Relying on remote resources makes XML processing susceptible to both planned and unplanned network downtime.
The XML catalog is a document describing a mapping between external entity references and locally cached equivalents.
[1] The following simple catalog shows how one might provide locally cached DTDs for an XHTML page validation tool, for example.
The following example shows how, in Java, a SAX parser may be created to parse some input source in which the org.apache.xml.resolver.tools.CatalogResolver is used to resolve external entities to locally cached instances.
This resolver originates from Apache Xerces but is now included with the Sun Java runtime.