Annotations on Java typesJSR 308 working document |
This document is available in PDF format at https://checkerframework.org/jsr308/java-annotation-design.pdf.
The JSR 308 webpage is https://checkerframework.org/jsr308/. It contains the latest version of this document, along with other information such as links to the prototype implementation and sample annotation processors.
JSR 308 proposes an extension to Java's annotation system [Bra04a] that permits annotations to appear on any use of a type. (By contrast, Java SE 6 permits annotations to appear only on class/method/field/variable declarations; JSR 308 is backward-compatible and continues to permit those annotations.) Such a generalization removes arbitrary limitations of Java's annotation system, and it enables new uses of annotations. This proposal also notes a few other possible extensions to annotations (see Section D).
This document specifies the syntax of extended Java annotations, but it makes no commitment as to their semantics. As with Java's existing annotations [Bra04a], the semantics is dependent on annotation processors (compiler plug-ins), and not every annotation is necessarily sensible in every location where it is syntactically permitted to appear. This proposal is compatible with existing annotations, such as those specified in JSR 250, “Common Annotations for the Java Platform” [Mor06], and JSR 305, “Annotations for Software Defect Detection” [Pug06]. (For a comparison of JSR 305 and JSR 308, see Section D.3.3.)
This proposal does not change the compile-time, load-time, or run-time semantics of Java. It does not change the abilities of Java annotation processors as defined in JSR 269 [Dar06]. The proposal merely makes annotations more general — and thus more useful for their current purposes, and also usable for new purposes that are compatible with the original vision for annotations [Bra04a].
This document has two parts: a normative part and a non-normative part. The normative part specifies the changes to the Java language syntax (Section 2), the class file format (Section 4), and the Java toolset (Section 3).
The non-normative part consists of appendices that discuss and explain the specification or deal with logistical issues. It motivates annotations on types by presenting one possible use, type qualifiers (Appendix A). It gives examples of and further motivation for the Java syntax changes (Appendix B) and lists tools that must be updated to accommodate the Java and class file modifications (Appendix C). Appendix D lists other possible extensions to Java annotations, some of which are within the scope of JSR 308 (and might be included in a future revision) and some of which are not. The document concludes with logistical matters relating to incorporation in the Sun JDK (Section E) and related work (Section F).
In Java SE 6, annotations can be written only on method parameters and the declarations of packages, classes, methods, fields, and local variables. JSR 308 extends Java to allow annotations on any use of a type. JSR 308 uses a simple prefix syntax for type annotations, with two exceptions that are necessitated by non-orthogonality in the Java grammar.
Section B.1 contains examples of the annotation syntax.
This section summarizes the Java language grammar changes, which correspond to the three rules of Section 2.1. Section 5 shows the grammar changes in detail. Additions are underlined.
| Type: |
| [Annotations] Identifier [TypeArguments] { . Identifier [TypeArguments]} {[]} |
| [Annotations] BasicType |
| VoidMethodDeclaratorRest: |
| FormalParameters [Annotations] [throws QualifiedIdentifierList] ( MethodBody | ; ) |
| Type: |
| [Annotations] Identifier [TypeArguments]{ . Identifier [TypeArguments]} {[ [Annotations] ]} |
| [Annotations] BasicType |
Java uses the @Target meta-annotation as a machine-checked way of expressing where an annotation is intended to appear. JSR 308 uses the existing ElementType.TYPE argument to indicate a type annotation:
@Target(ElementType.TYPE)
public @interface NonNull { ... }
An annotation that is meta-annotated with @Target(ElementType.TYPE) may appear on any use of a type, not just on a type declaration as in Java SE 6.
The compiler applies an annotation to every target that is consistent with its meta-annotation. The order of annotations is not used to disambiguate. As in Java SE 6, the compiler issues an error if a programmer places an annotation in a location not permitted by its Target meta-annotation.
When generating .class files, the compiler must emit the attributes described in Section 4.
The compiler is required to preserve annotations in the class file. More precisely, if a programmer places an annotation (with class file or runtime retention) on the type of an expression, and that expression is represented in the compiled class file, then the annotation must be present, in the compiled class file, on the type of the compiled representation of the expression. If the compiler optimizes away an expression, then it may also optimize away the annotation.
When creating bridge methods (an implementation strategy used when the erased signature of the actual method being invoked differs from that of the compile-time method declaration [GJSB05, §15.12.4.5]), annotations should be copied from the method being invoked. (As of Java SE 6, javac does not copy/transfer any annotations from original methods to the bridge methods; that is probably a bug in javac.)
The compiler must recognize a new -typeprocessor compiler option that specifies an annotation processor that runs after resolution/attribution. Such an annotation processor can depend on knowing the types of all expressions. By contrast, an annotation processor invoked by the -processor compiler option runs before method bodies are resolved.
Java annotations must be stored in the class file for two reasons. First, annotated signatures (public members) must be available to tools that read class files. For example, a type-checking compiler plug-in [Dar06] needs to read annotations when compiling a client of the class file. Second, annotated method bodies must be present to permit checking the class file against the annotations. This is necessary to give confidence in an entire program, since its parts (class files) may originate from any source. Otherwise, it would be necessary to simply trust annotated classes of unknown provenance. (A third non-goal is providing reflective access within method bodies.)
This document proposes conventions for storing the annotations described in Section 2, as well as for storing local variable annotations, which are permitted in Java syntax but currently discarded by the compiler. Class files already store annotations in the form of “attributes” [Bra04a, LY]. JVMs ignore unknown attributes. For backward compatibility, JSR 308 uses new attributes for storing the type annotations. In other words, JSR 308 merely reserves the names of a few attributes and specifies their layout. JSR 308 does not alter the way that existing annotations on classes, methods, method parameters, and fields are stored in the class file. Class files generated from programs that use no new annotations will be identical to those generated by a standard Java SE 6 (that is, pre-extended-annotations) compiler. Furthermore, the bytecode array will be identical between two programs that differ only in their annotations. Attributes have no effect on the bytecode array, because they exist outside it; however, they can represent properties of it by referring to the bytecode (including specific instructions, or bytecode offsets).
In Java SE 6, annotations are stored in the class file in attributes of the classes, fields, or methods they target. Attributes are sections of the class file that associate data with a program element (a method's bytecodes, for instance, are stored in a Code attribute). The RuntimeVisibleAnnotations attribute is used for annotations that are accessible at runtime using reflection, and the RuntimeInvisibleAnnotations attribute is used for annotations that are not accessible at runtime. These attributes contain arrays of annotation structure elements, which in turn contain arrays of element_value pairs. The element_value pairs store the names and values of an annotation's arguments.
JSR 308 introduces two new attributes: RuntimeVisibleTypeAnnotations and RuntimeInvisibleTypeAnnotations. These attributes are structurally identical to the RuntimeVisibleAnnotations and RuntimeInvisibleAnnotations attributes described above with one exception: rather than an array of annotation elements, RuntimeVisibleTypeAnnotations and RuntimeInvisibleTypeAnnotations contain an array of extended_annotation elements, which are described in section 4.1 below.
The Runtime[In]visibleTypeAnnotations attributes store annotations written in the new locations described in Section 2, and on local variables. For annotations on the type of a field, the field_info structure (see JVMS3 §4.6) corresponding to that field stores the Runtime[In]visibleTypeAnnotations attributes. For annotations on types in method signatures or bodies, the method_info structure (see JVMS3 §4.7) that corresponds to the annotations' containing method stores the Runtime[In]visibleTypeAnnotations attributes. For annotations on class type parameter bounds and class extends/implements types, the attributes structure (see JVMS3 §4.2) stores the Runtime[In]visibleTypeAnnotations attributes.
The extended_annotation structure has the following format, which adds target_type and reference_info to the annotation structure defined in JVMS3 §4.8.15:
extended_annotation {
u2 type_index;
u2 num_element_value_pairs;
{
u2 element_name_index;
element_value value;
} element_value_pairs[num_element_value_pairs];
u1 target_type; // new in JSR 308: where the annotation appears
{
...
} reference_info; // new in JSR 308: where the annotation appears
}
We briefly recap the fields of annotation, which are described in in JVMS3 §4.8.15.
The following sections describe the fields of the extended_annotation structure that differ from annotation.
The target_type field denotes the type of program element that the annotation targets. As described above, annotations in any of the following locations are written to Runtime[In]visibleTypeAnnotations attributes in the class file:
The corresponding values for each of these cases are shown in Figure 1. Some locations are assigned numbers even though annotations in those locations are prohibited or are actually written to Runtime[In]visibleAnnotations or Runtime[In]visibleParameterAnnotations. While those locations will never appear in a target_type field, including them in the enumeration may be convenient for software that processes extended annotations. They are marked * in Figure 1.
Annotation Target target_type Value typecast 0x00 typecast generic/array 0x01 type test (instanceof) 0x02 type test (instanceof) generic/array 0x03 object creation (new) 0x04 object creation (new) generic/array 0x05 method receiver 0x06 method receiver generic/array 0x07* local variable 0x08 local variable generic/array 0x09 method return type 0x0A* method return type generic/array 0x0B method parameter 0x0C* method parameter generic/array 0x0D field 0x0E* field generic/array 0x0F class type parameter bound 0x10 class type parameter bound generic/array 0x11 method type parameter bound 0x12 method type parameter bound generic/array 0x13 class extends/implements 0x14 class extends/implements generic/array 0x15 exception type in throws 0x16 exception type in throws generic/array 0x17* type argument in constructor call 0x18 type argument in constructor call generic/array 0x19 type argument in method call 0x1A type argument in method call generic/array 0x1B wildcard bound 0x1C wildcard bound generic/array 0x1D class literal 0x1E class literal generic/array 0x1F* method type parameter 0x20 method type parameter generic/array 0x21*
The reference_info field is used to reference the annotation's target in bytecode. The contents of the reference_info field is determined by the value of target_type.
When the annotation's target is a typecast, an instanceof expression, or a new expression, reference_info has the following structure:
{
u2 offset;
} reference_info;
The offset field denotes the offset (i.e., within the bytecodes of the containing method) of the checkcast bytecode emitted for the typecast, the instanceof bytecode emitted for the type tests, or of the new bytecode emitted for the object creation expression. Typecast annotations are attached to a single bytecode, not a bytecode range (or ranges): the annotation provides information about the type of a single value, not about the behavior of a code block. A similar argument applies to type tests and object creation.
For annotated typecasts, the attribute may be attached to a checkcast bytecode, or to any other bytecode. The rationale for this is that the Java compiler is permitted to omit checkcast bytecodes for typecasts that are guaranteed to be no-ops. For example, a cast from String to @NonNull String may be a no-op for the underlying Java type system (which sees a cast from String String). If the compiler omits the checkcast bytecode, the @NonNull attribute would be attached to the (last) bytecode that creates the target expression instead. This approach permits code generation for existing compilers to be unaffected.
See the end of this section for handling of generic type arguments and arrays.
When the annotation's target is a local variable, reference_info has the following structure:
{
u2 start_pc;
u2 length;
u2 index;
} reference_info;
The start_pc and length fields specify the variable's live range in the bytecodes of the local variable's containing method (from offset start_pc to offset start_pc + length). The index field stores the local variable's index in that method. These fields are similar to those of the optional LocalVariableTable attribute defined in JVMS3 §4.8.13.
Storing local variable annotations in the class file raises certain challenges. For example, live ranges are not isomorphic to local variables. Further, a local variable with no live range may not appear in the class file (but it is also irrelevant to the program).
When the annotation's target is a method receiver, reference_info is empty.
When the annotation's target is a bound of a type parameter of a class or method, reference_info has the following structure:
{
u1 param_index;
u1 bound_index;
} reference_info;
param_index specifies the index of the type parameter, while bound_index specifies the index of the bound. Consider the following example:
<T extends @A Object & @B Comparable, U extends @C Cloneable>
Here @A has param_index 0 and bound_index 0, @B has param_index 0 and bound_index 1, and @C has param_index 1 and bound_index 0.
When the annotation's target is a type in an extends or implements clause, reference_info has the following structure:
{
u1 type_index;
} reference_info;
type_index specifies the index of the type in the clause: -1 (255) is used if the annotation is on the superclass type, and the value i is used if the annotation is on the ith superinterface type.
When the annotation's target is a type in a throws clause, reference_info has the following structure:
{
u1 type_index;
} reference_info
type_index specifies the index of the exception type in the clause: the value i denotes an annotation on the ith exception type.
When the annotation's target is a generic type argument or array type, reference_info contains what it normally would for the raw type (e.g., offset for an annotation on a type argument in a typecast), plus the following fields at the end:
u2 location_length;
u1 location[location_length];
The location_length field specifies the number of elements in the variable-length location field. location encodes which type argument or array element the annotation targets. Specifically, the ith item in location denotes the index of the type argument or array dimension at the ith level of the hierarchy. Figure 2 shows the values of the location_length and location fields for the annotations in a sample field declaration.
Declaration: @A Map<@B Comparable<@C Object[@D][@E][@F]>, @G List<@H Document>>
Annotation location_length location @A not applicable @B 1 0 @C 2 0, 0 @D 3 0, 0, 0 @E 3 0, 0, 1 @F 3 0, 0, 2 @G 1 1 @H 2 1, 0
This section
gives detailed changes to the grammar of the Java
language [GJSB05, ch. 18], based on the conceptually
simple summary from
Section 2.2. Additions are
underlined, and and deletions crossed out.
This section is of interest primarily to language tool implementers, such as compiler writers. Most users can read just Sections 2.1 and B.1.
Infelicities in the Java grammar make this section longer than the simple summary of Section 2.2. Some improvements are possible (for instance, by slightly refactoring the Java grammar), but this version attempts to minimize changes to existing grammar productions.
| Type: |
| [Annotations] UnannType |
| UnannType: |
| Identifier [TypeArguments]{ . Identifier [TypeArguments]} {[ [Annotations] ]} |
| BasicType |
| FormalParameterDecls: |
| [final] |
| ForVarControl: |
| [final] |
| MethodOrFieldDecl: |
| UnannType Identifier MethodOrFieldRest |
| InterfaceMethodOrFieldDecl: |
| UnannType Identifier InterfaceMethodOrFieldRest |
| MethodDeclaratorRest: |
| FormalParameters {[ [Annotations] ]} [Annotations] [throws QualifiedIdentifierList] ( MethodBody | ; ) |
| VoidMethodDeclaratorRest: |
| FormalParameters [Annotations] [throws QualifiedIdentifierList] ( MethodBody | ; ) |
| InterfaceMethodDeclaratorRest: |
| FormalParameters {[ [Annotations] ]} [Annotations] [throws QualifiedIdentifierList] ; |
| VoidInterfaceMethodDeclaratorRest: |
| FormalParameters [Annotations] [throws QualifiedIdentifierList] ; |
| ConstructorDeclaratorRest: |
| FormalParameters [Annotations] [throws QualifiedIdentifierList] MethodBody |
| Primary: |
| ... |
| BasicType {[ [Annotations] ]} .class |
| IdentifierSuffix: |
| [ ( [Annotations] ] {[ [Annotations] ]} .class | Expression ]) |
| ... |
| VariableDeclaratorRest: |
| {[ [Annotations] ]} [= VariableInitializer] |
| ConstantDeclaratorRest: |
| {[ [Annotations] ]} [= VariableInitializer] |
| VariableDeclaratorId: |
| Identifier {[ [Annotations] ]} |
| FormalParameterDeclsRest: |
| VariableDeclaratorId [, FormalParameterDecls] |
| [Annotations] ... VariableDeclaratorId |
One example use of annotation on types is to create custom type qualifiers for Java, such as @NonNull, @ReadOnly, @Interned, or @Tainted. Type qualifiers are modifiers on a type; a declaration that uses a qualified type provides extra information about the declared variable. A designer can define new type qualifiers using Java annotations, and can provide compiler plug-ins to check their semantics (for instance, by issuing lint-like warnings during compilation). A programmer can then use these type qualifiers throughout a program to obtain additional guarantees at compile time about the program.
The type system defined by the type qualifiers does not change Java semantics, nor is it used by the Java compiler or run-time system. Rather, it is used by the checking tool, which can be viewed as performing type-checking on this richer type system. (The qualified type is usually treated as a subtype or a supertype of the unqualified type.) As an example, a variable of type Boolean has one of the values null, TRUE, or FALSE (more precisely, it is null or it refers to a value that is equal to TRUE or to FALSE). A programmer can depend on this, because the Java compiler guarantees it. Likewise, a compiler plug-in can guarantee that a variable of type @NonNull Boolean has one of the values TRUE or FALSE (but not null), and a programmer can depend on this. Note that a type qualifier such as @NonNull refers to a type, not a variable, though JSR 308 could be used to write annotations on variables as well.
Type qualifiers can help prevent errors and make possible a variety of program analyses. Since they are user-defined, developers can create and use the type qualifiers that are most appropriate for their software.
A system for custom type qualifiers requires extensions to Java's annotation system, described in this document; the existing Java SE 6 annotations are inadequate. Similarly to type qualifiers, other pluggable type systems [Bra04b] and similar lint-like checkers also require these extensions to Java's annotation system.
Our key goal is to create a type qualifier system that is compatible with the Java language, VM, and toolchain. Previous proposals for Java type qualifiers are incompatible with the existing Java language and tools, are too inexpressive, or both. The use of annotations for custom type qualifiers has a number of benefits over new Java keywords or special comments. First, Java already implements annotations, and Java SE 6 features a framework for compile-time annotation processing. This allows JSR 308 to build upon existing stable mechanisms and integrate with the Java toolchain, and it promotes the maintainability and simplicity of the modifications. Second, since annotations do not affect the runtime semantics of a program, applications written with custom type qualifiers are backward-compatible with the vanilla JDK. No modifications to the virtual machine are necessary.
Four compiler plug-ins that perform type qualifier type-checking, all built using JSR 308, are distributed at the JSR 308 webpage, https://checkerframework.org/jsr308/. The four checkers, respectively, help to prevent and detect null pointer errors (via a @NonNull annotation), equality-checking errors (via a @Interned annotation), mutation errors (via the Javari [BE04, TE05] type system), and mutation errors (vis the IGJ [ZPA+07] type system). A technical report [PAJ+07] discusses experience with these plug-ins, which revealed bugs in real programs.
The ability to place annotations on arbitrary occurrences of a type improves the expressiveness of annotations, which has many benefits for Java programmers. Here we mention just one use that is enabled by extended annotations, namely the creation of type qualifiers. (Figure 3 gives an example of the use of type qualifiers.)
Figure 3: The DAG class, which represents a directed acyclic graph, illustrates how type qualifiers might be written by a programmer and checked by a type-checking plug-in in order to detect or prevent errors. (1) The @NonNullDefault annotation (line 1) indicates that no reference in the DAG class may be null (unless otherwise annotated). It is equivalent to writing line 4 as “@NonNull Set<@NonNull Edge> edges;”, for example. This guarantees that the uses of edges on line 10, and e on lines 11 and 12, cannot cause a null pointer exception. Similarly, the (implicit) @NonNull return type of getNeighbors() (line 8) enables its clients to depend on the fact that it will always return a List, even if v has no neighbors. (2) The two @Readonly annotations on method getNeighbors (line 8) guarantee to clients that the method does not modify (respectively) its argument (a Vertex) or its receiver (a DAG). The lack of a @Readonly annotation on the return value indicates that clients are free to modify the returned List. (3) The @Interned annotation on line 8 (along with an @Interned annotation on the return type in the declaration of Edge.from(), not shown) indicates that the use of object equality (==) on line 11 is a valid optimization. In the absence of such annotations, use of the equals method is preferred to ==.
As an example of how JSR 308 might be used, consider a @NonNull type qualifier that signifies that a variable should never be assigned null [Det96, Eva96, DLNS98, FL03, CMM05]. A programmer can annotate any use of a type with the @NonNull annotation. A compiler plug-in would check that a @NonNull variable is never assigned a possibly-null value, thus enforcing the @NonNull type system.
@Readonly and @Immutable are other examples of useful type qualifiers [ZPA+07, BE04, TE05, GF05, KT01, SW01, PBKM00]. Similar to C's const, an object's internal state may not be modified through references that are declared @Readonly. A type qualifier designer would create a compiler plug-in (an annotation processor) to check the semantics of @Readonly. For instance, a method may only be called on a @Readonly object if the method was declared with a @Readonly receiver. @Readonly's immutability guarantee can help developers avoid accidental modifications, which are often manifested as run-time errors. An immutability annotation can also improve performance. For example, a programmer can indicate that a particular method (or all methods) on an Enterprise JavaBean is readonly, using the Access Intents mechanism of WebSphere Application Server.
Additional examples of useful type qualifiers abound. We mention just a few others. C uses the const, volatile, and restrict type qualifiers. Type qualifiers YY for two-digit year strings and YYYY for four-digit year strings helped to detect, then verify the absence of, Y2K errors [EFA99]. Range constraints, also known as ranged types, can indicate that a particular int has a value between 0 and 10; these are often desirable in realtime code and in other applications, and are supported in languages such as Ada and Pascal. Type qualifiers can indicate data that originated from an untrustworthy source [PØ95, VS97]; examples for C include user vs. kernel indicating user-space and kernel-space pointers in order to prevent attacks on operating systems [JW04], and tainted for strings that originated in user input and that should not be used as a format string [STFW01]. A localizable qualifier can indicate where translation of user-visible messages should be performed. Annotations can indicate other properties of its contents, such as the format or encoding of a string (e.g., XML, SQL, human language, etc.). An interned qualifier can indicate which objects have been converted to canonical form and thus may be compared via object equality. Type qualifiers such as unique and unaliased can express properties about pointers and aliases [Eva96, CMM05]; other qualifiers can detect and prevent deadlock in concurrent programs [FTA02, AFKT03]. Flow-sensitive type qualifiers [FTA02] can express typestate properties such as whether a file is in the open, read, write, readwrite, or closed state, and can guarantee that a file is opened for reading before it is read, etc. The Vault language's type guards and capability states are similar [DF01].
In Java SE 6, annotations can be written only on method parameters and the declarations of packages, classes, methods, fields, and local variables. Additional annotations are necessary in order to fully specify Java classes and methods.
This section gives examples of the annotation syntax specified in Sections 2.1 and 5. Section B.2 motivates annotating these locations by giving the meaning of annotations that need to be applied to these locations.
Map<@NonNull String, @NonEmpty List<@Readonly Document>> files;
o.<@NonNull String>m("...");
class Folder<F extends @Existing File> { ... }
Collection<? super @Existing File>
class UnmodifiableList<T> implements @Readonly List<@Readonly T> { ... }
void monitorTemperature() throws @Critical TemperatureException { ... }
myString = (@NonNull String) myObject;It is not permitted to omit the Java type, as in myString = (@NonNull) myObject;; see Sections B.2 and D.3.1.
boolean isNonNull = myString instanceof @NonNull String;It is not permitted to omit the Java type, as in myString instanceof @NonNull; see Sections B.2 and D.3.1.
new @NonEmpty @Readonly List<String>(myNonEmptyStringSet)
For generic constructors (JLS §8.8.4), the annotation follows the explicit type arguments (JLS §15.9):
new <String> @Interned MyObject()
public String toString() @Readonly { ... }
public void write() @Writable throws IOException { ... }
A method can express constraints on the generic parameters of the receiver (just as is possible for other formal parameters, albeit with a slightly different syntax):
public int size() @Readonly<@Readonly> { ... }
public void requiresNonNullKeys() <@NonNull,> { ... }
Class<@NonNull String> c = @NonNull String.class;
@NonNull Type.field
Document[@Readonly][] docs4 = new Document[@Readonly 2][12]; Document[][@Readonly] docs5 = new Document[2][@Readonly 12];
This syntax permits independent annotations for each distinct level of array, and for the elements.
This section gives examples of annotations that a programmer may wish to place on a type. Each of these uses is either impossible or extremely inconvenient in the absence of the new locations for annotations proposed in this document. For brevity, we do not give examples of uses for every type annotation. The specific annotation names used in this section, such as @NonNull, are examples only; this document does not define any annotations, merely specifying where they can appear in Java code.
It is worthwhile to permit annotations on all uses of types (even those for which no immediate use is apparent) for consistency, expressiveness, and support of unforeseen future uses. An annotation need not utilize every possible annotation location. For example, a system that fully specifies type qualifiers in signatures but infers them for implementations [GF05] may not need annotations on typecasts, object creation, local variables, or certain other locations. Other systems may forbid top-level (non-type-argument, non-array) annotations on object creation (new) expressions, such as new @Interned Object().
Annotations on generic type arguments and arrays are necessary so the programmer can fully specify types. Generic collection classes are declared one level at a time, so it is easy to annotate each level individually. It is desirable that the syntax for arrays be equally expressive, for uniformity.
Here are examples of uses for annotations on array levels:
Method receivers (this) are formal parameters and thus are an implicit mention of a type. For example, the method PrintStream.println(String) has two formal parameters (and at run time, its invocation involves two actual arguments). In Java's syntax, one of the formal parameters (the receiver) is implicit, but for consistency and expressiveness the implicit use of the receiver type should be annotatable just as the explicit parameters are. Such annotations require new syntax to distinguish them from annotations on the return value.
For example, this receiver annotation
Dimension getSize() @Readonly { ... }
indicates that getSize does not modify its receiver. This is different than saying the method has no side effects at all, so it is not appropriate as a method annotation (such as JML's pure annotation). This is also different than saying that a client should not modify the return value, so it is not appropriate as a return value annotation.
As with annotations on formal parameters (which are already permitted by Java), annotations on the receiver do not affect the Java signature, compile-time resolution of overloading, or run-time resolution of overriding. The Java type of every receiver in a class is the same — but their annotations, and thus their qualified type in a type qualifier framework, may differ.
There are two distinct reasons to annotate the type in a type cast: to fully specify the casted type (including annotations that are retained without change), or to indicate an application-specific invariant that is beyond the reasoning capability of the Java type system. Because a user can apply a type cast to any expression, a user can annotate the type of any expression. (This is different than annotating the expression itself; see Section D.3.1.)
@Readonly Object x; ... (@Readonly Date) x ...
the cast preserves the annotation part of the type and changes only the Java type. If a cast could not be annotated, then a cast would remove the annotation:
@Readonly Object x; ... (Date) x ... // annotation processor error due to casting away @Readonly
This cast changes the annotation; it uses x as a non-@Readonly object, which changes its type and would require a run-time mechanism to enforce type safety.
An annotation processor could permit the unannotated cast syntax but implicitly add the annotation, treating the cast type as @Readonly Date. This has the advantage of brevity, but the disadvantage of being less explicit and of interfering somewhat with the second use of cast annotations. Experience will indicate which design is better in practice.
As a trivial example, the following cast changes the annotation but is guaranteed to be safe at run time:
final Object x = new Object(); ... (@NonNull Object) x ...
An annotation processing tool could trust such type casts, perhaps issuing a warning to remind users to verify their safety by hand or in some other manner. An alternative approach would be to check the type cast dynamically, as Java casts are, but we do not endorse such an approach, because annotations are not intended to change the run-time behavior of a Java program and because there is not generally a run-time representation of the annotations.
Annotations on type tests (instanceof) allow the programmer to specify the full type, as in the first justification for annotations on type casts, above. However, the annotation is not tested at run time — the JVM only checks the base Java type. In the implementation, there is no run-time representation of the annotations on an object's type, so dynamic type test cannot determine whether an annotation is present. This abides by the intention of the Java annotation designers, that annotations should not change the run-time behavior of a Java program.
Annotation of the type test permits the idiom
if (x instanceof T) {
... (T) x ...
}
to be used with the same annotated type T in both occurrences. By contrast, using different types in the type test and the type cast might be confusing.
To prevent confusion caused by incompatible annotations, an annotation processor could require the annotation parts of the operand and the type to be the same:
@Readonly Object x;
if (x instanceof Date) { ... } // error: incompatible annotations
if (x instanceof @Readonly Date) { ... } // OK
Object y;
if (y instanceof Date) { ... } // OK
if (y instanceof @NonNull Date) { ... } // error: incompatible annotations
(As with type casts, an annotation processor could implicitly add a missing annotation; this would be more concise but less explicit, and experience will dictate which is better for users.)
As a consequence of the fact that the annotation is not checked at run time, in the following
if (x instanceof @A1 T) { ... }
else if (x instanceof @A2 T) { ... }
the second conditional is always dead code. An annotation processor may warn that one or both of the instanceof tests is a compile-time type error.
A non-null qualifier is a special case because it is possible to check at run time whether a given value can have a non-null type. A type-checker for a non-null type system could take advantage of this fact, for instance to perform flow-sensitive type analysis in the presence of a x != null test, but JSR 308 makes no special allowance for it.
Annotations on object creation (new) can indicate the type of the newly-created object, which could be statically (at compile time) verified to be compatible with the annotations on the constructor.
Annotations on type parameter bounds (extends) and wildcard bounds (extends and super) allow the programmer to fully constrain generic types. Creation of objects with constrained generic types could be statically verified to comply with the annotated bounds.
Annotations on class inheritance (extends and implements) are necessary to allow a programmer to fully specify a supertype. It would otherwise be impossible to extend the annotated version of a particular type t (which is often a valid subtype or supertype of t) without using an anonymous class.
These annotations also provide a convenient way to alias otherwise cumbersome types. For instance, a programmer might declare
final class MyStringMap extends
@Readonly Map<@NonNull String, @NonEmpty List<@NonNull @Readonly String>> {}
so that MyStringMap may be used in place of the full, unpalatable supertype. (However, also see Section D.3.4 for problems with this approach.)
Annotations in the throws clauses of method declarations allow programmers to enhance exception types. For instance, programs that use the @Critical annotation from the above examples could be statically checked to ensure that catch blocks for @Critical exceptions are not empty.
An annotation before a method declaration annotates either the return type, or the method declaration; similarly for field declarations. The @Target meta-annotation indicates the programmer intention.
Consider the following two field declarations.
@NonNegative int balance;
@GuardedBy("accessLock") long lastAccessedTime;
The annotation @NonNegative applies to the field type int, not to the whole variable declaration nor to the variable itself. The annotation @GuardedBy("accessLock") applies to the field.
As another example, in
@Override
@NonNull Dimension getSize() { ... }
@Override applies to the method and @NonNull applies to the return type. This is because Override is meta-annotated with ElementType.METHOD, and NonNull is meta-annotated with ElementType.TYPE (see Section 2.3).
As explained in Section 2.3, the compiler applies the annotation to every target that is consistent with its meta-annotation. This means that, for certain syntactic locations, which target (Java construct) is being annotated depends on the annotation, or an annotation might even be applied to two targets.
This section primarily discusses tool modifications that are consequences of JSR 308's changes to the Java syntax and class file format, as presented in Sections 2 and 4.
The syntax extensions described in Section 2 require the javac Java compiler to accept annotations in the proposed locations and to add them to the program's AST. The relevant AST node classes must also be modified to store these annotations.
Javac's -Xprint functionality reads a .class file and prints the interface (class declarations with signatures of all fields and methods). This must be updated to print the extended annotations as well. (The -Xprint functionality is similar to javap, but cannot provide any information about bytecodes or method bodies, because it is implemented internally as an annotation processor.)
Section 3 requires compilers to place certain annotations in the class file. This may change the compiler implementation of certain optimizations, such as common subexpression elimination, but this restriction on the compiler implementation is unobjectionable for three reasons.
The Tree API, which exposes the AST (including annotations) to authors of annotation processors (compile-time plug-ins), must be updated to reflect the modifications made to the internal AST node classes described in Section 2.
Like reflection, the JSR 269 (annotation processing) model does not represent constructs below the method level, such as individual statements and expressions. Therefore, it needs to be updated only with respect to annotations on class member declarations (also see Section D.3.6). The JSR 269 model, javax.lang.model.*, already has some classes representing annotations; see https://docs.oracle.com/javase/6/docs/api/javax/lang/model/element/package-summary.html. The annotation processing API in javax.annotation.processing must also be revised.
The java.lang.reflect.* and java.lang.Class APIs give access to annotations on public API elements such as classes, method signatures, etc. They must be updated to give the same access to the new extended annotations.
For example, new method Method.getReceiverAnnotation (for the receiver this) would parallel the existing Method.getAnnotations (for the return value) and Method.getParameterAnnotations (for the formal parameters). Reflection gives no access to method implementations, so no changes are needed to provide access to annotations on casts (or other annotations inside a method body), type parameter names, or similar implementation details.
Suppose that a method is declared as:
@NonEmpty List<@Interned String> foo(@NonNull List<@Opened File> files) @Readonly {...}
Then Method.getAnnotations() returns the @NonEmpty annotation, just as in Java SE 6, and likewise Method.getParameterAnnotations() returns the @NonNull annotation. New method Method.getReceiverAnnotations() returns the @Readonly annotation. We have not yet decided how to provide reflective access to annotations on generic types in a method's signature, such as the instances of @Interned and @Opened above.
The Mirror API com.sun.mirror.* need not be updated, as it has been superseded by JSR 269 [Dar06].
No modifications to the virtual machine are necessary.
The javap disassembler must recognize the new class file format and must output annotations.
The pack200/unpack200 tool must preserve the new attributes through a compress-decompress cycle.
Javadoc must output annotations at the new locations when those are part of the public API, such as in a method signature.
Similar modifications need to be made to tools outside the Sun JDK, such as IDEs (Eclipse, IDEA, JBuilder, jEdit, NetBeans), other tools that manipulate Java code (grammars for CUP, javacc), and tools that manipulate class files (ASM, BCEL). These changes need to be made by the authors of the respective tools.
A separate document, “Custom type qualifiers via annotations on Java types” (https://checkerframework.org/jsr308/java-type-qualifiers.pdf), explores implementation strategies for annotation processors that act as type-checking compiler plug-ins. It is not germane to this proposal, both because this proposal does not concern itself with annotation semantics and because writing such plug-ins does not require any changes beyond those described in this document.
A separate document, “Annotation File Specification” (https://checkerframework.org/jsr308/annotation-file-format.pdf), describes a textual format for annotations that is independent of .java or .class files. This textual format can represent annotations for libraries that cannot or should not be modified. We have built tools for manipulating annotations, including extracting annotations from and inserting annotations in .java and .class files. That file format is not part of this proposal for extending Java's annotations; it is better viewed as an implementation detail of our tools.
JSR 308 “Annotations on Java Types” [EC06] has the goal of refining the ideas presented here. This proposal serves as a starting point for the JSR 308 expert group, but the expert group has the freedom to modify this proposal or to explore other approaches. (A JSR, or Java Specification Request, is a proposed specification for some aspect of the Java platform — the Java language, virtual machine, libraries, etc. For more details, see the Java Community Process FAQ at https://jcp.org/en/introduction/faq.)
The Expert Group will consider whether the proposal should extend annotations in a few other ways that are not directly related to annotations on types. This is especially true if the additional changes are small, that there is no better time to add such an annotation, and the new syntax would permit unanticipated future uses. Two examples follow, for which the proposal does not currently include a detailed design. Then, the rest of this section presents extensions that are out of the scope of JSR 308.
Currently, array-valued annotations can be clumsy to write:
@Resources({
@Resource(name = "db1", type = DataSource.class)
@Resource(name = "db2", type = DataSource.class)
})
public class MyClass { ... }
Likewise, it may be desirable for some (but not all) annotations to be specified more than once at a single location. A cleaner syntax like
@Resource(name = "db1", type = DataSource.class)
@Resource(name = "db2", type = DataSource.class)
public class MyClass { ... }
may be desirable for both purposes.
Java does not currently permit annotations on statements. Annotations on statements (or on some subset of statements, such as blocks or loops) could be useful for properties such as atomicity/concurrency. Such an extension would require defining both Java syntax and a convention for storing the information in the class file.
This section of the document discusses several issues that are not in scope for JSR 308.
It is not a goal that JSR 308 is the last annotation-related JSR. It is acceptable to leave some issues to future language designers, just as JSR 175 (the previous annotations JSR [Bra04a]) did. It is a goal not to unnecessarily close off realistic future avenues of extension.
Annotating a type cast indicates a property of a value (the result of an expression). This is different than annotating the expression itself, which indicates some property of the entire computation, such as that it should be performed atomically, that it acquires no locks, or that it should be formatted specially by an IDE. JSR 308 does not support expression annotations, because we have not yet discovered compelling use cases for them that cannot be equally well supported by statement annotations. (A minor inconvenience is that the use of statement annotations may require the programmer to create a separate statement for the expression to be annotated.)
Arbitrary values can be annotated using an annotation on a cast:
(@Language("SQL") String) "select * from foo"
A possible shorthand would be to permit the Java type to be implicit:
(@Language("SQL")) "select * from foo"
This is not permitted (nor may a cast be omitted in a type test, as in @codex instanceof @NonNull), for several reasons. Erasing the annotations should leave a valid Java program. Stating the type reinforces that the annotation is a type annotation rather than an expression annotation. The benefit of omitting the type in the cast seems relatively minor. Especially in a type test, stating the type reinforces that the run-time effect has is to check and change the Java type; no run-time check of the annotation is possible in general.
An even shorter shorthand would drop the parentheses:
@Language("SQL") "select * from foo"
In addition to the benefits and problems noted above, such an annotation is syntactically ambiguous with an expression annotation. Whether an annotation applies to expressions or to types is clear from the annotation's documentation and its @Target meta-annotation, similarly to how it is determined whether an annotation applies to a type or to a declaration (Section B.3).
It would be possible to permit annotations only on blocks and/or loops, as a restricted special case of statements. This is less general than permitting annotations on statements, and uses are more syntactically cluttered (for instance, this requires a statement to be converted into a block before it can be annotated). Most declarations could not be annotated as statements because enclosing the declaration in a block to annotate it would change (and limit) the variable's scope. This limitation in flexibility does yield the advantage that there would be no syntactic ambiguity between (say) statement annotations and declaration or type annotations.
Similarly, permitting annotations on partial constructs (such as only the body of a loop) appears both more complex, and no more useful, than annotating complete constructs (such as a full statement).
Annotations cannot subclass one another, so it is difficult to share behavior or to express similarities or relationships among annotation types. (A meta-annotation can be used to work around this, but it is clumsy and indirect.)
In Java, it is not possible to define an annotation that takes an arbitrary annotation as a parameter, as in
@DefaultAnnotation(@MyAnnotation)
More generally, annotation types cannot have members of their own type. (An annotation whose parameter is an annotation of a specific type is explicitly permitted (JLS §9.6 and §9.7).) These limitations reduce the expressiveness of annotations, for instance making it impossible to define the @DefaultAnnotation above.
Annotation types cannot have positional arguments (except for the value argument, when it is the only argument). This limitation makes writing annotations with multiple arguments more verbose than necessary.
JSR 308 does not define any annotations nor take any position on their semantics. JSR 308 extends the Java and class file syntax to permit annotations to be written in more places, and thus makes existing and future annotations more useful to programmers.
By contrast, JSR 305 “Annotations for Software Defect Detection” aims to define a set of annotations for specific purposes, along with their semantics. Examples include type annotations such as non-nullness (@Nonnull), signedness (@Nonnegative), tainting, and string format; and also non-type annotations such as whether a method's return value should always be checked by the caller. For more details, see https://jcp.org/en/jsr/detail?id=305 and http://groups.google.com/group/jsr-305/.
The annotation type java.lang.annotation.Inherited (JLS §9.6.1.3) indicates that annotations on a class C corresponding to a given annotation type are inherited by subclasses of C. This implies that annotations on interfaces are not inherited, nor are annotations on members (methods, constructors, fields, etc.). It might be useful to provide a more fine-grained mechanism that applies different rules to classes, methods, fields, etc., or even to specify inheritance of annotations from interfaces. These semantic issue are out of the scope of JSR 308 but may be taken up by JSR 305 (“Annotations for Software Defect Detection” [Pug06]).
Specifying a default for annotations can reduce code size and (when used carefully and sparingly) increase code readability. For instance, Figure 3 uses @NonNullDefault to avoid the clutter of 5 @NonNull annotations. It would be nicer to have a general mechanism, such as
@DefaultAnnotation(NonNull.class, locations={ElementType.LOCAL_VARIABLE})
Defaults for annotations are a semantic issue that is out of the scope of JSR 308. It will be taken up by JSR 305 (“Annotations for Software Defect Detection” [Pug06]).
The defaulting syntax must also be able to specify the arguments to the default annotation (in the above example, the arguments to @NonNull).
A better syntax would use an annotation, not a class literal, as the argument to @DefaultAnnotation, as in
@DefaultAnnotation(@MyAnnotation(arg="foo"))
but in Java, it is not possible to define an annotation that takes an arbitrary annotation as a parameter; see Section D.3.2.
An issue for JSR 260 (Javadoc) and JSR 305 (Annotation semantics) is how inherited and defaulted annotations are handled in Javadoc: whether they are written out in full, or in some abbreviated form. Just as too many annotations may clutter source code, similar clutter-reduction ideas may need to be applied to Javadoc.
An annotated type may be long and hard to read; compare Map<String, Object> to @NonNull Map<@NonNull String, @NonNull Object>. Class inheritance annotations and subclassing provides a partial solution, as noted in Section B.2 with the following example:
final class MyStringMap extends
@Readonly Map<@NonNull String, @NonEmpty List<@NonNull @Readonly String>> {}
This approach limits reusability: if a method is declared to take a MyStringMap parameter, then a Map (even of the right type, including annotations) cannot be passed to it. (By contrast, a MyStringMap can always be used where a Map of the appropriate type is expected.) Goetz [Goe06] recommends exploiting Java's type inference to avoid some (but not all) instances of the long type name.
In summary, a built-in typedef mechanism might increase code readability.
Changes to the class file syntax are out of the scope of JSR 308, which, for backward compatibility, does not change the way that existing annotations are stored in the class file.
However, some changes to the class file syntax have significant benefits, and could be the subject of another, small, JSR whose focus is only the class file format. Class file syntax changes require modification of compilers, JVMs, javap, and other class file tools (see Sections C.4 and C.5).
Annotations could be stored in the constant pool, and use constant pool references from the annotation points? That would reduce class file size, especially if an annotation is used in many places in the same class, as is more likely with the annotations enabled by JSR 308 and those proposed in JSR 305.
The JSR 269 annotation processing API specifies that the process method is invoked on class, field, and method annotations. It does not process annotations on local variables, as it is not designed to access method bodies. JSR 269's limitations make it insufficient for a type-checking compiler plug-in (annotation processor), which must both process all annotations and also check at each use of a variable/method whose declaration is annotated. For example, if a variable x is declared as @NonNull Object x;, then every assignment to x must be checked, because any assignment x = null; would be illegal. Extending JSR 269 to process all annotations, for consistency and to support other types of annotation processors, is beyond the scope of JSR 308 but may be desirable in the future, after more experience is gained with JSR 308 annotation processors.
JSR 308 (“Annotations on Java types”) should be included under the Java SE 7 umbrella JSR (which lists the JSRs that are part of the Java SE 7 release). However, it should be a separate JSR because it needs a separate expert group. The expert group will have overlap with any others dealing with other added language features that might be annotatable (such as method-reference types or closures), to check impact.
The specification and the TCK will be freely available, most likely licensed under terms that permit arbitrary use. The prototype implementation is built on the OpenJDK Java implementation and is publicly available; our goal is for Sun to incorporate JSR 308 into the official OpenJDK release.
To ease the transition from standard Java SE 6 code to code with the extended annotations, the prototype implementation recognizes the extended annotations when surrounded by comment markers:
/*@Readonly*/ Object x;
This permits use of both standard Java SE 6 tools and the new annotations even
before Java SE 7 is released. However, it is not part of the proposal, and
the final Java SE 7 implementation will not recognize the new annotations when
embedded in comments.
The Spec# [BLS04] extension to C# can be made compilable by
a standard C# compiler in a similar way, by enclosing its annotations in
special /*^…\^*/ comment markers.
The /*@ comment syntax is a standard part of the
Splint [Eva96] and JML [LBR06] tools (that is, not
with the goal of backward compatibility).
JSR 308 will ship with a test suite (known as a TCK, or Technology Compatibility Kit).
Each tool that needs to be tested appears in Section 3; the TCK will include tests for each of them.
For each modified tool, we will test backward compatibility by passing all of its existing tests. (We may need to modify a few of them, for instance those that depend on specific bytecodes that are created by the compiler.)
We will test most other functionality by creating a set of Java programs that include annotations in every possible location. For instance, this can be used to test all aspects of the compiler (parsing, code generation, -Xprint).
We will provide multiple annotation processors (including at least one for checking @NonNull and one for checking @Interned) that utilize the new annotations, along with a test suite for each one. Each annotation processor's test suite consists of annotated code, along with expected output from the given annotation processor. Since the annotation processors utilize all aspects of JSR 308, this serves as an additional end-to-end test of the JSR 308 implementation. As a side benefit, the annotation processors will be useful in their own right, will thereby illustrate the utility of JSR 308, and will serve as examples for people who wish to create their own type-checking plug-ins.
Section A.1 gave many examples of how type qualifiers have been used in the past; also see [PAJ+07].
C#'s attributes [ECM06, chap. 24] play the same role as Java's annotations: they attach metadata to specific parts of a program, and are carried through to the compiled bytecode representation, where they can be accessed via reflection. The syntax is different: C# uses [AnnotationName] or [AnnotationName: data] where Java uses @AnnotationName or @AnnotationName(data); C# uses AttributeUsageAttribute where Java uses Target; and so forth. However, C# permits metadata on generic arguments, and C# permits multiple metadata instances of the same type to appear at a given location.
Like Java, C# does not permit metadata on elements within a method body. (The “[a]C#” language [CCC05], whose name is pronounced “annotated C sharp”, is an extension to C# that permits annotation of statements and code blocks.)
Harmon and Klefstad [HK07] propose a standard for worst-case execution time annotations.
Pechtchanski's dissertation [Pec03] uses annotations in the aid of dynamic program optimization. Pechtchanski implemented an extension to the Jikes compiler that supports stylized comments, and uses these annotations on classes, fields, methods, formals, local variable declarations, object creation (new) expressions, method invocations (calls), and program points (empty statements). The annotations are propagated by the compiler to the class file.
Mathias Ricken's LAPT-javac (https://ricken.us/research/laptjavac/) is a version of javac (version 1.5.0_06) that encodes annotations on local variables in the class file, in new Runtime{Inv,V}isibleLocalVariableAnnotations attributes. The class file format of LAPT-javac differs from that proposed in this document.
The Java Modeling Language, JML [LBR06], is a behavioral modeling language for writing specifications for Java code. It uses stylized comments as annotations, some of which apply to types.
Ownership types [CPN98, Boy04, Cla01, CD02, PNCB06, NVP98, DM05, LM04, LP06] permit programmers to control aliasing and access among objects. Ownership types can be expressed with type annotations and have been applied to program verification [LM04, Mül02, MPHL06], thread synchronization [BLR02, JPLS05], memory management [ACG+06, BSBR03], and representation independence [BN02].
JavaCOP [ANMM06] is a framework for implementing pluggable type systems in Java. Unlike JSR 308, JavaCOP does not use standard interfaces such as the Tree API and the JSR 269 annotation processing framework. JavaCOP provides a declarative pattern-matching and rule-based language; a type checker must be programmed in a combination of this language and in Java. JavaCOP's authors have defined over a dozen type-checkers in their language, but they do not report having run any of these type-checkers on a program to verify that the program satisfies the pluggable type system. As of this writing, no implementation is publicly available.
Matt Papi designed and implemented the JSR 308 compiler as modifications to Sun's javac compiler, and contributed to the JSR 308 design.
The members of the JSR 308 mailing list (https://types.cs.washington.edu/list-archives/jsr308/) provided valuable comments and suggestions. Additional feedback is welcome.
At the 5th annual JCP Program Awards (in May 2007), JSR 308 received the Most Innovative Java SE/EE JSR of the Year award.