Re: alternative to my ClassLoader hack

Mark Space <>
Thu, 02 Apr 2009 20:22:30 -0700
Mark Space wrote:

Just for the record, this runs, but gets an exception. I'm not sure
100% why. I'll try to clean it up then make it use your method. The

I have my version working now. The secret is to poke the object made
with the new classloader reflectively. I guess the exception I get
above is related to the fact that MyClassLoader is loaded by two
different classloaders, and therefore one class type isn't the same as
the other. Weird, but true.

This version does pass the first MyClassLoader.class to the second, and
then compares the two class types and determines they are not equal, as
expected. I think this program and Stevens programs are equivalent,
showing that both ideas are valid.

Again, must be run as a jar or the string mangling code used in the
beginning of main won't be able to produce a valid URL for the
URLClassLoader (actually a FubarLoader here).

  * 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 );
         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 );
         m.invoke( mcl, MyClassLoader.class );

     public void startApplication( Class<?> c )
// public void startApplication( )
         System.out.println( "Class files are equal: " + (c ==
                 MyClassLoader.class) );
         System.out.println( "Classloader: " + getClass().
                 getClassLoader() );
     // everything else here

class Launcher
     public void launch()
         System.out.println( "Classloader: " + getClass().
                 getClassLoader() );

  * classloader call trace:
  * I. loadClass( String )
  * II. loadClass( String, false )
  * 3. findLoadedClass prot
  * A. FindLoadClass0 -- native -- PRIVATE
  * 4. loadClass (String) on parent
  * 5. findBootstrapClass0 PRIVATE
  * 6. findClass prot
  * 7. resolveClass prot
  * A. resolveClass -- native -- PRIVATE
class FubarLoader extends URLClassLoader
     public FubarLoader( URL[] urls )
         super( urls );

     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 );

This produces the following output:

$ java -jar test.jar
String name: /fubar/MyClassLoader.class
path to jar file:/C:/Users/Brenden/Dev/misc/fubar/build/classes/test.jar
making FubarLoader:
Classloader: fubar.FubarLoader@19821f
finding fubar.MyClassLoader
finding java.lang.Object
trying super class...
findClass got it
trying super class...
finding fubar.FubarLoader
findClass got it
finding java.lang.Class
trying super class...
finding java.lang.String
trying super class...
finding java.lang.ClassNotFoundException
trying super class...
finding java.lang.InstantiationException
trying super class...
finding java.lang.IllegalAccessException
trying super class...
trying super class...
finding java.lang.NoSuchMethodException
trying super class...
finding java.lang.IllegalArgumentException
trying super class...
finding java.lang.reflect.InvocationTargetException
trying super class...
finding java.lang.System
trying super class...
finding java.lang.StringBuilder
trying super class...
trying super class...
Class files are equal: false
Classloader: fubar.FubarLoader@19821f

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