How can I make a URLClassLoader behave like this?

Here is what is happening. When you attempt to load Test$Thing, the defineClass method figures out that it needs to load the superclass of Test$Thing; i.e. Test$SuperThing.

The system classloader calls your classloader, which returns the Nothing class. The system class loader says "Woah! That class doesn't have the right fully-qualified classname!", and throws NoClassDefFoundError.

Basically, this system classloader is protecting against things that would destabilize the JVM. If it allowed a custom classloader to load the wrong class, nasty things could happen. For example, think about what might happen if some code invoked some method defined for the Test$SuperThing class after you had tricked classloader into loading the Nothing class.

If you want to do dirty tricks that substitute a dummy version of some class, you will need to generate bytecode on the fly, using the right fully qualified class name, the right superclass and interfaces, and methods with the right signatures. In short, you must satisfy the binary compatibility rules. If you don't, the system classloader will refuse to load your class ... no matter what you try to do in your custom classloader.

Frankly, you should be taking a completely different approach to whatever it is you are trying to do.

Thanks, this makes complete sense. – NellerLess Jan 25 '10 at 14:30.

Can a class named Test$Thing. Class found in the output of the compiler? As the code is given here it will be never directly referenced and maybe the compiler optimized the class away.In that case the reflection cannot find the class.

If that is the case you simply could add a (non-reflection) reference to the class, so that it will be compiled. EDIT: After trying it out myself, I think I found the source of the problem. If I removed the 'extends SuperThing' from the Thing-declaration it complains about not finding java.lang.Object.

Probably the classloader tries to load also all dependent classes (including the ones it extends) and cannot because they aren't on the whitelist. But that is a theory.

Yes, it's there. – NellerLess Jan 25 '10 at 11:36.

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