[JAVA] Annotation

@Documented

: javadoc을 만들 때 annotation 정보가 포함된다.

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

Document Annotation 코드를 보면 런타임시 참조할 수 있고 Annotation Type에만 선언이 가능하다.

▼ 예제

@Documented
public @interface MyAnnotation {

}

 

Annotation에 사용 가능하다.

 

@Retention

: annotation 정보를 언제까지 참조할 수 있는지 설정한다.

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}

▼ 예제

@Retention(RetentionPolicy.SOURCE)
public @interface MyAnnotation {

}

RetensionPolicy에는 SOURCE, CLASS, RUNTIME 세 종류가 있다.

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     */
    SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     */
    CLASS,

    /**
     * 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.
     *
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

 

RetentionPolicy.SOURCE // 컴파일시에만 사용.

RetentionPolicy.RUNTIME // default. RUNTIME 중 참조 가능.

RetentionPolicy.CLASS // 컴파일하면 생기는 class 파일에서까지 참조.

@MyAnnotation
public class MyClass {
    public static void main(String[] args) {
        Arrays.stream(MyClass.class.getAnnotations()).forEach(System.out::println);
        // @Retention(RetentionPolicy.SOURCE), @Retention(RetentionPolicy.CLASS)
        // 아무것도 출력X
        // @Retention(RetentionPolicy.RUNTIME) // default. 런타임에도 참조 가능.
        // 출력 : @me.whiteship.chapter05.item27.annotation.MyAnnotation()
    }
}

 

@Target

: 이 annotation 을 선언할 수 있는 위치를 설정한다.

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    /**
     * Returns an array of the kinds of elements an annotation interface
     * can be applied to.
     * @return an array of the kinds of elements an annotation interface
     * can be applied to
     */
    ElementType[] value();
}

▼ 예제

@Target({ElementType.TYPE, ElementType.METHOD})
public @interface MyAnnotation {

}

ElementType 종류를 한 번 보고 넘어가자.

 

public enum ElementType {
    /** Class, interface (including annotation interface), enum, or record
     * declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation interface declaration (Formerly known as an annotation type.) */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE,

    /**
     * Module declaration.
     *
     * @since 9
     */
    MODULE,

    /**
     * Record component
     *
     * @jls 8.10.3 Record Members
     * @jls 9.7.4 Where Annotations May Appear
     *
     * @since 16
     */
    RECORD_COMPONENT;
}

@MyAnnotation // ElementType.TYPE 선언시 위치 가능
public class MyClass {

    @MyAnnotation // ElementType.METHOD 선언시 위치 가능
    public static void main(String[] args) {
        Arrays.stream(MyClass.class.getAnnotations()).forEach(System.out::println);
    }
}