Re: alternative to my ClassLoader hack

From:

Mark Space <markspace@sbc.global.net>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 02 Apr 2009 19:13:18 -0700

Message-ID:

<7heBl.25213$yr3.803@nlpi068.nbdc.sbc.com>

Steven Simpson wrote:

I'd go with something like this:

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

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);
    }
}

Impressive! Obi Wan has taught you well. But you have a minor mistake
in the last line: args is interpetted as the parameter itself for
invoke, it should be wrapped up in an object array for the varargs.

Again, the following code must be run as a jar file to work correctly.

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

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

/**
  *
  * @author Brenden
  */
public class FubarLoader2 extends URLClassLoader
{
     public FubarLoader2( URL[] urls )
     {
         super( urls );
     }

     public static void main( String... args )
             throws Exception
     {
         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 FubarLoader2:" );
         URLClassLoader cl = new FubarLoader( urls );
         System.out.println( "class for name..." );
         Class<?> main =
                 Class.forName( "fubar.FubarLoader2", true, cl );

         System.out.println( "invoking launch" );
         Method mth = main.getMethod( "launch", args.getClass() );
         mth.invoke( null, new Object[] {args} );

     }

     public static void launch( String... args )
     {
         System.out.println( "LAUNCHED!" );
         System.out.println( FubarLoader2.class.getClassLoader() );
     }

     @Override
     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 );
     }
}

Output:

$ 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 FubarLoader2:
class for name...
finding fubar.FubarLoader2
trying super class...
findClass got it
trying super class...
trying super class...
trying super class...
trying super class...
finding fubar.FubarLoader
findClass got it
invoking launch
trying super class...
trying super class...
trying super class...
trying super class...
trying super class...
LAUNCHED!
fubar.FubarLoader@addbf1