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2.4MetaAnnotation of JDK

JDK's meta-Annotation is used to modify other Annotation definitions; JDK5.0 provides annotation types specifically for annotations, which are:

Retention

Target

Documented

Inherited

@ Retention: can only be used to modify an Annotation definition to specify how long the Annotation can be retained. @ Rentention contains a member variable of type RetentionPolicy, and a value must be specified for the value member variable when using @ Rentention:

RetentionPolicy.SOURCE: the compiler directly discards comments for this policy

RetentionPolicy.CLASS: the compiler will record comments in the class file. When you run the Java program, JVM does not retain comments. This is the default value.

RetentionPolicy.RUNTIME: the compiler will record comments in the class file. When you run the Java program, JVM retains comments. The program can get the comment through reflection

Target: used to modify the Annotation definition, to specify which program elements the modified Annotation can be used to modify. @ Target also contains a member variable named value.

Documented: used to specify that the Annotation class modified by the meta Annotation will be extracted into a document by the javadoc tool.

Annotations defined as Documented must set the Retention value to RUNTIME.

Inherited: the Annotation modified by it will have inheritance. If a class uses Annotation modified by @ Inherited, its subclasses will automatically have this annotation

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