code: public void stop() {
          Log.info(serverName() + " gracefully stopping...");
          for (EventLoopGroup g : eventLoopGroups) {
              g.shutdownGracefully().awaitUninterruptibly();
          }
          Log.info(serverName() + " gracefully stopped");
      }
docs: Stops the server gracefully.
  Do not call in action thread to avoid deadlock (start a new thread or use {@link #stopAtShutdown}).