This class extends the Catalog class provided by Norman Walsh's resolver library in xml-commons in order to add classpath entity and URI resolution. Since XMLCatalog already does classpath resolution, we simply add all CatalogEntry instances back to the controlling XMLCatalog instance. This is done via a callback mechanism. ApacheCatalog is only used for external catalog files. Inline entries (currently <dtd> and <entity>) are not added to ApacheCatalog. See XMLCatalog.java for the details of the entity and URI resolution algorithms.