title: dynamic message-mapping for websockets in Spring 4
body: <p>I want to develop a small chat with springs new websocket/stomp support.
  I guess i cannot use something like this:</p>
  
  <pre><code>@MessageMapping("/connect/{roomId}")
  @SendTo("/topic/newMessage")
  public String connectToChatRoom(@PathVariable String roomId, Principal p) {
      return getTimestamp() + " " + p.getName() + " connected to the room.";
  }
  </code></pre>
  
  <p>What are my options for dynamic mapping here? As a client i want to subscribe only to the room I'm in.</p>
  
  <p>Thanks in advance!</p>