Java linkbase

I'm working on a book about Java and XML, and I'm looking for an XLink
example that's a little more sophisticated than my ancient one at:

http://simonstl.com/projects/xlinkfilter/code/linkmap.htm

In particular, I'm curious if there's an open source Java linkbase out
there, something that stores XLink information persistently and permits
queries.  

(Commercial is possible, but the rest of the book is open source, so I'd
prefer to stay in that context.)

-- 
Simon St.Laurent
Ring around the content, a pocket full of brackets
Errors, errors, all fall down!
http://simonstl.com -- http://monasticxml.org