CometD Java Client Subscription

Java CometD Client API: Subscribing and Unsubscribing

Subscription and unsubscription is done very similarly to the JavaScript counterpart explained here.

The Java API is only slightly different since it must register a MessageListener before subscribing to a channel:

public class Example 
{
    private static final String CHANNEL = "/foo";
    private final MessageListener fooListener = new FooListener();

    public void attach()
    {
        HttpClient httpClient = ...
        BayeuxClient client = new BayeuxClient(httpClient, "http://localhost:8080/cometd");
        client.start();

        client.addListener(fooListener);
        client.subscribe(CHANNEL);
    }

    private static class FooListener implements MessageListener
    {
        public void deliver(Client from, Client to, Message message)
        {
            if (CHANNEL.equals(message.getChannel())
            {
                // Here we received a message on the channel
            }
        }
    }
}

Note
Calling subscribe() does not mean that you have completed the subscription with the server when subscribe() returns.

Differently from JavaScript, a MessageListener is notified for any type of channel (hence for meta channels as well as for normal channels).
Similarly, subscribe() only works properly for normal channels.

Unsubscription is straightforward: if you unsubscribe, messages on that channel will not be delivered to message listeners.
Using the Example class above:

public class Example 
{
    ...
    public void detach()
    {
        client.removeListener(fooListener);
        client.unsubscribe(CHANNEL);
    }
}

Tip
It is recommended that you remove also the MessageListener that you registered when you subscribed.
To make this simpler, avoid using the anonymous inner class style to register a message listener, since otherwise you will not have its reference when you want to remove it.