The Checker Framework Manual:
Custom pluggable types for Java

Version 3.7.0 (1 Oct 2020)

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

This manual is also available in PDF.


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 10)
  9. Tainting Checker for trust and security errors (see Chapter 11)
  10. Regex Checker to prevent use of syntactically invalid regular expressions (see Chapter 12)
  11. Format String Checker to ensure that format strings have the right number and type of % directives (see Chapter 13)
  12. Internationalization Format String Checker to ensure that i18n format strings have the right number and type of {} directives (see Chapter 14)
  13. Property File Checker to ensure that valid keys are used for property files and resource bundles (see Chapter 15)
  14. Internationalization Checker to ensure that code is properly internationalized (see Chapter 15.2)
  15. Signature String Checker to ensure that the string representation of a type is properly used, for example in Class.forName (see Chapter 16)
  16. GUI Effect Checker to ensure that non-GUI threads do not access the UI, which would crash the application (see Chapter 17)
  17. Units Checker to ensure operations are performed on correct units of measurement (see Chapter 18)
  18. Signedness Checker to ensure unsigned and signed values are not mixed (see Chapter 19)
  19. Aliasing Checker to identify whether expressions have aliases (see Chapter 20)
  20. Purity Checker to identify whether methods have side effects (see Chapter 21)
  21. Constant Value Checker to determine whether an expression’s value can be known at compile time (see Chapter 22)
  22. Reflection Checker to determine whether an expression’s value (of type Method or Class) can be known at compile time (see Chapter 24)
  23. Subtyping Checker for customized checking without writing any code (see Chapter 25)
  24. Third-party checkers that are distributed separately from the Checker Framework (see Chapter 26)

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 25 and 33.

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

Java’s built-in type-checker finds and prevents many errors — but it doesn’t find and prevent enough errors. The Checker Framework lets you define new type systems and run them as a plug-in to the javac compiler. Your code stays completely backward-compatible: your code compiles with any Java compiler, it runs on any JVM, and your coworkers don’t have to use the enhanced type system if they don’t want to. You can check part of your program, or the whole thing. Type inference tools exist to help you annotate your code; see Section 31.2.

Most programmers will use type systems created by other people, such as those listed at the start of the introduction (Chapter 1). Some people, called “type system designers”, create new type systems (Chapter 33). The Checker Framework is useful both to programmers who wish to write error-free code, and to type system designers who wish to evaluate and deploy their type systems.

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

1.3  Installation

This section describes how to install the Checker Framework.

Requirement: You must have JDK 8 or JDK 11 installed. (The Checker Framework should work with any JDK 8–12, but we only test with JDK 8 and JDK 11.)

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

  1. Download the Checker Framework distribution:
  2. Unzip it to create a checker-framework-3.7.0 directory.
  3. Configure your IDE, build system, or command shell to include the Checker Framework on the classpath. Choose the appropriate section of Chapter 35.

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
    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

      where javac is set as in Section 35.5.

    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 35).

    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: 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 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 30.1.
  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:

There is a tutorial that walks you through using the Checker Framework on the command line. There is also a Nullness Checker tutorial by David Bürgin (last updated in April 2016).

2.1  Where to write type annotations

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

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

You only need to write type annotations on method signatures, fields, and some type arguments. Most annotations within method bodies are inferred for you; for more details, see Section 28.7.

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

2.2  Running a checker

To run a checker, run the compiler javac as usual, but either pass the -processor plugin_class command-line option, or use auto-discovery as described in Section 2.2.3. A concrete example of using -processor to run the Nullness Checker is:

  javac -processor nullness

where javac is as specified in Section 35.5.

You can also run a checker from within your favorite IDE or build system. See Chapter 35 for details about build tools such as Ant (Section 35.3), Buck (Section 35.4), Gradle (Section 35.8), and Maven (Section 35.12); IDEs such as IntelliJ IDEA (Section 35.9), Eclipse (Section 35.7), NetBeans (Section 35.13), and tIDE (Section 35.14); and about customizing other IDEs and build tools.

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

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

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

2.2.1  Using annotated libraries

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

Some projects are already distributed with type annotations by their maintainers, so you do not need to do anything special. An example is all the libraries in 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 added annotations. (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 32.)

To use an annotated library:

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

2.2.2  Summary of command-line options

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

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


to pass different lint options to the Nullness and GUI Effect Checkers. A downside is that, in this example, each of the checkers will issue one “The following options were not recognized by any processor” warning.

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


Some checkers support additional options, which are described in that checker’s manual section. For example, -Aquals tells the Subtyping Checker (see Chapter 25) and the Fenum Checker (see Chapter 10) 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.

2.2.3  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 checkers to be used, listed one per line. For instance, to run the Nullness Checker and the Interning Checker automatically, the configuration file should contain:


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

2.2.4  Shorthand for built-in checkers

Ordinarily, javac’s -processor flag requires fully-qualified class names. When 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
  javac -processor NullnessChecker
  javac -processor nullness

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
  javac -processor nullness,regex

This feature does not apply to javac @argfiles.

2.3  What the checker guarantees

A checker guarantees that a particular property holds throughout the code.

For example, the Nullness Checker (Chapter 3) guarantees lack of null pointer exceptions; more precisely, it guarantees that only expressions whose type is a @NonNull type are dereferenced, and that such expressions never evaluate to null. The Interning Checker (Chapter 6) guarantees that every expression whose type is an @Interned type evaluates to an interned value, and thereby it prevents incorrect equality tests.

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

There are some limitations to the guarantee.

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. (Often, a single fix corrects all the warnings.)

If you find that a checker fails to issue a warning that it should, then please report a bug (see Section 37.2).

2.4  Tips about writing annotations

Section 32.1 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 36.1.2).

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.

The specification should state any facts that are relevant to callees. When checking a method, use only the specification, not the implementation, of other methods. (Equivalently, type-checking is “modular” or “intraprocedural”.)

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 32 tells you how to annotate libraries that your code uses. Section 2.4.5 and Chapter 29 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 27.1.1).

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


@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/

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 32.

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

    If there are complex facts in your proof that cannot be expressed as annotations, then that is a weakness in the type-checker. For example, the Nullness Checker cannot express “in list lst, elements stored at even indices are always non-null, but elements stored at odd elements might be null.” In this case, you have two choices. First, you can suppress the warning (Chapter 29); be sure to write a comment explaining your reasoning for suppressing the warning. You may wish to submit a feature request (Section 37.2) 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 29). 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 37.2).

If you have trouble understanding a Checker Framework warning message, you can search for its text in this manual. Also see Section 37.1.4 and Chapter 37, Troubleshooting. In particular, Section 37.1.4 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 run-time errors. 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 29.6 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. Options that enable checking modes:

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:

indicates a type that includes the null value. For example, the Java type Boolean is nullable: a variable of type Boolean always has one of the values TRUE, FALSE, or null. (Since @NonNull is the default type annotation, you would actually write this type as @Nullable Boolean.)
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).

indicates qualifier polymorphism. For a description of @PolyNull, see Section 27.2.
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, 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 28.7). 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.

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.
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.


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


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 28.4). 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 28.5.3. 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 28.8. More examples of the use of these annotations appear in the Javadoc for @EnsuresNonNullIf.

3.3.4  Nullness and array initialization

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

  Object [] oa = new Object[10];

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

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

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

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

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

3.3.5  Nullness and conversions from collections to arrays

The semantics of Collection.toArray(T[]) cannot be captured by the nullness type system syntax. The nullness type of the returned array depends on the size of the passed parameter. In particular, the returned array component is of type @NonNull if the following conditions hold:

Additionally, when you supply the -Alint=trustArrayLenZero command-line option, a call to Collection.toArray will be estimated to return an array with a non-null component type if the argument is a field access where the field declaration has a @ArrayLen(0) annotation. This trusts the @ArrayLen(0) annotation, but does not verify it. Run the Constant Value Checker (see Chapter 22) to verify that annotation.

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

3.3.6  Run-time checks for nullness

When you perform a run-time check for nullness, such as if (x != null) ..., then the Nullness Checker refines the type of x to @NonNull. The refinement lasts until the end of the scope of the test or until x may be side-effected. For more details, see Section 28.7.

3.3.7  Inference of @NonNull and @Nullable annotations

It can be tedious to write annotations in your code. Tools exist that can automatically infer annotations and insert them in your source code. (This is different than type qualifier refinement for local variables (Section 28.7), 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 29). An example use is

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

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

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

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

3.4.1  Suppressing warnings with assertions and method calls

Occasionally, it is inconvenient or verbose to use the @SuppressWarnings annotation. For example, Java does not permit annotations such as @SuppressWarnings to appear on statements. In such cases, you can use the @AssumeAssertion string in an assert message (see Section 29.2).

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:

  // 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.)

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 29.2.1.

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 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 29.2.1 for a discussion of the distinction between warning suppression and defensive programming.

3.4.2  Null arguments to collection classes

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

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

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

3.5  Examples

3.5.1  Tiny examples

To try the Nullness Checker on a source file that uses the @NonNull qualifier, use the following command (where javac is the Checker Framework compiler that is distributed with the Checker Framework, see Section 35.5 for details):

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

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/

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 36.10.1 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 30.1.1.

⇒  org.checkerframework.checker.nullness.qual.NonNull 
⇒  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 37.2) 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!

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, see Section 3.8.8.

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

  public class MyClass {
    private @NonNull Object f;
    public MyClass(int x) {
      // Error because constructor contains no assignment to this.f.
      // By the time the constructor exits, f must be initialized to a non-null value.
    public MyClass(int x, int y) {
      // Error because this.f is accessed before f is initialized.
      // At the beginning of the constructor's execution, accessing this.f
      // yields null, even though field f has a non-null type.
      f = new Object();
    public MyClass(int x, int y, int z) {
      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.

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:

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.
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).

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
@NonNull @UnderInitialization C b;b@NonNull
@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;

    // 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;

    // 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:
   void doSomething() {}

class B extends A {
   @NonNull String b;
   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.

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() {
    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: 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() {
    processed = new ArrayList<Integer>();
  public void process(int 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> {
    Node<T> sentinel;

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

    void insert(@Nullable T data) {

    public static void main() {
      List<Integer> l = new List<>();

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

    Node<T> next;

    List parent;

    T data;

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

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

    void insertAfter(@Nullable T data) {
      Node<T> n = new Node<>(this,, data); = n; = 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: error: call to initHelper() not allowed on the given receiver.
  found   : @UnderInitialization( @NonNull MyClass
  required: @Initialized @NonNull MyClass

The problem is that the current object (this) is under initialization, but the receiver formal parameter (Section 36.6.1) 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();
  private void helperMethod() {

the Nullness Checker issues the following warning: error: call to helperMethod() not allowed on the given receiver.
  found   : @UnderInitialization(MyClass.class) @NonNull MyClass
  required: @Initialized @NonNull MyClass
1 error

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

3.8.8  Suppressing warnings

To suppress most warnings related to partially-initialized values, use the “initialization.” (note the trailing period) warning suppression string. You can write @SuppressWarnings("initialization.") on a field, constructor, or class, or pass the command-line argument -AsuppressWarnings=initialization. when running the Nullness Checker. (For more about suppressing warnings, see Chapter 29). You will no longer get a guarantee of no null pointer exceptions, but you can still use the Nullness Checker to find most of the null pointer problems in your code.

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

It is not possible to completely disable initialization checking while retaining nullness checking. The “initialization” warning suppression string (without a period) has the same effect as “nullness”: both of them disable all nullness and 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.

3.8.9  More details about initialization checking

Use of method annotations

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

Source of the type system

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

Chapter 4  Map Key Checker

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

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

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

You can unsoundly suppress warnings related to map keys with @SuppressWarnings("keyfor"), or everywhere by using command-line option -AsuppressWarnings=keyfor; see Chapter 29.

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.
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.
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: 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 27.1.2):

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 28.7.

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

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

            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.

    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 ...

5.2  Optional annotations

These qualifiers make up the Optional type system:

The annotated Optional container may or may not contain a value. This is the default type.
The annotated Optional container definitely contains a (non-null) value.
indicates qualifier polymorphism (see Section 27.2).

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

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

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

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

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

  1. Interning. A factory method ensures that, globally, no two different interned objects are equals() to one another. (For some classes, every instance is interned; however, in other cases it is possible for two objects of the class to be equals() to one another, even if one of them is interned.) Interned objects should always be immutable.
  2. Global control flow. The program’s control flow is such that the constructor for class C is called a limited number of times, and with specific values that ensure the results are not equals() to one another. Objects of class C can always be compared with ==. Such objects may be mutable or immutable.
  3. Local control flow. Even though not all objects of the given type may be compared with ==, the specific objects that can reach a given comparison may be.

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:

indicates a type that includes only interned values (no non-interned values).
indicates a type such that each value is not equals() to any other Java value. This is a stronger (more restrictive) property than @Interned, but is a weaker property than writing @Interned on a class declaration. For details, see Section 6.3.3.
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.
indicates qualifier polymorphism (see Section 27.2).

6.1.2  Interning method and class annotations

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

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

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

6.2  Annotating your code with @Interned

Figure 6.1: Type hierarchy for the Interning type system.

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

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

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

6.3  Interned classes

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

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

For example:

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

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

6.3.1  The intern() methods

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

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

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

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

  if (!pool.contains(this)) {
    @Interned InternedClass internedThis = this;
  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
  @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 28.4).

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.

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

6.4.1  Imprecision (false positive warnings) of the Interning Checker

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

6.5  Examples

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

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

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/

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

The Daikon invariant detector ( 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 30.1.1.

⇒  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 36.4.5).

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

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 28.8 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 28.5.3). (Section 7.5.4 discusses this choice.) It is also the conservative default type qualifier for method parameters in unannotated libraries (see Chapter 32).

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 32).

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 28.5.3).

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

As with other qualifier-polymorphism annotations (Section 27.2), 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
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.

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.

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 32).

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


The overall rules for polymorphic qualifiers are given in Section 27.2.

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

An expression of type @GuardedBy(eset) may be dereferenced only if all locks in eset are held.
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:

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 28.7.5.

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
     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”})


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
  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 {

    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


    public void enter() {


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.
void myMethod() {
  myField = new Object();

int mySideEffectFreeMethod() {
  return 0;

void myUnlockingMethod() {

void myLockingMethod() {

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

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;
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 28.8 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 28.7.

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

  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 30.1.1.

⇒  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

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.
indicates qualifier polymorphism. This type combines @PolyLowerBound and @PolyUpperBound. For a description of qualifier polymorphism, see Section 27.2.
is a special polymorphic qualifier that combines @PolySameLen and @PolyValue from the Constant Value Checker (see Chapter 22). @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:

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.
The value is 0 or greater, so it is not too low to be used as an index.
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”.)
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 27.2.
There is no information about the value. It may not be used as an index for a sequence, because it might be too low.
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”.)
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 27.2.
@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”.)

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.
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 22). 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 22) 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.
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 27.2.
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.
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.
This is the bottom type, and programmers should rarely need to write it.
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).
This is the bottom type, and programmers should rarely need to write it.
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.
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.
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  Called Methods Checker

The Called Methods Checker tracks the names of methods that have definitely been called on an object. This checker is useful for checking any property of the form “call method A before method B”.

The Called Methods Checker provides built-in support for one such property: that clients of the builder pattern for object construction always provide all required arguments before calling build(). The builder pattern is a flexible and readable way to construct objects, but it is error-prone. Failing to provide a required argument causes a run-time error that manifests during testing or in the field, instead of at compile time as for regular Java constructors. The Called Methods Checker verifies at compile time that your code correctly uses the builder pattern, never omitting a required argument. The Called Methods Checker has built-in support for Lombok (see the caveats about Lombok in Section 9.2) and AutoValue.

You can verify other builders, or verify other properties of the form “foo() must be called before bar()”, by writing method specifications. Section 9.5 describes another example related to a security property.

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

9.1  How to run the Called Methods Checker

javac -processor CalledMethods ...

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

9.2  For Lombok users

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

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

9.3  Specifying your code

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

If you use AutoValue or Lombok, most specifications are automatically inferred by the Called Methods Checker, from field annotations such as @Nullable and field types such as Optional. Section 9.4 gives defaulting rules for Lombok and AutoValue.

Figure 9.1: The type hierarchy for the Called Methods type system, for an object with two methods: a() and b(). Types displayed in gray should rarely be written by the programmer.

In some cases, you may need to specify your code. You do so by writing one of the following type annotations (Figure 9.1):

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

Suppose that the method build is annotated as

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

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

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

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

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

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

@This may only be written on a method return type, and means that the method returns its receiver. This is helpful when type-checking fluent APIs. This annotation is defined by the Returns Receiver Checker (Chapter 23), but is particularly useful for the Called Methods Checker because many builders are fluent APIs.
The bottom type for the Called Methods hierarchy. Conceptually, this annotation means that all possible methods have been called on the object. Programmers should rarely, if ever, need to write this annotation—write an appropriate @CalledMethods annotation instead.

There are also method annotations:

This declaration annotation specifies a post-condition on a method, indicating the methods it guarantees to be called.

For example, this specification:

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

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

This declaration annotation specifies a post-condition on a method, indicating the methods it guarantees to be called if it returns a given result.

For example, this specification:

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

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

9.4  Default handling for Lombok and AutoValue

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

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

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

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

Minor notes/caveats on these rules:

9.5  Using the Called Methods Checker for properties unrelated to builders

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

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

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

Chapter 10  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 25), and that approach is sometimes more appropriate.

One common use for the Fake Enum Checker is the fake enumeration pattern (Section 10.6). 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 10.1, 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 10.1 shows part of the type hierarchy for the Fenum type system.

Figure 10.1: 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 10.1 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.

10.1  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
    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;
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    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 33.5.1.

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

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

10.2  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.)

10.3  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

10.4  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;

10.5  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.

10.6  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.

10.7  References

Chapter 11  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.

11.1  Tainting annotations

The Tainting type system uses the following annotations:

11.2  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 28.5 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 32 describes how to annotate external libraries):

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

11.3  @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 25). 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 33 for more details.

11.4  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:


That suggests that perhaps the function should be annotated as polymorphic:

  // UNSOUND annotation -- do not do this!
  public @PolyTainted StringBuffer append(@PolyTainted StringBuffer this, @PolyTainted String toAppend);

The problem with the above annotation is that it permits the undesirable invocation:

  usb.append(ts); // illegal invocation

This invocation is permitted because, in the expression, all @PolyTainted annotations on formal parameters are instantiated to @Tainted, the top annotation, and each argument is a subtype of the corresponding formal parameter.

Beware this problem both in code you write, and also in annotated libraries (such as stub files). The correct way to annotate this class is to add a class qualifier parameter; see Section 27.3.

(Side note: if append were purely functional (had no side effects and returned a new StringBuffer) the method call would be acceptable, because the return type is instantiated to @Tainted StringBuffer for the expression usb.append(ts). However, the append method works via side-effect, and only returns a reference to the buffer as a convenience for writing “fluent” client code.)

Chapter 12  Regex Checker for regular expression syntax

The Regex Checker prevents, at compile-time, use of syntactically invalid regular expressions and access of invalid capturing groups.

A regular expression, or regex, is a pattern for matching certain strings of text. In Java, a programmer writes a regular expression as a string. The syntax of regular expressions is complex, so it is easy to make a mistake. It is also easy to accidentally use a regex feature from another language that is not supported by Java (see section “Comparison to Perl 5” in the Pattern Javadoc). These problems cause run-time errors.

Regular expressions in Java also have capturing groups, which are delimited by parentheses and allow for extraction from text. If a programmer uses an incorrect index (larger than the number of capturing groups), an IndexOutOfBoundsException is thrown.

The Regex Checker warns about these problems at compile time, guaranteeing that your program does not crash due to incorrect use of regular expressions.

For further details, including case studies, see 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.

12.1  Regex annotations

These qualifiers make up the Regex type system:

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".
indicates qualifier polymorphism (see Section 27.2).

The subtyping hierarchy of the Regex Checker’s qualifiers is shown in Figure 12.1.

Figure 12.1: 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.

12.2  Annotating your code with @Regex

12.2.1  Implicit qualifiers

The Regex Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section 28.4). 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.

12.2.2  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 28.7), but you may need to add @Regex annotations with a capturing group count to Pattern and Matcher fields and parameters.

12.2.3  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 12.2: 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 12.2.

12.2.4  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 12.3 gives an example of the intended use of the RegexUtil methods.

returns true if its argument is a valid regular expression.
returns a String error message if its argument is not a valid regular expression, or null if its argument is a valid regular expression.
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 12.3: Example use of RegexUtil methods.

12.2.5  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 12.3 should support most cases so the asRegex method should be used rarely.

Chapter 13  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 13.3 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.

13.1  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.

13.2  Format String Checker annotations

The @Format qualifier on a string type indicates a valid format string. A programmer rarely writes the @Format annotation, as it is inferred for string literals. A programmer may need to write it on fields and on method signatures.

The @Format qualifier is parameterized with a list of conversion categories that impose restrictions on the format arguments. Conversion categories are explained in more detail in Section 13.2.1. 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 13.1: 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 13.2.2.

Figure 13.1 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 13.5.

13.2.1  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 13.3.2.) 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 13.2 summarizes the subset relationship among all conversion categories.

Figure 13.2: The subset relationship among conversion categories.

13.2.2  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);

13.3  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)

13.3.1  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.

13.3.2  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)

13.4  Implicit qualifiers

The Format String Checker adds implicit qualifiers, reducing the number of annotations that must appear in your code (see Section 28.4). The checker implicitly adds the @Format qualifier with the appropriate conversion categories to any String literal that is a valid format string.

13.5  @FormatMethod

Your project may contain methods that forward their arguments to a format method. Consider for example the following log method:

void log(String format, Object... args) {
    if (enabled) {
        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.

13.6  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(;
String fs =;
System.out.printf(fs, "hello", 1337);          // error: fs is not known to be a format string
Scanner s = new Scanner(;
@Format({GENERAL, INT}) String fs =;  // 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(;
String format =
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);
// 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 14  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 14.6.

The Internationalization Checker or I18n Checker (Chapter 15.2) 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.

14.1  Internationalization Format String Checker annotations

Figure 14.1: The Internationalization Format String Checker type qualifier hierarchy. The type qualifiers are applicable to CharSequence and its subtypes. The figure does not show the subtyping rules among different @I18nFormat(...) qualifiers; see Section 14.2. All @I18nFormatFor annotations are unrelated by subtyping, unless they are identical. The qualifiers in gray are used internally by the checker and should never be written by a programmer.

The MessageFormat documentation specifies the syntax of the i18n format string.

These are the qualifiers that make up the I18n Format String type system. Figure 14.1 shows their subtyping relationships.

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 14.2.
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 28.8. 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");
represents an invalid i18n format string. Programmers are not allowed to write this annotation. It is only used internally by the type checker.
represents any string. The string might or might not be a valid i18n format string. Programmers are not allowed to write this annotation.
indicates that the value is definitely null. Programmers are not allowed to write this annotation.

14.2  Conversion categories

In a message string, the optional second element within the curly braces is called a format type and must be one of number, date, time, and choice. These four format types correspond to different conversion categories. date and time correspond to DATE in the conversion categories figure. choice corresponds to NUMBER. The format type restricts what arguments are legal. For example, a date argument is not compatible with the number format type, i.e., MessageFormat.format("{0, number}", new Date()) will throw an exception.

The I18n Checker represents the possible arguments via conversion categories. A conversion category defines a set of restrictions or a subtyping rule.

Figure 14.2 summarizes the subset relationship among all conversion categories.

Figure 14.2: The subset relationship among i18n conversion categories.

14.3  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:

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).
means that any value can be supplied as an argument.
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.
means that the argument needs to be of Number type or a subclass: Number, AtomicInteger, AtomicLong, BigDecimal, BigInteger, Byte, Double, Float, Integer, Long, Short.

14.4  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 29).

    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}";

14.5  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 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 15.

14.6  Running the Internationalization Format Checker

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

14.7  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.

returns true if the given string has the given i18n format type.

For an example, see Section 14.8.

14.8  Examples of using the Internationalization Format Checker

Chapter 15  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 15.1) 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 15.2) verifies that code is properly internationalized. A Compiler Message Key Checker (Section 15.3) 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.

15.1  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 ...

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 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 ’:’.

15.2  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 14). 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.

15.2.1  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.

15.2.2  Running the Internationalization Checker

The Internationalization Checker can be invoked by running the following command:

  javac -processor org.checkerframework.checker.i18n.I18nChecker -Abundlenames=MyResource ...

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.

15.3  Compiler Message Key Checker

The Checker Framework uses compiler message keys to output error messages. These keys are substituted by localized strings for user-visible error messages. Using keys instead of the localized strings in the source code enables easier testing, as the expected message keys can stay unchanged while the localized strings can still be modified. We use the Compiler Message Key Checker to ensure that all internal keys are correctly localized. Instead of using the Property File Checker, we use a specialized checker, giving us more precise documentation of the intended use of Strings.

The single annotation used by this checker is @CompilerMessageKey. The Checker Framework is completely annotated; for example, class org.checkerframework.framework.source.Result uses @CompilerMessageKey in methods failure and warning. For most users of the Checker Framework there will be no need to annotate any Strings, as the checker looks up all String literals and adds annotations as appropriate.

The Compiler Message Key Checker can be invoked by running the following command:

  javac -processor org.checkerframework.checker.compilermsgs.CompilerMessagesChecker ...

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 16  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 16.1), 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.

16.1  Signature annotations

Java defines six formats for the string representation of a type. There is an annotation for each of these representations. Figure 16.1 shows how they are related; examples appear in a table below.

Figure 16.1: Partial type hierarchy for the Signature type system, showing string representations of a Java type. The type qualifiers are applicable to CharSequence and its subtypes. Programmers usually only need to write the boldfaced qualifiers; other qualifiers are included to improve the internal handling of String literals.

A fully qualified name (JLS §6.7), such as pakkage.Outer.Inner, is used in Java code and in messages to the user.
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.
A field descriptor (JVMS §4.3.2), such as Lpakkage/Outer$Inner;, is used in a .class file’s constant pool, for example to refer to other types. It abbreviates primitive types and array types. It uses internal form (binary names, but with / instead of .; see JVMS §4.2) for class names. See examples below.
A binary name (JLS §13.1), such as pakkage.Outer$Inner, is the conceptual name of a type in its own .class file.
The internal form (JVMS §4.2), such as pakkage/Outer$Inner, is how a class name is actually represented in its own .class file. It is also known as the “syntax of binary names that appear in class file structures”. It is the same as the binary name, but with periods (.) replaced by slashes (/). Programmers more often use the binary name, leaving the internal form as a JVM implementation detail.
The type representation returned by the Class.getSimpleName() method. This format is not required by any method in the JDK, so you will rarely write it in source code. The string can be empty. This is not the same as the “simple name” defined in (JLS §6.2), which is the same as @Identifier.
An extension of binary name format to represent primitives and arrays. It is like @FullyQualifiedName, but using “$” instead of “.” to separate nested classes from their containing classes. For example, "pkg.Outer$Inner" or "pkg.Outer$Inner[][]" or "int[]".
Syntactically identical to @FullyQualifiedName, but some classes have multiple fully-qualified names, only one of which is canonical (see JLS §6.7).

Other type qualifiers are the intersection of two or more qualifiers listed above; for example, a @BinaryNameWithoutPackage 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, notably qualified names (JLS §6.2). 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[][]
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$22;n/a for array typen/a for array type[]
java.lang.Integer[][Ljava.lang.Integer;[Ljava/lang/Integer;n/a for array typen/a for array typeInteger[]
pkg.Outer.Inner[][Lpkg.Outer$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:

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)

16.2  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 17  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.

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:

17.1  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


17.2  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).

17.3  Running the GUI Effect Checker

The GUI Effect Checker can be invoked by running the following command:

  javac -processor org.checkerframework.checker.guieffect.GuiEffectChecker ...

17.4  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.

17.5  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 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;.

17.5.1  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:

public interface Runnable {
    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.

17.5.2  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 = ...;;    // is @SafeEffect
@PolyUI Runnable p = ...;;    // is @PolyUIEffect (context-dependent)
@UI Runnable u = ...;  ;    // 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.

17.5.3  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.:

public class UIRunnable extends Runnable {...}
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.)

17.5.4  Subtyping with polymorphic effects

With three effect annotations, we must extend the static sub-effecting relationship:


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:


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 27.2).

@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 27.1.6), 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):

public interface Dispatcher {
    void dispatch(@PolyUI Runnable toRun);
public class SafeDispatcher implements Dispatcher {
    public void dispatch(@AlwaysSafe Runnable 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 = ...;

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.

17.6  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:

Chapter 18  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.

18.1  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:


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

18.2  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.

@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@UnitsMultiple(quantity=s.class, prefix=Prefix.nano)
public @interface ns {}

The @SubtypeOf meta-annotation specifies that this annotation introduces an additional unit of time. The @UnitsMultiple meta-annotation specifies that this annotation should be a nano multiple of the basic unit @s: @ns and @s(Prefix.nano) behave equivalently and interchangeably. Most annotation definitions do not have a @UnitsMultiple meta-annotation.

Note that all custom annotations must have the @Target(ElementType.TYPE_USE) meta-annotation. See section 33.5.1.

To take full advantage of the additional unit qualifier, you need to do two additional steps. (1) Provide constants that convert from unqualified types to types that use the new unit. See class UnitsTools for examples (you will need to suppress a checker warning in just those few locations). (2) Put the new unit in relation to existing units. Provide an implementation of the UnitsRelations interface as a meta-annotation to one of the units.

See demonstration docs/examples/units-extension/ for an example extension that defines Hertz (hz) as scalar per second, and defines an implementation of UnitsRelations to enforce it.

18.3  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.)

18.4  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.

18.5  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.

18.6  References

Chapter 19  Signedness Checker

The Signedness Checker guarantees that signed and unsigned integral values are not mixed together in a computation. In addition, it prohibits meaningless operations, such as division on an unsigned value.

Recall that a computer represents a number as a sequence of bits. Signedness indicates how to interpret the most significant bit. For example, the bits 10000010 ordinarily represent the value -126, but when interpreted as unsigned, those bits represent the value 130. The bits 01111110 represent the value 126 in signed and in unsigned interpretation. The range of signed byte values is -128 to 127. The range of unsigned byte values is 0 to 255.

Signedness is only applicable to the integral types byte, short, int, and long and their boxed variants Byte, Short, Integer, and Long. char and Character are always unsigned. Floating-point types float, double, Float, and Double are always signed.

Signedness is primarily about how the bits of the representation are interpreted, not about the values that it can represent. An unsigned value is always 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 22).

19.1  Annotations

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

Figure 19.1: 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:

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.
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.
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. (Equivalently, the most significant bit is guaranteed to be 0.) Programmers should usually write @Signed or @Unsigned instead.
indicates that a value is in the positive signed range, as with @SignedPositive. Furthermore, the value was derived from values that can be interpreted as @Unsigned. Programmers should rarely write this annotation.
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.
indicates qualifier polymorphism. For a description of qualifier polymorphism, see Section 27.2.
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.
indicates that the value is null. This annotation is used internally, and should not be written by the programmer.

19.1.1  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:

19.1.2  Widening conversions

Figure 19.1 shows the type qualifier hierarchy. When upcasting among integral types, the expression has type @SignedPositive because the value is guaranteed to be within the signed positive range. For example:

@Signed int sint;
@Unsigned int uint;
@SignedPositive long splong1 = sint;   // legal
@SignedPositive long splong2 = uint;   // legal
... (long) sint ...   // this expression has type @SignedPositive long
... (long) uint ...   // this expression has type @SignedPositive long

19.2  Prohibited operations

The Signedness Checker prohibits the following uses of operators:

There are some special cases where these operations are permitted; see Section 19.2.2.

Like every type-checker built with the Checker Framework, the Signedness Checker ensures that assignments and pseudo-assignments have consistent types. For example, it is not permitted to assign a @Signed expression to an @Unsigned variable or vice versa.

19.2.1  Rationale

The Signedness Checker prevents misuse of unsigned values in Java code. Most Java operations interpret operands as signed. If applied to unsigned values, those operations would produce unexpected, incorrect results.

Consider the following Java code:

public class SignednessManualExample {

    int s1 = -2;
    int s2 = -1;

    @Unsigned int u1 = 2147483646; // unsigned: 2^32 - 2, signed: -2
    @Unsigned int u2 = 2147483647; // unsigned: 2^32 - 1, signed: -1

    void m() {
        int w = s1 / s2; // OK: result is 2, which is correct for -2 / -1
        int x = u1 / u2; // ERROR: result is 2, which is incorrect for (2^32 - 2) / (2^32 - 1)

    int s3 = -1;
    int s4 = 5;

    @Unsigned int u3 = 2147483647; // unsigned: 2^32 - 1, signed: -1
    @Unsigned int u4 = 5;

    void m2() {
        int y = s3 % s4; // OK: result is -1, which is correct for -1 % 5
        int z = u3 % u4; // ERROR: result is -1, which is incorrect for (2^32 - 1) % 5 = 2

These examples illustrate why division and modulus with an unsigned operand are illegal. Other uses of operators are prohibited for similar reasons.

19.2.2  Permitted shifts

As exceptions to the rules given above, the Signedness Checker permits certain right shifts which are immediately followed by a cast or masking operation.

For example, right shift by 8 then mask by 0xFF evaluates to the same value whether the argument is interpreted as signed or unsigned. Thus, the Signedness Checker permits both ((myInt >> 8) & 0xFF) and ((myInt >>> 8) & 0xFF), regardless of the qualifier on the type of myInt.

Likewise, right shift by 8 then cast to byte evaluates to the same value whether the argument is interpreted as signed or unsigned, so the Signedness Checker permits both (byte) (myInt >> 8) and (byte) (myInt >>> 8), regardless of the type of myInt.

19.3  Utility routines for manipulating unsigned values

Class SignednessUtil provides static utility methods for working with unsigned values. They are properly annotated with @Unsigned where appropriate, so using them may reduce the number of annotations that you need to write.

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.

19.4  Local type refinement

Local type refinment/inference (Section 28.7) may be surprising for the Signedness type system. Ordinarily, an expression with unsigned type may not participate in a division, as shown in Sections 19.2 and 19.2.1. However, if a constant is assigned to a variable that was declared with @Unsigned type, then — just like the constant — the variable may be treated as either signed or unsigned. 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

19.5  Other signedness annotations

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

If your code is already annotated with a different annotation, the Checker Framework can type-check your code. It treats annotations from other tools as if you had written the corresponding annotation from the Signedness Checker, as described in Figure 19.2.

⇒  org.checkerframework.checker.signedness.qual.Unsigned 
Figure 19.2: Correspondence between other signedness annotations and the Checker Framework’s annotations.

Chapter 20  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 26.9) 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.

20.1  Aliasing annotations

Figure 20.1: 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:

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.)
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):

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.
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.

20.2  Leaking contexts

This section lists the expressions that create aliases. These are also called “leaking contexts”.

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 20.4.)
  @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
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.

20.3  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>.

20.4  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 28.7.

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 20.2 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 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 20.2: Example of the Aliasing Checker’s type refinement rules.

Chapter 21  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 28.7).

All checkers utilize purity annotations on called methods. You do not need to run the Purity Checker directly. However you may run just the Purity Checker by supplying the following command-line options to javac: -processor org.checkerframework.framework.util.PurityChecker.

21.1  Purity annotations

indicates that the method has no externally-visible side effects.
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()).
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.

21.2  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.

21.3  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: 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   : []

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.

21.4  Suppressing warnings

The command-line options -AassumeSideEffectFree, -AassumeDeterministic, -AassumePure make the Checker Framework unsoundly assume that every called method is side-effect-free, is deterministic, or is both, respectively. 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 22  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.

22.1  Annotations

The Constant Value Checker uses type annotations to indicate the value of an expression (Section 22.1.1), and it uses method annotations to indicate methods that the Constant Value Checker can execute at compile time (Section 22.2.2).

22.1.1  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 27.2). In addition, there are separate checkers for @ClassVal and @MethodVal annotations (see Section 24.2).

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 22.1 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 22.1: 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 22.2 illustrates how the Constant Value Checker infers type annotations (using flow-sensitive type qualifier refinement, Section 28.7).

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

22.2  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 22.3. If the other annotation is a declaration annotation, it may be moved; see Section 30.1.1. 
⇒  org.checkerframework.checker.common.value.qual.IntRange 
Figure 22.3: Correspondence between other constant value and range annotations and the Checker Framework’s annotations.

The Constant Value Checker trusts the @Positive, @NonNegative, and @GTENegativeOne annotations. If your code contains any of these annotations, then in order to guarantee soundness, you must run the Index Checker whenever you run the Constant Value Checker.

22.2.1  Compile-time execution of expressions

Whenever all the operands of an expression are compile-time constants (that is, their types have constant-value type annotations), the Constant Value Checker attempts to execute the expression. This is independent of any optimizations performed by the compiler and does not affect the code that is generated.

The Constant Value Checker statically executes (at compile time) operators that do not throw exceptions (e.g., +, -, <<, !=).

22.2.2  @StaticallyExecutable methods and the classpath

The Constant Value Checker statically executes (at compile time) methods annotated with @StaticallyExecutable.

@StaticallyExecutable @Pure
public static int myAdd(int a, int b) {
    return a + b;

public void bar() {
    int a = 5;            // a has type:  @IntVal({5}) int
    int b = 4;            // b has type:  @IntVal({4}) int
    int c = myAdd(a, b);  // c has type:  @IntVal({9}) int
Figure 22.4: The @StaticallyExecutable annotation enables constant propagation through method calls.

The static execution feature has some requirements:

22.3  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:

22.4  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 22.5 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 22.5: 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.

22.5  Strings can be null in concatenations

By default, the Constant Value Checker is sound with respect to string concatenation and nullness. It assumes that, in a string concatenation, every non-primitive argument might be null, except for String literals and compile-time constants. It ignores Nullness Checker annotations. (This behavior is conservative but sound.)

Consider a variable declared as @StringVal("a", "b") String x;. At run time, x evaluates to one of the values "a", "b", or null. Therefore, the type of “x + "c"” is @StringVal("ac", "bc", "nullc") String.

The -AnonNullStringsConcatenation command-line option makes the Constant Value Checker unsoundly assume that no arguments in a string concatenation are null. With the command-line argument, the type of “x + "c"” is @StringVal("ac", "bc") String.

Chapter 23  Returns Receiver Checker

The Returns Receiver Checker enables documenting and checking that a method returns its receiver (i.e., the this parameter).

There are two ways to run the Returns Receiver Checker.

23.1  Annotations

The qualifier @This on the return type of a method indicates that the method returns its receiver. Methods that return their receiver are common in so-called “fluent” APIs. Here is an example:

class MyBuilder {
  @This MyBuilder setName(String name) { = name;
    return this;

An @This annotation can only be written on a return type, a receiver type, or in a downcast.

As is standard, the Returns Receiver Checker has a top qualifier, @UnknownThis, and a bottom qualifier, @BottomThis. Programmers rarely need to write these annotations.

Here are additional details. @This is a polymorphic qualifier rather than a regular type qualifier ((see Section 27.2). Conceptually, a receiver type always has an @This qualifier. When a method return type also has an @This qualifier, the presence of the polymorphic annotation on both the method’s return and receiver type forces their type qualifiers to be equal. Hence, the method will only pass the type checker if it returns its receiver argument, achieving the desired checking.

23.2  AutoValue and Lombok Support

abstract class Animal {
  abstract String name();
  abstract int numberOfLegs();
  static Builder builder() {
    return new AutoValue_Animal.Builder();
  abstract static class Builder {
    abstract Builder setName(String value);       // @This is automatically added here
    abstract Builder setNumberOfLegs(int value);  // @This is automatically added here
    abstract Animal build();
Figure 23.1: User-written code that uses the @AutoValue.Builder annotation. Given this code, (1) AutoValue automatically generates a concrete subclass of Animal.Builder, see Figure 23.2, and (2) the Returns Receiver Checker automatically adds @This annotations on setters in both user-written and automatically-generated code.

    class AutoValue_Animal {
      static final class Builder extends Animal.Builder {
        private String name;
        private Integer numberOfLegs;
        @This Animal.Builder setName(String name) {
 = name;
          return this;
        @This Animal.Builder setNumberOfLegs(int numberOfLegs) {
          this.numberOfLegs = numberOfLegs;
          return this;
        Animal build() {
          return new AutoValue_Animal(, this.numberOfLegs);
Figure 23.2: Code generated by AutoValue for the example of Figure 23.1, including the @This annotations added by the Returns Receiver Checker.

The AutoValue and Lombok projects both support automatic generation of builder classes, which enable flexible object construction. For code using these two frameworks, the Returns Receiver Checker automatically adds @This annotations to setter methods in builder classes. All the @This annotations in Figures 23.1 and 23.2 are automatically added by the Returns Receiver Checker.

Chapter 24  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 24.1 gives an example of reflection resolution. Then, Section 24.2 describes the MethodVal and ClassVal Checkers, which reflection resolution uses internally.

24.1  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.

24.2  MethodVal and ClassVal Checkers

The implementation of reflection resolution internally uses the ClassVal Checker (Section 24.2.1) and the MethodVal Checker (Section 24.2.2). They are similar to the Constant Value Checker (Section 22) 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 24.2.3).

24.2.1  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).

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.
Type given to the null literal. It may not be written in source code.

Figure 24.1: 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 24.1 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.

24.2.2  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.

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.
Type given to the null literal. It may not be written in source code.

Figure 24.2: 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 24.2 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.

24.2.3  MethodVal and ClassVal inference

The developer rarely has to write @ClassVal or @MethodVal annotations, because the Checker Framework infers them according to Figure 24.3. Most readers can skip this section, which explains the inference rules.

Figure 24.3: 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 25  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 33). This includes type introduction rules via the @QualifierForLiterals meta-annotation, and other features such as type refinement (Section 28.7) and qualifier polymorphism (Section 27.2).

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 26.16. See Section 36.7.1 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 33.

25.1  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:

If the Checker Framework is on the processorpath, place the annotations on the processorpath instead of on the classpath.

25.1.1  Compiling your qualifiers and your project

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

25.1.2  Suppressing warnings from the Subtyping Checker

When suppressing a warning issued by the Subtyping Checker, as the “checker name” you may use the unqualified, uncapitalized name of any of the annotations passed to -Aquals. (See Section 29.1.1 for details about the @SuppressWarnings syntax.) As a matter of style, you should choose one of the annotations as the “checker name” part of the @SuppressWarnings string and use it consistently; this avoids confusion and makes it easier to search for uses.

25.2  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.
    @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.
    @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 33.5.1.

    Don’t forget to compile these classes:

    $ javac myPackage/qual/ myPackage/qual/

    The resulting .class files should either be on the same path (the classpath or processor path) as the Checker Framework.

  2. Write @Encrypted annotations in your program (say, in file
    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);
        // ...
    void sendPassword() {
        String password = getUserPassword();

    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 incompatible types.
    found   : @myPackage.qual.PossiblyUnencrypted java.lang.String
    required: @myPackage.qual.Encrypted java.lang.String
  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 \

    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:


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

25.3  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 18), the Fake Enum Checker (Chapter 10), 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 18) is useful for the particular case of units of measurement, such as kilometers verses miles.

The Fake Enum Checker (Chapter 10) 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 25.1.

Figure 25.1: 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) {

Because (implicit) upper bounds are interpreted as the top type (see Section 27.1.2), 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

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 26  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.

26.1  AWS crypto policy compliance checker

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

26.2  AWS KMS compliance checker

The AWS KMS compliance checker extends the Constant Value Checker (see Chapter 22) 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: '', name: 'aws-kms-compliance-checker', version: '1.0.2'

Other build systems are similar.

26.3  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

26.4  Glacier: Class immutability

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

26.5  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

26.6  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

26.7  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.

26.8  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.

26.9  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 The paper “Collaborative verification of information flow for a high-assurance app store” appeared in CCS 2014.

26.10  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

26.11  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

26.12  Generic Universe Types checker

A checker for Generic Universe Types [DEM11], a lightweight ownership type system, is available from

26.13  Safety-Critical Java checker

A checker for Safety-Critical Java (SCJ, JSR 302) [TPV10] is available at Developer resources are available at the project page

26.14  Thread locality checker

Loci [WPM+09], a checker for thread locality, is available at

26.15  Units and dimensions checker

A checker for units and dimensions is available at

Unlike the Units Checker that is distributed with the Checker Framework (see Section 18), 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.

26.16  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.

26.16.1  Comparison to flow-sensitive type refinement

The Checker Framework’s flow-sensitive type refinement (Section 28.7) 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 28.10) 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 27  Generics and polymorphism

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

Section 27.2 describes support for polymorphism over type qualifiers for methods. Section 27.3 describes support for polymorphism over type qualifiers for classes.

27.1  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 27.1.3); rather, the instantiation of the type parameter is restricted (see Section 27.1.2).

27.1.1  Raw types

Before running any pluggable type-checker, we recommend that you eliminate raw types from your code (e.g., your code should use List<...> as opposed to List). Your code should compile without warnings when using the standard Java compiler and the -Xlint:unchecked -Xlint:rawtypes command-line options. Using generics helps prevent type errors just as using a pluggable type-checker does, and makes the Checker Framework’s warnings easier to understand.

If your code uses raw types, then the Checker Framework will do its best to infer the Java type arguments and the type qualifiers. By default these inferred types are ignored in subtyping checks. If you supply the command-line option -AignoreRawTypeArguments=false you will see errors from raw types.

27.1.2  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 27.1.4.

For a concrete example, recall the type system of the Regex Checker (see Figure 12.1) 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


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 36.7.2.

27.1.3  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 27.1.2).

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.

27.1.4  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 28.5.5).

27.1.5  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/

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

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

27.1.6  Covariant type parameters

Java types are invariant in their type parameter. This means that A<X> is a subtype of B<Y> only if X is identical to Y. For example, ArrayList<Number> is a subtype of List<Number>, but neither ArrayList<Integer> nor List<Integer> is a subtype of List<Number>. (If they were, there would be a loophole in the Java type system.) For the same reason, type parameter annotations are treated invariantly. For example, List<@Nullable String> is not a subtype of List<String>.

When a type parameter is used in a read-only way — that is, when clients read values of that type from the class but never pass values of that type to the class — then it is safe for the type to be covariant in the type parameter. Use the @Covariant annotation to indicate this. When a type parameter is covariant, two instantiations of the class with different type arguments have the same subtyping relationship as the type arguments do.

For example, consider Iterator. A client can read elements but not write them, so Iterator<@Nullable String> can be a subtype of Iterator<String> without introducing a hole in the type system. Therefore, its type parameter is annotated with @Covariant. The first type parameter of Map.Entry is also covariant. Another example would be the type parameter of a hypothetical class ImmutableList.

The @Covariant annotation is trusted but not checked. If you incorrectly specify as covariant a type parameter that can be written (say, the class supports a set operation or some other mutation on an object of that type), then you have created an unsoundness in the type system. For example, it would be incorrect to annotate the type parameter of ListIterator as covariant, because ListIterator supports a set operation.

27.1.7  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.

27.1.8  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) { ... }

27.2  Qualifier polymorphism for methods

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 declaration. (It may be used on a field declaration, but it has a different meaning; see Section 27.3. Also, if it is written on a method in a class with a qualifier parameter, then it has a slightly different meaning, see Section 27.3.1.)

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 27.2.1.

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;

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

27.2.1  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.

27.2.2  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.

27.2.3  Using multiple polymorphic qualifiers in a method signature

Usually, it does not make sense to write only a single instance of a polymorphic qualifier in a method definition: if you write one instance of (say) @PolyNull, then you should use at least two. The main benefit of polymorphic qualifiers comes when one is used multiple times in a method, since then each instance turns into the same type qualifier. (Section 27.2.4 describes some exceptions to this rule: times when it makes sense to write a single polymorphic qualifier in a signature.)

Most frequently, the polymorphic qualifier appears on at least one formal parameter and also on the return type.

It can also be useful to have polymorphic qualifiers on (only) multiple formal parameters, especially if the method side-effects one of its arguments. For example, consider

void moveBetweenStacks(Stack<@PolyNull Object> s1, Stack<@PolyNull Object> s2) {

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) {

27.2.4  Using a single polymorphic qualifier in a method signature

As explained in Section 27.2.3, you will usually use a polymorphic qualifier multiple times in a signature. This section describes situations when it makes sense to write just one polymorphic qualifier in a method signature. Some of these situations can be avoided by writing a generic method, but in legacy code it may not be possible for you to change a method to be generic.

Using a single polymorphic qualifier on a return type

It is unusual, but permitted, to write just one polymorphic qualifier, on a return type.

This is just like it is unusual, but permitted, to write just one occurrence of a generic type parameter, on a return type. An example of such a method is Collections.emptyList().

Using a single polymorphic qualifier on an element type

It can make sense to use a polymorphic qualifier just once, on an array or generic element type.

For example, consider a routine that returns the index, in an array, of a given element:

  public static int indexOf(@PolyNull Object[] a, @Nullable Object elt) { ... }

If @PolyNull were replaced with either @Nullable or @NonNull, then one of these safe client calls would be rejected:

  @Nullable Object[] a1;
  @NonNull Object[] a2;

  indexOf(a1, someObject);
  indexOf(a2, someObject);

Of course, it would be better style to use a generic method, as in either of these signatures:

 public static <T extends @Nullable Object> int indexOf(T[] a, @Nullable Object elt) { ... }
 public static <T extends @Nullable Object> int indexOf(T[] a, T elt) { ... }

Another example is a method that writes bytes to a file. It accepts an array of signed or unsigned bytes, and it behaves identically for both:

  void write(@PolySigned byte[] b) { ... }

These examples use arrays, but there are similar examples that use collections.

Don’t use a single polymorphic qualifier on a formal parameter type

There is no point to writing

  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)

27.3  Class qualifier parameters

Class qualifier parameters permit you to supply a type qualifier (only, without a Java basetype) to any class (generic or not).

When a generic class represents a collection, a user can write a type qualifier on the type argument, as in List<@Tainted Character> versus List<@Untainted Character>. When a non-generic class represents a collection with a hard-coded type (as StringBuffer hard-codes Character), you can use a class qualifier parameter to distinguish StringBuffers that contain different types of characters.

To add a qualifier parameter to a class, annotate its declaration with @HasQualifierParameter and write the class of the top qualifier as its element.

class StringBuffer { ... }

A qualifier on a use of StringBuffer is treated as appearing both on the StringBuffer and on its conceptual type argument. That is:
@Tainted StringBuffer @Tainted Collection<@Tainted Character>
@Untainted StringBuffer
@Untainted Collection<@Untainted Character>

If two types have different qualifier arguments, they have no subtyping relationship. (This is “invariant subtyping”, also used by Java for generic classes.) In particular, @Untainted StringBuffer is not a subtype of @Tainted StringBuffer; an attempt to cast between them, in either direction, will yield an invariant.cast.unsafe error.

If a subclass extends a @HasQualifierParameter class (or implements a @HasQualifierParameter interface), then the subclass must also be marked @HasQualifierParameter.

Within a class with a qualifier parameter, the default qualifier for uses of that class is the polymorphic qualifier.

27.3.1  Resolving polymorphism when the receiver type has a polymorphic qualifier

Qualifier polymorphism changes the rules for instantiating polymorphic qualifiers (Section 27.2). If the receiver type has a qualifier parameter and is annotated with a polymorphic qualifier, then at a call site all polymorphic annotations are instantiated to the same qualifier as the type of the receiver expression of the method call. Otherwise, use the rules of Section 27.2

For example, consider

class Buffer {
  void append(@PolyTainted Buffer this, @PolyTainted String s) { ... }

Because @PolyTainted applies to a type (Buffer) with a qualifier parameter, all uses of @PolyTainted are instantiated to the qualifiers on the type of the receiver expression at call sites to append. For example,

@Untainted Buffer untaintedBuffer = ...;
@Tainted String taintedString = ...;
untaintedBuffer.append(taintedString); // error: argument.type.incompatible

The above append call is illegal because the @PolyTainted is instantiated to @Untainted and the type of the argument is @Tainted which is a supertype of @Untainted. If the type of untaintedBuffer were @Tainted then the call would be legal.

27.3.2  Using class qualifier parameters in the type of a field

To express that the type of a field should have the same qualifier as the class qualifier parameter, annotate the field type with the polymorphic qualifier for the type system.

class Buffer {
  @PolyTainted String field;

At a field access where the declared type of the field has a polymorphic qualifier, that polymorphic qualifier is instantiated to the qualifier on the type of the receiver of the field access (or in the case of type variables, the qualifier on the upper bound). That is, the qualifier on myBuffer.field is that same as that on myBuffer.

27.3.3  Local variable defaults for types with qualifier parameters

According to the CLIMB-to-top rule, local variables default to the top type (see Section 28.5.3). Type refinement determines if a variable can be treated as a suitable subtype, and annotations on local variables are rarely needed as a result. However, since qualifier parameters add invariant subtyping, type refinement is no longer valid. For example, suppose in the following code that StringBuffer is annotated with @HasQualifierParameter(Tainted.class).

    void method(@Untainted StringBuffer buffer) {
        StringBuffer local = buffer;

    void executeSql(@Untainted String code) {
        // ...

Normally, the framework would determine that local has type @Untainted StringBuffer and the call to executeSql would be valid. However, since by default local has type @Tainted StringBuffer, and @Untainted StringBuffer is not a subtype, no type refinement would be performed, leading to an error. Fixing this would require manually annotating local as an @Untainted StringBuffer, increasing the annotation burden on programmers.

For this reason, local variables with types that have a qualifier parameter use different defaulting rules. When a local variable has an initializer, the type of that initializer is used as the default type of that variable if no other annotations are written. For example, in the above code, the type of local would be @Untainted StringBuffer. This eliminates the need for type refinement.

27.3.4  Qualifier parameters by default

If many classes in a project should have @HasQualifierParameter, it’s possible to enable it on all classes in a package by default. Writing @HasQualifierParameter on a package is equivalent to writing @HasQualifierParameter on each class in that package and all subpackages with the same arguments.

For example, writing this annotation enables @HasQualifierParameter for all classes in mypackage.

package mypackage;

When using @HasQualifierParameter on a package, it’s possible to disable it for a specific class using @NoQualifierParameter. Writing this on a class indicates it has no class qualifier parameter and @HasQualifierParameter will not be enabled by default. Like @HasQualifierParameter, it takes one or more top annotations. It is illegal to explicitly write both @HasQualifierParameter and @NoQualifierParameter on the same class for the same hierarchy.

27.3.5  Types with qualifier parameters as type arguments

Types with qualifier parameters are only allowed as type arguments to type parameters whose upper bound have a qualifier parameter. If they were allowed for as type arguments for any type parameter, then unsound casts would be permitted. For example:

    interface Buffer {
        void append(@PolyTainted String s);

    public class ClassQPTypeVarTest {
        <T> @Tainted T cast(T param) {
            return param;

        void bug(@Untainted Buffer b, @Tainted String s) {
            cast(b).append(s);  // error

Chapter 28  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.

28.1  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.

28.2  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 12), 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"}).

28.3  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 — that is, by writing an annotation on a class declaration.

  @C class MyClass {}

This means that @B MyClass is an invalid type. (Annotations on class declarations may also specify default annotations for uses of the type; see Section 28.5.1)

If it is not possible to annotate the class’s definition (e.g., for primitives and some library classes), the type-system designer can specify an upper bound by using the meta-annotation @UpperBoundFor.

If no annotation is present on a type declaration and if no @UpperBoundFor mentions the type, then the bound is top. This can be changed by overriding AnnotatedTypeFactory#getTypeDeclarationBounds.

There are two exceptions.

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 were to forbid expressions and local variables from having types inconsistent with the class annotation, then important APIs and common coding paradigms would no longer type-check.

Consider the annotation

  @NonNull class Optional { ... }

and the client code

  Map<String, Optional> m;
  String key = ...;
  Optional value = m.get(key);
  if (value != null) {

The type of m.get(key) is @Nullable Optional, which is an illegal type. However, this is a very common paradigm. Programmers should not need to rewrite the code to test m.containsKey(key) nor suppress a warning in this safe code.

28.4  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 28.5. 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 28.7.

28.5  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 28.5.6 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 28.5.1)
  2. If no @NoDefaultQualifierForUse is written on the declaration of MyClass, the qualifier explicitly written on the declaration of MyClass. (Section 28.5.1)
  3. The qualifier with a meta-annotation @DefaultFor(types = MyClass.class). (Section 28.5.1)
  4. The qualifier with a meta-annotation @DefaultFor(typeKinds = KIND), where KIND is the TypeKind of MyClass. (Section 28.5.1)
  5. The qualifier in the innermost user-written @DefaultQualifier for the location of the use of MyClass. (Section 28.5.2)
  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 33.5.4); this is usually CLIMB-to-top (Section 28.5.3) (CLIMB means Casts, Locals, Instanceof, and (some) iMplicit Bounds).
  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. If rule 6 applies, it makes the type of local variables top so they can be refined.

28.5.1  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:

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:

28.5.2  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;

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

  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 file), then it applies to the given package and all subpackages.

28.5.3  Defaulting rules and CLIMB-to-top

Each type system defines a default qualifier (see Section 33.5.4). For example, the default qualifier for the Nullness Checker is @NonNull. When a user writes an unqualified type such as Date, the Nullness Checker interprets it as @NonNull Date.

The type system applies that default qualifier to most but not all type uses. In particular, unless otherwise stated, every type system uses the CLIMB-to-top rule. This rule states that the top qualifier in the hierarchy is the default for the CLIMB locations: Casts, Locals, 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 28.5.6.

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:

@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.

28.5.4  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. Such a feature would reduce annotation effort, but it would also reduce 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.

28.5.5  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 28.5.3). 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.

28.5.6  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 -AuseConservativeDefaultsForUncheckedCode=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 -AuseConservativeDefaultsForUncheckedCode=-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 32.4 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 32) or alternately suppress the warnings (see Chapter 29).

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.

28.6  Annotations on constructors

28.6.1  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.


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 11) 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.

28.6.2  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.

28.7  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 28.5) 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.

28.7.1  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 12):

  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"

28.7.2  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”.

28.7.3  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 27.1.6 and Section 28.1.) 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 37.2) 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:

28.7.4  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:

28.7.5  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 21 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
      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
      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

      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

  @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.

28.7.6  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 29.2.

28.8  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, with a few extensions, formal parameters like #1 and (for some type systems) <self>.

Limitations: It is not possible to write a quantification over all array components (e.g., to express that all array elements are non-null). There is no such Java expression, but it would be useful when writing specifications.

28.9  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) {
  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.

28.10  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 28.10.1), 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 26.16.

28.10.1  @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 {
  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 29  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.

29.1  @SuppressWarnings annotation

You can suppress specific errors and warnings by use of the @SuppressWarnings annotation, for example @SuppressWarnings("interning") or @SuppressWarnings("nullness"). Section 29.1.1 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 29.1.2 discusses where the annotation may be written in source code.

Section 29.1.3 gives best practices for writing @SuppressWarnings annotations.

29.1.1  @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 25.1.) Sometimes, a checker honors multiple checkername arguments; use the -AshowSuppressWarningsStrings command-line option to see them. Using "allcheckers" 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 string that can be used to suppress that warning. An example is “dereference.of.nullable” in error: [dereference.of.nullable] dereference of possibly-null reference myList

You are allowed to use any substring of a message key, so long as the substring extends at each end to a period or an end of the key. For example, to suppress a warning with message key "assignment.type.incompatible", you could use @SuppressWarnings("assignment"), @SuppressWarnings("assignment.type"), @SuppressWarnings("type.incompatible"), or other variants. We recommend using the longest possible message key; a short message might suppress more warnings than you expect.

The special messagekey “all” means to suppress all warnings.

If the checkername part is omitted, the @SuppressWarnings applies to all checkers. If the messagekey part is omitted, the @SuppressWarnings applies to all messages (it suppresses all warnings from the given checker).

With the -ArequirePrefixInWarningSuppressions command-line option, the Checker Framework only suppresses warnings when the string is in the "checkername" or "checkername:messagekey" format, as in @SuppressWarnings("nullness") or @SuppressWarnings("nullness:assignment.type.incompatible"). For example, @SuppressWarnings("assignment.type.incompatible") and @SuppressWarnings("all") have no effect (they are ignored) when -ArequirePrefixInWarningSuppressions is used. You can use @SuppressWarnings("allcheckers") to suppress all Checker Framework warnings.

29.1.2  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 29.2.)

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 29.2.

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 11). 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;

29.1.3  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 29.2.

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 29.1.2.

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 29.1.2:

  @SuppressWarnings("tainting")              // suppresses all tainting-related warnings
  @SuppressWarnings("cast")                  // suppresses warnings from all checkers about casts
  @SuppressWarnings("unsafe")                // suppresses warnings from all checkers about unsafe code
  @SuppressWarnings("cast.unsafe")           // suppresses warnings from all checkers about unsafe casts
  @SuppressWarnings("tainting:cast")         // suppresses tainting warnings about casts
  @SuppressWarnings("tainting:unsafe")       // suppresses tainting warnings about unsafe code
  @SuppressWarnings("tainting:cast.unsafe")  // suppresses tainting warnings about unsafe casts

The last one is the most specific, and therefore is the best style.

Justify why the warning is a false positive

A @SuppressWarnings annotation asserts that the programmer knows that the code is actually correct or safe (that is, no undesired behavior will occur), even though the type system is unable to prove that the code is correct or safe.

Whenever you write a @SuppressWarnings annotation, you should also write, typically on the same line, a code comment explaining why the code is actually correct. In some cases you might also justify why the code cannot be rewritten in a simpler way that would be amenable to type-checking. Also make it clear what error is being suppressed. (This is particularly important when the @SuppressWarnings is on a method declaration and the suppressed warning might be anywhere in the method body.)

This documentation will help you and others to understand the reason for the @SuppressWarnings annotation. It will also help you audit your code to verify all the warning suppressions. (The code is correct only if the checker issues no warnings and each @SuppressWarnings is correct.)

A suppression message like “a.f is not null” is not useful. The fact that you are suppressing the warning means that you believe that a.f is not null. The message should explain why you believe that; for example, “a.f was checked above and no subsequent side effect can affect it”.

Here are some terse examples from libraries in plume-lib:

@SuppressWarnings("cast") // cast is redundant (except when checking nullness)
@SuppressWarnings("interning") // FbType.FREE is interned but is not annotated
@SuppressWarnings("interning") // equality testing optimization
@SuppressWarnings("nullness") // used portion of array is non-null
@SuppressWarnings("nullness") // oi.factory is a static method, so null first argument is OK
@SuppressWarnings("purity") // side effect to local state of type BitSet

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

@SuppressWarnings("flowexpr.parse.error") //
@SuppressWarnings("keyfor") //

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).

29.2  @AssumeAssertion string in an assert message

You can suppress a warning by asserting that some property is true, and placing the string @AssumeAssertion(checkername) 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 28.7), if the expression appeared in a conditional test. Each type system has its own rules about what type refinement it performs.

The value in parentheses is a lowercase checker name, exactly as in the @SuppressWarnings annotation (Section 29.1.1). The same good practices apply as for @SuppressWarnings annotations, such as writing a comment justifying why the assumption is safe (Section 29.1.3). 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 28.7.6) 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.

29.2.1  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.

29.3  -AsuppressWarnings command-line option

Supplying the -AsuppressWarnings command-line option is equivalent to writing a @SuppressWarnings annotation on every class that the compiler type-checks. The argument to -AsuppressWarnings is a comma-separated list of warning suppression strings, as in -AsuppressWarnings=purity,uninitialized.

When possible, it is better to write a @SuppressWarnings annotation with a smaller scope, rather than using the -AsuppressWarnings command-line option.

29.4  -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 29.5).

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.

29.5  -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 29.4.)

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.

29.6  -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 (and possibly more, see the checker’s documentation):

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.

29.7  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.

29.8  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 10.4) and the Units Checker (Section 18.5).

Chapter 30  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 2.2.2.

30.1  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 32 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 29).
  3. Suppress all warnings related to uses of unannotated_method via the skipUses processor option (see Section 29.4). 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 32 discusses adding annotations to signatures when you do not have source code available. Chapter 29 discusses suppressing warnings.

30.1.1  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,

  @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 31  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 31.1.
  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 31.2.

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

31.1  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 28.7.

31.2  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:

31.2.1  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 28.7.5.

Type inference for any type system

“Whole program inference”, or WPI, utilizes the -Ainfer=outputformat command-line option. See Section 31.3.

“Checker Framework Inference”, or CFI, is a type inference framework built on the Checker Framework. You need to slightly rewrite your type system to work with CFI. The CFI repository contains rewritten versions of some of the type systems that are distributed with the Checker Framework.

Cascade [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

31.3  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/ 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
cd plume-lib
make jar
$CHECKERFRAMEWORK/checker/bin/ \
    "LockChecker,NullnessChecker" java/plume.jar:java/lib/junit-4.12.jar:$JAVA_HOME/lib/tools.jar \
    `find java/src/plume/ -name "*.java"`
# View the results
git diff

You may need to wait a few minutes for the command to complete. You can ignore warnings that the command outputs while it tries different annotations in your code.

It is recommended that you run 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 28.7) 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.

31.3.1  Whole-program inference ignores some code

Whole-program inference ignores code within the scope of a @SuppressWarnings annotation with an appropriate key (Section 29.1). 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.

31.3.2  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.

31.3.3  How whole-program inference works

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

The script repeatedly runs the following steps:

  1. Run the checker with the -Ainfer=jaifs 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 (

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 cannot typecheck (either because contains a defect, or because it contains subtle code that is beyond the capabilities of the type system). However, each of the inferred annotations is sound, and this reduces your manual effort in annotating the program.

The iterative process is required because type-checking is modular: it processes each class and each method only once, independently. Modularity enables you to run type-checking on only part of your program, and it makes type-checking fast. However, it has some disadvantages:

Chapter 32  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 28.5.6 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 29.429.5.)

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 35). Now, when you compile the library, the resulting .jar file contains type annotations.

    When checking a client of the library, put the annotated library’s .jar file on the classpath, as explained in Section 2.2.1.

    With this compilation approach, the syntax of the library annotations is validated ahead of time. Thus, this compilation approach is less error-prone, and the type-checker runs faster. You get correctness guarantees about the library in addition to your code.

    For instructions, see Section 32.2.

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

    Then, when check a client of the library, supply the “stub file” textually to the Checker Framework.

    This approach does not require you to compile the library source code. A stub file is applicable to multiple versions of a library, so the stub file does not need to be updated when a new version of the library is released, unless the API has changed (such as defining a new method).

    For instructions, see Section 32.5.

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 32.1.4, you should annotate an entire class at a time. You may find type inference tools (Chapter 31.2) helpful when getting started, but you should always examine their results.

32.1  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.

32.1.1  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.

32.1.2  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.

32.1.3  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.

32.1.4  Fully annotate the library, or indicate which parts you did not

If you do not annotate all the files in the library, then use @AnnotatedFor to indicate what files you have annotated.

Whenever you annotate any part of a file, fully annotate the file! That is, write annotations for all the methods and fields, based on their documentation. Here are reasons for this rule:

32.1.5  Verify your annotations

Ideally, after you annotate a file, you should type-check the file to verify the correctness and completeness of your annotations.

An alternative is to only annotate method signatures. The alternative is quicker but more error-prone. There is no difference from the point of view of clients, who can only see annotations on public methods and fields. When you compile the library, the type-checker will probably issue warnings; you can supply -Awarns so that the compiler will still produce .class files.

32.2  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. Add an @AnnotatedFor({"checkername"}) annotation to each class declaration, or augment an existing @AnnotatedFor annotation.

  3. Build the library.

    Because of the changes that you made in step 1, this will run pluggable type-checkers. If there are any compiler warnings, fix them and re-compile.

    Now you have a .jar file that you can use while type-checking and at run time.

  4. Tell other people about your work so that they can benefit from it.

32.3  Creating an annotated JDK

When you create a new checker, you need to also supply annotations for parts of the JDK. You can do so either as stub files or in a copy of the JDK source code, as described in Section 33.10. Section 37.2.1 tells how to improve JDK annotations for an existing type system.

32.4  Compiling partially-annotated libraries

If you completely annotate a library, then you can compile it using a pluggable type-checker. You get a guarantee that the library contains no errors (that is, it is consistent with the specifications you wrote).

The rest of this section tells you how to compile a library if you partially annotate it: that is, you write annotations for some of its classes but not others. (There is another type of partial annotation, which is when you annotate method signatures but do not type-check the bodies. See Section 32.1.)

Here are two concerns you may have when partially annotating a library:

32.4.1  The -AuseConservativeDefaultsForUncheckedCode=source,bytecode command-line argument

When compiling a library that is not fully annotated, use command-line argument -AuseConservativeDefaultsForUncheckedCode=source,bytecode. This causes the checker to behave normally for classes with a relevant @AnnotatedFor annotation. For classes without @AnnotatedFor, the checker uses conservative defaults (see Section 28.5.6) 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 -AuseConservativeDefaultsForUncheckedCode=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 2.2.4). 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 -AuseConservativeDefaultsForUncheckedCode=source,bytecode command-line argument, the resulting .class files are fully-annotated; each type use in the .class file has an explicit type qualifier for any checker that is run.

32.5  Using stub classes

A “stub class” is Java source code that is allowed to omit certain parts, such as method bodies. (See Section 32.5.3 for details about the stub file format.) You use a stub file to write annotations for a library, when you cannot edit and recompile the library. A checker uses the annotated signatures at compile time, instead of or in addition to annotations that appear in the library’s .class files.

Section 32.3 explains how you should choose between creating stub classes or creating an annotated library. Section 32.5.4 describes how to create stub classes. Section 32.5.1 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.

32.5.1  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 ...

Each stub path entry is a stub file (ending with .astub), directory, or .jar file; specifying a directory or .jar file is equivalent to specifying every file in it whose name ends with .astub. The stub path entries are delimited by File.pathSeparator (‘:’ for Linux and Mac, ‘;’ for Windows).

A checker automatically reads some of its own stub files, even without a -Astubs command-line argument; see Section 33.10.

32.5.2  Multiple specifications for a method

There are three possible sources of information for a given element: source files, stub files, and bytecode files. Usually source files take precedence over the other two, and stub files take precedence over bytecode. In other words:

If a method appears in more than one stub file (or twice in the same stub file), then, for each type hierarchy, the last annotation read is used.

The annotated JDK is read as a stub file.

The command-line option -AmergeStubsWithSource tells the checker to use both source files and stub files. The checker permits only values that are permitted by both the source and stub annotations. (This is called the greatest lower bound, or GLB, of the types.) Do not use -AmergeStubsWithSource if the default qualifier is other than top (e.g., the Nullness Checker).

As an example of GLB, suppose the source annotation says the value is in the range [1..20] and the stub file says the value is in the range [11..30]. Since both sources of information are trusted, the value must be in the range [11..20]. Equivalently, the GLB of @IntRange(from=1, to=20) and @IntRange(from=11, to=30) is @IntRange(from=11, to=20). As another example, the GLB of @KeyFor({"map1", "map2"}) and @KeyFor({"map2", "map3"}) is @KeyFor({"map1", "map2", "map3"}) since the value is known to be a key for all three maps.

32.5.3  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.

32.5.4  Creating a stub file

If you have access to the Java source code

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

Alternatively, you can minimize source files to make them more suitable as stub files. Use the JavaStubifier to convert, in-place, all .java files in given directories into minimal stub files.

  mkdir project-stubs
  cp -R project/src project-stubs
  java -cp $CHECKERFRAMEWORK/checker/dist/checker.jar org.checkerframework.framework.stub.JavaStubifier project-stubs
  find project-stubs -type f -name "*.java" -exec rename 's/.java$/.astub/' {} \;

Supply it a list of directories to process. Replacement happens in-place, so make sure to process a copy of your sources.

You can now provide project-stubs as a stub directory using -Astubs=project-stubs as an additional command-line option.

If you wish to use a single stub file, rather than a stub directory, you can concatenate all the minimized .java files into a single .astub file:

find project-stubs/ -name "*.java" -type f | xargs cat > project.astub

If you do not have access to the Java source code

If you do not have access to the library source code, then you can create a stub file from the class file (Section 32.5.4), 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 32.5.3.

32.5.5  Distributing stub files

If you are writing stub files but are not writing a checker, you can place the stub files anywhere that is convenient. However, please consider providing your stub files to the checker author, so that other users can also benefit from the stub files you wrote.

If you are distributing stub files with a checker implementation, the stub files appear in the same directory as the checker class, which is a subtype of BaseTypeChecker. A @StubFiles annotation on the checker class lists stub files that are always used. For stub files whose use is optional (for example, because the behavior is unsound, or unsound except in certain circumstances), users must supply the -Astubs=... command-line option.

If a stub file contains annotations that are used by the framework rather than by any specific checker (such as purity annotations), and you wish to distribute it with the Checker Framework, put the stub file in directory checker/resources/. You can also do this if the stub file has annotations for multiple checkers. To use a stub file in directory checker/resources/, users must supply the -Astubs=checker.jar/stub-file-name.astub command-line option.

32.5.6  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:

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.
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.

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.

32.6  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 33  How to create a new checker

This chapter describes how to create a checker — a type-checking compiler plugin that detects bugs or verifies their absence. After a programmer annotates a program, the checker verifies that the code is consistent with the annotations. If you only want to use a checker, you do not need to read this chapter. 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.Documented;
import java.lang.annotation.Target;
import java.lang.annotation.ElementType;
import org.checkerframework.common.subtyping.qual.Unqualified;
import org.checkerframework.framework.qual.SubtypeOf;

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

This checker is so short because it builds on the Subtyping Checker (Chapter 25). See Section 25.2 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

If you write a new checker and wish to advertise it to the world, let us know so we can mention it in Chapter 26 or even include it in the Checker Framework distribution.

33.1  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.











Base Checker

(enforces subtyping rules)





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.

33.2  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:

  | qual/                               type qualifiers
  |                  [optional] interface to the compiler
  |                  [optional] type rules
  |     [optional] type introduction and dataflow rules

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

Sections 33.533.9 describe the individual components of a type system as written using the Checker Framework:

33.5 Type qualifiers and hierarchy. You define the annotations for the type system and the subtyping relationships among qualified types (for instance, @NonNull Object is a subtype of @Nullable Object). This is also where you specify the default annotation that applies whenever the programmer wrote no annotation and no other defaulting rule applies.
33.6 Interface to the compiler. The compiler interface indicates which annotations are part of the type system, which command-line options and @SuppressWarnings annotations the checker recognizes, etc.
33.7 Type rules. You specify the type system semantics (type rules), violation of which yields a type error. A type system has two types of rules.
33.8 Type introduction rules. You specify the type of some expressions where the rules differ from the built in framework rules
33.9 Dataflow rules. These optional rules enhance flow-sensitive type qualifier inference (also known as local variable type inference).

33.3  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 Another choice is to write it in a stand-alone repository. Here is a template for a stand-alone repository:; 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

Note that your custom checker’s .class files must be on the same path (the classpath or processorpath) as the Checker Framework. Invoking a custom checker that builds on the Subtyping Checker is slightly different (Section 25.1).

33.4  Tips for creating a checker

To make your job easier, we recommend that you build your type-checker incrementally, testing at each phase rather than trying to build the whole thing at once.

Here is a good way to proceed.

  1. Write the user manual. Do this before you start coding. The manual explains the type system, what it guarantees, how to use it, etc., from the point of view of a user. Writing the manual will help you flesh out your goals and the concepts, which are easier to understand and change in text than in an implementation. Section 33.13 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. It is more efficient to find problems now rather than after coding up your design.

  2. Implement the type qualifiers and hierarchy (Section 33.5).

    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 25).

  3. Write the checker class itself (Section 33.6).

    Ensure that you can still type-check your test files and that the results are the same. You will not use the Subtyping Checker any more; you will call the checker directly, as in

      javac -processor mypackage.MyChecker ...
  4. Test infrastructure. If your checker source code is in a clone of the Checker Framework repository, integrate your checker with the Checker Framework’s Gradle targets for testing (Section 33.11). 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 33.10).

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

  6. Implement type rules, if any (Section 33.7). (Some type systems need JDK annotations but don’t have any additional type rules.)

    Before implementing type rules (or any other code in your type-checker), read the Javadoc to familiarize yourself with the utility routines in the org.checkerframework.javacutil package, especially AnnotationBuilder, AnnotationUtils, ElementUtils, TreeUtils, TypeAnnotationUtils, and TypesUtils. You will learn how to access needed information and avoid reimplementing existing functionality.

    Write simple test cases to test the type rules, and ensure that the type-checker behaves as expected on those test files. For example, if your type system forbids indexing an array by a possibly-negative value, then you would write a test case such as:

      void testArrayIndexing(String[] myArray, @UnknownSign int us, @NonNegative int nn) {
        myArray[us];  // expected error on this line
  7. Implement type introduction rules, if any (Section 33.8).

    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 33.9).

    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

33.5  Annotations: Type qualifiers and hierarchy

A type system designer specifies the qualifiers in the type system (Section 33.5.1) and the type hierarchy that relates them. The type hierarchy — the subtyping relationships among the qualifiers — can be defined either declaratively via meta-annotations (Section 33.5.2), or procedurally through subclassing QualifierHierarchy or TypeHierarchy (Section 33.5.3).

33.5.1  Defining the type qualifiers

Type qualifiers are defined as Java annotations. In Java, an annotation is defined using the Java @interface keyword. Here is how to define a two-qualifier hierarchy:

package mypackage.qual;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
import org.checkerframework.framework.qual.SubtypeOf;
 * The run-time value of the integer is unknown.
 * @checker_framework.manual #nonnegative-checker Non-Negative Checker
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
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
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
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. The Checker Framework automatically treats any annotation that is declared in the qual package as a type qualifier. (See Section 33.6.1 for more details.) For example, the Nullness Checker’s source file is located at .../nullness/ The @NonNull qualifier is defined in file .../nullness/qual/

Your type system should include a top qualifier and a bottom qualifier (Section 33.5.7). The top qualifier is conventionally named CheckerNameUnknown. Most type systems should also include a polymorphic qualifier @PolyMyTypeSystem (Section 27.2).

Choose good names for the qualifiers, because users will write these in their source code. The Javadoc of every type qualifier should include a precise English description and an example use of the qualifier.

33.5.2  Declaratively defining the qualifier hierarchy

Declaratively, the type system designer uses two meta-annotations (written on the declaration of qualifier annotations) to specify the qualifier hierarchy.

The declarative and procedural mechanisms for specifying the hierarchy can be used together. If any of the annotations representing type qualifiers have elements, then the relationships between those qualifiers must be defined procedurally.

33.5.3  Procedurally defining the qualifier hierarchy

The declarative syntax suffices for most cases. More complex type hierarchies can be expressed by overriding, in your subclass of BaseAnnotatedTypeFactory, either createQualifierHierarchy or createTypeHierarchy (typically only one of these needs to be overridden).

For more details, see the Javadoc of those methods and of the classes QualifierHierarchy and TypeHierarchy.

The QualifierHierarchy class represents the qualifier hierarchy (not the type hierarchy). A type-system designer may subclass QualifierHierarchy to express customized qualifier relationships (e.g., relationships based on annotation arguments).

The TypeHierarchy class represents the type hierarchy — that is, relationships between annotated types, rather than merely type qualifiers, e.g., @NonNull Date is a subtype of @Nullable Date. The default TypeHierarchy uses QualifierHierarchy to determine all subtyping relationships. The default TypeHierarchy handles generic type arguments, array components, type variables, and wildcards in a similar manner to the Java standard subtype relationship but with taking qualifiers into consideration. Some type systems may need to override that behavior. For instance, the Java Language Specification specifies that two generic types are subtypes only if their type arguments are identical: for example, List<Date> is not a subtype of List<Object>, or of any other generic List. (In the technical jargon, the generic arguments are “invariant” or “novariant”.)

33.5.4  Defining the default annotation

A type system applies the default qualifier where the user has not written a qualifier (and no other default qualifier is applicable), as explained in Section 28.5.

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 28.5.3 other defaulting rules are automatically added to every checker. Also, Section 28.5 describes other meta-annotations used to specify default annotations.

33.5.5  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.

33.5.6  Do not re-use type qualifiers

Every annotation should belong to only one type system. No annotation should be used by multiple type systems. This is true even of annotations that are internal to the type system and are not intended to be written by the programmer.

Suppose that you have two type systems that both use the same type qualifier @Bottom. In a client program, a use of type T may require type qualifier @Bottom for one type system but a different qualifier for the other type system. There is no annotation that a programmer can write to make the program type-check under both type systems.

This also applies to type qualifiers that a programmer does not write, because the compiler outputs .class files that contain an explicit type qualifier on every type — a defaulted or inferred type qualifier if the programmer didn’t write a type qualifier explicitly.

33.5.7  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.

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

Sometimes, an annotation needs to refer to a Java expression. Section 28.8 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 28.8. 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.

33.5.9  Repeatable annotations

Some pre- and post-condition annotations that have multiple elements (that is, annotations that take multiple arguments) should be repeatable, so that programmers can specify them more than once. An example is @EnsuresNonNullIf; it could not be defined with each of its elements being a list, as (for example) @KeyFor is.

Make an annotation A repeatable by defining a nested annotation (within A’s definition) named List, and writing @Repeatable(A.List.class) on A’s definition.

33.6  The checker class: Compiler interface

A checker’s entry point is a subclass of SourceChecker, and is usually a direct subclass of either BaseTypeChecker or AggregateChecker. This entry point, which we call the checker class, serves two roles: an interface to the compiler and a factory for constructing type-system classes.

Because the Checker Framework provides reasonable defaults, oftentimes the checker class has no work to do. Here are the complete definitions of the checker classes for the Interning Checker and the Nullness Checker:

  package my.package;
  import org.checkerframework.common.basetype.BaseTypeChecker;
  public final class InterningChecker extends BaseTypeChecker {}

  package my.package;
  import org.checkerframework.common.basetype.BaseTypeChecker;
  @SupportedLintOptions({"flow", "cast", "cast:redundant"})
  public class NullnessChecker extends BaseTypeChecker {}

(The @SupportedLintOptions annotation is optional, and many checker classes do not have one.)

The checker class bridges between the Java compiler and the checker. It invokes the type-rule check visitor on every Java source file being compiled. The checker uses SourceChecker.reportError and SourceChecker.reportWarning to issue errors.

Also, the checker class follows the factory method pattern to construct the concrete classes (e.g., visitor, factory) and annotation hierarchy representation. It is a convention that, for a type system named Foo, the compiler interface (checker), the visitor, and the annotated type factory are named as FooChecker, FooVisitor, and FooAnnotatedTypeFactory. BaseTypeChecker uses the convention to reflectively construct the components. Otherwise, the checker writer must specify the component classes for construction.

A checker can customize the default error messages through a Properties-loadable text file named that appears in the same directory as the checker class. The property file keys are the strings passed to reportError and reportWarning (like "type.incompatible") and the values are the strings to be printed ("cannot assign ..."). The 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 29.1.3 discusses best practices when using a message key in a @SuppressWarnings annotation.

33.6.1  Indicating supported annotations

A checker must indicate the annotations that it supports (that make up its type hierarchy).

By default, a checker supports all type annotations located in a subdirectory called qual that’s located in the same directory as the checker. A type annotation is meta-annotated with either @Target(ElementType.TYPE_USE) or @Target(ElementType.TYPE_USE, ElementType.TYPE_PARAMETER).

To indicate support for annotations that are located outside of the qual subdirectory, annotations that have other ElementType values, checker writers can override the createSupportedTypeQualifiers method (see its Javadoc for details). It is required to define createSupportedTypeQualifiers if you are mixing qualifiers from multiple directories (including when extending an existing checker that has its own qualifiers) and when using the Buck build tool, whose classloader cannot find the qualifier directory.

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

33.6.2  Bundling multiple checkers

Sometimes, multiple checkers work together and should always be run together. There are two different ways to bundle multiple checkers together, by creating either an “aggregate checker” or a “compound checker”.

  1. An aggregate checker runs multiple independent, unrelated checkers. There is no communication or cooperation among them.

    The effect is the same as if a user passes multiple processors to the -processor command-line option.

    For example, instead of a user having to run

      javac -processor DistanceUnitChecker,VelocityUnitChecker,MassUnitChecker

    the user can write

      javac -processor MyUnitCheckers

    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,

    An example of an aggregate checker is I18nChecker (see Chapter 15.2), 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.

33.6.3  Providing command-line options

A checker can provide two kinds of command-line options: boolean flags and named string values (the standard annotation processor options).

Boolean flags

To specify a simple boolean flag, add:


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:


The value of the option can be queried using


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.

33.7  Visitor: Type rules

A type system’s rules define which operations on values of a particular type are forbidden. These rules must be defined procedurally, not declaratively. Put them in a file that extends BaseTypeVisitor.

BaseTypeVisitor performs type-checking at each node of a source file’s AST. It uses the visitor design pattern to traverse Java syntax trees as provided by Oracle’s jdk.compiler API, and it issues a warning (by calling reportError or reportWarning) whenever the type system is violated.

Most type-checkers override only a few methods in BaseTypeVisitor. A checker’s visitor overrides one method in the base visitor for each special rule in the type qualifier system. The last line of the overridden version is “return super.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:

33.7.1  AST traversal

The Checker Framework needs to do its own traversal of the AST even though it operates as an ordinary annotation processor [Dar06]. Java provides a visitor for Java code that is intended to be used by annotation processors, but that visitor only visits the public elements of Java code, such as classes, fields, methods, and method arguments — it does not visit code bodies or various other locations. The Checker Framework hardly uses the built-in visitor — as soon as the built-in visitor starts to visit a class, then the Checker Framework’s visitor takes over and visits all of the class’s source code.

Because there is no standard API for the AST of Java code1, the Checker Framework uses the javac implementation. This is why the Checker Framework is not deeply integrated with Eclipse or IntelliJ IDEA, but runs as an external tool (see Section 35.7).

33.7.2  Avoid hardcoding

If a method’s contract is expressible in the type system’s annotation syntax, then you should write annotations, in a stub file or annotated JDK (Chapter 32).

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.

33.8  Type factory: Type introduction rules

The annotated type of expressions and types are defined via type introduction rules in the type factory. For most expressions and types, these rules are the same regardless of the type system. For example, the type of a method invocation expression is the return type of the invoked method, viewpoint-adapted for the call site. The framework implements these rules so that all type systems automatically use them. For other expressions, such as string literals, their (annotated) types depend on the type system, so the framework provides way to specify what qualifiers should apply to these expressions.

Defaulting rules are type introduction rules for computing the annotated type for an unannotated type; these rules are explained in Section 33.5.4. The meta-annotation @QualifierForLiterals can be written on an annotation declaration to specify that that annotation should be applied to the type of literals listed in the meta-annotation.

33.8.1  Procedurally specifying type introduction rules

If the meta-annotations are not sufficiently expressive, then you can write your own type introduction rules. There are three ways to do so. Each makes changes to an AnnotatedTypeMirror, which is the Checker Framework’s representation of an annotated type.

  1. Define a subclass of TreeAnnotator, typically as a private inner class of your AnnotatedTypeFactory. There is a method of TreeAnnotator for every AST node, and the visitor has access to both the tree (the AST node) and its type. In your subclass of AnnotatedTypeFactory, override createTreeAnnotator to return a ListTreeAnnotator containing that annotator, as in
      protected TreeAnnotator createTreeAnnotator() {
          return new ListTreeAnnotator(super.createTreeAnnotator(), new MyTreeAnnotator(this));

    (or put your TreeAnnotator first; several tree annotators are run by default, and among them, PropagationTreeAnnotator adds annotations to AnnotatedTypeMirrors that do not have an annotation, but has no effect on those that have an annotation).

  2. Define a subclass of a TypeAnnotator, typically as a private inner class of your AnnotatedTypeFactory. There is a method of TypeAnnotator for every kind of type, and the visitor has access to only the type. In your subclass of AnnotatedTypeFactory, override createTypeAnnotator to return a ListTypeAnnotator containing that annotator, as in
      protected TypeAnnotator createTypeAnnotator() {
          return new ListTypeAnnotator(new MyTypeAnnotator(this), super.createTypeAnnotator());

    (or put your TypeAnnotator last).

  3. Create a subclass of AnnotatedTypeFactory and override two addComputedTypeAnnotations methods: addComputedTypeAnnotations(Tree,AnnotatedTypeMirror) (or addComputedTypeAnnotations(Tree,AnnotatedTypeMirror,boolean) if extending GenericAnnotatedTypeFactory) and addComputedTypeAnnotations(Element,AnnotatedTypeMirror). The methods can make arbitrary changes to the annotations on a type.

    Recall that AnnotatedTypeFactory, when given a program expression, returns the expression’s type. This should include not only the qualifiers that the programmer explicitly wrote in the source code, but also default annotations and type refinement (see Section 28.4 for explanations of these concepts).

    The approach of overriding addComputedTypeAnnotations is a last resort, if your logic cannot be implemented using a TreeAnnotator or a TypeAnnotator. The implementation of addComputedTypeAnnotations in GenericAnnotatedTypeFactory calls the tree annotator and the type annotator (in that order), but by overriding the method you can cause your logic to be run even earlier or even later.

33.9  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 28.7.

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

The steps to customizing type refinement are:

  1. §33.9.1 Determine which expressions will be refined.
  2. §33.9.2 Create required class and configure its use.
  3. §33.9.3 Override methods that handle Nodes of interest.
  4. §33.9.4 Implement the refinement.

The Regex Checker’s dataflow customization for the RegexUtil.asRegex run-time check is used as a running example.

If needed, you can find more details about the implementation of type refinement, and the control flow graph (CFG) data structure that it uses, in the Dataflow Manual.

33.9.1  Determine expressions to refine the types of

A run-time check or run-time operation involves multiple expressions (arguments, results). Determine which expression the customization will refine. This is usually specific to the type system and run-time test. There is no code to write in this step; you are merely determining the design of your type refinement.

For the program operation op(a,b), you can refine the types in either or both of the following ways:

  1. Change the result type of the entire expression op(a,b).

    As an example (and as the running example of implementing a dataflow refinement), the RegexUtil.asRegex method is declared as:

      @Regex(0) String asRegex(String s, int groups) { ... }

    This annotation is sound and conservative: it says that an expression such as RegexUtil.asRegex(myString, myInt) has type @Regex(0) String. However, this annotation is imprecise. When the group argument is known at compile time, a better estimate can be given. For example, RegexUtil.asRegex(myString, 2) has type @Regex(2) String.

  2. Change the type of some other expression, such as a or b.

    As an example, consider an equality test in the Nullness type system:

      @Nullable String s;
        if (s != null) {
        } else {

    The type of s != null is always boolean. However, in the true branch, the type of s can be refined to @NonNull String.

If you are refining the types of arguments or the result of a method call, then you may be able to implement your flow-sensitive refinement rules by just writing @EnsuresQualifier and/or @EnsuresQualifierIf annotations. When this is possible, it is the best approach.

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

33.9.2  Create required class

In the same directory as, 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 a new instance of the class.)

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

33.9.3  Override methods that handle Nodes of interest

Decide what source code syntax is relevant to the run-time checks or run-time operations you are trying to support. The CFG (control flow graph) represents source code as Node, a node in the abstract syntax tree of the program being checked (see “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 33.14 for more information about Trees. As an example, the statement String a = ""; is represented as this abstract syntax tree:

  name: "a"
      name: String
      value: ""

33.9.4  Implement the refinement

Each visitor method in CFAbstractTransfer returns a TransferResult. A TransferResult represents the refined information that is known after an operation. It has two components: the result type for the Node being evaluated, and a map from expressions in scope to estimates of their types (a Store). Each of these components is relevant to one of the two cases in Section 33.9.1:

  1. Changing the TransferResult’s result type changes the type that is returned by the AnnotatedTypeFactory for the tree corresponding to the Node that was visited. (Remember that BaseTypeVisitor uses the AnnotatedTypeFactory to look up the type of a Tree, and then performs checks on types of one or more Trees.)

    For example, When RegexTransfer evaluates a RegexUtils.asRegex invocation, it updates the TransferResult’s result type. This changes the type of the RegexUtils.asRegex invocation when its Tree is looked up by the