code: public void sendToDirect(String topicURI, Object event,
  			Set<String> webSocketSessionIds) {
  		if (webSocketSessionIds != null) {
  			for (String webSocketSessionId : webSocketSessionIds) {
  				EventMessage eventMessage = new EventMessage(topicURI, event);
  				eventMessage.setWebSocketSessionId(webSocketSessionId);
  				sendDirect(eventMessage);
  			}
  		}
  	}
docs: Send an EventMessage directly to each client listed in the webSocketSessionId set
  parameter. If parameter webSocketSessionIds is null or empty no messages are sent.
  <p>
  In contrast to {@link #sendTo(String, Object, Set)} this method does not check if
  the receivers are subscribed to the destination. The
  {@link SimpleBrokerMessageHandler} is not involved in sending these messages.
  
  @param topicURI the name of the topic
  @param event the payload of the {@link EventMessage}
  @param webSocketSessionIds list of receivers for the EVENT message