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!

Are you 100% positive that `java -jar test.jar 1 20" outputs something? Have you tried to also redirect the error stream? — assylias
– assylias, Commented Nov 12, 2015 at 9:05
@ManojKrishna good hint, but preferably write your comment that way: "you can follow this example in stackoverflow" — Binkan Salaryman
– Binkan Salaryman, Commented Nov 12, 2015 at 9:19
Is there a constraint to run a second JVM? Why not running the code in the current JVM using another class loader? — SubOptimal
– SubOptimal, Commented Nov 12, 2015 at 10:01

SubOptimal · Accepted Answer · 2015-11-12 12:19:10Z

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);
}

answered Nov 12, 2015 at 12:19

SubOptimal

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4 Comments

firstpostcommenter Over a year ago

But could you capture the output of that jar without using processBuilder?

SubOptimal Over a year ago

@firstpostcommenter You mean to redirect the output of test.jar to something else then System.out?

firstpostcommenter Over a year ago

yes, this code doesn't allow to consume the System.out where as processBuilder makes it possible (We could use a shell script to pipe the first jar output to second jar input if required but that is different solution)

SubOptimal Over a year ago

@firstpostcommenter It depends what are your requirements and what you want to achieve. Instead of starting another JVM from within the current JVM using a loggin framework would be a less memory consuming and probaly faster (related to the startup time for the additional JVM) solution.

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