How to Redirect Subprocess Output Asynchronously in Java Without Blocking the Main Thread?

Redirecting Process Output Without Blocking the Main Thread

Question

In a Java application, how can one capture the stdout and stderr output of a subprocess launched using ProcessBuilder and redirect it to System.out asynchronously, allowing the subprocess and output redirection to run in the background?

The current approach involves manually creating a new thread to continuously read from stdout and write to System.out, but it feels cumbersome and requires additional thread management. Is there a more elegant solution?

Answer

By leveraging ProcessBuilder.inheritIO, you can set the subprocess's standard I/O to mirror that of the current Java process. This redirects the subprocess's output directly to System.out without requiring a separate thread.

Process p = new ProcessBuilder().inheritIO().command("command1").start();

If using Java 7 is not feasible, another option is to manually redirect using threads:

public static void main(String[] args) throws Exception {
    Process p = Runtime.getRuntime().exec("cmd /c dir");
    inheritIO(p.getInputStream(), System.out);
    inheritIO(p.getErrorStream(), System.err);
}

private static void inheritIO(final InputStream src, final PrintStream dest) {
    new Thread(new Runnable() {
        public void run() {
            Scanner sc = new Scanner(src);
            while (sc.hasNextLine()) {
                dest.println(sc.nextLine());
            }
        }
    }).start();
}

This approach utilizes threads that automatically terminate when the subprocess completes, as the input/error streams reach their end.

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