title: Gwt how to make session listener?
body: <p>I'm making gwt + spring application. It's kind of game and i need to know if user leave the game. Is there any chance to check all sessions on the server side? 
  I don't know if there is any already implemented mechanism, but i thought only about some event checker witch sends AsyncCallback to server once for 5 sec and i'll store list of this users somewhere on server.
  Do you know any better idea to do it?</p>