There are a zillion libraries for this - the one that's best depends on your needs. From memory, some terms to Google for are JAXB (Sun's system), or JAX-RPC if you want a full Web services kit. Apache has Commons Digester, a simple but less powerful system, and XMLBeans, a JAXB implementation (I think!)
These are only some of many options - if you tell us what you intend to do with the Java classes / XML files, perhaps we can give more specific recommendations.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com