Java Runtime exec() get stuck after a while

Question

I'm building a simple UI for ffmpeg launching ffmpeg.exe with parameters using exec(). it works on Os X but on Windows 7-8 after few seconds the ffmpeg process suspends itself and resumes only when I kill the father process. (also ddlhost.exe is created) Ffmpeg.exe converts successfully the same video from cmd.

Searching on the internet I've found this answer but I have the same problem running a simple test program which is not using the Input and Error streams.

Here is the test program code which has the same problem of the main one:

public class Main {

static String param_ffmpeg_1 = "./data/ffmpeg.exe";
static String param_ffmpeg_2 = "-i";

static String in = "./data/source.mov";
static String out = "./data/out.flv";

static Process p;

public static void main(String[] args) {

    /*File f = new File(out);

    if (f.exists()){
        f.delete();
    }*/


    Runtime rt = Runtime.getRuntime() ;
    //String cmd1 =  param_ffmpeg_1 + param_ffmpeg_2 +  in_path +  param_ffmpeg_3 + out_path ;
    System.out.println(in);
    System.out.println(out);
    String[] cmd1 = new String[] { param_ffmpeg_1, param_ffmpeg_2, in, "-ar", "44100", "-vb", "2500k", "-s", "882x496", "-f", "flv", out};

    try {
        p = rt.exec(cmd1);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    int r = 123456;
    try {
        r = p.waitFor();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    System.out.println(r);


}

}

score 4 · Accepted Answer · answered Dec 20 '13 at 15:58

Does ffmpg write anything to the stdout or stderr? If yes you have to consume that. (In seperate threads as you need to consume the stderr and the stdout in parallel) see Process.getInputStream() and Process.getErrorStream() for the details. When the buffer is buffer is full your called programm is stopped and hangs.

The fact that it works in OS/X but not Windows might be caused by different buffer sizes.

score 3 · Answer 2 · edited May 23 '17 at 12:33

You should call getInputStream() and getErrorStream() on the Process returned by Runtime.exec and drain that all the time the process is running. If you do not then it will eventually fill up and block and stop the process from running.

See Java Process with Input/Output Stream for examples.

From the accepted answer

ProcessBuilder builder = new ProcessBuilder("/bin/bash");
builder.redirectErrorStream(true);
Process process = builder.start();
InputStream itsOutput = process.getInputStream();
// Wrap the stream in a Reader ...
while ((line = reader.readLine ()) != null) {
    System.out.println ("Stdout: " + line);
}

Java Runtime exec() get stuck after a while

2 Answers2