How to Capture and Redirect Process Output Asynchronously in Java?

Capturing and Redirecting Process Output Asynchronously

When launching processes using ProcessBuilder, sometimes the need arises to forward their stdout and stderr output without blocking the main thread. By default, reading from process streams blocks the caller, making it unsuitable for tasks that require concurrency.

One approach to achieve asynchronous output redirection is to manually create a thread that continuously monitors the stdOut stream and writes its contents to System.out. However, this approach can be cumbersome and requires additional thread management.

To streamline this process, ProcessBuilder offers the inheritIO method, which seamlessly sets the standard I/O streams of the subprocess to be the same as those of the calling Java process. This ensures that any output generated by the subprocess is directly sent to the console without the need for manual thread creation:

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

Alternatively, for older Java versions that do not support inheritIO, a custom implementation using threads can be employed:

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 implementation creates threads that monitor the subprocess's input and error streams, forwarding their data to System.out and System.err, respectively. The threads automatically terminate when the subprocess completes, as the input streams will reach their end.

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