A posting from Norman Walsh (Sun Microsystems) announces the release of a set of Java classes originally written to implement the OASIS XML Catalogs Committee Specification for SAX entityResolver() and JAXP URIResolver(). These classes "greatly simplify the task of using Catalog files to perform entity resolution. You can use these classes directly 'out of the box' with their applications (such as Xalan and Saxon) or customize them to suit your particular needs. Developers will also be interested in the included JavaDoc API Documentation. The distribution package includes Java classes, JavaDoc API documentation, and step-by-step instructions explaining how to use and customize the resolver components." The Preview Version 0.2 requires JDK 1.2 or later. The package with binaries and sample code is available for download from the Sun XML Developer Connection.
For background, see the section "Catalogs, Formal Public Identifiers, Formal System Identifiers." Also, (1) "SGML Entity Types" and (2) "SGML/XML Entity Sets and Entity Management."