How to Efficiently Redirect ProcessBuilder Output Asynchronously in Java?

Asynchronous Output Redirection for ProcessBuilder Processes

When constructing a process with ProcessBuilder, it can be desirable to capture and redirect stdout and/or stderr to System.out asynchronously. Traditional approaches, such as manually spawning a thread to continuously read from stdOut, are cumbersome and inefficient.

Fortunately, Java 7 introduced the ProcessBuilder.inheritIO method, which conveniently sets the subprocess standard I/O to be the same as the current Java process. This eliminates the need for additional threads or complex output redirection logic.

For Java 7 and later, simply invoke:

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

For earlier versions of Java, a custom solution is required:

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();
}

In this custom solution, threads are automatically terminated when the subprocess finishes, as the src stream reaches EOF. This ensures proper resource management without the need for explicit thread handling.

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