## XMLCatalog

An XMLCatalog is a catalog of public resources such as DTDs or entities that are referenced in an XML document. Catalogs are typically used to make web references to resources point to a locally cached copy of the resource.

This allows the XML Parser, XSLT Processor or other consumer of XML documents to efficiently allow a local substitution for a resource available on the web.

This data type provides a catalog of resource locations based on the OASIS "Open Catalog" standard. The catalog entries are used both for Entity resolution and URI resolution, in accordance with the `org.xml.sax.EntityResolver` and `javax.xml.transform.URIResolver` interfaces as defined in the Java API for XML Processing (JAXP) Specification.

For example, in a `web.xml` file, the DTD is referenced as:

    <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
      "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">

The XML processor, without XMLCatalog support, would need to retrieve the DTD from the URL specified whenever validation of the document was required.

This can be very time consuming during the build process, especially where network throughput is limited. Alternatively, you can do the following:

1. Copy `web-app_2_2.dtd` onto your local disk somewhere (either in the filesystem or even embedded inside a jar or zip file on the classpath).
2. Create an `<xmlcatalog>` with a `<dtd>` element whose `location` attribute points to the file.
3. Success! The XML processor will now use the local copy instead of calling out to the internet.

XMLCatalogs can appear inside tasks that support this feature or at the same level as `target` - i.e., as children of `project` for reuse across different tasks, e.g. XML Validation and XSLT Transformation. The XML Validate task uses XMLCatalogs for entity resolution. The XSLT Transformation task uses XMLCatalogs for both entity and URI resolution.

XMLCatalogs are specified as either a reference to another XMLCatalog, defined previously in a build file, or as a list of `dtd` or `entity` locations. A separate classpath for entity resolution may be specified inline via nested `classpath` elements; otherwise the system classpath is used for this as well.

XMLCatalogs can also be nested inside other XMLCatalogs. For example, a "superset" XMLCatalog could be made by including several nested XMLCatalogs that referred to other, previously defined XMLCatalogs.

Currently, only `<dtd>` and `<entity>` elements may be specified inline; these roughly correspond to OASIS catalog entry types `PUBLIC` and `URI` respectively.

### Entity/DTD/URI Resolution Algorithm

When an entity, DTD, or URI is looked up by the XML processor, the XMLCatalog searches its list of entries to see if any match. That is, it attempts to match the `publicId` attribute of each entry with the PublicID or URI of the entity to be resolved. Assuming a matching entry is found, XMLCatalog then executes the following steps:

#### 1. Filesystem lookup

The `location` is first looked up in the filesystem. If the `location` is a relative path, the ant project basedir attribute is used as the base directory. If the `location` specifies an absolute path, it is used as is. Once we have an absolute path in hand, we check to see if a valid and readable file exists at that path. If so, we are done. If not, we proceed to the next step.

#### 2. Classpath lookup

The `location` is next looked up in the classpath. Recall that jar files are merely fancy zip files. For classpath lookup, the `location` is used as is (no base is prepended). We use a Classloader to attempt to load the resource from the classpath. For example, if hello.jar is in the classpath and it contains `foo/bar/blat.dtd` it will resolve an entity whose `location` is `foo/bar/blat.dtd`. Of course, it will not resolve an entity whose `location` is `blat.dtd`.

#### 3. URL-space lookup

Finally, we attempt to make a URL out of the `location`. At first this may seem like this would defeat the purpose of XMLCatalogs -- why go back out to the internet? But in fact, this can be used to (in a sense) implement HTTP redirects, substituting one URL for another. The mapped-to URL might also be served by a local web server. If the URL resolves to a valid and readable resource, we are done. Otherwise, we give up. In this case, the XML processor will perform its normal resolution algorithm. Depending on the processor configuration, further resolution failures may or may not result in fatal (i.e. build-ending) errors.

### XMLCatalog attributes

Attribute | Description | Required
id | a unique name for an XMLCatalog, used for referencing the XMLCatalog's contents from another XMLCatalog | No
refid | the `id` of another XMLCatalog whose contents you would like to be used for this XMLCatalog | No

### XMLCatalog nested elements

#### dtd/entity

The `dtd` and `entity` elements used to specify XMLCatalogs are identical in their structure

Attribute | Description | Required
publicId | The public identifier used when defining a dtd or entity, e.g. `"-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"` | Yes
location | The location of the local replacement to be used for the public identifier specified. This may be specified as a file name, resource name found on the classpath, or a URL. Relative paths will be resolved according to the base, which by default is the Ant project basedir. | Yes

#### classpath

The classpath to use for entity resolution. The nested `<classpath>` is a path-like structure.

### Examples

Set up an XMLCatalog with a single dtd referenced locally in a user's home directory:

>         <xmlcatalog>
>             <dtd
>                 publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
>                 location="/home/dion/downloads/docbook/docbookx.dtd"/>
>         </xmlcatalog>

Set up an XMLCatalog with a multiple dtds to be found either in the filesystem (relative to the Ant project basedir) or in the classpath:

>         <xmlcatalog id="commonDTDs">
>             <dtd
>                 publicId="-//OASIS//DTD DocBook XML V4.1.2//EN"
>                 location="docbook/docbookx.dtd"/>
>             <dtd
>                 publicId="-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
>                 location="web-app_2_2.dtd"/>
>         </xmlcatalog>

Set up an XMLCatalog with a combination of DTDs and entities as well as a nested XMLCatalog:

>         <xmlcatalog id="allcatalogs">
>             <dtd
>                 publicId="-//ArielPartners//DTD XML Article V1.0//EN"
>                 location="com/arielpartners/knowledgebase/dtd/article.dtd"/>
>             <entity
>                 publicId="LargeLogo"
>                 location="com/arielpartners/images/ariel-logo-large.gif"/>
>     	<xmlcatalog refid="commonDTDs"/>
>         </xmlcatalog>

To reference the above XMLCatalog in an `xslt` task:

>         <xslt basedir="${source.doc}"
>                destdir="${dest.xdocs}"
>                extension=".xml"
>                style="${source.xsl.converter.docbook}"
>                includes="**/*.xml"
>                force="true">
>             <xmlcatalog refid="allcatalogs"/>
>         </xslt>

Copyright © 2002 Apache Software Foundation. All rights Reserved.