code: public void onOpen(final Session session, final String channelName) {
          final SynchronizeFXWebsocketChannel channel;
          synchronized (channels) {
              channel = getChannelOrFail(channelName);
              clients.put(session, channel);
          }
          channel.newClient(session);
      }
docs: Pass {@link OnOpen} events of the Websocket API to this method to handle new clients.
  
  @param session The client that has connected.
  @param channelName The name of the channel this client connected too.
  @throws IllegalArgumentException If the channel passed as argument does not exist.