java注解Annotation

@Target({ElementType.TYPE,ElementType.FIELD}) //注解作用在哪里，这里是类上和变量上
@Retention(RetentionPolicy.RUNTIME) //注解生命周期，这里是运行时
@Inherited //注解的子类和动态代理类也会拥有这个注解
public @interface MyAnn {
    //默认值value，这个属性名比较特殊@MyAnn("xxx")，这里xxx就是这个value
    String value();
    //注解值有很多(可以是数组，枚举或者其他注解等),还可以设置默认值
    int age() default 0;
    String[] parentsName() default {};
}

@MyAnn(value = "haha",age = 3,parentsName = {"zhangsan","lisi"})
public class Client {

    @MyAnn(value = "hehe",age = 5,parentsName = {"wangwu","zhaoliu"})
    private static String name;

    public static void main(String[] args) throws NoSuchFieldException {
        //获取类上的注解
        final Class<Client> clazz = Client.class;
        final MyAnn annotationClass = clazz.getAnnotation(MyAnn.class);
        System.out.println(annotationClass.age());//3
        System.out.println(Arrays.toString(annotationClass.parentsName()));//[zhangsan, lisi]
        System.out.println(annotationClass.value());//haha

        //获取成员变量上的注解
        final Field name = clazz.getDeclaredField("name");
        final MyAnn annotationField = name.getAnnotation(MyAnn.class);

        System.out.println(annotationField.age());//5
        System.out.println(Arrays.toString(annotationField.parentsName()));//[wangwu, zhaoliu]
        System.out.println(annotationField.value());//hehe
    }
}