// retrofit, 基于Okhttp，考慮到項目中經常會用到retrofit，就導入這個了。
  compile 'com.squareup.retrofit2:retrofit:2.1.0'
// ButterKnife
  compile 'com.jakewharton:butterknife:7.0.1'
// rxjava 本例中線程切換要用到，代替handler
  compile 'io.reactivex:rxjava:1.1.6'
  compile 'io.reactivex:rxandroid:1.2.1'

// 參考okhttp的官方demo，此類當中我們主要把注意力放在ProgressListener和read方法中。在這里獲取文件總長我寫在了構造方法里，這樣免得在source的read方法中重復調用或判斷。讀者也可以根據個人需要定制自己的監聽器。
public class ProgressResponseBody extends ResponseBody {

  public interface ProgressListener {
    void onPreExecute(long contentLength);
    void update(long totalBytes, boolean done);
  }

  private final ResponseBody responseBody;
  private final ProgressListener progressListener;
  private BufferedSource bufferedSource;

  public ProgressResponseBody(ResponseBody responseBody,
                ProgressListener progressListener) {
    this.responseBody = responseBody;
    this.progressListener = progressListener;
    if(progressListener!=null){
      progressListener.onPreExecute(contentLength());
    }
  }

  @Override
  public MediaType contentType() {
    return responseBody.contentType();
  }

  @Override
  public long contentLength() {
    return responseBody.contentLength();
  }

  @Override
  public BufferedSource source() {
    if (bufferedSource == null) {
      bufferedSource = Okio.buffer(source(responseBody.source()));
    }
    return bufferedSource;
  }

  private Source source(Source source) {
    return new ForwardingSource(source) {
      long totalBytes = 0L;
      @Override
      public long read(Buffer sink, long byteCount) throws IOException {
        long bytesRead = super.read(sink, byteCount);
        // read() returns the number of bytes read, or -1 if this source is exhausted.
        totalBytes += bytesRead != -1 ? bytesRead : 0;
        if (null != progressListener) {
          progressListener.update(totalBytes, bytesRead == -1);
        }
        return bytesRead;
      }
    };
  }
}

//帶進度監聽功能的輔助類
public class ProgressDownloader {

  public static final String TAG = "ProgressDownloader";

  private ProgressListener progressListener;
  private String url;
  private OkHttpClient client;
  private File destination;
  private Call call;

  public ProgressDownloader(String url, File destination, ProgressListener progressListener) {
    this.url = url;
    this.destination = destination;
    this.progressListener = progressListener;
    //在下載、暫停后的繼續下載中可復用同一個client對象
    client = getProgressClient();
  }
  //每次下載需要新建新的Call對象
  private Call newCall(long startPoints) {
    Request request = new Request.Builder()
        .url(url)
        .header("RANGE", "bytes=" + startPoints + "-")//斷點續傳要用到的，指示下載的區間
        .build();
    return client.newCall(request);
  }

  public OkHttpClient getProgressClient() {
  // 攔截器，用上ProgressResponseBody
    Interceptor interceptor = new Interceptor() {
      @Override
      public Response intercept(Chain chain) throws IOException {
        Response originalResponse = chain.proceed(chain.request());
        return originalResponse.newBuilder()
            .body(new ProgressResponseBody(originalResponse.body(), progressListener))
            .build();
      }
    };

    return new OkHttpClient.Builder()
        .addNetworkInterceptor(interceptor)
        .build();
  }

// startsPoint指定開始下載的點
  public void download(final long startsPoint) {
    call = newCall(startsPoint);
    call.enqueue(new Callback() {
          @Override
          public void onFailure(Call call, IOException e) {

          }

          @Override
          public void onResponse(Call call, Response response) throws IOException {
            save(response, startsPoint);
          }
        });
  }

  public void pause() {
    if(call!=null){
      call.cancel();
    }
  }

  private void save(Response response, long startsPoint) {
    ResponseBody body = response.body();
    InputStream in = body.byteStream();
    FileChannel channelOut = null;
    // 隨機訪問文件，可以指定斷點續傳的起始位置
    RandomAccessFile randomAccessFile = null;
    try {
      randomAccessFile = new RandomAccessFile(destination, "rwd");
      //Chanel NIO中的用法，由于RandomAccessFile沒有使用緩存策略，直接使用會使得下載速度變慢，親測緩存下載3.3秒的文件，用普通的RandomAccessFile需要20多秒。
      channelOut = randomAccessFile.getChannel();
      // 內存映射，直接使用RandomAccessFile，是用其seek方法指定下載的起始位置，使用緩存下載，在這里指定下載位置。
      MappedByteBuffer mappedBuffer = channelOut.map(FileChannel.MapMode.READ_WRITE, startsPoint, body.contentLength());
      byte[] buffer = new byte[1024];
      int len;
      while ((len = in.read(buffer)) != -1) {
        mappedBuffer.put(buffer, 0, len);
      }
    } catch (IOException e) {
      e.printStackTrace();
    }finally {
      try {
        in.close();
        if (channelOut != null) {
          channelOut.close();
        }
        if (randomAccessFile != null) {
          randomAccessFile.close();
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

public class MainActivity extends AppCompatActivity implements ProgressResponseBody.ProgressListener {

  public static final String TAG = "MainActivity";
  public static final String PACKAGE_URL = "http://gdown.baidu.com/data/wisegame/df65a597122796a4/weixin_821.apk";
  @Bind(R.id.progressBar)
  ProgressBar progressBar;
  private long breakPoints;
  private ProgressDownloader downloader;
  private File file;
  private long totalBytes;
  private long contentLength;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ButterKnife.bind(this);
  }

  @OnClick({R.id.downloadButton, R.id.cancel_button, R.id.continue_button})
  public void onClick(View view) {
    switch (view.getId()) {
      case R.id.downloadButton:
      // 新下載前清空斷點信息
        breakPoints = 0L;
        file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS), "sample.apk");
        downloader = new ProgressDownloader(PACKAGE_URL, file, this);
        downloader.download(0L);
        break;
      case R.id.pause_button:
        downloader.pause();
        Toast.makeText(this, "下載暫停", Toast.LENGTH_SHORT).show();
        // 存儲此時的totalBytes，即斷點位置。
        breakPoints = totalBytes;
        break;
      case R.id.continue_button:
        downloader.download(breakPoints);
        break;
    }
  }

  @Override
  public void onPreExecute(long contentLength) {
    // 文件總長只需記錄一次，要注意斷點續傳后的contentLength只是剩余部分的長度
    if (this.contentLength == 0L) {
      this.contentLength = contentLength;
      progressBar.setMax((int) (contentLength / 1024));
    }
  }

  @Override
  public void update(long totalBytes, boolean done) {
    // 注意加上斷點的長度
    this.totalBytes = totalBytes + breakPoints;
    progressBar.setProgress((int) (totalBytes + breakPoints) / 1024);
    if (done) {
    // 切換到主線程
      Observable
          .empty()
          .observeOn(AndroidSchedulers.mainThread())
          .doOnCompleted(new Action0() {
            @Override
            public void call() {
              Toast.makeText(MainActivity.this, "下載完成", Toast.LENGTH_SHORT).show();
            }
          })
          .subscribe();
    }
  }
}