Catalog Resolver
This article is part of the Advanced User's Guide. It clarifies how to deal with external DTD declarations 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 xhtml1-strict.dtd
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.
Usage
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 com.sun.org.apache.xml.internal.resolver.*
).
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:
<catalog prefer="system" xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"> <rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD/" rewritePrefix="file:///path/to/dtds/" /> </catalog>
This rewrites all systemIds starting with: http://www.w3.org/TR/xhtml1/DTD/
to file:///path/to/dtds/
. 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"> <html xmlns="http://www.w3.org/1999/xhtml"/>
The XHTML DTD xhtml1-transitional.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.
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 DTD
option and assign the path to your XML catalog file to the CATFILE
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.