Java library for creating straight skeleton?

I wrote this a little while back. Not sure if it's robust enough. https://github.com/twak/campskeleton

(edited for 2018...)

See http://www.sable.mcgill.ca/~dbelan2/roofs/roofs.html which contains an applet.

You may be able to use the JTS Topology Suite. It is a very capable library that I've used on a number of projects - never for straight skeleton, but it may be possible.