- XML Catalog
XML documents typically refer to external entities, for example the public and/or system ID for theDocument Type Definition . These external relationships are expressed using URIs, typically as URLs.However, if they're absolute URLs, they only work when your network can reach them. Relying on remote resources makes XML processing susceptible to both planned and unplanned network downtime.
Conversely, if they're relative URLs, they're only useful in the context where the were initially created. For example, the URL "../../xml/dtd/docbookx.xml" will usually only be useful in very limited circumstances.
One way to avoid these problems is to use an entity resolver (a standard part of
SAX ) or a URI Resolver (a standard part ofJAXP ). A resolver can examine the URIs of the resources being requested and determine how best to satisfy those requests. The XML catalog is a document describing a mapping between external entity references and locally-cached equivalents.Example Catalog.xml
The following simple catalog shows how one might provide locally-cached DTDs for an XHTML page validation tool, for example.
This catalog makes it possible to resolve -//W3C//DTD XHTML 1.0 Strict//EN to the local URI dtd/xhtml1/xhtml1-strict.dtd. Similarly, it provides local URIs for two other public IDs.
Note that the document above includes a DOCTYPE - this may cause the parser to attempt to access the system ID URL for the DOCTYPE (i.e. http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd) before the catalog resolver is fully functioning, which is probably undesirable. To prevent this, simply remove the DOCTYPE declaration.
The following example shows this, and also shows the equivalent <system/> declarations as an alternative to <public/> declarations.
Using a Catalog - Java SAX Example
Catalog resolvers are available for various programming languages. 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 ApacheXerces but is now included with the Sun Java runtime.Simply create a SAXParser in the normal way, using factories. Obtain the XML reader and set the entity resolver to the standard one (CatalogResolver) or another of your own.
final SAXParser saxParser = SAXParserFactory.newInstance().newSAXParser(); final XMLReader reader = saxParser.getXMLReader();
final ContentHandler handler = ...; final InputSource input = ...;
reader.setEntityResolver( new CatalogResolver() ); reader.setContentHandler( handler ); reader.parse( input );
It is important to call the parse method on the reader, "not" on the SAX parser.
ee also
* [http://www.oasis-open.org/committees/entity/spec-2001-08-06.html Oasis XML Catalogs specification]
* [http://java.sun.com/webservices/docs/1.6/jaxb/catalog.html XML Entity and URI Resolvers] , Sun
* [http://xmlcatmgr.sourceforge.net/ XML Catalog Manager] project on Sourceforge
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