Java Annotations

External

• https://www.developer.com/java/other/article.php/3556176
• https://docs.oracle.com/javase/6/docs/technotes/guides/language/annotations.html
• https://www.theregister.co.uk/2006/02/24/java_annotations/
• https://web.archive.org/web/20140223113106/http://www.somanyword.com/2014/02/how-to-create-and-implement-custom-annotations-in-java/
• http://www.tutorialsdesk.com/2014/10/java-annotations-tutorial-with-examples.html
• https://www.javaquery.com/2015/10/understanding-annotations-in-java.html

Internal

• Java

Overview

Introduced in Java 5.

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.

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

@Retention(RUNTIME)
@Target({METHOD, FIELD, TYPE})
public @interface Encrypted {
}

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})

@Target is a meta-annotation that indicates the context in which the annotated annotation is applicable. Possible values:

• ElementType.TYPE - for Class, interface (including annotation type) or enum declaration.
• ElementType.FIELD - for field declaration (includes enum constants).
• ElementType.METHOD - for method declaration.
• ElementType.PARAMETER - for parameter declaration.
• ElementType.CONSTRUCTOR - for constructor declaration.
• ElementType.LOCAL_VARIABLE - for local variable declaration.
• ElementType.ANNOTATION_TYPE - for annotation type declaration.
• ElementType.PACKAGE - for method declaration.
• ElementType.TYPE_PARAMETER - for package declaration.
• ElementType.TYPE_USE - for use of a type.
• ElementType.MODULE - for module declaration.

Meta-Annotation

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