Catalog Resolver

This article is part of the Advanced User's Guide. It clarifies how to deal with mapping system IDs (DTD locations) and URIs to local resources when parsing and transforming XML data.

Introduction
XML documents often rely on Document Type Definitions (DTDs). Entities can be resolved with respect to that particular DTD. By default, the DTD is only used for entity resolution.

XHTML, for example, defines its doctype via the following line:

 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Fetching  from the W3C’s server obviously involves network traffic. When dealing with single files, this may seem tolerable, but importing large collections benefits from caching these resources. Depending on the remote server, you will experience significant speed improvements when caching DTDs locally.

To address these issues, the XML Catalogs Standard defines an entity catalog that maps both external identifiers and arbitrary URI references to URI references.

Another application for XML catalogs is to provide local resources for reusable XSLT stylesheet libraries that are imported from a canonical location. This is described in greater detail in the following section.

System ID (DTD Location) Rewrites
BaseX relies on the Apache-maintained XML Commons Resolver. The xml-resolver-1.2.jar library is included in the full distributions of BaseX. If the resolver is not found in the classpath, and if Java 8 is used, Java’s built-in resolver will be applied (via ).

To enable entity resolving you have to provide a valid XML Catalog file, so that the parser knows where to look for mirrored DTDs.

A simple working example for XHTML might look like this:

  

This rewrites all systemIds starting with:  to. For example, if the following XML file is parsed:

 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 

The XHTML DTD  and all its linked resources will now be loaded from the specified path.

The catalog file etc/w3-catalog.xml in the full distributions can be used out of the box. It defines rewriting for some common W3 DTD files.

URI Rewrites
Consider a library of reusable XSLT stylesheets. For performance reasons, this library will be cached locally. However, the import URI for a given stylesheet should always be the same, independent of the accidental relative or absolute path that it is stored at locally. Example:

 

The XSLT stylesheet might not even be available from this location. The URI serves as a canonical location identifier for this XSLT stylesheet. A local copy of the  directory is expected to reside somewhere, and the location of this directory relative to the local XML catalog file is specified in an entry in this catalog, like this:

 

This way, XSLT import URIs don’t have to be adjusted for the relative or absolute locations of the XSLT library’s local copy.

The same URI rewriting works for resources retrieved by the  function from within an XSLT stylesheet. See XSLT Module for details on how to invoke XSLT stylesheets from within BaseX.

NOTE: This URI rewriting is currently restricted to XSLT stylesheets. It has neither been enabled yet for the XQuery function  nor for XSD schema locations.

GUI Mode
When running BaseX in GUI mode, enable DTD parsing and provide the path to your XML Catalog file in the Parsing Tab of the Database Creation Dialog.

Console & Server Mode
To enable Entity Resolving in Console Mode, enable the option and assign the path to your XML catalog file to the  option. All subsequent commands for adding documents will use the specified catalog file to resolve entities.

Paths to your catalog file and the actual DTDs are either absolute or relative to the current working directory. When using BaseX in client-server mode, they are resolved against the working directory of the server.

Additional Notes
Entity resolving only works if the internal XML parser is switched off (which is the default case).

The runtime properties of the catalog resolver can be changed by setting system properties, or adding a CatalogManager.properties file to the classpath. By default, and if the system property xml.catalog.ignoreMissing is not assigned, no warnings will be output to standard error if the properties file or resources linked from that file are not found. See Controlling the Catalog Resolver for more information.

When using a catalog within an XQuery Module, the global  option may not be set in this module. You can set it via pragma instead:

 (# db:catfile xmlcatalog/catalog.xml #) { xslt:transform(db:open('acme_content')[1], '../acmecustom/acmehtml.xsl') }

It is assumed that this stylesheet  (location relative to the current XQuery script or module) imports   by its canonical URI that will be resolved to a local URI by the catalog resolver.

Please note that since catalog-based URI rewriting does not work yet within URIs accessed from XQuery, you cannot give a canonical location that needs to be catalog-resolved as the second argument of.

The catalog location in the pragma can be given relative to the current working directory (the directory that is returned by ) or as an absolute operating system path. The catalog location in the pragma is not an XQuery expression; no concatenation or other operations may occur in the pragma, and the location string must not be surrounded by quotes.

Links

 * XML Catalogs. OASIS Standard, Version 1.1. 07-October-2005
 * Wikipedia on Document Type Definitions
 * Apache XML Commons Article on Entity Resolving
 * XML Entity and URI Resolvers, Sun