android - URLConnection类的获取内容长度方法（getContentLength()）返回负1（-1）

Question

下面是我AsyncTask从CloudBoost数据库中下载 PDF 文件。代码工作并下载文件；但是，进度条更新不起作用，因为返回的文件长度为-1. 有人可以给我一个提示，告诉我如何处理这个问题。

顺便说loading.setProgress(progress[0]);一下，进度更新方法中的行正在AsyncTask该类嵌套的类的顶部初始化。

class DownloadPdfFromInternet extends AsyncTask<String, Integer, String> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected String doInBackground(String... strings) {
        int count;
        try {
            URL url = new URL(strings[0]);
            URLConnection connection = url.openConnection();
            connection.connect();
            // get length of file
            int lengthOfFile = connection.getContentLength();
            Log.d("dozer74", "LengthOf File: " + lengthOfFile);

            InputStream input = new BufferedInputStream(url.openStream(), 10 * 1024);
            OutputStream output = new FileOutputStream(Environment.getExternalStorageDirectory().getPath() + "/" + pubName);
            byte data[] = new byte[1024];
            long total = 0;
            while ((count = input.read(data)) != -1) {
                total += count;
                publishProgress((int) (total * 100 / lengthOfFile));
                // write data to file
                output.write(data, 0, count);
            }
            output.flush();
            output.close();
            input.close();
        } catch (Exception e) {
            Log.e("Error: ", e.getMessage());
        }
        return null;
    }

    protected void onProgressUpdate(Integer... progress) {
        loading.setProgress(progress[0]);
    }

    @Override
    protected void onPostExecute(String url) {
        loading.dismiss();
        openPdfFile();
    }

}