title: GWT server side method should only be called by one client at the same time
body: <p>Is it possible to synchronize server-side methods, so that they can only be called by one client at once?</p>
  
  <p>I tried this using the keyword "synchronized" on the server-side method. But it is still possible, that more than one client can call the method at the same time.</p>
  
  <p>EDIT: The class containing the synchronized method is a thread-safe singleton.</p>
  
  <p>Thanks in advance.</p>