code: public synchronized static void closeClientIOEventGroup() {
          if (clientIOEventLoopGroup != null) {
              AtomicInteger ref = refCounter.get(clientIOEventLoopGroup);
              if (ref.decrementAndGet() <= 0) {
                  if (!clientIOEventLoopGroup.isShutdown() && !clientIOEventLoopGroup.isShuttingDown()) {
                      clientIOEventLoopGroup.shutdownGracefully();
                  }
                  refCounter.remove(clientIOEventLoopGroup);
                  if (LOGGER.isDebugEnabled()) {
                      LOGGER.debug("Closing Client EventLoopGroup, ref : 0");
                  }
              } else {
                  if (LOGGER.isDebugEnabled()) {
                      LOGGER.debug("Client EventLoopGroup still has ref : {}", ref.get());
                  }
              }
          }
          clientIOEventLoopGroup = null;
      }
docs: 关闭客户端IO线程池