使用Retrofit下载文件并实现进度监听的示例

public interface DownloadListener {
  void onStart();//下载开始

  void onProgress(int progress);//下载进度

  void onFinish(String path);//下载完成

  void onFail(String errorInfo);//下载失败
}

public interface DownloadService {

  @Streaming
  @GET
  Call download(@Url String url);
}

public class DownloadUtil {

  public static void download(String url, final String path, final DownloadListener downloadListener) {

    Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("http://www.xxx.com")
        //通过线程池获取一个线程，指定callback在子线程中运行。
        .callbackExecutor(Executors.newSingleThreadExecutor())
        .build();

    DownloadService service = retrofit.create(DownloadService.class);

    Call call = service.download(url);
    call.enqueue(new Callback() {
      @Override
      public void onResponse(@NonNull Call call, @NonNull final Response response) {
        //将Response写入到从磁盘中，详见下面分析
        //注意，这个方法是运行在子线程中的
        writeResponseToDisk(path, response, downloadListener);
      }

      @Override
      public void onFailure(@NonNull Call call, @NonNull Throwable throwable) {
        downloadListener.onFail("网络错误～");
      }
    });
  }
}

private static void writeResponseToDisk(String path, Response response, DownloadListener downloadListener) {
    //从response获取输入流以及总大小
    writeFileFromIS(new File(path), response.body().byteStream(), response.body().contentLength(), downloadListener);
  }

  private static int sBufferSize = 8192;

  //将输入流写入文件
  private static void writeFileFromIS(File file, InputStream is, long totalLength, DownloadListener downloadListener) {
    //开始下载
    downloadListener.onStart();

    //创建文件
    if (!file.exists()) {
      if (!file.getParentFile().exists())
        file.getParentFile().mkdir();
      try {
        file.createNewFile();
      } catch (IOException e) {
        e.printStackTrace();
        downloadListener.onFail("createNewFile IOException");
      }
    }

    OutputStream os = null;
    long currentLength = 0;
    try {
      os = new BufferedOutputStream(new FileOutputStream(file));
      byte data[] = new byte[sBufferSize];
      int len;
      while ((len = is.read(data, 0, sBufferSize)) != -1) {
        os.write(data, 0, len);
        currentLength += len;
        //计算当前下载进度
        downloadListener.onProgress((int) (100 * currentLength / totalLength));
      }
      //下载完成，并返回保存的文件路径
      downloadListener.onFinish(file.getAbsolutePath());
    } catch (IOException e) {
      e.printStackTrace();
      downloadListener.onFail("IOException");
    } finally {
      try {
        is.close();
      } catch (IOException e) {
        e.printStackTrace();
      }
      try {
        if (os != null) {
          os.close();
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }

String url = "";
      String path = "";
      DownloadUtil.download(url, path, new DownloadListener() {
        @Override
        public void onStart() {
          //运行在子线程
        }

        @Override
        public void onProgress(int progress) {
          //运行在子线程
        }

        @Override
        public void onFinish(String path) {
          //运行在子线程
        }

        @Override
        public void onFail(String errorInfo) {
          //运行在子线程
        }
      });