Question

Just assume there is a program which takes inputs from the standard input.

For example:

cin>>id;

What I want to figure out is how to execute the process and give some input to its standard input. Getting the output of the process is not an issue for me. It works properly. The question is how to feed inputs for such processes using java.lang.Process class.

If there are any other third party libraries like Apache commons please mention them also.

Thanks in advance!

Answer 1

You need to start a separate thread which reads from the output of one process and writes it as input to the other process.

Something like this should do:

class DataForwarder extends Thread {

    OutputStream out;
    InputStream in;

    public DataForwarder(InputStream in, OutputStream out) {
        this.out = out;
        this.in = in;
    }

    @Override
    public void run() {
        byte[] buf = new byte[1024];
        System.out.println("Hej");
        try {
            int n;
            while (-1 != (n = in.read(buf)))
                out.write(buf, 0, n);
            out.close();
        } catch (IOException e) {
            // Handle in some suitable way.
        }
    }
}

Which would be used for prod >> cons as follows:

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

        Process prod = new ProcessBuilder("ls").start();
        Process cons = new ProcessBuilder("cat").start();

        // Start feeding cons with output from prod.
        new DataForwarder(prod.getInputStream(), cons.getOutputStream()).start();
    }
}