XML Schema validation with Xerces-J

Hiranya Jayathilaka raised an interesting discussion some time ago on xerces-dev list, that how we could validate an XML Schema 1.0 document using Xerces-J. Hiranya was looking for a solution using a Java API with Xerces-J. We were looking for verifying the correctness of the Schema document, and not doing an XML instance document validation.

I'm providing a summary of the discussion we had on the list, and the conclusions we made.

There are basically three ways of doing this:

1. Using a JAXP SchemaFactory
Using this technique, we do something like below:

SchemaFactory sf = SchemaFactory.newInstance ..
sf.setErrorHandler ..
Schema s = sf.newSchema(new StreamSource(schemapath));

The 'SchemaFactory.newSchema' call would not succeed if XML Schema has a grammar error.

2. Using XSLoader
Using this technique, we do something like below:

XSLoaderImpl xsLoader = new XSLoaderImpl();
XSModel xsModel = xsLoader.loadURI(xsdUri);

Michael Glavassevich suggested, how we could add an error handler to this mechanism:

DOMErrorHandler myErrorHandler = ...;

XSImplementation xsImpl = (XSImplementation) registry.getDOMImplementation("XS-Loader");
XSLoader xsLoader = xsImpl.createXSLoader(null);

DOMConfiguration config = xsLoader.getConfig();
config.setParameter("error-handler", myErrorHandler); // <-- set the error handler

3. Using XMLGrammarPreparser
Using this technique, we do something like below (thanks to Hiranya Jayathilaka for sharing this code):

XMLGrammarPreparser preparser = new XMLGrammarPreparser();
preparser.registerPreparser(XMLGrammarDescription.XML_SCHEMA, null);
preparser.setFeature("http://xml.org/sax/features/namespaces", true);
preparser.setFeature("http://xml.org/sax/features/validation", true);
preparser.setFeature("http://apache.org/xml/features/validation/schema", true);
preparser.setErrorHandler(new MyErrorHandler());
Grammar g = preparser.preparseGrammar(XMLGrammarDescription.XML_SCHEMA, new XMLInputSource(null, xsdUrl, null));

Michael Glavassevich provided a nice comparison of these three approaches:
SchemaFactory - it is an entry point into the JAXP Validation API for loading schemas for validation. If it was a user asking I'd recommend SchemaFactory of the three choices since it's in Java 5+ and would work in environments where Xerces isn't available.
XSLoader- it is an entry point into the XML Schema API for obtaining an XSModel for analysis/processing of the component model.
XMLGrammarPreparser - it provides API for preparsing schemas and DTDs for use in grammar caching (i.e. a lower-level alternative to SchemaFactory).