Well, seems to me the first thing you would need would be a Schematron implementation written in Java. The Schematron site provides links to somebody who has written one. Following those links might also lead to information about how to use said implementation.
My Google keywords were "schematron java" by the way.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com