//创建下载任务,downloadUrl就是下载链接
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(downloadUrl));
//指定下载路径和下载文件名
request.setDestinationInExternalPublicDir("/download/", fileName);
//获取下载管理器
DownloadManager downloadManager= (DownloadManager) mContext.getSystemService(Context.DOWNLOAD_SERVICE);
//将下载任务加入下载队列，否则不会进行下载
downloadManager.enqueue(request); 

 //使用系统下载器下载
private void downloadAPK(String versionUrl, String versionName) {
    //创建下载任务
    DownloadManager.Request request = new DownloadManager.Request(Uri.parse(versionUrl));
    request.setAllowedOverRoaming(false);//漫游网络是否可以下载

    //设置文件类型，可以在下载结束后自动打开该文件
    MimeTypeMap mimeTypeMap = MimeTypeMap.getSingleton();
    String mimeString = mimeTypeMap.getMimeTypeFromExtension(MimeTypeMap.getFileExtensionFromUrl(versionUrl));
    request.setMimeType(mimeString);

    //在通知栏中显示，默认就是显示的
    request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE);
    request.setVisibleInDownloadsUi(true);

    //sdcard的目录下的download文件夹，必须设置
    request.setDestinationInExternalPublicDir("/download/", versionName);
    //request.setDestinationInExternalFilesDir(),也可以自己制定下载路径

    //将下载请求加入下载队列
    downloadManager = (DownloadManager) mContext.getSystemService(Context.DOWNLOAD_SERVICE);
    //加入下载队列后会给该任务返回一个long型的id，
    //通过该id可以取消任务，重启任务等等，看上面源码中框起来的方法
    mTaskId = downloadManager.enqueue(request);

    //注册广播接收者，监听下载状态
    mContext.registerReceiver(receiver,
        new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
  } 

   //广播接受者，接收下载状态
  private BroadcastReceiver receiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
      checkDownloadStatus();//检查下载状态
    }
  }; 

 //检查下载状态
  private void checkDownloadStatus() {
    DownloadManager.Query query = new DownloadManager.Query();
    query.setFilterById(mTaskId);//筛选下载任务，传入任务ID，可变参数
    Cursor c = downloadManager.query(query);
    if (c.moveToFirst()) {
      int status = c.getInt(c.getColumnIndex(DownloadManager.COLUMN_STATUS));
      switch (status) {
        case DownloadManager.STATUS_PAUSED:
          MLog.i(">>>下载暂停");
        case DownloadManager.STATUS_PENDING:
          MLog.i(">>>下载延迟");
        case DownloadManager.STATUS_RUNNING:
          MLog.i(">>>正在下载");
          break;
        case DownloadManager.STATUS_SUCCESSFUL
          MLog.i(">>>下载完成");
          //下载完成安装APK
          //downloadPath = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getAbsolutePath() + File.separator + versionName;
          installAPK(new File(downloadPath));
          break;
        case DownloadManager.STATUS_FAILED:
          MLog.i(">>>下载失败");
          break;
      }
    }
  } 

   //下载到本地后执行安装
  protected void installAPK(File file) {
    if (!file.exists()) return;
    Intent intent = new Intent(Intent.ACTION_VIEW);
    Uri uri = Uri.parse("file://" + file.toString());
    intent.setDataAndType(uri, "application/vnd.android.package-archive");
    //在服务中开启activity必须设置flag,后面解释
    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    mContext.startActivity(intent);
  }