This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
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.