You may find the Cassowary Constraint Solver of interest. I've reworked the libraries and an example program for Java 5+ compatibility. I'll be releasing the revised sources soon via the original authors, Greg Badros and Alan Borning.
I took an interest in constraint solvers in the early 1980s and was especially interested in the work of Alan Borning and his students as it relates to computer graphics.
The temporary link for the demo is at http://www.cyberthinkers.com/cassowary/cda.html