code: public int getConnectedClientCount(final SynchronizeFxServer server) {
          synchronized (channels) {
              final SynchronizeFXWebsocketChannel channel = servers.get(server);
              if (channel == null) {
                  throw new IllegalArgumentException(
                          "The SynchronizeFXServer passed as argument was not created by this instance "
                                  + "or has already been shut down.");
              }
              return channel.getCurrentlyConnectedClientCount();
          }
      }
docs: The number of clients that are currently connected to the given {@link SynchronizeFxServer}.
  
  @param server The server thats connection count should be checked.
  @throws IllegalArgumentException If the {@link SynchronizeFxServer} passed as argument wasn't started by this
  servlet or if it has already been shut down.
  @return The client count