code: @OnOpen
      public void uiClientSessionOpen(Session session) {
          log.infoWsSessionOpened(session.getId(), endpoint);
          wsEndpoints.getUiClientSessions().addSession(session.getId(), session);
          WelcomeResponse welcomeResponse = new WelcomeResponse();
          // FIXME we should not send the true sessionIds to clients to prevent spoofing.
          welcomeResponse.setSessionId(session.getId());
          try {
              new WebSocketHelper().sendBasicMessageSync(session, welcomeResponse);
          } catch (IOException e) {
              log.warnf(e, "Could not send [%s] to UI client session [%s].", WelcomeResponse.class.getName(),
                      session.getId());
          }
      }
docs: When a UI client connects, this method is called. This will immediately send a welcome
  message to the UI client.
  
  @param session the new UI client's session