Re: alternative to my ClassLoader hack

Mark Space <>
Sun, 05 Apr 2009 13:19:08 -0700
<6n8Cl.22708$> wrote:

Hi again,

I tried to modify your "startApplication" method so
I could pass it both the original "String[] args"
*and* the URLClassLoader. I didn't succeed however.

May I bother you a little bit more and ask you how
I could change this:

public void startApplication( Class<?> c )

to that:

public void startApplication( Class<?> c, String[] args,
URLClassLoader ucl)

OK, I think the problem was that I wasn't setting the parent
classloader, so there was no way for my classloader to commune with the
system classloader, and it was loading all classes itself.

Dealing with reflection isn't hard. The first part is that you have to
look for the method you want. That means the name, and a list of
..class's that match the signature.

      Method m = mcl.getClass().getMethod( "startApplication",
              Class.class, String[].class, URLClassLoader.class );

So there it is. Your method takes a Class, a String[], and a
URLClassloader. Add .class to each of those, and you get the class
literal for each.

Next you have to invoke the method with the parameters. That's pretty
obvious I think.

      m.invoke( mcl, MyClassLoader.class, args, cl );

Those are pretty much random arguments that I had, just to prove the
arguments could be passed.

And of course you have to modify startApplication to actually accept
that argument list.

Here's what worked for me:

  * To change this template, choose Tools | Templates
  * and open the template in the editor.
package fubar;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

  * @author Brenden
public class MyClassLoader
     public static void main( String... args )
             throws ClassNotFoundException, InstantiationException,
             IllegalAccessException, MalformedURLException,
             NoSuchMethodException, IllegalArgumentException,

         URL[] urls = new URL[1];

         String classResource =
                 "/" +
                 MyClassLoader.class.getName().replaceAll( "\\.", "/" ) +
         System.out.println( "String name: " + classResource );
         URL myClass =
                 MyClassLoader.class.getResource( classResource );

         System.out.println( "URL: " + myClass );

         String pathToClass = myClass.toString();
         int index = pathToClass.indexOf( '!' );
         pathToClass = pathToClass.substring( 4, index );
         System.out.println( "path to jar " + pathToClass );
         URL jarURL = new URL( pathToClass );

         urls[0] = jarURL;

         System.out.println( "making FubarLoader:" );
         URLClassLoader cl = new FubarLoader( urls,
                 MyClassLoader.class.getClassLoader() );
         System.out.println( "Classloader: " + cl );
         @SuppressWarnings( "unchecked" )
         Class<MyClassLoader> main = (Class<MyClassLoader>) cl.
                 "fubar.MyClassLoader" );
         /*Exception in thread "main" java.lang.ClassCastException:
fubar.MyClassLoader can
         not be cast to fubar.MyClassLoader
         at fubar.MyClassLoader.main(
// MyClassLoader mcl = main.newInstance();
// mcl.startApplication( MyClassLoader.class );
         Object mcl = main.newInstance();
         Method m = mcl.getClass().getMethod( "startApplication",
                 Class.class, String[].class, URLClassLoader.class );
         m.invoke( mcl, MyClassLoader.class, args, cl );

     public void startApplication( Class<?> c, String[] args,
             URLClassLoader ucl )
// public void startApplication( )
         System.out.println( "Class files are equal: " + (c ==
                 MyClassLoader.class) );
         System.out.println( "Classloader: " + getClass().
                 getClassLoader() );
     // everything else here
      * classloader call trace:
      * I. loadClass( String )
      * II. loadClass( String, false )
      * 1. findLoadedClass prot
      * A. FindLoadClass0 -- native -- PRIVATE
      * 2. loadClass (String) on parent
      * 3. findBootstrapClass0 PRIVATE
      * 4. findClass prot
      * 5. resolveClass prot
      * A. resolveClass -- native -- PRIVATE

     static class FubarLoader extends URLClassLoader
         public FubarLoader( URL[] urls, ClassLoader parent )
             super( urls, parent );

         public Class<?> loadClass( String className )
                 throws ClassNotFoundException
             System.out.println( "finding " + className );
             if( className.startsWith( "fubar" ) ) {
                 Class<?> c = null;
                 try {
                     c = findClass( className );
                 catch( ClassNotFoundException ex ) {
                 if( c != null ) {
                     System.out.println( "findClass got it" );
                     return c;
             System.out.println( "trying super class..." );
             return super.loadClass( className );

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