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9.2.4.2. About Remoting Callbacks

When a Remoting client requests information from the server, it can block and wait for the server to reply, but this is often not the ideal behavior. To allow the client to listen for asynchronous events on the server, and continue doing other work while waiting for the server to finish the request, your application can ask the server to send a notification when it has finished. This is referred to as a callback. One client can add itself as a listener for asynchronous events generated on behalf of another client, as well. There are two different choices for how to receive callbacks: pull callbacks or push callbacks. Clients check for pull callbacks synchronously, but passively listen for push callbacks.
In essence, a callback works by the server sending an InvocationRequest to the client. Your server-side code works the same regardless of whether the callback is synchronous or asynchronous. Only the client needs to know the difference. The server's InvocationRequest sends a responseObject to the client. This is the payload that the client has requested. This may be a direct response to a request or an event notification.
Your server also tracks listeners using an m_listeners object. It contains a list of all listeners that have been added to your server handler. The ServerInvocationHandler interface includes methods that allow you to manage this list.
The client handles pull and push callback in different ways. In either case, it must implement a callback handler. A callback handler is an implementation of interface org.jboss.remoting.InvokerCallbackHandler, which processes the callback data. After implementing the callback handler, you either add yourself as a listener for a pull callback, or implement a callback server for a push callback.
Pull Callbacks

For a pull callback, your client adds itself to the server's list of listeners using the Client.addListener() method. It then polls the server periodically for synchronous delivery of callback data. This poll is performed using the Client.getCallbacks().

Push Callback

A push callback requires your client application to run its own InvocationHandler. To do this, you need to run a Remoting service on the client itself. This is referred to as a callback server. The callback server accepts incoming requests asynchronously and processes them for the requester (in this case, the server). To register your client's callback server with the main server, pass the callback server's InvokerLocator as the second argument to the addListener method.

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