Running a java program from another java program

Question

I am working on a simple java program. It simply compiles and executes another java program. I am using Runtime.exec() function to compile and run. There is no problem with compilation. but when it runs, if the second program needs an input to read from keyboard, I can't give it from the master process. I used getOutputStream() function. but it couldn't help. I will provide my code.

public class sam {  
    public static void main(String[] args) throws Exception {  
        try { 
             Process p = Runtime.getRuntime().exec("javac sam2.java");
             Process p2 = Runtime.getRuntime().exec("java sam2");
             BufferedReader in = new BufferedReader(  
                                new InputStreamReader(p2.getInputStream()));

             OutputStream out = p.getOutputStream();
             String line = null; 
             line = in.readLine();
             System.out.println(line);
             input=input+"\n";
             out.write(input.getBytes());
             p.wait(10000);
             out.flush();
        }catch (IOException e) {  
             e.printStackTrace();  
        }  
    }  
}

This is my master program(sam.java).

The following is the code of sam2.java

public class sam2 {  
public static void main(String[] args) throws Exception {  

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String str; 
    System.out.println("Enter the number..\n");
    str = br.readLine(); 
    System.out.println(Integer.parseInt(str));

    }  
}

There is no problem, if my second program has only printing statements. But the problem arises when I have to read something from the other.

you would have to do some kind of pipe from the STDIN of sam to sam2 but that may be more trouble that its worth. — Alex Gittemeier
– Alex Gittemeier, Commented Mar 5, 2013 at 8:22
Please learn java naming conventions and stick to them, here: class names should start with an uppercase letter. — kleopatra
– kleopatra, Commented Mar 5, 2013 at 10:20
i was trying to present a simple example. thanks for your advice.. :) — AKA
– AKA, Commented Mar 5, 2013 at 12:30

András Tóth · Accepted Answer · 2013-03-05 08:30:09Z

22

It is a bit strange but you can run the second program without forking it. Just calling the main method in it. So forget the runtime section and do this:

sam2.main(new String[0]);

Of course this way you must compile sam2 at compile time

answered Mar 5, 2013 at 8:30

András Tóth

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4 Comments

AKA Over a year ago

i couldnt understand what u mean by this statement..!! :(

AKA Over a year ago

It works.. :) thanks a lot.. But when i use this way.. the entire control will be gone to the second program. so master will execute only after the completion of the second program.. should i use threads to solve this problem..??

AKA Over a year ago

Problem..!!! when i store the class name of second program to a string variable it encounters the following error.. cannot find symbol tempfile.main(new String[0]); symbol: method main(String[]) location: variable tempfile of type String What should i do..??

András Tóth Over a year ago

What do you want? You can not use a String as a class instance. Maybe you should use java reflection to this work. But reflection is usually not needed.

MadProgrammer · Accepted Answer · 2013-03-05 09:40:21Z

Each process needs to be allowed to run and finish. You can use Process#waitFor for this purpose. Equally, you need to consume any output from the process at the same time. waitFor will block so you will need use a Thread to read the input (and if you need to, write output to the process)

Depending on the location of the java/class file, you may also need to specify a starting folder from which the execution of the process can start.

Most of this significantly easier using ProcessBuilder

import java.io.File;
import java.io.IOException;
import java.io.InputStream;

public class CompileAndRun {

    public static void main(String[] args) {
        new CompileAndRun();
    }

    public CompileAndRun() {
        try {
            int result = compile("compileandrun/HelloWorld.java");
            System.out.println("javac returned " + result);
            result = run("compileandrun.HelloWorld");
        } catch (IOException | InterruptedException ex) {
            ex.printStackTrace();
        }
    }

    public int run(String clazz) throws IOException, InterruptedException {        
        ProcessBuilder pb = new ProcessBuilder("java", clazz);
        pb.redirectError();
        pb.directory(new File("src"));
        Process p = pb.start();
        InputStreamConsumer consumer = new InputStreamConsumer(p.getInputStream());
        consumer.start();

        int result = p.waitFor();

        consumer.join();

        System.out.println(consumer.getOutput());

        return result;
    }

    public int compile(String file) throws IOException, InterruptedException {        
        ProcessBuilder pb = new ProcessBuilder("javac", file);
        pb.redirectError();
        pb.directory(new File("src"));
        Process p = pb.start();
        InputStreamConsumer consumer = new InputStreamConsumer(p.getInputStream());
        consumer.start();

        int result = p.waitFor();

        consumer.join();

        System.out.println(consumer.getOutput());

        return result;        
    }

    public class InputStreamConsumer extends Thread {

        private InputStream is;
        private IOException exp;
        private StringBuilder output;

        public InputStreamConsumer(InputStream is) {
            this.is = is;
        }

        @Override
        public void run() {
            int in = -1;
            output = new StringBuilder(64);
            try {
                while ((in = is.read()) != -1) {
                    output.append((char) in);
                }
            } catch (IOException ex) {
                ex.printStackTrace();
                exp = ex;
            }
        }

        public StringBuilder getOutput() {
            return output;
        }

        public IOException getException() {
            return exp;
        }
    }
}

Now obviously, you should check the return results of the processes, and may be produce a better mechanism for interacting with the processes, but that's the basic idea...

Jay Q. · Accepted Answer · 2013-03-05 08:32:02Z

1

You can just call the main method of the second class. The main method is just like any other static method.

answered Mar 5, 2013 at 8:32

Jay Q.

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Comments

Stephen Rauch · Accepted Answer · 2018-09-03 01:08:01Z

0

This is what worked for me:

try {
    single.main(new String[0]);
} catch (Exception e) {
    JOptionPane.showMessageDialog(null, e);
}

edited Sep 3, 2018 at 1:08

Stephen Rauch♦

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answered Sep 3, 2018 at 0:44

denis

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Comments

Malakai · Accepted Answer · 2019-01-23 23:19:29Z

0

Just call the main class file. For example, if your java class file name is xyz.java, you can call and execute the same in java swing application on click of a JButton, code is

private void Btn_createdatabaseActionPerformed(java.awt.event.ActionEvent evt) {                                                   
       xyz.main(new String[0]);
}

That's it...

edited Jan 23, 2019 at 23:19

Malakai

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answered Jan 23, 2019 at 22:04

Srinivasan. B

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