Java Annotations

Internal

• Java

Custom Annotations

Annotation type declarations are similar to normal interface declarations. An at-sign (@) precedes the interface keyword.

Each method declaration defines an element of the annotation type. Method declarations must not have any parameters or a throws clause. Return types are restricted to primitives, String, Class, enums, annotations, and arrays of the preceding types.

Methods can have default values.

Annotations themselves may be annotated to indicate where and when they can be used"

@Retention

@Retention(RetentionPolicy.RUNTIME)

@Retention is a meta-annotation that specifies how long the annotations annotated with it are to be retained. Possible values:

• RetentionPolicy.SOURCE: annotations are to be discarded by the compiler.
• RetentionPolicy.CLASS (default): Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.
• RetentionPolicy.RUNTIME: Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.

A Retention meta-annotation has effect only if the meta-annotated type is used directly for annotation.

@Target

@Target({ElementType.METHOD})

      // This annotation can only be applied to class methods.

The compiler reserves a set of special annotations (including @Deprecated, @Override and @SuppressWarnings) for syntactic purposes.

Meta-Annotation

Annotations that are applied to other annotations: @Retention, @Target.