Java Programming/Annotations/Meta-Annotations

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There are four annotation types in the java.lang.annotation package. These so-called meta-annotations are used to annotate other annotation types.

[edit] Documented

If a member is annotated with a type itself marked as @Documented, then that member will be documented as annotating that type. An example will be useful.

@interface Secret { }
 
@Documented
@interface NotSecret { }
 
@Secret
@NotSecret
public class Example {
}

In the documentation for the Example class, such as the JavaDoc, Example will be shown as annotated with @NotSecret, but not @Secret.

[edit] Inherited

Exactly as the name sounds, an @Inherited annotation type is inherited by subclasses of an annotated type.

@Inherited
@interface ForEveryone { }
 
@interface JustForMe { }
 
@ForEveryone
@JustForMe
class Superclass { }
 
class Subclass extends Superclass { }

In this example, Superclass has been explicitly annotated with both @ForEveryone and @JustForMe. Subclass hasn't been explicitly marked with either one; however, it inherits @ForEveryone because the latter is annotated with @Inherited. @JustForMe isn't annotated, so it isn't inherited by Subclass.

[edit] Retention

Different annotation types have different purposes. Some are intended for use with the compiler; others are meant to be reflected dynamically at runtime. There's no reason for a compiler annotation to be available at runtime, so the @Retention meta-annotation specifies how long an annotation type should be retained. The value attribute is one of the java.lang.annotation.RetentionPolicy enum constants. The possible values, in order from shortest to longest retention, are as follows:

RetentionPolicy.SOURCE

The annotation will not be included in the class file. This is useful for annotations which are intended for the compiler only.

RetentionPolicy.CLASS

The annotation will be included in the class file, but cannot be read reflectively.

RetentionPolicy.RUNTIME

The annotation can be reflected at runtime.

If no @Retention policy is specified, it defaults to RetentionPolicy.CLASS.

[edit] Target

The @Target meta-annotation determines what may be marked by the annotation. The value attribute is one or more of the java.lang.annotation.ElementType enum constants. Those constants are ElementType.ANNOTATION_TYPE, CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, and TYPE.