@Override
    public void run() {
        // TODO Auto-generated method stub
        while (!isStop) {
            DownloadTask downloadTask = downloadTaskManager.getDownloadTask();
            if (downloadTask != null) {
                pool.execute(downloadTask);
            } else {  //如果当前未有downloadTask在任务队列中
                try {
                    // 查询任务完成失败的,重新加载任务队列
                    // 轮询,
                    Thread.sleep(SLEEP_TIME);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        }
        if (isStop) {
            pool.shutdown();
        }

    }