Starting a process with inherited stdin/stdout/stderr in Java 6

Question

If I start a process via Java's ProcessBuilder class, I have full access to that process's standard in, standard out, and standard error streams as Java InputStreams and OutputStreams. However, I can't find a way to seamlessly connect those streams to System.in, System.out, and System.err.

It's possible to use redirectErrorStream() to get a single InputStream that contains the subprocess's standard out and standard error, and just loop through that and send it through my standard out—but I can't find a way to do that and let the user type into the process, as he or she could if I used the C system() call.

This appears to be possible in Java SE 7 when it comes out—I'm just wondering if there's a workaround now. Bonus points if the result of isatty() in the child process carries through the redirection.

Answer 1

You will need to copy the Process out, err, and input streams to the System versions. The easiest way to do that is using the IOUtils class from the Commons IO package. The copy method looks to be what you need. The copy method invocations will need to be in separate threads.

Here is the basic code:

// Assume you already have a processBuilder all configured and ready to go
final Process process = processBuilder.start();
new Thread(new Runnable() {public void run() {
  IOUtils.copy(process.getOutputStream(), System.out);
} } ).start();
new Thread(new Runnable() {public void run() {
  IOUtils.copy(process.getErrorStream(), System.err);
} } ).start();
new Thread(new Runnable() {public void run() {
  IOUtils.copy(System.in, process.getInputStream());
} } ).start();

Answer 2

A variation on John's answer that compiles and doesn't require you to use Commons IO:

private static void pipeOutput(Process process) {
    pipe(process.getErrorStream(), System.err);
    pipe(process.getInputStream(), System.out);
}

private static void pipe(final InputStream src, final PrintStream dest) {
    new Thread(new Runnable() {
        public void run() {
            try {
                byte[] buffer = new byte[1024];
                for (int n = 0; n != -1; n = src.read(buffer)) {
                    dest.write(buffer, 0, n);
                }
            } catch (IOException e) { // just exit
            }
        }
    }).start();
}