JDK 1.4 embeds a number of important XML jars inside it's rt.jar (or otherwise inside the JDK). Which should be able to detect which classes are coming from a JDK-supplied jar (normally older versions of things like JAXP parsers/processors) and which are coming from use of the Endorsed Standards Override Mechanism at http://java.sun.com/j2se/1.4/docs/guide/standards/ (Note: we should consider using reflection to keep JDK 1.1.x compatible for the time being; or perhaps encapsulate this in a specific separate class that can be used or not used depending on JDK version)