code: public int sendMessageToTopic(Collection<Session> sessionTargets, MessageToClient mtc, Object payload) {
  		int sended = 0;
  		logger.debug("Sending message to topic {}...", mtc);
  		Collection<Session> doublons = topicManager.getSessionsForTopic(mtc.getId());
  		if (doublons != null && !doublons.isEmpty()) {
  			Set<Session> sessions = new HashSet<>(doublons);
  			if (sessionTargets != null) {
  				sessions.retainAll(sessionTargets);
  			}
  			sended = sendMessageToTopicForSessions(sessions, mtc, payload);
  		} else {
  			logger.debug("No client for topic '{}'", mtc.getId());
  		}
  		return sended;
  	}
docs: Send message to all sessions for topic mtc.getId() and intersection of sessionTargets
  return number sended
  @param sessionTargets
  @param mtc
  @param payload
  @return