The Apache XML Commons Resolver classes implement Catalog-based entity and URI resolution.
The resolver class implements the full semantics of OASIS Technical Resolution 9401:1997 (Amendment 2 to TR 9401) catalogs and the 06 Aug 2001 Committee Specification of OASIS XML Catalogs.
It also includes a framework of classes designed to read catalog files in a number of formats:
The tools classes provide "out-of-the-box" SAX1 Parser, SAX2 XMLReader, and JAXP URIResolvers. If you're just interested in hooking a parser or URI resolver into your application, this is the place to start.