作用

为修饰者提供元数据

注解本质

/**Annotation接口源码*/
public interface Annotation {

    boolean equals(Object obj);

    int hashCode();

    Class<? extends Annotation> annotationType();
}

元注解：提供给注解使用的注解

. @Retention
. @Target
. @Documented
. @Inherited
. @Repeatable

定义与使用

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyTestAnnotation {
    String name() default "mao";
    int age() default 18;
}

@MyTestAnnotation(name = "father",age = 50)
public class Father {
}

/****************************************
* 注解获取方法
***/
/**是否存在对应 Annotation 对象*/
public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass) {
        return GenericDeclaration.super.isAnnotationPresent(annotationClass);
}
 /**获取 Annotation 对象*/
public <A extends Annotation> A getAnnotation(Class<A> annotationClass) {
        Objects.requireNonNull(annotationClass);
        return (A) annotationData().annotations.get(annotationClass);
}
 /**获取所有 Annotation 对象数组*/   
public Annotation[] getAnnotations() {
        return AnnotationParser.toArray(annotationData().annotations);
}

注解获取实例

        /**
         * 获取类注解属性
         */
        Class<Father> fatherClass = Father.class;
        boolean annotationPresent = fatherClass.isAnnotationPresent(MyTestAnnotation.class);
        if(annotationPresent){
            MyTestAnnotation annotation = fatherClass.getAnnotation(MyTestAnnotation.class);
            System.out.println(annotation.name());
            System.out.println(annotation.age());
        }

        /**
         * 获取方法注解属性
         */
        try {
            Field age = fatherClass.getDeclaredField("age");
            boolean annotationPresent1 = age.isAnnotationPresent(Age.class);
            if(annotationPresent1){
                Age annotation = age.getAnnotation(Age.class);
                System.out.println(annotation.value());
            }

            Method play = PlayGame.class.getDeclaredMethod("play");
            if (play!=null){
                People annotation2 = play.getAnnotation(People.class);
                Game[] value = annotation2.value();
                for (Game game : value) {
                    System.out.println(game.value());
                }
            }
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }

【参阅】