具体内容如下

DownloadSubscribe（被观察者）中执行下载存入本地操作

核心还是：addHeader("RANGE", "bytes=" + downloadLength + "-" + contentLength)

DownLoadObserver（观察者）通过onnext(DownloadInfo downloadInfo)方法回调下载进度

下面上主要代码：

/** * 开始下载 * @param url 下载请求的网址 * @param downFileCallback 用来回调的接口 */ public void download(final String url, final DownFileCallback downFileCallback) { if (url == null || downCalls.get(url) != null) { return; } Observable.just(url) .filter(new Predicate() {  @Override  public boolean test(String s) throws Exception {  //过滤条件 若map中存在，则这次不下载  return !downCalls.containsKey(s);  } }) .flatMap(new Function>() {  @Override  public ObservableSource apply(String s) throws Exception {  //创建下载实体类  return Observable.just(createDownInfo(s));  } }) .map(new Function() {  @Override  public DownloadInfo apply(DownloadInfo s) throws Exception {  //根据本地是否存在此文件，来设置文件名及文件初始下载大小  return getRealFileName(s);  } }) .flatMap(new Function>() {  @Override  public ObservableSource apply(DownloadInfo downloadInfo) throws Exception {  //创建被观察者  return Observable.create(new DownloadSubscribe(downloadInfo));  } })//下载 .observeOn(AndroidSchedulers.mainThread())//在主线程回调 .subscribeOn(Schedulers.io())//在子线程执行 .subscribe(new DownLoadObserver() {//添加观察者  @Override  public void onNext(DownloadInfo downloadInfo) {  super.onNext(downloadInfo);  downFileCallback.onProgress(downloadInfo.getTotal(), downloadInfo.getProgress());  }   @Override  public void onError(Throwable e) {  super.onError(e);  if (!(e instanceof SocketException)) {  downFileCallback.onFail(e.getMessage());  }   }   @Override  public void onComplete() {  downFileCallback.onSuccess(url);  }  }); }

/** * 根据url暂停下载操作 * @param url */ public void cancel(String url) { Call call = downCalls.get(url); if (call != null) { call.cancel();//取消 } downCalls.remove(url); }

/** * 创建被观察者DownloadSubscribe */ private class DownloadSubscribe implements ObservableOnSubscribe { private DownloadInfo downloadInfo;  public DownloadSubscribe(DownloadInfo downloadInfo) { this.downloadInfo = downloadInfo; }  @Override public void subscribe(ObservableEmitter e) throws Exception { String url = downloadInfo.getUrl(); long downloadLength = downloadInfo.getProgress();//已经下载好的长度 long contentLength = downloadInfo.getTotal();//文件的总长度 //初始进度信息 e.onNext(downloadInfo);  Request request = new Request.Builder()  //断点续传的核心  .addHeader("RANGE", "bytes=" + downloadLength + "-" + contentLength)  .url(url)  .build(); Call call = mClient.newCall(request); //根据下载url，把call存放在map中，取消的时候就可以通过call.cancle()来实现 downCalls.put(url, call); Response response = call.execute();  File file = new File(getTemporaryPath(), downloadInfo.getFileName()); InputStream is = null; FileOutputStream fileOutputStream = null; try { is = response.body().byteStream(); fileOutputStream = new FileOutputStream(file, true); byte[] buffer = new byte[2048];//缓冲数组2kB int len; while ((len = is.read(buffer)) != -1) {  fileOutputStream.write(buffer, 0, len);  downloadLength += len;  downloadInfo.setProgress(downloadLength);  e.onNext(downloadInfo); } fileOutputStream.flush(); downCalls.remove(url); } finally { //关闭IO流 IOUtil.closeAll(is, fileOutputStream);  } e.onComplete();//完成 } }

/** * 从服务器获取文件长度 * * @param downloadUrl * @return */ private long getContentLength(String downloadUrl) { Request request = new Request.Builder() .url(downloadUrl) .build(); try { Response response = mClient.newCall(request).execute(); if (response != null && response.isSuccessful()) { long contentLength = response.body().contentLength(); response.close(); return contentLength == 0 ? DownloadInfo.TOTAL_ERROR : contentLength; } } catch (IOException e) { e.printStackTrace(); } return DownloadInfo.TOTAL_ERROR; }

从服务器获取文件长度的时候注意一下，Android P之后，也就是api 28以上禁止明文网络传输。需要在你的AndroidManifest中的application标签中声明"android:usesCleartextTraffic="true",允许应用进行明文传输。

使用方法：首先要获取sd卡权限

DownloadManager.getInstance().downloadPath(本地存在地址).download(url1, new DownFileCallback() {  @Override  public void onProgress(long totalSize, long downSize) {  progress1.setMax((int) totalSize);  progress1.setProgress((int) downSize);  }   @Override  public void onSuccess(String url) {  Toast.makeText(MainActivity.this, url1 + "下载完成", Toast.LENGTH_SHORT).show();  }   @Override  public void onFail(String msg) {  Toast.makeText(MainActivity.this, url1 + "下载失败", Toast.LENGTH_SHORT).show();  } });