code: private static synchronized void decrementUseCount() {
          final String methodName = "decrementUseCount";
          logger.entry(methodName);
  
          --useCount;
          if (useCount <= 0) {
              if (bootstrap != null) {
                bootstrap.group().shutdownGracefully(0, 500, TimeUnit.MILLISECONDS);
              }
              bootstrap = null;
              useCount = 0;
          }
  
          logger.exit(methodName);
      }
docs: Decrement the use count of the workerGroup and request a graceful
  shutdown once it is no longer being used by anyone.