1. 导入依赖包

// retrofit, 基于Okhttp，考虑到项目中经常会用到retrofit，就导入这个了。
    compile ‘com.squareup.retrofit2:retrofit:2.1.0‘
// ButterKnife
    compile ‘com.jakewharton:butterknife:7.0.1‘
//  rxjava 本例中线程切换要用到，代替handler
    compile ‘io.reactivex:rxjava:1.1.6‘
    compile ‘io.reactivex:rxandroid:1.2.1‘

2. 继承ResponseBody，生成带进度监听的ProgressResponseBody

// 参考okhttp的官方demo，此类当中我们主要把注意力放在ProgressListener和read方法中。在这里获取文件总长我写在了构造方法里，这样免得在source的read方法中重复调用或判断。读者也可以根据个人需要定制自己的监听器。
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;
            }
        };
    }
}

3.创建ProgressDownloader

//带进度监听功能的辅助类
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;
        //在下载、暂停后的继续下载中可复用同一个client对象
        client = getProgressClient();
    }
    //每次下载需要新建新的Call对象
    private Call newCall(long startPoints) {
        Request request = new Request.Builder()
                .url(url)
                .header("RANGE", "bytes=" + startPoints + "-")//断点续传要用到的，指示下载的区间
                .build();
        return client.newCall(request);
    }

    public OkHttpClient getProgressClient() {
    // 拦截器，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指定开始下载的点
    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;
        // 随机访问文件，可以指定断点续传的起始位置
        RandomAccessFile randomAccessFile = null;
        try {
            randomAccessFile = new RandomAccessFile(destination, "rwd");
            //Chanel NIO中的用法，由于RandomAccessFile没有使用缓存策略，直接使用会使得下载速度变慢，亲测缓存下载3.3秒的文件，用普通的RandomAccessFile需要20多秒。
            channelOut = randomAccessFile.getChannel();
            // 内存映射，直接使用RandomAccessFile，是用其seek方法指定下载的起始位置，使用缓存下载，在这里指定下载位置。
            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();
            }
        }
    }
}

4. 测试demo

清单文件中添加网络权限和文件访问权限

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

MainActivity

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:
            // 新下载前清空断点信息
                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, "下载暂停", Toast.LENGTH_SHORT).show();
                // 存储此时的totalBytes，即断点位置。
                breakPoints = totalBytes;
                break;
            case R.id.continue_button:
                downloader.download(breakPoints);
                break;
        }
    }

    @Override
    public void onPreExecute(long contentLength) {
        // 文件总长只需记录一次，要注意断点续传后的contentLength只是剩余部分的长度
        if (this.contentLength == 0L) {
            this.contentLength = contentLength;
            progressBar.setMax((int) (contentLength / 1024));
        }
    }

    @Override
    public void update(long totalBytes, boolean done) {
        // 注意加上断点的长度
        this.totalBytes = totalBytes + breakPoints;
        progressBar.setProgress((int) (totalBytes + breakPoints) / 1024);
        if (done) {
        // 切换到主线程
            Observable
                    .empty()
                    .observeOn(AndroidSchedulers.mainThread())
                    .doOnCompleted(new Action0() {
                        @Override
                        public void call() {
                            Toast.makeText(MainActivity.this, "下载完成", Toast.LENGTH_SHORT).show();
                        }
                    })
                    .subscribe();
        }
    }
}

最后是动态效果图