How to do a session pattern with callbacks?

I know how to do a session pattern without callbacks like here How to organize RMI Client-Server architecture. But i don't know how to do one with callbacks. Do I need another session object for the client to pass to the server when logging in? If yes where do i keep it in the server side: hold a reference of the client session in the server session, or a collection of all client sessions somewhere else like the login server? If no what is the object that implements the client callback methods?

Maybe someone can give a code example like in the question i linked, because that one was clear and I didn't find one for the callbacks.

The answer doesn't have to be strictly RMI but I saw a lot of questions on this with no answers so i added the tag.