JAVA: Annotations

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JAVA: Annotations

· java

Define

An annotation is a form of syntactic metadata that can be added to Java source code.
Classes, methods, variables, parameters and Java packages may be annotated.

―Replacement of marker interfaces:

//Before
public class Foo implements MarkerInterface {}
//Now
@MyAnnotation
public class Foo {}

―Better metadata management:

//Before:in Javadoc
/**
* @deprecated As of JDK version 1.1,
*/
public class DeprecatedApi {}
//Now:with annotation
@Deprecated(since="1.2", forRemoval=true)
public class DeprecatedApi {}

Creating an annotation

Annotations require:
1) A target: this defines where the annotation can be set
2) A retention: this describes up to which step in the compilation process the annotation will be available

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
@interface Bar {}

Annotation parameters

Annotations can define parameters.
Parameters allow to add some level of configuration at the time the annotation is used.

@Target(ElementType.CLASS)
@interface Foo {
int bar();
Class<? extends Collection> baz() default List.class;
String[] qux();
}

@Foo(bar = 1, qux = { "a", "b", "c" })
class MyClass {}

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