Java自定义注解

//表示适用于方法或类
@Target({ElementType.METHOD,ElementType.TYPE})
//运行时注解
@Retention(RetentionPolicy.RUNTIME)
public @interface Description {
    String value();
    int age() default 18;
}

@Description(value = "i am class annotation",age=12)
public class Child {
    //age采用default值
    @Description(value = "i am method annotation")
    public String name(){
        return null;
    }
    public int age(){
        return 0;
    }
}

public class Main {
    public static void main(String[] args){
        //RetentionPolicy.CLASS或RetentionPolicy.SOURCE时无输出
        try{
            Class c = new Child().getClass();
            boolean isExist = c.isAnnotationPresent(Description.class);
            if(isExist){
                Description d = (Description) c.getAnnotation(Description.class);
                System.out.println(d.value());
                System.out.println(d.age());
            }
            for(Method m : c.getMethods()){
                for(Annotation a : m.getAnnotations()){
                    if(a instanceof Description){
                        Description d = (Description) a;
                        System.out.println((d.value()));
                        System.out.println((d.age()));
                    }
                }
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}
i am class annotation
12
i am method annotation
18