code: public synchronized void stop() {
      if (isStopped()) {
        throw new IllegalStateException("Failed to stop Socket.IO server: server already stopped");
      }
  
      log.info("Socket.IO server stopping");
  
      timer.stop();
      bootstrap.config().group().shutdownGracefully().syncUninterruptibly();
      state = State.STOPPED;
  
      log.info("Socket.IO server stopped");
    }
docs: Stops Socket.IO server.
  
  @throws IllegalStateException
  if server already stopped