code: public void logout(final BooleanCallback callback) {
  		GwtCommand command = new GwtCommand(logoutCommandName);
  		GwtCommandDispatcher.getInstance().execute(command, new AbstractCommandCallback<SuccessCommandResponse>() {
  
  			public void execute(SuccessCommandResponse response) {
  				if (response.isSuccess()) {
  					userToken = null;
  					Authentication.this.userId = null;
  					if (callback != null) {
  						callback.execute(true);
  					}
  					GwtCommandDispatcher.getInstance().logout();
  					manager.fireEvent(new LogoutSuccessEvent());
  				} else {
  					if (callback != null) {
  						callback.execute(false);
  					}
  					manager.fireEvent(new LogoutFailureEvent());
  				}
  			}
  		});
  	}
docs: Logs the user out.
  
  @param callback
  A possible callback to be executed when the logout has been done (successfully or not). Can be null.