android中的多线程下载怎么利用AsyncTask实现

public class DownloadTask extends AsyncTask<String, Integer, Integer> {  public static final int TYPE_SUCCESS = 0;  public static final int TYPE_FAILURE = 1;  public static final int TYPE_PAUSE = 2;  public static final int TYPE_CANCEL = 3;  public int positionDownload;  private boolean isPaused = false;  private boolean isCancelled = false;  private DownloadListener downloadListener;  private int lastProgress;  public DownloadTask(DownloadListener downloadListener){    this.downloadListener = downloadListener;  }  public void setDownloadListener(DownloadListener downloadListener){    this.downloadListener = downloadListener;  }  @Override  protected Integer doInBackground(String... params) {    InputStream is = null;    RandomAccessFile savedFile = null;    File file = null;    long downloadLength = 0;    String downloadUrl = params[0];    positionDownload = Integer.parseInt(params[1]);    String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));    String directory = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();    file = new File(directory + fileName);    if(file.exists()){      downloadLength = file.length();    }    long contentLength = getContentLength(downloadUrl);    if(contentLength == 0){      return TYPE_FAILURE;    } else if(contentLength == downloadLength){      return TYPE_SUCCESS;    }    OkHttpClient client = new OkHttpClient();    Request request = new Request.Builder()                  .addHeader("RANGE", "bytes="+downloadLength+"-")                  .url(downloadUrl)                  .build();    try {      Response response = client.newCall(request).execute();      if(response != null){        is = response.body().byteStream();        savedFile = new RandomAccessFile(file, "rw");        savedFile.seek(downloadLength);        byte[] buffer = new byte[1024];        int total = 0;        int length;        while((length = is.read(buffer)) != -1){          if(isCancelled){            response.body().close();            return TYPE_CANCEL;          } else if(isPaused) {            response.body().close();            return TYPE_PAUSE;          }          total += length;          savedFile.write(buffer, 0, length);          int progress = (int) ((total + downloadLength) * 100 / contentLength);          int currentDownload = (int) (total + downloadLength);          publishProgress(positionDownload, progress, currentDownload, (int) contentLength);        }        response.body().close();        return TYPE_SUCCESS;      }    } catch (IOException e) {      e.printStackTrace();    } finally {      try {        if(is != null) is.close();        if(savedFile != null) savedFile.close();        if(isCancelled && file != null) file.delete();      } catch (IOException e) {        e.printStackTrace();      }    }    return TYPE_FAILURE;  }  @Override  protected void onProgressUpdate(Integer... values) {    int progress = values[1];    if(progress > lastProgress){      downloadListener.onProgress(values[0], progress, values[2], values[3]);      lastProgress = progress;    }  }  @Override  protected void onPostExecute(Integer status) {    switch (status){      case TYPE_SUCCESS:        downloadListener.onSuccess(positionDownload);        break;      case TYPE_FAILURE:        downloadListener.onFailure();        break;      case TYPE_PAUSE:        downloadListener.onPause();        break;      case TYPE_CANCEL:        downloadListener.onCancel();        break;    }  }  public void pauseDownload(){    isPaused = true;  }  public void cancelDownload(){    isCancelled = true;  }  private long getContentLength(String downloadUrl) {    OkHttpClient client = new OkHttpClient();    Request request = new Request.Builder()                  .url(downloadUrl)                  .build();    Response response = null;    try {      response = client.newCall(request).execute();      if(response != null && response.isSuccessful()){        long contentLength = response.body().contentLength();        response.body().close();        return contentLength;      }    } catch (IOException e) {      e.printStackTrace();    }    return 0;  }}