Question

I am creating a GUI using Java. This GUI launches a program from the command line using the ProcessBuilder class.

A little information on the process being launched: from the command line, it creates another window and prints information to said window.

In my GUI window, I have a text area to where I would like to redirect said output. I originally intended to use a SwingWorker object to constantly check for more output and not hold up the GUI. To test and make sure I had the original syntax down (without even bringing the GUI into things) I thought I would print the output from the secondary process’ window to System.out. However, something seems to be wrong as I can see the output in the secondary process’ window, but not the terminal from which I am working.

Excerpt of code is as follows:

Process p = pb.start(); 
Scanner s = new Scanner(p.getInputStream());

SwingWorker pipe = new SwingWorker<String, Void> (){
    public String doInBackground(){
        while(run){
            if(s.hasNextLine()){
                System.out.println("S has next!");
                System.out.println(s.nextLine());
            }
        }
        return null;
    }
};
pipe.execute();

The boolean run is defined elsewhere in the program and is set to false when the process p exits or is force quit (additional question: is that a really bad idea? I feel like it might be…).

Does anyone have an idea as to why I am never getting any output when I see it being printed to the other window? Initially my reaction was to use p.getOutputStream() but Scanner does not take an outputStream as a paramter.

Thank you for your time.

Answer 1

You should also scan p.getErrorStream() – some programs write to STDERR which is indistinguishable from STDOUT when run from the command line. It is generally good practice to consume both streams, as if either one is not consumed it can cause the external process to hang.