Annotation Types in Java

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Annotation Types
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It is a compile-time error if an annotation type T contains an element of type
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T, either directly or indirectly
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For example, this is illegal:
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and so is this:
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// Illegal circularity!! @interface Ping { Pong value();
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@interface Pong { Ping value(); }
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// Illegal nested array!! @interface Verboten { String[][] value(); }
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An annotation type element may have a default value speci ed for it This is done by following its (empty) parameter list with the keyword default and the default value of the element Defaults are applied dynamically at the time annotations are read; default values are not compiled into annotations Thus, changing a default value affects annotations even in classes that were compiled before the change was made (presuming these annotations lack an explicit value for the defaulted element) An ElementValue is used to specify a default value It is a compile-time error if the type of the element is not commensurate ( 97) with the default value speci ed An ElementValue is always FP-strict ( 154)
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Note also that this speci cation precludes elements whose types are nested arrays For example, this annotation type declaration is illegal:
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// Illegal self-reference!! @interface SelfRef { SelfRef value(); }
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The following annotation type declaration de nes an annotation type with several elements:
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// Normal annotation type declaration with several elements /** * Describes the "request-for-enhancement" (RFE) * that led to the presence of * the annotated API element
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*/ public @interface RequestForEnhancement { int id(); // Unique ID number associated with RFE String synopsis(); // Synopsis of RFE String engineer(); // Name of engineer who implemented RFE String date(); // Date RFE was implemented }
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The following annotation type declaration de nes an annotation type with no elements, termed a marker annotation type:
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// Marker annotation type declaration
/** * Annotation with this type indicates that the specification of the * annotated API element is preliminary and subject to change */ public @interface Preliminary { }
By convention, the name of the sole element in a single-element annotation type is value
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Linguistic support for this convention is provided by the single element annotation construct ( 97); one must obey the convention in order to take advantage of the construct
Annotation Types
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The convention is illustrated in the following annotation type declaration:
// Single-element annotation type declaration /** * Associates a copyright notice with the annotated API element */ public @interface Copyright { String value(); }
The following annotation type declaration de nes a single-element annotation type whose sole element has an array type:
// Single-element annotation type declaration with array-typed // element /** * Associates a list of endorsers with the annotated class */ public @interface Endorsers { String[] value(); }
Here is an example of complex annotation types, annotation types that contain one or more elements whose types are also annotation types
// Complex Annotation Type
/** * A person s name This annotation type is not designed to be used * directly to annotate program elements, but to define elements * of other annotation types */ public @interface Name { String first(); String last(); }
/** * Indicates the author of the annotated program element */ public @interface Author { Name value(); } /** * Indicates the reviewer of the annotated program element */ public @interface Reviewer { Name value(); }
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Prede ned Annotation Types
The following annotation type declaration provides default values for two of its four elements:
// Annotation type declaration with defaults on some elements public @interface RequestForEnhancement { int id(); // No default - must be specified in // each annotation String synopsis(); // No default - must be specified in // each annotation String engineer() default "[unassigned]"; String date() default "[unimplemented]"; }
The following annotation type declaration shows a Class annotation whose value is restricted by a bounded wildcard
// Annotation type declaration with bounded wildcard to // restrict Class annotation // The annotation type declaration below presumes the existence // of this interface, which describes a formatter for Java // programming language source code public interface Formatter { } // Designates a formatter to pretty-print the annotated class public @interface PrettyPrinter { Class< extends Formatter> value(); }
Note that the grammar for annotation type declarations permits other element declarations besides method declarations For example, one might choose to declare a nested enum for use in conjunction with an annotation type:
// Annotation type declaration with nested enum type declaration public @interface Quality { enum Level { BAD, INDIFFERENT, GOOD } Level value(); }
961 Prede ned Annotation Types Several annotation types are prede ned in the libraries of the Java platform Some of these prede ned annotation types have special semantics These semantics are speci ed in this section This section does not provide a complete speci cation for the prede ned annotations contained here in; that is the role of the appropriate API speci cations Only those semantics that require special behavior on the part of the Java compiler or virtual machine are speci ed here