Help: ClassLoader and Casting??

[John Davison]

I need to load a different version of a class than what is being used
by the overall system. My test code works fine for loading the class,
but fails when I try to cast the Object. For example, the following
throws a java.lang.NoClassDefFoundError when trying to cast.

public class URLTester
{
public static void main( String args[] ) throws Exception
{
// Create an array of URLs
// From directory name specified as args[0]
URL urlList[] = {new File( args[ 0 ] ).toURL()};

// Create a URLClassLoader
ClassLoader loader = new URLClassLoader( urlList );
// Thread.currentThread().setContextClassLoader( loader );

// Load class from class loader
// Use args[1] as class name
Class c = loader.loadClass( args[ 1 ] );

// Create an instance of the class just loaded
Object obj = c.newInstance();

// This fails with a NoClassDefFoundError
Tester tester = (Tester)obj;
}
}

My hunch is that the system ClassLoader is doing the casting. I've
tried, with no success, to add
Thread.currentThread().setContextClassLoader( loader ) to change the
thread's ClassLoader.

Any help would be greatly appreciated!

-- John

 

[Roedy Green]

"load a different version of a class"

do you mean you have two class files with the exact same package and
name ? but different internal workings ?
you can't do this. If there are in different packages no problem.

You can if you load the second with a different classloader. Then Java
treats them as different classes.

see http://mindprod.com/jgloss/classforname.html
http://mindprod.com/jgloss/classloader.html