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==Overview==XML documents often rely on Document Type Definitions (This article is part of the [[Advanced User's Guide]]. It clarifies how to deal with external DTD). While declarations when parsing a document with BaseX elements and entities can be checked for validity with respect to that particular DTD.Currently the DTD is used only for entity resolutiontransforming 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:
XHTML for example defines its doctype via the following line:
<pre class="brush:xml">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
</pre>
Fetching the <code>xhtml1-strict.dtd</code> obviously involves network traffic. When dealing with single files , this may seem tolerable, but importing large collections might benefit benefits from caching these resources locally. Depending on your connection the remote server, you will experience significant speed improvementswhen caching DTDs locally. To address these issues, the [https://www.oasis-open.org/committees/download.php/14809/xml-catalogs.html XML Catalogs Standard] defines an entity catalog that maps both external identifiers and arbitrary URI references to URI references.
== XML Entity and URI Resolvers in BaseX Usage== BaseX comes with a default URI relies on the Apache-maintained [http://xml.apache.org/commons XML Commons Resolver]. The ''xml-resolver that -1.2.jar'' library is usable out included in the full distributions of BaseX. If the boxresolver is not found in the classpath, and if Java 8 is used, Java’s built-in resolver will be applied (via <code>com.sun.org.apache.xml.internal.resolver.*</code>). 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:
To enable entity resolving you have to provide a valid XML Catalog file.
A simple working example for XHTML might look like this:
<pre class="brush:xml" start="0">
<?xml version="1.0"?>
<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>
</pre>
This rewrites all SystemIds starting with: ''<nowiki>http://www.w3.org/TR/xhtml1/DTD/</nowiki>'' to ''file:///path/to/dtds/''.
This rewrites all systemIds starting with: <code><nowiki>http://www.w3.org/TR/xhtml1/DTD/</nowiki></code> to <code>file:///path/to/dtds/</code>. For example, if the following XML file is parsed: <pre class="brush:xml" start="0"><!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"/></pre> The XHTML DTD <code>xhtml1-stricttransitional.dtd</code> 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===
[[File:catalog-file.jpg|thumb|Location for the Catalog File]]When running BaseX in GUI mode simply , 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 specify the following [[options]]:
* <code>SET CATFILE [path]</code>
Now entity resolving is active for the current session. All subsequent <code>ADD</code> commands will use the catalog file to resolve entities.
The To enable Entity Resolving in Console Mode, enable the {{Option|DTD}} option and assign the path to your XML catalog file and to the {{Option|CATFILE}} option. All subsequent commands for adding documents will use the actual specified catalog file to resolve entities may be either absolute or are relative to the current working directory.
Paths to your catalog file and the actual DTDs are either absolute or relative to the ''current working directory'Please note''' that entity resolving only works with option: <code>SET INTPARSE false</code>. <code>INTPARSE</code> is set to ''false'' by When using BaseX in client-server mode, they are resolved against the working directory of the ''defaultserver''.
Using the internal parser let's you specify manually whether you want to parse DTDs and entities or not.===Additional Notes===
== Using other Resolvers ==There might be some cases when you do not want to use Entity resolving only works if the [[Parsers#XML Parsers|internal XML parser]] is switched off (which is the built-in resolver that Java provides by default (via <code>com.sun.org.apache.xml.internal.resolver.*</code>case).
BaseX offers support for 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 {{Code|xml.catalog.ignoreMissing}} is not assigned, no warnings will be output to standard error if the Apache maintained properties file or resources linked from that file are not found. See [httphttps://xmlxerces.apache.org/xml-commons XML Commons Resolver] available for download [http:/components/xerces.apache.orgresolver/mirrorsresolver-article.cgi herehtml#ctrlresolver Controlling the Catalog Resolver]for more information.
To use it add '''resolver.jar''' to the classpath when [[Startup|starting BaseX]]:<pre class="brush:bash">java -cp basex.jar:resolver.jar org.basex.BaseXServer</pre>=Links==
== More Information ==* [https://www.oasis-open.org/committees/download.php/14809/xml-catalogs.html XML Catalogs. OASIS Standard, Version 1.1. 07-October-2005]* [http://en.wikipedia.org/wiki/Document_Type_Definition Wikipedia on Document Type Definitions]*[http://xml.apache.org/commons/components/resolver/resolver-article.html Apache XML Commons Article on Entity Resolving]*[http://java.sun.com/webservices/docs/1.6/jaxb/catalog.html XML Entity and URI Resolvers], Sun*[http://www.oasis-open.org/committees/download.php/14810/xml-catalogs.pdf XML Catalogs. OASIS Standard, Version 1.1. 07-October-2005.]
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