UnsupportedOperationException and XML Parser versions

I lost a day struggling with this Java error. I had been trying to parse a XML document in a Java web application. It had worked in previous deployments so I knew the code was sound. But when I deployed the WAR file onto my Tomcat6 server on Ubuntu Linux 9.10 I hit an obscure error I had not encountered before.

Apache Commons Digester was throwing an error when it tried to load a XML parser.

java.lang.UnsupportedOperationException: This parser does not support specification "null" version "null"
	at javax.xml.parsers.SAXParserFactory.setXIncludeAware(SAXParserFactory.java:448)
	at org.apache.commons.digester.Digester.getFactory(Digester.java:534)

This caused another exception in turn:

Digester.getParser:
NullPointerException

I tried everything: switching parsers from xerces to crimson; running tomcat with different JVMs; overriding classloaders; all to no avail. Finally I came upon a blog post which revealed that “[the] message This parser does not support specification “null” version “null” [is] the secret. That’s the default error message of the XML parser if you turn on XInclude but that isn’t a supported feature.”

The reason for the UnsupportedOperationException error was that the XML parser implementation I was using did not support the features Commons Digester was trying to set on it. I upgraded my version of xercesImpl.jar from 2.6.2 to 2.9.1, restarted tomcat, and the problem was solved!