code: public void sendToDirect(String topicURI, Object event, String webSocketSessionId) {
  		Assert.notNull(webSocketSessionId, "WebSocket session id must not be null");
  
  		sendToDirect(topicURI, event, Collections.singleton(webSocketSessionId));
  	}
docs: Send an EventMessage directly to the client specified with the webSocketSessionId
  parameter.
  <p>
  In contrast to {@link #sendTo(String, Object, String)} this method does not check
  if the receiver is subscribed to the destination. The
  {@link SimpleBrokerMessageHandler} is not involved in sending this message.
  
  @param topicURI the name of the topic
  @param event the payload of the {@link EventMessage}
  @param webSocketSessionId receiver of the EVENT message