Run external program from Java, read output, allow interruption

Question

I want to launch a process from Java, read its output, and get its return code. But while it\'s executing, I want to be able to cancel it. I start out by launching the proce

Answer 1

A helper class like this would do the trick:

public class ProcessWatcher implements Runnable {

    private Process p;
    private volatile boolean finished = false;

    public ProcessWatcher(Process p) {
        this.p = p;
        new Thread(this).start();
    }

    public boolean isFinished() {
        return finished;
    }

    public void run() {
        try {
            p.waitFor();
        } catch (Exception e) {}
        finished = true;
    }

}

You would then implement your loop exactly as you describe:

Process p = Runtime.getRuntime().exec("whatever command");
ProcessWatcher pw = new ProcessWatcher(p);
InputStream output = p.getInputStream();
while(!pw.isFinished()) {
    processOutput(output);
    if(shouldCancel()) p.destroy();
    Thread.sleep(500);
}

Depending upon what conditions would make you want to destroy the process, you might want to do that in a separate thread. Otherwise, you may block while waiting for more program output to process and never really get the option to destroy it.

EDIT: McDowell is 100% right in his comment below, so I've made the finished variable volatile.