Defining Custom Java Annotations with @interface

@interface, meta-annotations, annotation attributes

Defining a Custom Annotation: The @interface Keyword

@interface name {
    definition
}
  name: annotation name
  definition: annotation definition

You can define your own annotation using the @interface keyword. For example, the following defines a Version annotation.

Version.java

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Inherited
public @interface Version {
    double value();
}

Meta-Annotations

In an annotation definition, annotations can be added to define information about the annotation itself. Such annotations are called meta-annotations.

The types of meta-annotations are as follows.

@Retention
@Target
@Inherited

@Retention

The range in which annotation information is retained.

Annotation	Overview
`@Retention(SOURCE)`	The annotation can be used only in source code and disappears after compilation.
`@Retention(CLASS)`	The annotation exists in the `.class` file but disappears at runtime.
`@Retention(RUNTIME)`	The annotation can be used by the compiler and at runtime.

@Target

Specifies the targets to which the annotation can be applied.

Annotation	Overview
`@Target(ElementType.TYPE)`	Can be applied to any element of a class; default value.
`@Target(ElementType.FIELD)`	A specific field of a class.
`@Target(ElementType.METHOD)`	A method of a class.
`@Target(ElementType.PARAMETER)`	A method parameter.
`@Target(ElementType.CONSTRUCTOR)`	A constructor.
`@Target(ElementType.LOCAL_VARIABLE)`	A local variable.
`@Target(ElementType.ANNOTATION_TYPE)`	An annotation type.

@Inherited

Whether annotation information can be inherited by subclasses.

Annotation Attributes

Under the @interface keyword, specify attributes that can be used in the annotation. Attributes are represented as a set of type and name, like abstract methods.

Attributes defined this way can generally be specified in the form @AnnotationName(attributeName=value, ...). However, when the attribute name is value, the name can be omitted and written as @AnnotationName(value).

For example, the following is an example of an annotationTest method that specifies the Version annotation.

AnnotationSample.java

package com.example.mynavi.object;
 
public class AnnotationSample {
    @Version(1.0)
    public void annotationTest() {
        System.out.println("Released");
    }
}