Re: alternative to my ClassLoader hack

From:

Mark Space <markspace@sbc.global.net>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 02 Apr 2009 18:49:29 -0700

Message-ID:

<NWdBl.25206$yr3.2748@nlpi068.nbdc.sbc.com>

Steven Simpson wrote:

public class MyClassLoader {
    public static void main(String... args) throws Exception {
        // Using subdir of current directory as example.
        File curDir = new File(System.getProperty("user.dir"));
        curDir = new File(curDir, "subdir").getCanonicalFile();
        URL[] urls = { curDir.toURI().toURL() };

        ClassLoader cl = new URLClassLoader(urls);
        Class<?> main = Class.forName("TheRealMainClass", true, cl);

        Method mth = main.getMethod("main", args.getClass());
        mth.invoke(null, (Object[]) args);
    }
}

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
code below must be executed as a jar file or it won't reproduce the error.

$ java -jar test.jar
String name: /fubar/MyClassLoader.class
URL:
jar:file:/C:/Users/Brenden/Dev/misc/fubar/build/classes/test.jar!/fubar/MyClassLoader.class
path to jar file:/C:/Users/Brenden/Dev/misc/fubar/build/classes/test.jar
making FubarLoader:
Classloader: fubar.FubarLoader@addbf1
finding fubar.Launcher
trying super class...
findClass got it
Exception in thread "main" java.lang.IllegalAccessException: Class
fubar.MyClass
Loader can not access a member of class fubar.Launcher with modifiers ""
         at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)
         at java.lang.Class.newInstance0(Unknown Source)
         at java.lang.Class.newInstance(Unknown Source)
         at fubar.MyClassLoader.main(MyClassLoader.java:53)

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

import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class MyClassLoader
{
     public static void main( String... args )
             throws ClassNotFoundException, InstantiationException,
             IllegalAccessException, MalformedURLException
     {

         URL[] urls = new URL[1];

         String classResource = "/" +
                 MyClassLoader.class.getName().replaceAll( "\\.", "/" )
                 + ".class";
         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<Launcher> main = (Class<Launcher>) cl.loadClass(
                 "fubar.Launcher" );
         Launcher mcl = main.newInstance();
         mcl.launch();
     }

     private void startApplication( Class<?> c )
     {
         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 );
     }

     @Override
     public Class<?> loadClass( String className )
             throws ClassNotFoundException
     {
         if( className.startsWith( "fubar" ) ) {
             System.out.println( "finding " + className );
             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 );
     }
}