.ElementType;
  import java.lang.annotation.Target;
  import org.checkerframework.framework.qual.PolymorphicQualifier;

  @PolymorphicQualifier
  @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
  public @interface PolyNull {}

See Section 25.2.3 for a way you can sometimes avoid defining a new polymorphic qualifier.

25.2.1  Examples of using polymorphic qualifiers 🔗

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.

25.2.2  Relationship to subtyping and generics 🔗

Qualifier polymorphism has the same purpose and plays the same role as Java’s generics. You use them in the same cases, such as:

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.

25.2.3  The @PolyAll qualifier applies to every type system 🔗

Each type system may have its own polymorphic type qualifier. If some method is qualifier-polymorphic over every type qualifier hierarchy, then you can use @PolyAll. This is better than trying to write every @Poly* qualifier on that method.

For example, a method that only performs == on array elements will work no matter what the array’s element types are:

  /**
   * Searches for the first occurrence of the given element in the array,
   * testing for equality using == (not the equals method).
   */
  public static int indexOfEq(@PolyAll Object[] a, @Nullable Object elt) {
    for (int i=0; i<a.length; i++) {
      if (elt == a[i]) {
        return i;
      }
    }
    return -1;
  }

25.2.4  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 25.2.5 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());
}

25.2.5  Using a single polymorphic qualifier in a method signature 🔗

As explained in Section 25.2.4, 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

  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)

Using a single @PolyAll qualifier to indicate all arguments are legal 🔗

A single @PolyAll annotation can indicate that any possible value is permitted to be passed. For example:

  boolean eq(@PolyAll Object other) {
    return this == other;
  }

The @PolyAll annotation applies to all type systems. It would be infeasible to write the top qualifier for every possible type system and to update this method’s annotation whenever a new type system is defined.

By contrast, a declaration of eq without @PolyAll:

  boolean eq(Object other) {
    return this == other;
  }

would reject some calls, in type systems where the default type qualifier applied to Object is not the top type.

A related use of a single polymorphic qualifier is to override a generic type. For example, the annotation on Comparable.compareTo() is:

  public interface Comparable<T extends @NonNull Object> {
    @Pure int compareTo(@PolyAll @NonNull T a1);
  }

which indicates that, for every type system other than the nullness type system, every value is permitted as an argument, regardless of how the Comparable type was instantiated. For example, this call is legal:

  Comparable<@MyBottom String> cble;
  @MyTop String s;
  ...
  cble.compareTo(s);

Chapter 26  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.

26.1  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.

26.2  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 11), 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"}).

26.3  Upper bound of qualifiers on uses of a given type 🔗

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.

  @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 26.5.1)

An upper bound can also be specified by the type-system designer 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.

Due to existing type rules, an expression of type A MyClass can only be used in limited ways.

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 forbid expressions and local variables from having types inconsistent with the class annotation, then important APIs and common coding paradigms will 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.

26.4  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. In particular, qualifiers in method bodies are rare (except occasionally 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 26.5. 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 26.7.

26.5  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 26.5.6 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 the MyClass. (Section 26.5.1)
  2. If no @NoDefaultQualifierForUse is written on the declaration of MyClass, the qualifier explicitly written on the declaration of MyClass. (Section 26.5.1)
  3. The qualifier with a meta-annotation @DefaultFor(types = MyClass.class). (Section 26.5.1)
  4. The qualifier with a meta-annotation @DefaultFor(typeKinds = KIND), where KIND is the TypeKind of MyClass. (Section 26.5.1)
  5. The qualifier in the innermost user-written @DefaultQualifier for the location of the use of MyClass. (Section 26.5.2)
  6. 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 31.4.4); this is usually CLIMB-to-top (Section 26.5.3).
  7. 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. This makes the type local variables top so they can be refined.

26.5.1  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 of types, using the types element, or type kinds, using the typeKinds elements.

Interaction between qualifier bounds and DefaultQualifierForUse:

26.5.2  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.

26.5.3  Defaulting rules and CLIMB-to-top 🔗

Each type system defines a default qualifier (see Section 31.4.4). 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, Instanceof, 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).

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 26.5.6.

The rest of this section explains the rationale and implementation of CLIMB-to-top.

Here is the rationale for CLIMB-to-top:

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.

26.5.4  Inherited defaults 🔗

In certain situations, it would be convenient for an annotation on a superclass member to be automatically inherited by subclasses that override it. This feature would reduce both annotation effort and program comprehensibility. In general, a program is read more often than it is edited/annotated, so the Checker Framework does not currently support this feature.

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.

Computation is necessary 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, an error would be thrown if no such annotations existed.

In the future, this feature may be added optionally, and each type-checker implementation can enable it if desired.

26.5.5  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.type.incompatible 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 26.5.3). 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.

26.5.6  Default qualifiers for .class files (library defaults) 🔗

(Note: Currently, the conservative library defaults presented in this section are off by default and can be turned on by supplying the -AuseDefaultsForUncheckedcode=bytecode command-line option. In a future release, they will be turned on by default and it will be possible to turn them off by supplying a -AuseDefaultsForUncheckedCode=-bytecode command-line option.)

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.

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.) When you are performing pluggable type-checking, it is best to ensure that the compiler only reads such .class files. Section 30.4 discusses how to create annotated libraries.

If the checker was not run during the compiler execution that created the .class file, then the .class file contains only the type qualifiers that the programmer wrote explicitly. (Furthermore, there is no guarantee that these qualifiers are correct, since they have not been checked.) In this case, each checker decides what qualifier to use for the locations where the programmer did not write an annotation. Unless otherwise noted, the choice is:

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 30) or alternately suppress the warnings (see Chapter 27).

For example, an unannotated method

  String concatenate(String p1, String p2)

in a classfile would be interpreted as

  @Top String concatenate(@Bottom String p1, @Bottom String p2)

There is no single possible default that is sound for fields. In the rare circumstance that there is a mutable public field in an unannotated library, the Checker Framework may fail to warn about code that can misbehave at run time.

26.6  Annotations on constructors 🔗

26.6.1  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 10) 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.

26.6.2  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.

26.7  Type refinement (flow-sensitive type qualifier inference) 🔗

The checkers treat expressions within a method body as having a subtype of their declared or defaulted (Section 26.5) type.

Type refinement eliminates some false positive warnings. Type refinement also reduces your burden of writing type qualifiers in your program, since you do not need to write type qualifiers on local variables.

26.7.1  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: posible dereference of null

Type refinement applies to every checker, including new checkers that you write. Here is an example for the Regex Checker (Chapter 11):

  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"
  }

26.7.2  Type refinement behavior 🔗

The checker treats a variable or expression as a subtype of its declared type:

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: automatic type refinement, flow-sensitive type qualifier inference, local type inference, and sometimes just “flow”.

26.7.3  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 25.1.6 and Section 26.1.) 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 34.2) 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:

26.7.4  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:

Type systems that do not currently support a run-time test, but could do so with some additional implementation work, are

Type systems that cannot support a run-time test are:

26.7.5  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 20 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 it as
      @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 myField as @MonotonicNonNull.
      @MonotonicNonNull Object myField;
    
  3. If computeValue sets myField to a non-null value, declare it as
      @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() { ... }
    

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.

26.7.6  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 or -da 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 are enabled. If you supply the -AassumeAssertionsAreDisabled command-line option, then the Checker Framework assumes assertions are disabled. 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 27.2.

26.8  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:

The set of permitted expressions is a subset of all Java expressions:

Limitations: The following Java expressions may not currently be written:

Additionally, it is not possible to write 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.

26.9  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.

26.10  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 26.10.1), 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 24.17.

26.10.1  @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 27  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:

You can suppress warnings throughout the codebase by using the following mechanisms:

Some type checkers can suppress warnings via

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.

27.1  @SuppressWarnings annotation 🔗

You can suppress specific errors and warnings by use of the @SuppressWarnings annotation, for example @SuppressWarnings("interning") or @SuppressWarnings("nullness"). Section 27.1.1 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 27.1.2 discusses where the annotation may be written in source code.

Section 27.1.3 gives best practices for writing @SuppressWarnings annotations.

27.1.1  @SuppressWarnings syntax 🔗

The @SuppressWarnings annotation takes a string argument, in one of the following forms: "checkername:messagekey", "checkername", or "messagekey".

The argument checkername is in lower case and is derived from the way you invoke the checker. 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 23.1.) Sometimes, a checker honors checkername arguments; use the -AshowSuppressWarningKeys command-line option to see them. Using "all" as the checkername applies to all checkers; this is not recommended, except for messages common to all checkers such as purity-related messages when using -AcheckPurityAnnotations.

The argument messagekey is the message key for the error. Each warning message from the compiler gives the most specific suppression key 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. Beware writing a very short messagekey, because it might suppress more warnings than you expect.

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, a warning is only suppressed if the key is in the "checkername" or "checkername:messagekey" format, as in @SuppressWarnings("nullness") or @SuppressWarnings("nullness:assignment.type.incompatible").

27.1.2  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. It cannot be used on statements, expressions, or types. (assert plus @AssumeAssertion can be used between statements and can affect arbitrary expressions; see Section 27.2.)

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 27.2.

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 10). 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;

27.1.3  Good practices when suppressing warnings 🔗

Suppress warnings in the smallest possible scope 🔗

Prefer @SuppressWarnings on a local variable to one on a method, and prefer one on a method to one on a class.

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 27.2.

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 27.1.2.

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 27.1.2:

  @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("purity") // side effect to local state of type BitSet
@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

A particularly good (and concise) justification is to reference an issue in the issue tracker, as in these two from Daikon:

@SuppressWarnings("keyfor") // https://tinyurl.com/cfissue/877
@SuppressWarnings("flowexpr.parse.error") // https://tinyurl.com/cfissue/862

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).

27.2  @AssumeAssertion string in an assert message 🔗

You can suppress a warning by asserting that some property is true, and placing the string @AssumeAssertion(warningkey) in the assertion message.

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 26.7), if the expression appeared in a conditional test. Each type system has its own rules about what type refinement it performs.

The warning key is exactly as in the @SuppressWarnings annotation (Section 27.1). The same good practices apply as for @SuppressWarnings annotations, such as writing a comment justifying why the assumption is safe (Section 27.1.3). Sometimes, writing an assertion and @AssumeAssertion is a less disruptive code change than refactoring to create a location where @SuppressWarnings can be written.

The -AassumeAssertionsAreEnabled and -AassumeAssertionsAreDisabled command-line options (Section 26.7.6) 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.

27.2.1  Suppressing warnings and defensive programming 🔗

This section explains the distinction between two different uses for assertions (and for related methods like JUnit’s Assert.assertNotNull).

Assertions are commonly used for two distinct purposes: documenting how the program works and debugging the program when it does not work correctly. By default, the Checker Framework assumes that each assertion is used for debugging: the assertion might fail at run time, and the programmer wishes to be informed at compile time about such possible run-time errors. On the other hand, if you write the @AssumeAssertion string in the assert message, then the Checker Framework assumes that you have used some other technique to verify that the assertion can never fail at run time, so the checker assumes the assertion passes and does not issue a warning.

Distinguishing the purpose of each assertion is important for precise type-checking. 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.

On the other hand, assertions sometimes document facts that a programmer has independently verified to be true, and 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. Only do so if you are sure that the assertion always succeeds at run time.

Sometimes methods such as NullnessUtil.castNonNull are used instead of assertions. Just as for assertions, you can treat them as debugging aids or as documentation. If you know that a particular codebase uses a nullness-checking method not for defensive programming but to indicate facts that are guaranteed to be true (that is, these assertions will never fail at run time), then you can suppress warnings related to it. Annotate its definition just as NullnessUtil.castNonNull is annotated (see 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 you are annotating a codebase that already contains precondition checks, such as:

  public String get(String key, String def) {
    checkNotNull(key, "key"); // NOI18N
    ...
  }

then you should mark the appropriate parameter as @NonNull (which is the default). This will prevent the checker from issuing a warning about the checkNotNull call.

27.3  -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 keys, 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.

27.4  -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 or -AonlyDefs command-line option, see 27.5).

Set the -AskipUses command-line option to a regular expression that matches 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 class names (not file names) for which warnings and errors should be emitted; warnings about uses of all other classes will be suppressed.

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.

27.5  -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 27.4.)

Set the -AskipDefs command-line option to a regular expression that matches 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 class names (not file names) whose definitions should be type-checked.

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.

27.6  -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:

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.

27.7  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.

27.8  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 17.5).


Chapter 28  Handling legacy code 🔗

Section 2.4.2 describes a methodology for applying annotations to legacy code. This chapter tells you what to do if, for some reason, you cannot change your code in such a way as to eliminate a checker warning.

Also recall that you can convert checker errors into warnings via the -Awarns command-line option; see Section 2.2.3.

28.1  Checking partially-annotated programs: handling unannotated code 🔗

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.

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 library call never returns null, there are several ways to eliminate the compiler warnings.

  1. Annotate unannotated_method in full. This approach provides the strongest guarantees, but may require you to annotate additional methods that unannotated_method calls. See Chapter 30 for a discussion of how to annotate libraries for which you have no source code.
  2. Annotate only the signature of unannotated_method, and suppress warnings in its body. Two ways to suppress the warnings are via a @SuppressWarnings annotation or by not running the checker on that file (see Chapter 27).
  3. Suppress all warnings related to uses of unannotated_method via the skipUses processor option (see Section 27.4). Since this can suppress more warnings than you may expect, it is usually better to annotate at least the method’s signature. If you choose the boundary between the annotated and unannotated code wisely, then you only have to annotate the signatures of a limited number of classes/methods (e.g., the public interface to a library or package).

Chapter 30 discusses adding annotations to signatures when you do not have source code available. Chapter 27 discusses suppressing warnings.

28.1.1  Declaration annotations 🔗

When a declaration annotation is an alias for a type annotation, then 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.

Users who are using old Java 5–7 declaration annotations (for instance, from the no-longer-supported FindBugs tool) 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.


Chapter 29  Type inference 🔗

There are two different tasks that are commonly called “type inference”:

  1. Type inference during type-checking: The type-checker fills in an appropriate type where the programmer didn’t write one, but does not change the source code. See Section 29.1.
  2. Type inference to annotate a program: As a separate step before type-checking, a type inference tool inserts type qualifiers into the source code. See Section 29.2.

Each variety has its own advantages, discussed below. Advantages of all varieties of type inference include:

29.1  Local type inference during type-checking 🔗

During type-checking, if certain variables have no programmer-written type qualifier, the type-checker determines whether there is some type qualifier that would permit the program to type-check. If so, the type-checker uses that type qualifier, but does not change the source code. Each time the type-checker runs, it re-infers the type qualifier for that variable. If no type qualifier exists that permits the program to type-check, the type-checker issues a warning.

Local type inference is built into the Checker Framework. Every checker automatically uses it. As a result, a programmer typically does not have to write any qualifiers inside the body of a method (except occasionally on type arguments). However, it primarily works within a method, not across method boundaries. The source code must already contain annotations for method signatures (arguments and return values) and fields.

Advantages of this variety of type inference include:

For more details about local type inference during type-checking, also known as “flow-sensitive type refinement”, see Section 26.7.

29.2  Type inference to annotate a program 🔗

As a separate step before type-checking, a type inference tool takes the program as input, and outputs a set of type qualifiers that would make the program type-check. (If no such set exists, for example because the program is not type-correct, then the inference tool does its best but makes no guarantees.) These qualifiers are inserted into the source code or the class file. They can be viewed and adjusted by the programmer, and can be used by tools such as the type-checker.

Advantages of this variety of type inference include:

29.2.1  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.

Type inference for specific type systems 🔗

Section 3.3.7 lists several tools that infer annotations for the Nullness 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 26.7.5.

Type inference for any type system 🔗

By supplying the -Ainfer command-line option, any type-checker can infer annotations. See Section 29.3.

Cascade [VPEJ14] 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.

29.3  Whole-program inference 🔗

Whole-program inference is an interprocedural inference that 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 inserted into your program. 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.

To use 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 trying 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.

Whole-program inference differs from type refinement (Section 26.7) in three ways. First, type refinement only works within a method body. Second, type refinement always refines the current type, regardless of whether the value already has an annotation in the source code. Third, whole-program inference can infer a subtype or a supertype of the default type, by contrast with type refinement which always refines the current type to a subtype.

29.3.1  Whole-program inference ignores some code 🔗

Whole-program inference ignores code within the scope of a @SuppressWarnings annotation with an appropriate key (Section 27.1). 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 below, whole-program inference requires invocations of your code, or assignments to your methods, to generalize from. If a field is set via reflection (such as via injection), then whole-program inference would 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 (pseudo-)assignments where the right-hand-side is the null literal.

29.3.2  Manually checking whole-program inference results 🔗

As with any type inference tool, it is a good idea to manually examine the results.

Poor whole-program inference results due to non-representative uses 🔗

Whole-program inference 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.

29.3.3  How whole-program inference works 🔗

This section explains how the infer-and-annotate.sh script works. If you merely want to run the script and you are not encountering trouble, you can skip this section.

The infer-and-annotate.sh script repeatedly runs the following steps:

  1. Run the checker with the -Ainfer command-line option to infer types for fields and method signatures. The output of this step is a .jaif file that records the inferred types.
  2. Insert the inferred annotations in the program. This step uses the Annotation File Utilities (https://checkerframework.org/annotation-file-utilities/).

The process halts when there are no more changes to the inference results, that is, the .jaif file is unchanged between two runs. On each iteration through the process, there may be new annotations in the .jaif file, and some type-checking errors may be eliminated (though others might be introduced).

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 contains a defect. 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:


Chapter 30  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 26.5.6 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, you can instead suppress all warnings related to an unannotated library, or to part of your codebase, by use of the -AskipUses or -AonlyUses command-line option; see Sections 27.427.5.)

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 32).

    Then, when doing pluggable type-checking, 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 30.2.

  2. Write annotations in a “stub file”, if you do not have access to the source code.

    Then, when doing pluggable type-checking, 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 method).

    For instructions, see Section 30.5.

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 distributed with 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 30.1.4, you should annotate an entire class at a time. You may find type inference tools (Chapter 29.2) helpful when getting started, but you should always examine their results.

30.1  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.

30.1.1  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.

All of these changes increase the difference between upstream (the original version) and your annotated version. This makes it harder for others to understand what you have done, and they make it harder to pull changes from upstream into the annotated library.

30.1.2  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 null, nor that it permits it. (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.

30.1.3  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.

30.1.4  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. If you don’t annotate the whole file, then other people won’t know whether a particular method is unannotated because you didn’t get to it yet or because you decided that it didn’t need any annotations.

30.1.5  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, and 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, but if you supply -Awarns, you don’t have to suppress those warnings and the compiler will still produce .class files.

30.2  Creating an annotated library 🔗

This section describes how to create an annotated library.

  1. See the the org.checkerframework.annotatedlib group in the Central Repository to find out whether an annotated version of the library already exists.
  2. Annotate some files.

    When you annotate a file, annotate the whole thing, not just a few of its methods. Once the file is fully annotated, add an @AnnotatedFor({"checkername"}) annotation to its class(es), 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.

30.3  Creating an annotated JDK 🔗

This section tells how to create an annotated JDK for a new type system. Section 34.2.1 tells how to improve annotations for the JDK for an existing type system.

When you create a new checker, you need to also supply annotations for parts of the JDK, either as stub files or as source code that will be compiled and its annotations inserted into the JDK. This section describes the latter approach. It assumes that you have already followed the instructions for building the Checker Framework from source (Section 34.3).

If you are adding annotations to an existing annotated JDK, then you only need to follow the last two steps.

  1. Get Java 8 source code (must be version 8)
    1. Download JDK8 from Oracle
    2. Open the downloaded tar (tar -xvzf)
    3. Unzip the contained src.zip (resulting in folders: com/, java/, javax/, launcher/, org/)
  2. Set up the Checker Framework (replace mychecker with the name of your checker)
    1. mkdir $CHECKERFRAMEWORK/checker/jdk/mychecker/
      cd $CHECKERFRAMEWORK/checker/jdk/mychecker/
      echo "include ../Makefile.jdk" > Makefile
    2. Add mychecker to $CHECKERFRAMEWORK/checker/jdk/MakeFile in the definition of CHECKER_DIRS and in the definition of ANNOTATED_CLASSES (don’t forget to add a closing parenthesis at the end of the definition of ANNOTATED_CLASSES!).
  3. For each file you want to annotate, copy the JDK version into the directory $CHECKERFRAMEWORK/checker/jdk/mychecker/src/, using the same directory structure as the JDK.

    Whenever you add a file, fully annotate it, as described in Section 30.1.

    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 built using the -Awarns command-line option.

  4. Build the annotated JDK. There are two ways:

    (First, be sure you have followed the instructions for building the Checker Framework from source (Section 34.3) including running ./gradlew cloneAndBuildDependencies.)

    You may receive a compilation error because your files use a part of the JDK that is not in the annotated JDK. If the missing part is relatively small, please add it. If the missing part is large, then you can make small changes to the JDK source code, such as commenting out a method body. (Use /* and */ on their own lines, so that the diffs are minimal.)

    To upload a compiled version of the annotated JDK, for use by other people, see https://github.com/typetools/annotated-libraries.

30.4  Compiling partially-annotated libraries 🔗

If you completely annotate a library, then you can compile it using a pluggable type-checker, and include the resulting .jar file on your classpath. You get a guarantee that the library contains no errors.

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 30.1.)

There are two concerns:

30.4.1  The -AuseDefaultsForUncheckedCode=source,bytecode command-line argument 🔗

When compiling a library that is not fully annotated, use command-line argument -AuseDefaultsForUncheckedCode=source,bytecode. This causes the checker to behave normally for classes with a relevant @AnnotatedFor annotation. For classes without @AnnotatedFor, the checker uses unchecked code defaults (see Section 26.5.6) for any type use with no explicit user-written annotation, and the checker issues no warnings.

The @AnnotatedFor annotation, written on a class, indicates that the class has been annotated for certain type systems. For example, @AnnotatedFor({"nullness", "regex"}) means that the programmer has written annotations for the Nullness and Regular Expression type systems. If one of those two type-checkers is run, the -AuseDefaultsForUncheckedCode=source,bytecode command-line argument has no effect and this class is treated normally: unannotated types are defaulted using normal source-code defaults and type-checking warnings are issued. @AnnotatedFor’s arguments are any string that may be passed to the -processor command-line argument: the fully-qualified class name for the checker, or a shorthand for built-in checkers (see Section 2.2.5). Writing @AnnotatedFor on a class doesn’t necessarily mean that you wrote any annotations, but that the user examined the source code and verified that all appropriate annotations are present.

Whenever you compile a class using the Checker Framework, including when using the -AuseDefaultsForUncheckedCode=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.

30.5  Using stub classes 🔗

A stub file contains “stub classes” that contain annotated signatures, but no method bodies. A checker uses the annotated signatures at compile time, instead of or in addition to annotations that appear in the library.

Section 30.3 explains how you should choose between creating stub classes or creating an annotated library. Section 30.5.3 describes how to create stub classes. Section 30.5.1 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.

30.5.1  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=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).

The -Astubs argument is for stub files that users create. A checker automatically reads some of its own stub files; see Section 31.9.

If a method appears in more than one stub file (or twice in the same stub file), then the annotations are merged. If any of the methods have different annotations from the same hierarchy on the same type, then the annotation from the last declaration is used.

If both bytecode and a stub file provide information for the same element, the stub file information is used. In particular, an un-annotated type variable in a stub file is used instead of annotations on a type variable in bytecode. This feature allows stub files to change the effective annotations in all possible situations. Use the -AstubWarnIfOverwritesBytecode command-line option to get a warning whenever a stub file overwrites bytecode annotations. Use the -AstubWarnIfRedundantWithBytecode command-line option to get a warning whenever a stub file specification is redundant with bytecode annotations.

A file being compiled takes precedence over a stub file. If file A.java is being compiled, then any stub for class A is ignored.

When a stub file is provided by the author of a checker, the stub file is used automatically, with no need for the user to supply a command-line option.

30.5.2  Stub file format 🔗

Every Java file is a valid stub file. However, you can omit information that is not relevant to pluggable type-checking; this makes the stub file smaller and easier for people to read and write. Also note that 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 typically the library is unannotated.)
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.
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.
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.

30.5.3  Creating a stub file 🔗

Stub files are generally stored together with the checker implementation, in the same directory as the checker’s .java source code.

If you have access to the Java source code 🔗

Every Java file is a stub file. If you have access to the Java file, rename 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.

Optionally (but highly recommended!), run the type-checker to verify that your annotations are correct. 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 the JDK itself, then you should use the -Aignorejdkastub command-line option.

This 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 this 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.

If you do not have access to the Java source code 🔗

If you do not have access to the library source code, then you can create a stub file from the class file (Section 30.5.3), and then annotate it. The rest of this section describes this approach.

  1. Create a stub file by running the stub class generator. (checker.jar must be on your classpath.)
      cd nullness-stub
      java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar org.checkerframework.framework.stub.StubGenerator java.lang.String > String.astub
    

    Supply it with the fully-qualified name of the class for which you wish to generate a stub class. The stub class generator prints the stub class to standard out, so you may wish to redirect its output to a file.

  2. Add import statements for the annotations. So you would need to add the following import statement at the beginning of the file:
      import org.checkerframework.checker.interning.qual.*;
    

    The stub file parser silently ignores any annotations that it cannot resolve to a type, so don’t forget the import statement. Use the -AstubWarnIfNotFound command-line option to see warnings if an entry could not be found.

  3. Add annotations to the stub class. For example, you might annotate the String.intern() method as follows:
      @Interned String intern();
    

    You may also remove irrelevant parts of the stub file; see Section 30.5.2.

30.5.4  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
Warn whenever some element of a stub file cannot be found. The @NoStubParserWarning annotation on a package or type in a stub file overrides the -AstubWarnIfNotFound command-line option, and no warning will be issued.
-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.
-AstubWarnIfOverwritesBytecode
Warn whenever some element of a stub file overwrites annotations contained in bytecode.

Finally, use command-line option -AstubDebug to output debugging messages while parsing stub files, including about unknown classes, methods, and annotations. This overrides the @NoStubParserWarning 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

StubParser: 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

StubParser: 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.

30.6  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 two flags that may help you in determining whether library files are read, and if they are read whether the library’s annotations are parsed.

-verbose Outputs info about compile phases — when the compiler reads/parses/attributes/writes any file. Also outputs the classpath and sourcepath paths.
-XDTA:parser (which is equivalent to -XDTA:reader plus -XDTA:writer) Sets the internal debugJSR308 flag, which outputs information about reading and writing.

Chapter 31  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 plugin 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. There is also a developer manual for people who wish to edit the Checker Framework source code or make pull requests.

Writing a simple checker is easy! For example, here is a complete, useful type-checker:

import java.lang.annotation.Target;
import java.lang.annotation.ElementType;
import org.checkerframework.common.subtyping.qual.Unqualified;
import org.checkerframework.framework.qual.SubtypeOf;

@SubtypeOf(Unqualified.class)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
public @interface Encrypted {}

This checker is so short because it builds on the Subtyping Checker (Chapter 23). See Section 23.2 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:

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 24 or even include it in the Checker Framework distribution.

31.1  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)

(.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.

31.2  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                  [optional] interface to the compiler
  | MyPropVisitor.java                  [optional] type rules
  | MyPropAnnotatedTypeFactory.java     [optional] type introduction and dataflow rules

Note that MyPropChecker.java is required unless you are building on the Subtyping Checker.

Sections 31.431.8 describe the individual components of a type system as written using the Checker Framework:

31.4 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.
31.5 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.
31.6 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.
31.7 Type introduction rules. You specify the type of some expressions where the rules differ from the built in framework rules
31.8 Dataflow rules. These optional rules enhance flow-sensitive type qualifier inference (also known as local variable type inference).

31.3  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 Java classpath. Invoking a custom checker that builds on the Subtyping Checker is slightly different (Section 23.1).

31.3.1  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. Before you start coding, first write the user manual. 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 31.12 gives a suggested structure for the manual chapter, which will help you avoid omitting any parts. 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.

  2. Implement the type qualifiers and hierarchy (Section 31.4).

    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:

      void testHierarchy(@UnknownSign int us, @NonNegative int nn) {
        @UnknownSign int a = us;
        @UnknownSign int b = nn;
        // :: error: assignment.type.incompatible
        @NonNegative int c = us;  // expected error on this line
        @NonNegative int d = nn;
      }
    

    Type-check your test files using the Subtyping Checker (Chapter 23).

  3. Write the checker class itself (Section 31.5).

    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 ...
    
  4. 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 31.10). This will make it much more convenient to run tests, and to ensure that they are passing, as your work proceeds.
  5. Annotate parts of the JDK, if relevant (Section 31.9).

    Write test cases for at least some of the annotated JDK methods to ensure that the annotations are being properly read by your checker.

  6. Implement type rules, if any (Section 31.6). (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 31.7).

    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 31.8).

    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
      }
    

31.4  Annotations: Type qualifiers and hierarchy 🔗

A type system designer specifies the qualifiers in the type system (Section 31.4.1) 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 31.4.2), or procedurally through subclassing QualifierHierarchy or TypeHierarchy (Section 31.4.3).

31.4.1  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 annotations should be placed within a directory called qual, and qual should be placed in the same directory as your checker’s source file.

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.

The Checker Framework automatically treats any annotation that is declared in the qual package as a type qualifier. (See Section 31.5.1 for more details.)

Your type system should include a top qualifier and a bottom qualifier (Section 31.4.7).

You should also define a polymorphic qualifier @PolyMyTypeSystem (Section 25.2).

The Javadoc of every type qualifier should include an example use of the qualifier.

31.4.2  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.

The declarative and procedural mechanisms for specifying the hierarchy can be used together. In particular, when using the @SubtypeOf meta-annotation, further customizations may be performed procedurally (Section 31.4.3) by overriding the isSubtype method in the checker class (Section 31.5). However, the declarative mechanism is sufficient for most type systems.

31.4.3  Procedurally defining the qualifier hierarchy 🔗

While the declarative syntax suffices for many cases, more complex type hierarchies can be expressed by overriding, in your subclass of BaseTypeVisitor, 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”.)

31.4.4  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 26.5.

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 26.5.3 other defaulting rules are automatically added to every checker. Also, Section 26.5 describes other meta-annotations used to specify default annotations.

31.4.5  Relevant Java types 🔗

Sometimes, a checker only processes certain Java types. For example, the Format String Checker is relevant only to CharSequence and its subtypes such as String. The @RelevantJavaTypes annotation on the checker class indicates that its qualifiers may only be written on those types and no others. All irrelevant types are defaulted to the top annotation.

31.4.6  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.

31.4.7  Completeness of the type hierarchy 🔗

When you define a type system, its type hierarchy must be a lattice: every set of types has a unique least upper bound and a unique greatest lower bound. This implies that there must be a top type that is a supertype of all other types, and there must be a bottom type that is a subtype of all other types. Furthermore, the top type and bottom type 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 type hierarchies. An example is the Nullness Checker, which has three separate type hierarchies, one each for nullness, initialization, and map keys. In this case, each type 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 type hierarchy must have a bottom qualifier — a qualifier that is a (direct or indirect) subtype of every other qualifier.

null is 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”.

The hierarchy shown in Figure 3.3 lacks a bottom qualifier, but the actual implementation does contain a (non-user-visible) bottom qualifier.

Top qualifier 🔗

Your type hierarchy must have a top qualifier — a qualifier that is a (direct or indirect) supertype of every other qualifier. Here is one reason. The default type for local variables is the top qualifier (that type is then flow-sensitively refined depending on what values are stored in the local variable). If there is no single top qualifier, then there is no unambiguous choice to make for local variables.

31.4.8  Annotations whose argument is a Java expression (dependent type annotations) 🔗

Sometimes, an annotation needs to refer to a Java expression. Section 26.8 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 26.8. 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 FlowExpressions.Receiver object by calling internalReprOf(AnnotationProvider, ExpressionTree) or internalReprOf(AnnotationProvider, Node). Then, call toString() on the FlowExpressions.Receiver object.

31.5  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, and provides a simple API, SourceChecker.report, to issue errors using the compiler error reporting mechanism.

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 report (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 27.1.3 discusses best practices when using a message key in a @SuppressWarnings annotation.

31.5.1  Indicating supported annotations 🔗

A checker must indicate the annotations that it supports (that make up its type hierarchy), including whether it supports the polymorphic qualifier @PolyAll.

By default, a checker supports PolyAll, and all type annotations located in a subdirectory called qual that’s located in the same directory as the checker. 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, or to indicate whether a checker supports the polymorphic qualifier @PolyAll, checker writers can override the createSupportedTypeQualifiers method (see its Javadoc for details).

An aggregate checker (which extends AggregateChecker) does not need to specify its type qualifiers, but each of its component checkers should do so.

31.5.2  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.

    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 getSupportedTypeCheckers method, like the following:

      public class MyUnitCheckers extends AggregateChecker {
        protected Collection<Class<? extends SourceChecker>> getSupportedCheckers() {
          return Arrays.asList(DistanceUnitChecker.class,
                               VelocityUnitChecker.class,
                               MassUnitChecker.class);
        }
      }
    

    An example of an aggregate checker is I18nChecker (see Chapter 14.2), 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 getTypeFactoryOfSubchecker.

31.5.3  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 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 MyChecker -Amyoption=p1,p2

The value is returned as a single string and you have to perform the required parsing of the option.

31.6  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 MyCheckerVisitor.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 SourceChecker.report) 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.visitTreeType(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:

31.6.1  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 32.5).

31.6.2  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 30).

Only if the contract is not expressible should you write a type-checking rule for method invocation, where your rule checks the name of the method being called and then treats the method in a special way.

31.7  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 31.4.4. The meta-annotation @QualifierForLiterals can be written on an annotation declaration to specifiy that that annotation should be applied to the type of literals listed in the meta-annotation.

31.7.1  Procedurally specifying type introduction rules 🔗

If the meta-annotations are not sufficiently expressive, then you can write your own type introduction rules. To do so, create a subclass of AnnotatedTypeFactory and override its two addComputedTypeAnnotations methods.

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 26.4 for explanations of these concepts).

To add type introduction rules, you should override addComputedTypeAnnotations(Tree,AnnotatedTypeMirror) (or addComputedTypeAnnotations(Tree,AnnotatedTypeMirror,boolean) if extending GenericAnnotatedTypeFactory) and addComputedTypeAnnotations(Element,AnnotatedTypeMirror). The methods operate on AnnotatedTypeMirror, which is the Checker Framework’s representation of an annotated type. The methods can make arbitrary changes to the annotations on a type.

31.8  Dataflow: enhancing flow-sensitive type refinement 🔗

By default, every checker performs flow-sensitive type refinement, also known as local type inference, as described in Section 26.7.

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 26.7.4 gives examples of type systems with and without run-time tests.

The steps to customizing type refinement are:

  1. §31.8.1 Determine which expressions will be refined.
  2. §31.8.2 Create required class and configure its use.
  3. §31.8.3 Override methods that handle Nodes of interest.
  4. §31.8.4 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.

31.8.1  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 31.8.231.8.4 explain how to create a transfer class when the @EnsuresQualifier and @EnsuresQualifierIf annotations are insufficient.

31.8.2  Create required class 🔗

In the same directory as MyCheckerChecker.java, create a class named MyCheckerTransfer that extends CFTransfer.

Leave the class body empty for now. 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 MyCheckerTransfer class, you will also need to override the createFlowTransferFunction method in your type factory to return to return a new instance of the class.)

(As a reminder, use of @EnsuresQualifier and @EnsuresQualifierIf may obviate the need for a transfer class.)

31.8.3  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 “Program representation” below).

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 of a run-time test method call. 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 31.13 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: ""

31.8.4  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 31.8.1:

  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,
                     null, "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 21).

      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.

31.8.5  Disabling flow-sensitive inference 🔗

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 31.8), put the following two lines at the beginning of the constructor for your subtype of BaseAnnotatedTypeFactory:

        // disable flow inference
        super(checker, /*useFlow=*/ false);

31.9  Annotated JDK 🔗

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. As described in Section 30, there are two general ways to supply an annotated library: in Java files that will be compiled to .class files, or as stub files (partial Java source files).

It’s easier to start out with stub files. If you need to annotate many classes (say, more than 20 or so), then you should create an annotated JDK.

To supply an annotated JDK that will be compiled, see Section 30.3.

To supply an annotated JDK as a stub file, create a file jdk.astub in the checker’s main source directory. It 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.

31.10  Testing framework 🔗

The Checker Framework provides a convenient way to write tests for your checker. It is extensively documented in file checker-framework/checker/tests/README.

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. Here are some of the tasks you will perform:

31.11  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.

31.11.1  Amount of detail in messages 🔗

31.11.2  Stub and JDK libraries 🔗

31.11.3  Progress tracing 🔗

31.11.4  Saving the command-line arguments to a file 🔗

31.11.5  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:

javac -processor myProcessor -Aflowdotdir=. MyClass.java
for dotfile in *.dot; do dot -Tpdf -o $dotfile.pdf $dotfile; done

where the first command creates file someDirectory/myClass.dot that represents the CFG, and the last command draws the CFG in a PDF file. The dot program is part of Graphviz.

31.11.6  Miscellaneous debugging options 🔗

31.11.7  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

31.11.8  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

31.12  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; see ManualTaglet.

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 26.7.4 And add the file to docs/manual/Makefile. Keep the lists appear 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.

31.13  javac implementation survival guide 🔗

Since this section of the manual was written, the useful “The Hitchhiker’s Guide to javac” has become available at http://openjdk.java.net/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.

31.13.1  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 class/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, like 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-loop statement.

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.

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 to determine the class of the object, because you cannot necessarily predict the run-time type of the object that implements an interface. Instead, use the getKind() method. The method returns TypeKind, ElementKind, and Tree.Kind for the three interfaces, respectively.

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. Using equals with one object created by the Checker Framework and one by javac will always return false even if they represent the same annotation. Therefore, never use any method that compares AnnotationMirrors using equals. AnnotationUtils has various methods that should be used instead. Also, AnnotationMirrorMap and AnnotationMirrorSet can be used.

31.13.2  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 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 31.6.1.

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 http://wiki.netbeans.org/Java_DevelopersGuide and the compiler mailing list archives at http://mail.openjdk.java.net/pipermail/compiler-dev/ (subscribe at http://mail.openjdk.java.net/mailman/listinfo/compiler-dev).

31.14  Integrating a checker with the Checker Framework 🔗

To integrate a new checker with the Checker Framework release, perform the following:



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 32  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:

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 32.8. 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.

32.1  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 annotations in the main dex, but strips out class-retention annotations.

32.2  Android Studio 3.0 and the Android Gradle Plugin 3.0 🔗

Android Studio 3.0 and Android Gradle Plugin 3.0.0 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 app’s build.gradle file.

  1. In your module’s build.gradle file, set the source and target compatibility to JavaVersion.VERSION_1_8
    android {
        ...
        compileOptions {
            sourceCompatibility JavaVersion.VERSION_1_8
            targetCompatibility JavaVersion.VERSION_1_8
        }
    }
    
  2. 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", "")
            }
        }
    }
    
  3. Add a dependency configuration for the annotated JDK:
    configurations {
        checkerFrameworkAnnotatedJDK {
            description = 'a copy of JDK classes with Checker Framework type qualifiers inserted'
        }
    }
    
    
  4. Declare the Checker Framework dependencies:
    dependencies {
        ... existing dependencies...
        ext.checkerFrameworkVersion = '2.11.1'
        implementation "org.checkerframework:checker-qual-android:${checkerFrameworkVersion}"
        annotationProcessor "org.checkerframework:checker:${checkerFrameworkVersion}"
        checkerFrameworkAnnotatedJDK "org.checkerframework:jdk8:${checkerFrameworkVersion}"
    }
    
  5. Direct all tasks of type JavaCompile used by the CheckTypes build variant to use the annotated JDK.
    gradle.projectsEvaluated {
        tasks.withType(JavaCompile).all { compile ->
            if (compile.name.contains("CheckTypes")) {
                compile.options.compilerArgs += [
                        "-Xbootclasspath/p:${configurations.checkerFrameworkAnnotatedJDK.asPath}"
                ]
            }
        }
    }
    
  6. To run the checkers, build using the checkTypes variant.
    gradlew checkTypes
    

32.3  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.

  1. Set the jsr308javac property:
      <property environment="env"/>
    
      <property name="checkerframework" value="${env.CHECKERFRAMEWORK}" />
    
      <!-- On Mac/Linux, use the javac shell script; on Windows, use javac.bat -->
      <condition property="cfJavac" value="javac.bat" else="javac">
          <os family="windows" />
      </condition>
    
      <presetdef name="jsr308.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 jsr308.javac instead of javac -->
        <jsr308.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"/>
          ...
        </jsr308.javac>
      </target>
    

    Fill in each ellipsis (…) from the original compilation target.

    In the example, the target is named check-nullness, but you can name it whatever you like.

32.3.1  Explanation 🔗

This section explains each part of the Ant task.

  1. Definition of jsr308.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 the Checker Framework compiler that is distributed with the Checker Framework.

    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.

32.4  Buck 🔗

Buck is a build system maintained by Facebook.

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:

prebuilt_jar(
    name = 'checker-framework',
    binary_jar = 'checker-2.11.1.jar',
    visibility = [ 'PUBLIC' ]
)

prebuilt_jar(
    name = 'checker-qual',
    binary_jar = 'checker-qual-2.11.1.jar',
    visibility = [ 'PUBLIC' ]
)

java_library (
 name = 'hello',
 srcs = glob(['src/main/java/**/*.java']),
 java_version = '1.8',
 provided_deps = [ ':checker-framework', ':checker-qual' ],
# To add annotation processing
 annotation_processors = [ 'org.checkerframework.checker.units.UnitsChecker' ],
 annotation_processor_deps = [ ':checker-framework', ':checker-qual' ],
 annotation_processor_params = [ 'nocheckjdk' ]
)

32.5  Eclipse 🔗

You need to run the Checker Framework via Ant or via another build tool, 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 a type-checking plug-in 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 32.5.1 gives details for Ant, but other build systems are similar.

32.5.1  Using an Ant task 🔗

Add an Ant target as described in Section 32.3. You can run the Ant target by executing the following steps (instructions copied from http://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.

32.5.2  Troubleshooting Eclipse 🔗

Eclipse issues an “Unhandled Token in @SuppressWarnings” warning if you write a @SuppressWarnings annotation containing a string that 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.

32.6  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.

32.7  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 such as Gradle, these instructions will not work. 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 Gradle, possibly not).

To run a checker within IntelliJ IDEA on every compile:

  1. Change the project SDK to 1.8, as explained at https://www.jetbrains.com/help/idea/sdk.html#change-project-sdk.
  2. Change the language level for your project to 8, as explained at https://www.jetbrains.com/help/idea/project-page.html.
  3. Add the annotated JDK library to the bootclasspath. On the Java compiler settings page, see https://www.jetbrains.com/help/idea/java-compiler.html, in the “Additional command line parameters”: section, add:
    -Xbootclasspath/p:"<path to checker framework>/checker/dist/jdk8.jar"
  4. Configure an annotation profile.
    1. Create an annotation profile.
    2. Associate any desired modules with that profile.
    3. Configure that profile:
      1. Add checker.jar to the processor path. (It’s located in checker-framework/checker/dist/.)
      2. Add checkers to be run during compilation by writing the fully-qualified name of the checker in the “Processor FQ Name” section.
  5. Add checker-qual.jar, located in 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.

32.8  Javac compiler (command line) 🔗

To perform pluggable type-checking, run the javac compiler with the Checker Framework on the classpath. There are three ways to achieve this. You can use any one of them. However, if you are using the Windows command shell, you must use the last one.

32.9  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 String getRole() { return role; }
  public void setRole(String role) { this.role = role; }

Unfortunately, the generated code is lacking @Regex annotations on the return type of getRole and the argument type of setRole. You need to tell Lombok to copy type annotations from the field, so that clients that use the generated methods will type-check properly.

In your lombok.config file, add to the lombok.copyableAnnotations configuration key any type annotations that might interact with a Lombok feature (e.g., a type use annotation on a field annotated with @Getter or @Setter). For example:

  lombok.copyableAnnotations += org.checkerframework.checker.regex.qual.Regex

Directory docs/examples/lombok contains an example Gradle project that augments the configuration key.

(Note: Lombok does support a limited set of annotations without the need for the lombok.copyableAnnotations configuration key, but as of version 1.18.4 (October 2018), Lombok does not yet support any of the Checker Framework annotations; see variable BASE_COPYABLE_ANNOTATIONS in https://github.com/rzwitserloot/lombok/blob/master/src/core/lombok/core/handlers/HandlerUtil.java.)

32.10  Maven 🔗

If you use the Maven tool, then you can enable Checker Framework checkers by following the instructions below.

See the directory docs/examples/MavenExample/ for examples of the use of Maven build files that run a checker. These examples 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 31.11.4) and shorthands for built-in checkers (see Section 2.2.5) 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.

  1. Declare a dependency on the Checker Framework artifacts, either from Maven Central or from a local directory. Find the existing <dependencies> section and add the following new <dependency> items:
    1. To obtain artifacts from Maven Central:
        <dependencies>
          ... existing <dependency> items ...
      
          <!-- Annotations from the Checker Framework: nullness, interning, locking, ... -->
          <dependency>
            <groupId>org.checkerframework</groupId>
            <artifactId>checker-qual</artifactId>
            <version>2.11.1</version>
          </dependency>
          <dependency>
            <groupId>org.checkerframework</groupId>
            <artifactId>jdk8</artifactId>
            <version>2.11.1</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. To use a local version of the Checker Framework (for example, one that you have built from source):
        <dependencies>
          ... existing <dependency> items ...
      
          <!-- Annotations from the Checker Framework: nullness, interning, locking, ... -->
          <dependency>
            <groupId>org.checkerframework</groupId>
            <artifactId>checker-qual</artifactId>
            <version>2.11.1</version>
            <scope>system</scope>
            <systemPath>${env.CHECKERFRAMEWORK}/checker/dist/checker-qual.jar</systemPath>
          </dependency>
          <dependency>
            <groupId>org.checkerframework</groupId>
            <artifactId>checker</artifactId>
            <version>2.11.1</version>
            <scope>system</scope>
            <systemPath>${env.CHECKERFRAMEWORK}/checker/dist/checker.jar</systemPath>
          </dependency>
          <!-- The annotated JDK to use. -->
          <dependency>
            <groupId>org.checkerframework</groupId>
            <artifactId>jdk8</artifactId>
            <version>2.11.1</version>
            <scope>system</scope>
            <systemPath>${env.CHECKERFRAMEWORK}/checker/dist/jdk8.jar</systemPath>
          </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. Use Maven properties to hold the locations of the annotated JDK. It was declared as Maven dependencies above. To set the value of this property automatically, you will use the Maven Dependency plugin.

    First, create the property in the properties section of the POM:

    <properties>
      <!-- These properties will be set by the Maven Dependency plugin -->
      <annotatedJdk>${org.checkerframework:jdk8:jar}</annotatedJdk>
    </properties>
    

    Change the reference to the maven-dependency-plugin within the <plugins> section, or add it if it is not present.

      <plugin>
        <!-- This plugin will set properties values using dependency information -->
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <executions>
          <execution>
            <goals>
              <goal>properties</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    
  3. If you are using Java 8, use the javac from Error Prone.
      <!-- using github.com/google/error-prone-javac is required when running on JDK 8 -->
      <profiles>
        <profile>
          <id>jdk8</id>
          <activation>
            <jdk>1.8</jdk>
          </activation>
          <properties>
            <javac.version>9+181-r4173-1</javac.version>
          </properties>
          <dependencies>
            <dependency>
              <groupId>com.google.errorprone</groupId>
              <artifactId>javac</artifactId>
              <version>9+181-r4173-1</version>
            </dependency>
          </dependencies>
          <build>
            <plugins>
              <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                  <fork>true</fork>
                  <compilerArgs combine.children="append">
                    <arg>-J-Xbootclasspath/p:${settings.localRepository}/com/google/errorprone/javac/${javac.version}/javac-${javac.version}.jar</arg>
                  </compilerArgs>
                </configuration>
              </plugin>
            </plugins>
          </build>
        </profile>
      </profiles>
    
  4. Direct the Maven compiler plugin to use the desired checkers. Change the reference to the maven-compiler-plugin within the <plugins> section, or add it if it is not present.

    For example, to use the org.checkerframework.checker.nullness.NullnessChecker:

      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.0</version>
        <configuration>
          <fork>true</fork>
          <source>1.8</source>
          <target>1.8</target>
          <compilerArguments>
            <Xmaxerrs>10000</Xmaxerrs>
            <Xmaxwarns>10000</Xmaxwarns>
          </compilerArguments>
          <annotationProcessorPaths>
            <path>
              <groupId>org.checkerframework</groupId>
              <artifactId>checker</artifactId>
              <version>2.11.1</version>
            </path>
          </annotationProcessorPaths>
          <annotationProcessors>
            <!-- Add all the checkers you want to enable here -->
            <annotationProcessor>org.checkerframework.checker.nullness.NullnessChecker</annotationProcessor>
          </annotationProcessors>
          <compilerArgs combine.children="append">
            <!-- location of the annotated JDK, which comes from a Maven dependency -->
            <arg>-Xbootclasspath/p:${annotatedJdk}</arg>
            <!-- -Awarns turns type-checking errors into warnings. -->
            <!-- <arg>-Awarns</arg> -->
          </compilerArgs>
        </configuration>
      </plugin>
    

    Now, building with Maven will run the checkers during compilation.

    If you want to allow Maven to compile your code without running the checkers, you may want to move the declarations above to within a Maven profile, so that the checkers run only if the profile was enabled.

32.11  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 whereas this functionality has not yet been incorporated into NetBeans 8.2. Additionally, JDK annotations are currently unavailable on NetBeans 8.1 and 8.2.

32.11.1  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.

32.11.2  Adding a checker via an ant target 🔗

  1. Set the jsr308javac property:
      <property environment="env"/>
    
      <property name="checkerframework" value="${env.CHECKERFRAMEWORK}" />
    
      <!-- On Mac/Linux, use the javac shell script; on Windows, use javac.bat -->
      <condition property="cfJavac" value="javac.bat" else="javac">
          <os family="windows" />
      </condition>
    
      <presetdef name="jsr308.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 jsr308.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="${empty.dir}"/>
                <mkdir dir="@{apgeneratedsrcdir}"/>
                <jsr308.javac debug="@{debug}" deprecation="${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/>
                </jsr308.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.

32.12  tIDE 🔗

tIDE, an open-source Java IDE, supports the Checker Framework. You can download it from https://sourceforge.net/projects/tide/.

32.13  Type inference tools 🔗

There are two different tasks that are commonly called “type inference”. You are probably interested in type inference to annotate a program’s source code; see Section 29.2. This section explains the two different varieties of type inference.

  1. Type inference during type-checking (Section 26.7): During type-checking, if certain variables have no programmer-written type qualifier, the type-checker determines whether there is some type qualifier that would permit the program to type-check. If so, the type-checker uses that type qualifier, but never tells the programmer what it was. Each time the type-checker runs, it re-infers the type qualifier for that variable. If no type qualifier exists that permits the program to type-check, the type-checker issues a type warning.

    This variety of type inference is built into the Checker Framework. Every checker can take advantage of it at no extra effort. However, it only works within a method, not across method boundaries.

    Advantages of this variety of type inference include:

  2. Type inference to annotate a program (Section 29.2): As a separate step before type-checking, a type inference tool takes the program as input, and outputs a set of type qualifiers that would type-check. These qualifiers are inserted into the source code or the class file. They can be viewed and adjusted by the programmer, and can be used by tools such as the type-checker.

    This variety of type inference must be provided by a separate tool. It is not built into the Checker Framework.

    Advantages of this variety of type inference include:

Advantages of both varieties of inference include:

Each variety of type inference has its place. When using the Checker Framework, type inference during type-checking is performed only within a method (Section 26.7). Every method signature (arguments and return values) and field must have already been explicitly annotated, either by the programmer or by a separate type-checking tool (Section 29.2). This approach enables modular checking (one class or method at a time) and gives documentation benefits. The programmer still has to put in some effort, but much less than without inference: typically, a programmer does not have to write any qualifiers inside the body of a method (except sometimes on type arguments).


Chapter 33  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:

33.1: Motivation for pluggable type-checking
33.1.1: I don’t make type errors, so would pluggable type-checking help me?
33.1.2: Should I use pluggable types (type qualifiers), or should I used Java subtypes?

33.2: Getting started
33.2.1: How do I get started annotating an existing program?
33.2.2: Which checker should I start with?
33.2.3: How can I join the checker-framework-dev mailing list?

33.3: Usability of pluggable type-checking
33.3.1: Are type annotations easy to read and write?
33.3.2: Will my code become cluttered with type annotations?
33.3.3: Will using the Checker Framework slow down my program? Will it slow down the compiler?
33.3.4: How do I shorten the command line when invoking a checker?
33.3.5: Method pre-condition contracts, including formal parameter annotations, make no sense for public methods.

33.4: How to handle warnings
33.4.1: What should I do if a checker issues a warning about my code?
33.4.2: What does a certain Checker Framework warning message mean?
33.4.3: Can a pluggable type-checker guarantee that my code is correct?
33.4.4: What guarantee does the Checker Framework give for concurrent code?
33.4.5: How do I make compilation succeed even if a checker issues errors?
33.4.6: Why does the checker always say there are 100 errors or warnings?
33.4.7: Why does the Checker Framework report an error regarding a type I have not written in my program?
33.4.8: Why does the Checker Framework accept code on one line but reject it on the next?
33.4.9: How can I do run-time monitoring of properties that were not statically checked?

33.5: False positive warnings
33.5.1: What is a “false positive” warning?
33.5.2: How can I improve the Checker Framework to eliminate a false positive warning?
33.5.3: Why doesn’t the Checker Framework infer types for fields and method return types?
33.5.4: Why doesn’t the Checker Framework track relationships between variables?
33.5.5: Why isn’t the Checker Framework path-sensitive?

33.6: Syntax of type annotations
33.6.1: What is a “receiver”?
33.6.2: What is the meaning of an annotation after a type, such as @NonNull Object @Nullable?
33.6.3: What is the meaning of array annotations such as @NonNull Object @Nullable []?
33.6.4: What is the meaning of varargs annotations such as @English String @NonEmpty ...?
33.6.5: What is the meaning of a type qualifier at a class declaration?
33.6.6: How are type qualifiers written on upper and lower bounds?
33.6.7: Why shouldn’t a qualifier apply to both types and declarations?
33.6.8: How do I annotate a fully-qualified type name?
33.6.9: What is the difference between type annotations and declaration annotations?

33.7: Semantics of type annotations
33.7.1: How can I handle typestate, or phases of my program with different data properties?
33.7.2: Why are explicit and implicit bounds defaulted differently?
33.7.3: Why are type annotations declared with @Retention(RetentionPolicy.RUNTIME)?

33.8: Creating a new checker
33.8.1: How do I create a new checker?
33.8.2: What properties can and cannot be handled by type-checking?
33.8.3: Why is there no declarative syntax for writing type rules?

33.9: Tool questions
33.9.1: How does pluggable type-checking work?
33.9.2: What classpath is needed to use an annotated library?
33.9.4: Is there a type-checker for managing checked and unchecked exceptions?

33.10: Relationship to other tools
33.10.1: Why not just use a bug detector (like FindBugs or Error Prone)?
33.10.2: How does the Checker Framework compare with Eclipse’s Null Analysis?
33.10.3: How does the Checker Framework compare with NullAway?
33.10.4: How does the Checker Framework compare with the JDK’s Optional type?
33.10.5: How does pluggable type-checking compare with JML?
33.10.6: Is the Checker Framework an official part of Java?
33.10.7: What is the relationship between the Checker Framework and JSR 305?
33.10.8: What is the relationship between the Checker Framework and JSR 308?

33.1  Motivation for pluggable type-checking 🔗

33.1.1  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.

33.1.2  Should I use pluggable types (type qualifiers), or should I used 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. Brian Goetz expands on this issue in an article on the pseudo-typedef antipattern [Goe06]. Even if compatibility is not a concern, a code change may introduce bugs, whereas adding annotations does not change the run-time behavior. It is possible to add annotations to existing code, including code you do not maintain or cannot change. For code that strictly cannot be changed, you can write library annotations (see Chapter 30).
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.
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 inference (Section 26.5), 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 33.1.2.

33.2  Getting started 🔗

33.2.1  How do I get started annotating an existing program? 🔗

See Section 2.4.2.

33.2.2  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 24) have known bugs that limit their usability. (Report the ones that affect you, so the developers will prioritize fixing them.)

33.2.3  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, which is for user-focused discussions.

Anyone is welcome to join checker-framework-discuss@googlegroups.com and send mail to it.

33.3  Usability of pluggable type-checking 🔗

33.3.1  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.

33.3.2  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:

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.

33.3.3  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 compiler emits the identical bytecodes as the Java 8 compiler and 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. In theory it should only add a few percent overhead, but our current implementation can double the compilation time — or more, if you run many pluggable type-checkers at once. This is especially true if you run pluggable type-checking on every file (as we recommend) instead of just on the ones that have recently changed. Nonetheless, compilation with pluggable type-checking still feels like compilation, and you can do it as part of your normal development process.

33.3.4  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 .... See Section 2.2.4 for a way to avoid the need for the -processor command-line option.

33.3.5  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.

33.4  How to handle warnings and errors 🔗

33.4.1  What should I do if a checker issues a warning about my code? 🔗

For a discussion of this issue, see Section 2.4.5.

33.4.2  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 ask on the mailing list. 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.

33.4.3  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, 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.

33.4.4  What guarantee does the Checker Framework give for concurrent code? 🔗

The Lock Checker (see Chapter 7) 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 26.7) that the Checker Framework can do.

33.4.5  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.

33.4.6  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.

33.4.7  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 26.5. 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 26.7. Note that an invocation of an impure method may cause the loss of all information that was determined via type refinement; see Section 26.7.5.

33.4.8  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 26.7), which also tells how to specify the side effects of someMethod (Section 26.7.5).

33.4.9  How can I do run-time monitoring of properties that were not statically checked? 🔗

Some properties are not checked statically (see Chapter 27 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 11.2.4). But, it would be better to have more general support that does not require the user to explicitly insert method calls.

33.5  False positive warnings 🔗

33.5.1  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 34.2), 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 33.5.2).

33.5.2  How can I improve the Checker Framework to eliminate a false positive warning? 🔗

As noted in Section 33.5.1, 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.

33.5.3  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 26.7), 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 29.3.) 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.

33.5.4  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 26.8.

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 26.7):

  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 8), 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
  }

33.5.5  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 33.5.4).

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.

33.6  Syntax of type annotations 🔗

There is also a separate FAQ for the type annotations syntax (https://checkerframework.org/jsr308/jsr308-faq.html).

33.6.1  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.

33.6.2  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 33.6.3).

33.6.3  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 28.1.1 explains how the Checker Framework handles declaration annotations in the meanwhile.

33.6.4  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.

33.6.5  What is the meaning of a type qualifier at a class declaration? 🔗

See Section 26.3.

33.6.6  How are type qualifiers written on upper and lower bounds? 🔗

See Section 25.1.2.

33.6.7  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 33.6.1.

Another example of a type-and-declaration annotation that represents poor design is JCIP’s @GuardedBy annotation [GPB+06]. As discussed in Section 7.6.1, it means two different things when applied to a field or a method. To reduce confusion and increase expressiveness, the Lock Checker (see Chapter 7) 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, and should not be necessary any longer.

33.6.8  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

33.6.9  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.

33.7  Semantics of type annotations 🔗

33.7.1  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 24.17), but they have not been as extensively tested.

33.7.2  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 26.5.3):

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 11.1.

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:

33.7.3  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).

For the purpose of static checking at compile time, CLASS retention would be 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.

33.8  Creating a new checker 🔗

33.8.1  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 31 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 23.2 explains this checker and tells you how to run it.

33.8.2  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.

33.8.3  Why is there no declarative syntax for writing type rules? 🔗

A type system implementer can declaratively specify the type qualifier hierarchy (Section 31.4.2) and the type introduction rules (Section 31.7). However, the Checker Framework uses a procedural syntax for specifying type-checking rules (Section 31.6). 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.

33.9  Tool questions 🔗

33.9.1  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.

33.9.2  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.

33.9.3  Why do .class files contain more annotations than the source code? 🔗

You may notice annotations such as @Pure and @SideEffectFree in compiled .class files even though the source code contains no annotations. These annotations are propagated from annotated libraries, such as the JDK, to your code.

When an overridden method has a side effect annotation, the overriding method must have one too. If you do not write one explicitly, the Checker Framework could issue a warning, or it could automatically add the missing annotation. It does the latter, for annotations declared with the @InheritedAnnotation meta-annotation.

In addition, defaulted and inferred type qualifiers are written into .class files; see Section 26.4.

33.9.4  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 16) is a type-and-effect system that is distributed with the Checker Framework.

33.10  Relationship to other tools 🔗

33.10.1  Why not just use a bug detector (like FindBugs 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], Jlint [Art01], or PMD [Cop05], or the tools built into Eclipse (see Section 33.10.2) and IntelliJ.

A pluggable type-checker or verifier differs from a bug detector in several ways:

For a case study that compared the nullness analysis of FindBugs, 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 and IntelliJ’s nullness analyses found 0 and 3 out of the 9 errors that the Nullness Checker found.

The JSR 308 [Ern08] documentation also contains a discussion of related work.

33.10.2  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 33.4.4).

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.

33.10.3  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.

33.10.4  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.

33.10.5  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.

33.10.6  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 are part of Java 8. See the Type Annotations (JSR 308) FAQ for more details.

33.10.7  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, FindBugs, IntelliJ, and other tools; see Section 3.7.

33.10.8  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 34  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 33). If not, you can use the mailing list, checker-framework-discuss@googlegroups.com, to ask other users for help. For archives and to subscribe, see https://groups.google.com/forum/#!forum/checker-framework-discuss. To report bugs, please see Section 34.2. If you want to help out, you can give feedback (including on the documentation), choose a bug and fix it, or select a project from the ideas list at https://rawgit.com/typetools/checker-framework/master/docs/developer/gsoc-ideas.html.

34.1  Common problems and solutions 🔗

34.1.1  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.

34.1.2  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 34.2.) This section describes some possible problems and solutions.

34.1.3  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.

34.1.4  Unexpected type-checking results 🔗

This section describes possible problems that can lead the type-checker to give unexpected results.

34.1.5  Unexpected compilation output when running javac without a pluggable type-checker 🔗

On rare occasions, javac may issue an error when compiling against the Checker Framework’s annotated JDK that you do not get when using the regular JDK. (Recall that the Checker Framework compiler uses the annotated JDK.) This may occur even when you are not running any annotation processor. The error is:

  unchecked method invocation: method myMethod in class C is applied to given types

The reason for this is that there are some wildcards in the annotated JDK that have been given an explicit upper bound — they have been changed from, for example, List<?> to List<? extends Object>. The JDK treats these two types as identical in almost all respects. However, they have a different representation in bytecode. More importantly, List<?> is reifiable but List<? extends Object> is not; this means that the compiler permits uses of the former in some locations where it issues a warning for the latter. You can suppress the warning.

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;

A message of the form

  Error: cannot find symbol

for an annotation type that is imported, and is a applied to a statically-imported enum, is caused by a javac bug, unrelated to the Checker Framework. See https://bugs.openjdk.java.net/browse/JDK-7101822 and https://bugs.openjdk.java.net/browse/JDK-8169095.

34.2  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 34.2.1.

Alternately (especially if your communication is not a bug report), you can send mail to checker-framework-dev@googlegroups.com. We welcome suggestions, annotated libraries, bug fixes, new features, new checker plugins, 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 will be most helpful if you:

A particularly useful format for a test case is as a new file, or a diff to an existing file, for the existing Checker Framework test suite. For instance, for the Nullness Checker, see directory checker-framework/checker/tests/nullness/. But, please report your bug even if you do not report it in this format.

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.

34.2.1  Problems with annotated libraries 🔗

Sometimes, a checker will report a warning because a library is not annotated or because the library contains incorrect annotations. Please do not open GitHub issues for these. 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 30.5.

If the library is annotated but has some incorrect or missing annotations, then please open a pull request that adds the annotations. You can find the annotated source for non-JDK libraries in https://github.com/typetools/. You can find the annotated JDK source in the Checker Framework GitHub repository, either in subdirectories of checker/jdk or in files named jdk.astub in the checker’s implementation directory.

If the library is not annotated at all, you can fork it, add annotations, and propose that your annotated version be moved into https://github.com/typetools/.

For either approach, please see Chapter 30 for tips about writing library annotations.

Thanks for your contribution to the annotated libraries!

34.3  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.

The Checker Framework can be easily built on Unix (Linux and Mac). You might have trouble building on Windows. If so, you can switch to Unix (at least for building, if not for running, the Checker Framework) or you can report the problems to the Checker Framework developers.

34.3.1  Install prerequisites 🔗

You need to install several packages in order to build the Checker Framework. Follow the instructions for your operating system.

Ubuntu
Run the following commands:
sudo apt-get -qqy update
sudo apt-get -qqy install ant cpp git gradle junit4 libcurl3-gnutls \
 make maven mercurial unzip wget openjdk-8-jdk \
 dia hevea latexmk librsvg2-bin maven python-pip \
 texlive-font-utils texlive-fonts-recommended \
 texlive-latex-base texlive-latex-extra texlive-latex-recommended
Mac OS
If you employ homebrew to install packages, run the following commands (you may need to update the version number 2.31):
brew update
brew install git ant hevea maven mercurial librsvg unzip make
brew cask install java
brew cask install mactex

Make a latex directory and copy hevea.sty file into it:

mkdir -p $HOME/Library/texmf/tex/latex
cp -p /usr/local/Cellar/hevea/2.31/lib/hevea/hevea.sty $HOME/Library/texmf/tex/latex/

Note: If copy permission is denied, try sudo.

Also set the JAVA_HOME environment variable in your .bashrc file; for example,

export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0_144.jdk/Contents/Home
Windows
To build on Windows 10, run bash to obtain a version of Ubuntu (provided by the Windows Subsystem for Linux) and follow the Ubuntu instructions.

34.3.2  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.

Set the JAVA_HOME environment variable to the location of your JDK 8 installation (not the JRE installation, and not JDK 7 or earlier). This needs to be an Oracle JDK. (The JAVA_HOME environment variable might already be set, because it is needed for Ant to work.)

Ubuntu
If you use the bash shell, put the following command in your .bashrc file. (This might not work if java refers to an executable in the JRE rather than in the JDK, but you need the JDK installed to build the Checker Framework anyway.)
  export JAVA_HOME=${JAVA_HOME:-$(dirname $(dirname $(dirname $(readlink -f $(/usr/bin/which java)))))}
Mac OS X 10.5 or later
Add one of the following two commands in your .bash_profile or .profile file according to the version of your JDK.
  export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)
  export JAVA_HOME=$(/usr/libexec/java_home -v 1.9)

34.3.3  Build the Checker Framework 🔗

  1. Run ./gradlew cloneAndBuildDependencies to build the Checker Framework dependencies:
      cd $CHECKERFRAMEWORK
      ./gradlew cloneAndBuildDependencies
    
  2. Run ./gradlew assemble to build the Checker Framework:
      cd $CHECKERFRAMEWORK
      ./gradlew assemble
    
  3. 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 Type Annotations compiler.

    Putting the Checker Framework’s javac earlier in your PATH will ensure that the Checker Framework’s version is used.

  4. If you are developing a checker within the Checker Framework, there is a developer version of javac in the bin-devel directory. This version will use compiled classes from dataflow/build, javacutil/build, framework/build, and checker/build in the checker-framework directory instead of the compiled jar files, and by default will print stack traces for all errors.

    To use it, set your PATH to use javac in the bin-devel directory:

    export PATH=$CHECKERFRAMEWORK/checker/bin-devel:${PATH}
    

    The developer version of javac allows you to not have to rebuild the jar files after every code change, in turn allowing you to test your changes faster. Source files can be compiled using command ./gradlew compileJava, or can be automatically compiled by an IDE such as Eclipse.

  5. Test that everything works:

34.3.4  Build the Checker Framework Manual (this document) 🔗

  1. Install needed packages; see Section 34.3.1 for instructions.
  2. Run make in the docs/manual directory to build both the PDF and HTML versions of the manual.

34.3.5  Code style, IDE configuration, pull requests, etc. 🔗

Please see the Checker Framework Developer Manual.

34.3.6  Enable continuous integration builds 🔗

A continuous integration system runs tests every time you push a commit to GitHub.

If you have forked any of the projects rather than just creating a clone, then we recommend that you enable CI builds, so that you learn quickly of any errors that creep into your fork.

Follow the Azure Pipelines getting started directions.

To save time, by default the CI jobs download a pre-built version of the JDK rather than building the JDK from the sources in the Checker Framework repository. This means that if you make a change to the annotated JDK, your tests will fail. [TODO: Explain how to deal with this issue:]

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-jdk8
docker run -it mdernst/cf-ubuntu-jdk8 /bin/bash

Then, you can run arbitrary commands, including those that appear in the CI configuration files.

34.4  Contributing fixes (creating a pull request) 🔗

Please see the document How to create and review a GitHub pull request for instructions about how to create a pull request, which is the way you can contribute code to the Checker Framework project. Thanks in advance for your participation!

For writing new test cases, see file checker-framework/checker/tests/README.

34.5  Publications 🔗

Here are two technical papers about the Checker Framework itself:

Here are some papers about type systems that were implemented and evaluated using the Checker Framework:

Nullness (Chapter 3)
See the two papers about the Checker Framework, described above.
Rawness initialization (Section 24.8)
“Inference of field initialization” (ICSE 2011, https://homes.cs.washington.edu/~mernst/pubs/initialization-icse2011-abstract.html) describes inference for the Rawness Initialization Checker.
Interning (Chapter 6)
See the two papers about the Checker Framework, described above.
Locking (Chapter 7)
“Locking discipline inference and checking” (ICSE 2016, https://homes.cs.washington.edu/~mernst/pubs/locking-inference-checking-icse2016-abstract.html) describes the Lock Checker.
Fake enumerations, type aliases, and typedefs (Chapter 9)
See the ICSE 2011 paper about the Checker Framework, described above.
Regular expressions (Chapter 11)
“A type system for regular expressions” [SDE12] (FTfJP 2012, https://homes.cs.washington.edu/~mernst/pubs/regex-types-ftfjp2012-abstract.html) describes the Regex Checker.
Format Strings (Chapter 12)
“A type system for format strings” [WKSE14] (ISSTA 2014, https://homes.cs.washington.edu/~mernst/pubs/format-string-issta2014-abstract.html) describes the Format String Checker.
Signature strings (Chapter 15)
See the ICSE 2011 paper about the Checker Framework, described above.
GUI Effects (Chapter 16)
“JavaUI: Effects for controlling UI object access” [GDEG13] (ECOOP 2013, https://homes.cs.washington.edu/~mernst/pubs/gui-thread-ecoop2013-abstract.html) describes the GUI Effect Checker.
“Verification games: Making verification fun” (FTfJP 2012, https://homes.cs.washington.edu/~mernst/pubs/verigames-ftfjp2012-abstract.html) describes a general inference approach that, at the time, had only been implemented for the Nullness Checker (Section 3).
Thread locality (Section 24.15)
“Loci: Simple thread-locality for Java” [WPM+09] (ECOOP 2009, http://janvitek.org/pubs/ecoop09.pdf)
Security (Section 24.10)
“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) describes the SPARTA toolset and information flow type-checker.

“Boolean formulas for the static identification of injection attacks in Java” [ELM+15] (LPAR 2015, https://homes.cs.washington.edu/~mernst/pubs/detect-injections-lpar2015-abstract.html)

Generic Universe Types (Section 24.13)
“Tunable static inference for Generic Universe Types” (ECOOP 2011, https://homes.cs.washington.edu/~mernst/pubs/tunable-typeinf-ecoop2011-abstract.html) 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, https://homes.cs.washington.edu/~mernst/pubs/infer-ownership-ecoop2012-abstract.html).

Approximate data (Section 24.12)
“EnerJ: Approximate Data Types for Safe and General Low-Power Computation” [SDF+11] (PLDI 2011, https://homes.cs.washington.edu/~luisceze/publications/Enerj-pldi2011.pdf)
Information flow and tainting (Section 24.10)
“Collaborative Verification of Information Flow for a High-Assurance App Store” [EJM+14] (CCS 2014, https://homes.cs.washington.edu/~mernst/pubs/infoflow-ccs2014.pdf) describes the SPARTA information flow type system.
IGJ and OIGJ immutability (Section 24.9)
“Object and reference immutability using Java generics” [ZPA+07] (ESEC/FSE 2007, https://homes.cs.washington.edu/~mernst/pubs/immutability-generics-fse2007-abstract.html) and “Ownership and immutability in generic Java” [ZPL+10] (OOPSLA 2010, https://homes.cs.washington.edu/~mernst/pubs/ownership-immutability-oopsla2010-abstract.html) describe the IGJ and OIGJ immutability type systems. For further case studies, also see the ISSTA 2008 paper about the Checker Framework, described above.
Javari immutability (Section 24.9)
“Javari: Adding reference immutability to Java” [TE05] (OOPSLA 2005, https://homes.cs.washington.edu/~mernst/pubs/ref-immutability-oopsla2005-abstract.html) describes the Javari type system. For inference, see “Inference of reference immutability” [QTE08] (ECOOP 2008, https://homes.cs.washington.edu/~mernst/pubs/infer-refimmutability-ecoop2008-abstract.html) and “Parameter reference immutability: Formal definition, inference tool, and comparison” [AQKE09] (J.ASE 2009, https://homes.cs.washington.edu/~mernst/pubs/mutability-jase2009-abstract.html). For further case studies, also see the ISSTA 2008 paper about the Checker Framework, described above.
ReIm immutability
“ReIm & ReImInfer: Checking and inference of reference immutability and method purity” [HMDE12] (OOPSLA 2012, https://homes.cs.washington.edu/~mernst/pubs/infer-refimmutability-oopsla2012-abstract.html) describes the ReIm immutability type system.

In addition to these papers that discuss use the Checker Framework directly, other academic papers use the Checker Framework in their implementation or evaluation. Most educational use of the Checker Framework is never published, and most commercial use of the Checker Framework is never discussed publicly.

(If you know of a paper or other use that is not listed here, please inform the Checker Framework developers so we can add it.)

34.6  Credits and changelog 🔗

Differences from previous versions of the checkers and framework can be found in the changelog.txt file. This file is included in the Checker Framework distribution and is also available on the web at https://checkerframework.org/changelog.txt.

Developers who have contributed code to the Checker Framework include Abraham Lin, Akash Srivastava, Alvin Abdagic, Anatoly Kupriyanov, Andy Turner, Arie van Deursen, 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, Christopher Mackie, Colin S. Gordon, Dan Brotherston, Dan Brown, David Lazar, David McArthur, Dilraj Singh, Eric Spishak, Felipe R. Monteiro, Gautam Korlam, Google Inc. (via @wmdietlGC), Haaris Ahmed, Javier Thaine, Jeff Luo, Jenny Xiang, Jianchu Li, Jiasen (Jason) Xu, Joe Schafer, John Vandenberg, Jonathan Burke, Jonathan Nieder, Kartikeya Goswami, Kivanc Muslu, Konstantin Weitz, Liam Miller-Cushon, Luqman Aden, Lázaro Clapp, Mahmood Ali, Manu Sridharan, Mark Roberts, Martin Kellogg, Matt Mullen, Maximilian Gama, Michael Bayne, Michael Coblenz, Michael Ernst, Michael Sloan, Mier Ta, Nhat Dinh, Nhat Nguyen, Nikhil Shinde, Oleg Shchelykalnov, Pascal Wittmann, Patrick Meiring, Paul Vines, Paulo Barros, Philip Lai, Rashmi Mudduluru, Ravi Roshan, Renato Athaydes, René Just, Ruturaj Mohanty, Ryan Oblak, Sadaf Tajik, Sagar Tewari, Sean McLaughlin, Shinya Yoshida, Shubham Raj, Stefan Heule, Steph Dietzel, Stuart Pernsteiner, Suzanne Millstein, Tony Wang, Trask Stalnaker, Travis Haagen, Utsav Oza, Vatsal Sura, Vlastimil Dort, Weitian Xing, Werner Dietl. In addition, too many users to list have provided valuable feedback, which has improved the toolset’s design and implementation. Thanks for your help!

34.7  License 🔗

Two different licenses apply to different parts of the Checker Framework.

For details, see file LICENSE.txt.


References

[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.
[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.
[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.
[Cop05]
Tom Copeland. PMD Applied. Centennial Books, November 2005.
[Cro06]
Jose Cronembold. JSR 198: A standard extension API for Integrated Development Environments. http://jcp.org/en/jsr/detail?id=198, May 8, 2006.
[Dar06]
Joe Darcy. JSR 269: Pluggable annotation processing API. http://jcp.org/en/jsr/detail?id=269, May 17, 2006. Public review version.
[DDE+11]
Werner Dietl, Stephanie Dietzel, Michael D. Ernst, Kivanç 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.
[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.
[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+15]
Michael D. Ernst, Alberto Lovato, Damiano Macedonio, Ciprian Spiridon, and Fausto Spoto. Boolean formulas for the static identification of injection attacks in Java. In LPAR 2015: Proceedings of the 20th International Conference on Logic for Programming, Artificial Intelligence, and Reasoning, pages 130–145, Suva, Fiji, November 2015.
[Ern08]
Michael D. Ernst. Type Annotations specification (JSR 308). https://checkerframework.org/jsr308/, September 12, 2008.
[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://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.
[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.
[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.
[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.
[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.
[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.
[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.
[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.
[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.
[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.
[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.
[VPEJ14]
Mohsen Vakilian, Amarin Phaosawasdi, Michael D. Ernst, and Ralph E. Johnson. Cascade: A universal type qualifier inference tool. Technical report, University of Illinois at Urbana-Champaign, Urbana, IL, USA, September 2014.
[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.
[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.