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发布订阅概念

发布/订阅者模式最大的特点就是实现了松耦合，也就是说你可以让发布者发布消息、订阅者接受消息，而不是寻找一种方式把两个分离 的系统连接在一起。当然这种松耦合也是发布/订阅者模式最大的缺点，因为需要中间的代理，增加了系统的复杂度。而且发布者无法实时知道发布的消息是否被每个订阅者接收到了，增加了系统的不确定性。

发布订阅 vs 监听者

参考文章：https://hackernoon.com/observer-vs-pub-sub-pattern-50d3b27f838c, 理解发布订阅模式和观察者模式的区别

Let’s list out the differences as a quick Summary:

• In the Observer pattern, the Observers are aware of the Subject, also the Subject maintains a record of the Observers. Whereas, in Publisher/Subscriber, publishers and subscribers don’t need to know each other. They simply communicate with the help of message queues or broker.

• In Publisher/Subscriber pattern, components are loosely coupled as opposed to Observer pattern.

• Observer pattern is mostly implemented in a synchronous way, i.e. the Subject calls the appropriate method of all its observers when some event occurs. The Publisher/Subscriber pattern is mostly implemented in an asynchronous way (using message queue).

• Observer pattern needs to be implemented in a single application address space. On the other hand, the Publisher/Subscriber pattern is more of a cross-application pattern.

发布订阅解偶，通常使用消息中间件，能实现跨平台、异步通信

例子代码

• 发布者发布消息

public interface Publisher {// 发布消息void publishMessage(String msg);}

• 订阅者订阅并响应消息

public interface Subscriber {// 进行订阅void subscribe();// 响应消息void onMessage(String message);
}

• 第三方中间件做消息中转并通知订阅者

public class Broker {List<Subscriber> subscriberList = new ArrayList<>();List<Publisher> publishList = new ArrayList<>();List<String> messageList = new ArrayList<>();public void notifySubscribers(){while(!messageList.isEmpty()){String message = messageList.remove(0);for(Subscriber subscriber :subscriberList){subscriber.onMessage(message);}}}public void publishMessage(String message){messageList.add(message);}public void subscribt(Subscriber subscriber){subscriberList.add(subscriber);}public void publish(Publisher publisher){publishList.add(publisher);}public static void main(String[] args) {// 发布订阅中心, 消息中间件一样Broker broker = new Broker();// 发布者注册Publisher1 publisher = new Publisher1();publisher.setBroker(broker);// 订阅者注册Subscriber1 subscriber = new Subscriber1();subscriber.setBroker(broker);subscriber.subscribe();// 发布者发布消息publisher.publishMessage("hello");// 中间间会通知所有订阅者broker.notifySubscribers();}
}