Contents - Cultural View

Java annotation 168
Java annotation
An annotation, in the Java computer programming language, is a special form of syntactic metadata that can be
added to Java source code. [1] Classes, methods, variables, parameters and packages may be annotated. Unlike
Javadoc tags, Java annotations can be reflective in that they can be embedded in class files generated by the compiler
and may be retained by the Java VM to be made retrievable at run-time. [2]
Examples
// @Twizzle is an annotation to method toggle().
@Twizzle
public void toggle() {
}
// Declares the annotation Twizzle.
public @interface Twizzle {
}
Annotations may include an optional list of key-value pairs:
// Same as: @Edible(value = true)
@Edible(true)
Item item = new Carrot();
public @interface Edible {
}
boolean value() default false;
@Author(first = "Oompah", last = "Loompah")
Book book = new Book();
public @interface Author {
}
String first();
String last();
Annotations themselves may be annotated to indicate where and when they can be used:
@Retention(RetentionPolicy.RUNTIME) // Make this annotation
accessible at runtime via reflection.
@Target({ElementType.METHOD}) // This annotation can only be
applied to class methods.
public @interface Tweezable {
}
The compiler reserves a set of special annotations (including @Deprecated, @Override and
@SuppressWarnings) for syntactic purposes.
Annotations are often used by frameworks as a way of conveniently applying behaviours to user-defined classes and
methods that must otherwise be declared in some external source (such as an XML configuration file) or