Class Loading to an interface

Question

Hi,

I'm quite restricted in the platform I'm currently working on (JDK 1.3, BD-J). One JAR file I would like to use attempts to perform a self-integrity check on load and if it fails it goes into an inoperable state. It's quite difficult to find out why this is happening but most sources point to that it cannot find/access it self through the BD-J structure, so it dies.

This rules out using it at load time and instead to load it in the application itself. This is quite a large library so I have to create quite an amount of interfaces so I can cast a loaded object to it and potentially use it. This is where my problem lies.

The interfaces are loaded on normal load time and the library is then loaded during run time and casted to the previously loaded interfaces, is this a problem? I'm receiving ClassCastException

I've based the interfaces off the libraries public methods as best I can, but when I attempt to cast to an interface I receive the ClassCastException. Note: It all loads fine, I can access constructors and read the method names. Just when casting it for it to be useable it fails.

The interface packages are different in my project to that of the toolkit, does this matter?

I'm running out of ideas, is there something I have overlooked?

Thanks.

Answer 1

I'm not sure I fully grok what your problem is - maybe some more details about what the class hierarchy looks like would help in figuring out the situation. From what you wrote I can guess two possible scenarios:

.1. The classes you want to use do not implement any interface.

In this case no matter what you name your interfaces, it will not work, since the classes you're loading do not implement them. You're stuck with using reflection if you can't load that jar as part of the boot classpath.

.2. The classes you want implement some interface that you're trying to replicate.

In this case you interface implementation must match the exact qualified name of the interface the classes are implementing. Normally, when loading the classes from the jar, the class loader will pick up the interfaces from the system class loader first, thus loading your interfaces, and everything should work.

If they use some crazy internal class loader, though, they might still try to load their own interfaces. You could try to figure out if that's the case by using "-XX:+TraceClassLoading", although I don't know if the 1.3 jre will understand that option.

Now if you're willing to experiment more, you could also try another approach. Write your own class loader that loads both the classes from that jar and the code you want to run. That way, your code would be able to directly refer to the classes in that jar, but to start your application the "main" method will have to be one that initializes this classloader, loads the "real" main class using reflection, and executes its main() method also via reflection.

Answer 2

Most probably the classes are loaded by different class loaders. http://mindprod.com/jgloss/classloader.html may give some idea.