code: private void shutdownGracefully(final Iterator<EventExecutorGroup> iterator) {
      if (iterator.hasNext()) {
        EventExecutorGroup group = iterator.next();
        if (!group.isShuttingDown()) {
          group.shutdownGracefully().addListener(future -> {
            if (!future.isSuccess()) {
              log.debug("shutdown of {} resulted in exception", group, future.cause());
            }
            shutdownGracefully(iterator);
          });
        }
      }
    }
docs: Shutdown executor in order.
  
  @param iterator Executors to shutdown.