Running a Jar with Java ProcessBuilder gives me null output

Question

I have been trying to get the following code to work.

package com.compressor;

 import java.io.BufferedReader;
 import java.io.File;
 import java.io.InputStreamReader;

 public class JSCompressor {

 public static void main(String[] args) {

    try {

        String currentDir = System.getProperty("user.dir");

        String[] commands = { "java", "-jar","yuicompressor.jar"};

        ProcessBuilder pb = new ProcessBuilder(commands);
        pb.directory(new File(currentDir));

        Process p = pb.start();

        BufferedReader output = new BufferedReader(new InputStreamReader(p.getInputStream()));
        System.out.println("Result : " + output.readLine());                            


    } catch (Exception ex) {
        ex.printStackTrace();
    }

}

 }

My project directory looks like what is shown in the image below :

But still when I run the program inside eclipse it gives me null output like given below :

Result : null

I have tried googling several options without any success. Could anyone please point out what I am doing wrong here ?.

I the jar I am testing is indeed runnable and gives an output when run normally in the command line. But I need to be able to run this jar programetically. Could anyone please help ?.

Answer 1

I think you want to change

String[] commands = { "java", "-jar","yuicompressor.jar"};

to

String[] commands = { "java", "-jar", jarPath};

Since that's the path to your yuicompressor.jar. Also, you should use another thread to read the process output - and wait for the process to complete.

final Process p = pb.start();
// then start a thread to read the output.
new Thread(new Runnable() {
  public void run() {
    BufferedReader output = new BufferedReader(
        new InputStreamReader(p.getInputStream()));
    String line;
    System.out.print("Result : ");
    while ((line = output.readLine()) != null) {
     System.out.println(line);
    }
  }
}).start();
p.waitFor();