Java annotation

In the Java computer programming language, an annotation is a form of syntactic metadata that can be added to Java source code. Classes, methods, variables, parameters and packages may be annotated. Unlike Javadoc tags, Java annotations can be reflective by means of being embedded in class files, which are generated by the compiler, and may be retained by the Java VM to be made retrievable at run-time. It is possible to create meta-annotations out of the existing ones in Java.

The Java platform has various ad-hoc annotation mechanisms—for example, the transient modifier, or the @deprecated javadoc tag. JSR-175 introduced the general-purpose annotation (also known as metadata) facility to the Java Community Process in 2002; it gained approval in September 2004. Annotations became available in the language itself beginning with version 1.5 of the JDK. The apt tool provided a provisional interface for compile-time annotation processing in JDK version 1.5; JSR-269 formalized this, and it became integrated into the javac compiler in version 1.6.

Java defines a set of annotations that are built into the language. Of the seven standard annotations, three are part of java.lang, and the remaining four are imported from java.lang.annotation.

Annotations applied to Java code:

Annotations applied to other annotations (also known as "Meta Annotations"):

Since Java 7, three additional annotations have been added to the language.

This example demonstrates the use of the @Override annotation. It instructs the compiler to check parent classes for matching methods. In this case, an error is generated because the gettype() method of class Cat doesn't in fact override getType() of class Animal like is desired. If the @Override annotation was absent, a new method of name gettype() would be created in class Cat.

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.

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