XMLResolver: An enhanced XML resolver with XML Catalog support

Attention: On 22 May 2021, I moved this repository from my personal account to a new xmlresolver organization. GitHub should provide redirects, but you may want to update your git remote if you forked it before the move.

Attention: On 7 August 2020, I changed the default branch in this repository to main. If you’ve got a clone of this repo, you may want to update it. Apologies for the inconvenience.

The xmlresolver project provides an advanced implementation of the SAX EntityResolver, the Transformer URIResolver, and a new NamespaceResolver. The implementation uses the OASIS XML Catalogs V1.1 Standard to provide a mapping from public identifiers to local resources.

The xmlresolver can be found on Maven Central and has the coordinates:

<groupId>org.xmlresolver</groupId>
<artifactId>xmlresolver</artifactId>

In addition to enhanced support for RDDL-based namespace resolution, the implementation supports automatic local caching of resources. This provides the advantages of the catalog specification without requiring users to manage the mapping by hand.

Applications can use the resolver directly or they can instantiate one of a set of convenience classes to access parsers that automatically implement these resolvers.

The goal of this project is to produce a clean, reasonably simple API and a robust, thread-safe implementation.

See also: https://xmlresolver.org/

For guidelines about how to migrate your application from the Apache Commons resolver to this resolver, see documentation and examples in https://github.com/xmlresolver/resolver-migration

ChangeLog

API Changes

Version 2.0.x introduces some API changes. The most common user-level APIs (new Catalog(), new Resolver(), etc. are unchanged), but if you’re extending or integrating with the resolver directly, you may have to change a few things. I’ve added a ResolverFeature type to track features in a more typesafe way.

The Configuration object has been renamed XMLResolverConfiguration and now implements the ResolverConfiguration interface.

Several methods on the configuration object have been removed. Their values can be obtained by requesting the corresponding feature.

Behavior changes

The resolver class can be configured with either system properties or a properties file. In version 1.x, if a property file is used, the values specified in that file always take precedence.

This means that you can’t, for example, selectively override the catalog file list for a single application by specifying a system property.

On reflection, that seems backwards. In 2.x, if a property is specified in both places, the system property wins.

To keep the existing behavior, set the boolean property prefer-property-file to true in the property file. That will preserve the former behavior. (The system property equivalent is xml.catalog.preferPropertyFile.)

Support for data: URIs

It is now possible to use data: URIs in the catalog. Data URIs are defined by RFC 2397. For example, this catalog entry:

  <uri name="http://example.com/example.xml"
       uri="data:application/xml;base64,PGRvYz5JIHdhcyBhIGRhdGEgVVJJPC9kb2M+Cg=="/>

maps the URI http://example.com/example.xml to a short XML document defined by that data URI (<doc>I was a data URI</doc>).

Support for classpath: URIs

It is now possible to use classpath: URIs in the catalog. It is also possible to use classpath: URIs in the catalog list. The classpath: URI scheme seems to be defined somewhat informally by the Spring framework.

In brief, a classpath: URI is resolved by attempting to find a document with the specified path in the classpath, including within JAR files on the classpath.

This catalog entry:

  <uri name="http://example.com/example.xml"
       uri="classpath:path/example-doc.xml"/>

maps the URI http://example.com/example.xml to a document with the path path/example-doc.xml on the classpath. (Searches always begin at the root of the classpath segments, so path/example-doc.xml and /path/example-doc.xml are equivalent.)

Suppose, for example, that your classpath includes /home/ndw/java/libs/example.jar:

$ jar vtf /home/ndw/java/libs/example.jar
     0 Wed May 05 14:51:50 BST 2021 META-INF/
    25 Wed May 05 14:51:50 BST 2021 META-INF/MANIFEST.MF
     0 Wed May 05 14:51:48 BST 2021 org/
     0 Wed May 05 14:51:50 BST 2021 org/example/
  3262 Wed May 05 14:51:50 BST 2021 org/example/DWIM.class
  1831 Wed May 05 14:51:50 BST 2021 path/example-doc.xml
   219 Wed May 05 14:51:50 BST 2021 path/something-else.txt

Assuming that this JAR file is the first place on your classpath where path/example-doc.xml occurs, then that’s the document that will be returned.

Lies, damned lies, and URIs

At this point, we expect the resolver to return that resource with a base URI of classpath:path/example-doc.xml. Unfortunately, if we do that, any attempt to resolve a URI against that document’s base URI (for example, if example-doc.xml contains an XInclude with a relative href value), will immediately fail. It fails constructing the URI long before it calls the resolver to attempt to retrieve it.

To avoid this, the resolver lies. It returns the resource with the base URI set to the resolved location, jar:file:///home/ndw/java/libs/example.jar!path/example-doc.xml. The URI class doesn’t resolve relative URIs against that base URI either, but at least it doesn’t throw an exception.

The practical consequence of this is that the resolver never gets asked to resolve URIs made absolute against either of these forms of URI. If you put a document in a JAR file, make sure that all of its relative references (includes, imports, etc.) will resolve correctly. You can’t re-interpret them in the resolver.

Support for additional catalog files

If a project uses a particular schema, or set of schemas, it may be useful to add an additional catalog (or catalogs) to the user’s default catalog file path. That’s not currently practical: if both system properties and a property file are used to configure a resolver, and the same setting appears in both places, either the system property value is used (the default in 2.x) or the property file value is used (the default in 1.x).

So in order to add additional catalog files, you’d have to work out the current value, construct a new value incorporating both that default and your new catalog(s), and specify that new value in the xml.catalog.files system property.

To make this easier, the 2.0 release adds a new system property, xml.catalog.additions, and a new property file key, catalog-additions. Both properties take a list of catalog files. Those files will be added to the list defined by the normal catalog settings.

Support for validating catalog files

The resolver is tolerant of errors in catalog files by design. A production application shouldn’t fall over because someone adds a bad catalog file to the catalog path. Instead, the errant file is simply ignored.

During development, it may be useful to take a more restrictive view. Putting a typo in a catalog file is one common source of resolution failures. More than once, I’ve spent time trying to track down a resolver bug only to discover that I’d typed systemid instead of systemId in a catalog file, or made some other similar error.

Obviously, editing your catalog files with a validating editor is one simply remedy to this problem, but experience suggests that’s not always what actually happens.

You can use the catalog-loader-class property to specify an alternate catalog loader. If you specify, org.xmlresolver.loaders.ValidatingXmlLoader, the catalog files will be validated as they are loaded and validation errors will raise an exception.

In order to use this feature, you must have Jing version 20181222 on your classpath.