Design patterns Publish-Subscribe Publish-Subscribe in Java
Example
The publisher-subscriber is a familiar concept given the rise of YouTube, Facebook and other social media services. The basic concept is that there is a Publisher who generates content and a Subscriber who consumes content. Whenever the Publisher generates content, each Subscriber is notified. Subscribers can theoretically be subscribed to more than one publisher.
Usually there is a ContentServer that sits between publisher and subscriber to help mediate the messaging
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) {
...
}
}
}
Usually, the pub-sub design pattern is implemented with a multithreaded view in mind. One of the more common implementations sees each Subscriber as a separate thread, with the ContentServer managing a thread pool
