Hi,
Welcome to Javaranch!
What you're describing sounds very similar to what
Struts framework does. It has a struts-config xml file where you describe the command name and the associated action class. The action classes are application provided. The struts front controller intercepts the request, extracts the command name from the URL, looks it up in the xml file, gets the correspoding Action class name, instantiates it using reflection API, and calls a method in it. Struts-config XML has more than just these mappings.
So one option is you can use the Struts framework. It's quite easy to integrate into an existing app if it already has a FrontController + Commands + dispatcher-to-views architecture.
If for some reason you want to rollout your own implementation, then a simple properties file with 'command -> command-classname' key value mappings should be sufficient isn't it? Lookup the command, get corresponding action classname, use reflection to instantiate and call a method on the instance. I can't think of any added value in using XML format file over a plain properties file, in this case.