用 Java 发布 - 订阅

鉴于 YouTube，Facebook 和其他社交媒体服务的兴起，发布商订阅者是一个熟悉的概念。基本概念是产生内容的 Publisher 和消费内容的 Subscriber。每当 Publisher 生成内容时，都会通知每个 Subscriber。从理论上讲，Subscribers 可以订阅多个发布者。

通常在发布者和订阅者之间有一个 ContentServer 来帮助调解消息

public class Publisher {
    ...
    public Publisher(Topic t) {
        this.topic = t;
    }

    public void publish(Message m) {
        ContentServer.getInstance().sendMessage(this.topic, m);
    }
}

public class ContentServer {
    private Hashtable<Topic, List<Subscriber>> subscriberLists;

    private static ContentServer serverInstance;

    public static ContentServer getInstance() {
        if (serverInstance == null) {
            serverInstance = new ContentServer();
        }
        return serverInstance;
    }

    private ContentServer() {
        this.subscriberLists = new Hashtable<>();
    }
    
    public sendMessage(Topic t, Message m) {
        List<Subscriber> subs = subscriberLists.get(t);
        for (Subscriber s : subs) {
            s.receivedMessage(t, m);
        }
    }

    public void registerSubscriber(Subscriber s, Topic t) {
        subscriberLists.get(t).add(s);
    }

public class Subscriber {
    public Subscriber(Topic...topics) {
        for (Topic t : topics) {
            ContentServer.getInstance().registerSubscriber(this, t);
        }
    }
    
    public void receivedMessage(Topic t, Message m) {
        switch(t) {
            ...
        }
    }
}

通常，pub-sub 设计模式在实现时考虑了多线程视图。其中一个更常见的实现将每个 Subscriber 视为一个单独的线程，ContentServer 管理一个线程池