Question

I have the following code;

String[] cmd = { "bash", "-c", "~/path/to/script.sh" };
Process p = Runtime.getRuntime().exec(cmd);

PipeThread a = new PipeThread(p.getInputStream(), System.out);
PipeThread b = new PipeThread(p.getErrorStream(), System.err);

p.waitFor();

a.die();
b.die();

The PipeThread class is quite simple so I will include it in full;

public class PipeThread implements Runnable {

    private BufferedInputStream in;
    private BufferedOutputStream out;

    public Thread thread;

    private boolean die = false;

    public PipeThread(InputStream i, OutputStream o) {
        in = new BufferedInputStream(i);
        out = new BufferedOutputStream(o);
        thread = new Thread(this);
        thread.start();
    }

    public void die() { die = true; }

    public void run() {
        try {
            byte[] b = new byte[1024];
            while(!die) {
                int x = in.read(b, 0, 1024);
                if(x > 0) out.write(b, 0, x);
                else die();
                out.flush();
            }
        }
        catch(Exception e) { e.printStackTrace(); }

        try { 
            in.close();
            out.close();
        }
        catch(Exception e) { }
    }
}

My problem is this; p.waitFor() blocks endlessly, even after the subprocess has terminated. If I do not create the pair of PipeThread instances, then p.waitFor() works perfectly. What is it about the piping of io streams that is causing p.waitFor() to continue blocking?

I’m confused as I thought the IO streams would be passive, unable to keep a process alive, or to make Java think the process is still alive.

Answer 1

So after much head-scratching, I realised what is happening. p.waitFor() actually isn’t blocking indefinitely, rather my method of checking it is failing; a System.out.println() statement after p.waitFor().

PipeThread is a class that I had lying around from a previous project, and I have often used it to pipe one stream onto another in a separate thread.

This must be the first time I have used it with System.out. PipeThread closes whatever streams it is passed upon reading EOF. Both streams in this case included my standard output, so debugging with System.out.println becomes impossible… 🙁

Weirdly, no IOException was thrown by System.out.println(), I will investigate.