如何使用FFMPEG转码文件并在Java servlet的响应中流式传输输出文件？

Question

Basically, this is what I'm trying to do: 1. User passes a URL as a GET parameter to my servlet. 2. Servlet uses a ProcessBuilder to convert the media contained in that URL to a valid media format (ie: MP3). 3. The servlet streams the output file being transcoded by FFMPEG back to the browser.

1 and 2 work fine, but it is 3 I am having a problem with. The best I can do is create a FileInputStream to the output file being transcoded and send that as the response but it is not working. My guess is that it is because the file is being written as I'm trying to stream it.

Is there anyway to intercept the output file argument in FFMPEG and read it into an InputStream? In my mind it does not seem that it should be difficult to take input file A, transcode it to output file B, and then stream output file B back to the client, on the fly.

ProcessBuilder pb = new ProcessBuilder("ffmpeg.exe", "-i", url, "file.mp3");
    Process p = pb.start();

    final InputStream inStream = p.getErrorStream();

    new Thread(new Runnable() {

    public void run() {
            InputStreamReader reader = new InputStreamReader(inStream);
            Scanner scan = new Scanner(reader);
            while (scan.hasNextLine()) {
                System.out.println(scan.nextLine());
            }
        }
    }).start();

    ServletOutputStream stream = null;
    BufferedInputStream buf = null;
    try {

        stream = response.getOutputStream();
        File mp3 = new File(file.mp3");

        //set response headers
        response.setContentType("audio/mpeg");

        response.addHeader("Content-Disposition", "attachment; filename=file.mp3");

        response.setContentLength(-1);

        //response.setContentLength((int) mp3.length());

        FileInputStream input = new FileInputStream(mp3);
        buf = new BufferedInputStream(input);
        int readBytes = 0;
        //read from the file; write to the ServletOutputStream
        while ((readBytes = buf.read()) != -1) {
            stream.write(readBytes);
        }
    } catch (IOException ioe) {
        throw new ServletException(ioe.getMessage());
    } finally {
        if (stream != null) {
            stream.close();
        }
        if (buf != null) {
            buf.close();
        }
    }

Answer 1

FFmpeg doesn't have to write to a file. It can write to stdout:

ffmpeg -i $input -f mp3 -

- means standard output. Since there is no file name, you need to specify the format with -f .

If you invoke it like this, you can read mp3 stream directly from Process 's InputStream .

Answer 2

I figured it out.

The trick is not to create a file on the file system and try to stream that back to the client but to pipe the ffmpeg output to an InputStream in Java and f-in voila!