code: public void sendToUsersDirect(String topicURI, Object event, Set<String> users) {
  
  		Set<String> webSocketSessionIds = null;
  		if (users != null && !users.isEmpty()) {
  			webSocketSessionIds = new HashSet<>(users.size());
  
  			for (String user : users) {
  				for (SimpSession session : this.simpUserRegistry.getUser(user)
  						.getSessions()) {
  					webSocketSessionIds.add(session.getId());
  				}
  			}
  		}
  
  		this.eventMessenger.sendToDirect(topicURI, event, webSocketSessionIds);
  	}
docs: Send an EventMessage directly to each client listed in the users set parameter. A
  user is ignored if there is no entry in the {@link SimpUserRegistry} for his
  username.
  <p>
  In contrast to {@link #sendToUsers(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 users list of receivers for the EVENT message