如果未下载完文件，如何检查活动，请重新下载文件

Question

我的问题是：下载开始时，文件“ music.mp3”将出现在目录中，但它不完整，我想检查文件是否不完整，然后重新下载 这是我的代码：

字符串根= android.os.Environment.getExternalStorageDirectory（）+“ /folderName/music.mp3”;             文件wav =新文件（根）；             长长度= wav.length（）;

        if (!wav.isFile()) {

            // instantiate it within the onCreate method
            mProgressDialog = new ProgressDialog(TV_LG.this); // Downloading = activity name
            mProgressDialog.setMessage("downloading... . .");
            mProgressDialog.setIndeterminate(true);
            mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            mProgressDialog.setCancelable(true);

            // execute this when the downloader must be fired
            final DownloadTask downloadTask = new DownloadTask(TV_LG.this); // Downloading = activity name

downloadTask.execute（“ http://dl.music.ml/1398/03/16/music.mp3”）; //您要下载的文件的网址

            mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
                @Override
                public void onCancel(DialogInterface dialog) {
                    downloadTask.cancel(true);
                }
            });
            return;

        }
    }

私有类DownloadTask扩展了AsyncTask {

    private Context context;

    public DownloadTask(Context context) {
        this.context = context;
    }

    @Override
    protected String doInBackground(String... sUrl) {
        // take CPU lock to prevent CPU from going off if the user
        // presses the power button during download
        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
                getClass().getName());
        wl.acquire();

        try {
            InputStream input = null;
            OutputStream output = null;
            HttpURLConnection connection = null;
            try {
                URL url = new URL(sUrl[0]);
                connection = (HttpURLConnection) url.openConnection();
                connection.connect();

                // expect HTTP 200 OK, so we don't mistakenly save error report
                // instead of the file
                if (connection.getResponseCode() != HttpURLConnection.HTTP_OK)
                    return "Server returned HTTP " + connection.getResponseCode()
                            + " " + connection.getResponseMessage();

                // this will be useful to display download percentage
                // might be -1: server did not report the length
                int fileLength = connection.getContentLength();

                // download the file
                input = connection.getInputStream();

                File root = android.os.Environment.getExternalStorageDirectory();
                File dir = new File(root.getAbsolutePath() + "/folderName");
                if (!dir.exists()) {
                    dir.mkdirs(); // build directory
                }
                output = new FileOutputStream(root.getAbsolutePath() + "/folderName/music.mp3");

                byte data[] = new byte[4096];
                long total = 0;
                int count;
                while ((count = input.read(data)) != -1) {
                    // allow canceling with back button
                    if (isCancelled())
                        return null;
                    total += count;
                    // publishing the progress....
                    if (fileLength > 0) // only if total length is known
                        publishProgress((int) (total * 100 / fileLength)); 

                    output.write(data, 0, count);
                }
            } catch (Exception e) {
                return e.toString();
            } finally {
                try {
                    if (output != null)
                        output.close();
                    if (input != null)
                        input.close();
                } catch (IOException ignored) {
                }

                if (connection != null)
                    connection.disconnect();
            }
        } finally {
            wl.release();
        }
        return null;
    }