public enum RetentionPolicy {
    SOURCE,
    CLASS,
    RUNTIME
}

import java.lang.annotation.Annotation;
@Deprecated
class Person{
}
class Demo{
    public static void main(String[] args) {
        Annotation[] annotations = Person.class.getAnnotations();
        for (Annotation a: annotations){
            System.out.println(a);
            // @java.lang.Deprecated()
        }
    }
}

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
// 定义运行时策略
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
    public String name();
    public int age() default 23; // 设置默认值
}
class Message {
    @MyAnnotation(name="Tom")
    public void send(String msg){
        System.out.println(msg);
    }
}
class Demo{
    public static void main(String[] args) throws Exception {
         Method method = Message.class.getDeclaredMethod("send", String.class);
        MyAnnotation annotation = method.getAnnotation(MyAnnotation.class);
        String name = annotation.name(); // Tom
        int age = annotation.age();  // 23
        String msg = String.format("[%s] %s ", name, age);
        method.invoke(Message.class.getDeclaredConstructor().newInstance(), msg);
        // [Tom] 23 
    }
}

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
// 消息接口
interface IMessage {
    public void send(String msg);
}
// 消息实现类
class MessageImpl implements IMessage {
    public void send(String msg) {
        System.out.println("消息发送");
    }
}
// 动态代理类
class MessageProxy implements InvocationHandler {
    private Object target;
    public Object bind(Object target) {
        this.target = target;
        return Proxy.newProxyInstance(
                target.getClass().getClassLoader(),
                target.getClass().getInterfaces(),
                this
        );
    }
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object obj = null;
        if (this.connect()) {
            obj = method.invoke(this.target, args);
        }
        this.close();
        return obj;
    }
    public boolean connect() {
        System.out.println("打开连接");
        return true;
    }
    public void close() {
        System.out.println("关闭连接");
    }
}
// 工厂类
class Factory {
    private Factory() {
    }
    public static <T> T getInstance(Class<T> clazz) {
        try {
            return (T) new MessageProxy().bind(clazz.getDeclaredConstructor().newInstance());
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}
@Retention(RetentionPolicy.RUNTIME)
@interface UseMessage {
    public Class<?> clazz();
}
// 利用注解实现类的使用
@UseMessage(clazz = MessageImpl.class)
class MessageService {
    private IMessage message;
    public MessageService() {
        UseMessage use = MessageService.class.getAnnotation(UseMessage.class);
        this.message = (IMessage) Factory.getInstance(use.clazz());
    }
    public void send(String msg) {
        this.message.send(msg);
    }
}
class Demo {
    public static void main(String[] args) {
        MessageService message = new MessageService();
        message.send("发送消息");
    }
}