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

Annotations are a a form of metadata providing data about a program that is not part of the program it self. Annotations have no direct effect on the operation of the code they annotate. Annotations have been Introduced to the language 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.