code: public void shutdown() {
  	    if (shutdown) 
  	    {
  	        return;
  	    }
  	    shutdown = true;
  	    synchronized (workers) 
  	    {
  	        for (int i = workers.size(); i > 0; i--) 
  	        {
  	        	systemExecute(exitTask);
  	        }
  	        unSafe_singallAllWhenBlockingWorker();
  	    }
  	}
docs: 使用WorkerExitTask结束任务,防止之前的任务因停止而丢失