Running a JAR file from a java program

[igwe kalu kalu ogba]

Hello, could anyone help me with this. I have a java program and I want to be able to execute a JAR file from within that program. Using Runtime.exec I can easily execute .exe files, how can I do the same with my jar files. Thank you.

[Ernest Friedman-Hill]

With Runtime.exec("java -jar myjar.jar") ?

Or you could use a java.net.URLClassLoader to load the main class, and execute it directly.

[Pradeep Kadambar]

Hey ... what on earth are you thinking of. Put the jar in the classpath and all the methods in them can be accessed.

So call the main program of the jar in your java code. That it !!!

[Geoffrey Falk]

Putting the jar file in the classpath is not going to help if you don't have the jar file in advance. I assume you want to do this without restarting your program.

So use a URLClassLoader to reference the jar file. Find out the main class from the Manifest. This will tell you the entry point. Use the URLClassLoader to load the main class, then invoke its main method with some arguments.

import java.io.File;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.Attributes;
import java.util.jar.JarFile;
import java.util.jar.Manifest;
 
/**
 * Call this class with the name of the jar file.
 *
 * Example:
 * java -classpath . Launcher foo.jar [arguments]
 */
public class Launcher {
 
  public static void main(String args[]) throws Exception {
    File jarfile = new File(args[0]);
    JarFile jar = new JarFile(jarfile);
    Manifest manifest = jar.getManifest();
    Attributes attrs = manifest.getMainAttributes();
    String mainClassName = attrs.getValue("Main-Class");
    URL url = new URL("file", null, jarfile.getAbsolutePath());
    ClassLoader cl = new URLClassLoader(new URL[] {url});
    Class mainClass = cl.loadClass(mainClassName);
    Method mainMethod = mainClass.getMethod("main", new Class[] {String[].class});
    String[] args2 = new String[args.length - 1];
    System.arraycopy(args, 1, args2, 0, args.length - 1);
    mainMethod.invoke(mainClass, new Object[] {args2});
  }
}

You will want to add exception handling to this example.

Geoffrey
[ May 10, 2005: Message edited by: Geoffrey Falk ]