Question

I wrote an application with java and convert it to execute jar file, my project is depend on “jxl.jar”.
Is there any way to copy “jxl.jar” to my jar file and after that call it.

I think that called “Nested Jars” ?!?

Thanks a lot

Best Regards,

Mike

Answer 1

The simplest way is to unpack jxl.jar and pack it into single jar together with your class files.

You can also put jxl.jar into your jar, load it using JarClassLoader and call using reflection, but it’s probably not worth the effort and added complication.

This should look something like this:

import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

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

        Class<Preloader> c= Preloader.class;

        URLClassLoader loader = new URLClassLoader(new URL[]{
            c.getResource("program.jar"),
            c.getResource("library.jar")});

        Class<?> main_class=loader.loadClass("Main");
        Method main_method = main_class.getMethod("main", args.getClass());
        main_method.invoke(null, new Object[]{args});

    }
}

Unfortunately it doesn’t really work with nested jars. URLClassLoader has problems with them. You can extract your nested jars to temporary files and pass them to URLClassLoader. You can also use custom URLStreamHandler as described here, it may even be a working solution.