Java ProcessBuilder() runs a program, but the program doesn't return any output

Question

I run the program like this:

    Process process;
    try {
        process = new ProcessBuilder("java", "-jar", "test.jar", "1", "20").start();
        BufferedReader in = new BufferedReader(new InputStreamReader(process.getInputStream()));
        String line;
        while ((line = in.readLine()) != null) {
          System.out.println(line);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

The program I call uses standard output System.out.println("Hello!"); However, the the calling program gets nothing back. Am I using ProcessBuilder() wrong? Thanks!

Answer 1

If there is no constraint to start another JVM (for example: usage of System.exit() in test.jar) you could load and run the test.jar inside the current JVM.

Following snippet shows the principle.

File file = new File("/tmp/test.jar");
URLClassLoader loader = new URLClassLoader(
        new URL[]{file.toURI().toURL()}
);

String className = new JarFile(file)
        .getManifest()
        .getMainAttributes()
        .getValue(Attributes.Name.MAIN_CLASS);

Method main = loader
        .loadClass(className)
        .getDeclaredMethod("main", String[].class);

Object arg = new String[]{"1", "20"};

try {
    main.invoke(null, arg);
} catch (Exception e) {
    // do appropriate exception handling here
    e.printStackTrace(System.err);
}