發布訂閱概念

發布/訂閱者模式最大的特點就是實現了松耦合，也就是說你可以讓發布者發布消息、訂閱者接受消息，而不是尋找一種方式把兩個分離 的系統連接在一起。當然這種松耦合也是發布/訂閱者模式最大的缺點，因為需要中間的代理，增加了系統的復雜度。而且發布者無法實時知道發布的消息是否被每個訂閱者接收到了，增加了系統的不確定性。

發布訂閱 vs 監聽者

參考文章：https://hackernoon.com/observer-vs-pub-sub-pattern-50d3b27f838c, 理解發布訂閱模式和觀察者模式的區別

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.

發布訂閱解偶，通常使用消息中間件，能實現跨平臺、異步通信

例子代碼

• 發布者發布消息

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

• 訂閱者訂閱并響應消息

public interface Subscriber {// 進行訂閱void subscribe();// 響應消息void onMessage(String message);
}

• 第三方中間件做消息中轉并通知訂閱者

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) {// 發布訂閱中心, 消息中間件一樣Broker broker = new Broker();// 發布者注冊Publisher1 publisher = new Publisher1();publisher.setBroker(broker);// 訂閱者注冊Subscriber1 subscriber = new Subscriber1();subscriber.setBroker(broker);subscriber.subscribe();// 發布者發布消息publisher.publishMessage("hello");// 中間間會通知所有訂閱者broker.notifySubscribers();}
}