Compiling and executing Java code using Runtime#exec()

Question

import java.io.*;

public class Auto {

    /**
     * @param args
     * @throws IOException
     */
    public static void main(String[] args) throws IOException {

        try {
            Runtime.getRuntime().exec("javac C:/HelloWorld.java");
            Runtime.getRuntime().exec("java C:/HelloWorld > C:/out.txt");
            System.out.println("END");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

This program is able to compile the 'HelloWorld.java' file, but not execute it(HelloWorld). Could anyone please suggest me how to make it work? Thanks in Advance! :) Also, if the output could be able to be taken in another text file say 'output.txt'.

It wasnt the fault I was interested in , I had already rectified it but posted the old erroneous code by mistake.Sorry :) — ivorykoder
– ivorykoder, Commented Aug 26, 2010 at 17:37

Community · Accepted Answer · 2017-05-23 12:13:30Z

4

When your run the java program, you must be in your project root directory, and run java package.to.ClassWhichContainsMainMethod

Runtime.getRuntime().exec() will give you a Process which contains an OutputStream and an InpuStream to the executed application.

You can redirect the InputStream content to your log file.

In your case I would use this exec : public Process exec(String command, String[] envp, File dir) like this :

exec("java HelloWorld", null, new File("C:/"));

To copy data from the inputStream to the file (code stolen on this post) :

public runningMethod(){
    Process p = exec("java HelloWorld", null, new File("C:/"));
    pipe(p.getInputStream(), new FileOutputStream("C:/test.txt"));
}

public void pipe(InputStream in, OutputStream out) {
    byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
    int writtenBytes;
    while((writtenBytes = in.read(buf)) >= 0) {
        out.write(buf, 0, writtenBytes);
    }
}

edited May 23, 2017 at 12:13

CommunityBot

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answered Aug 26, 2010 at 17:31

Colin Hebert

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1 Comment

ivorykoder Over a year ago

Hi Andreas_D I tried this:- Runtime.getRuntime().exec("java HelloWorld", null, new File("C:/")); But still could not get the output in out.txt file. I didnt know how to use getInputStream and getOutputStreams. can u please tell me how to get the output in out.txt file?

Andrew Thompson · Accepted Answer · 2010-08-26 18:02:44Z

3

3 Points.

The JavaCompiler was introduced in Java 1.6 to allow direct compilation of Java source from within Java code.
ProcessBuilder (1.5+) is an easier/more robust way to launch a Process.
For dealing with any process, make sure you read and implement all the points of When Runtime.exec() won't.

answered Aug 26, 2010 at 18:02

Andrew Thompson

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Comments

Thierry Roy · Accepted Answer · 2010-08-26 17:31:42Z

0

You do not execute a ".java" in java. You execute a class file. So change the second line to:

Runtime.getRuntime().exec("cd c:\;java HelloWorld > C:/out.txt");

As for the output, instead of redirecting to a file, you might want to use the inputStream:

InputStream is = Runtime.getRuntime().exec("cd c:\;java HelloWorld").getInputStream();

answered Aug 26, 2010 at 17:31

Thierry Roy

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