Question

is it possible to run a process inside my code and then see both output streams (the process and mine) on the screen?

I need to see what the other process doing in parallel with my own code!

Answer 1

You need to start a thread which reads data from the Process output stream, and prints it on System.out.

You can use a class like this:

class ProcessOutputStreamPrinter extends Thread {

    BufferedReader reader;

    public ProcessOutputStreamPrinter(Process p) {
        reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
    }

    public void run() {
        try {
            String line;
            while (null != (line = reader.readLine()))
                System.out.println("Process output: " + line);
        } catch (IOException e) {
            // handle somehow
        }
    }
}

Here’s some test code for that class:

class Test {
    public static void main(String[] args) throws IOException {

        // Start external process. (Replace "cat" with whatever you want.)
        ProcessBuilder pb = new ProcessBuilder("cat");
        Process p = pb.start();

        // Start printing it's output to System.out.
        new ProcessOutputStreamPrinter(p).start();

        // Just for testing:

        // Print something ourselves:
        System.out.println("My program output: hello");

        // Give cat some input (which it will echo as output).
        PrintWriter pw = new PrintWriter(new PrintStream(p.getOutputStream()));
        pw.println("hello");
        pw.flush();

        // Close stdin to terminate "cat".
        pw.close();
    }
}

Output:

My program output: hello
Process output: hello