code: public void resume() {
      if (taskInfo == null || taskInfo.getChunkInfoList() == null) {
        //下载进度信息不存在
        start(true);
        return;
      }
      synchronized (taskInfo) {
        if (taskInfo.getStatus() == HttpDownStatus.RUNNING
            || taskInfo.getStatus() == HttpDownStatus.DONE) {
          return;
        }
        if (!FileUtil.exists(HttpDownUtil.getTaskFilePath(this))) {
          close();
          start();
        } else {
          if (taskInfo.getDownSize() >= response.getTotalSize()) {
            taskInfo.setStatus(HttpDownStatus.DONE);
            return;
          }
          long time = System.currentTimeMillis();
          for (ChunkInfo chunkInfo : taskInfo.getChunkInfoList()) {
            if (chunkInfo.getStatus() == HttpDownStatus.PAUSE) {
              chunkInfo.setPauseTime(chunkInfo.getPauseTime() + (time - chunkInfo.getLastPauseTime()));
            }
          }
          commonStart();
          for (ConnectInfo connectInfo : taskInfo.getConnectInfoList()) {
            if (connectInfo.getStatus() == HttpDownStatus.RUNNING) {
              reConnect(connectInfo);
            }
          }
        }
      }
      if (callback != null) {
        callback.onResume(this);
      }
    }
docs: 继续下载