You can write your own XPath lib in Java to make certain things easier and re-usable. For an example see here.
From the documentation of 12C, nothing indicated that things would have been changed so I stuck in an old library and restarted JDev. When I opened the xslt mapper, I didn’t see my functions under User Defined. This made me wonder if maybe something did have changed. Last week I attended the OFM Summercamp in Lisbon and Wilfred van der Deijl mentioned that JDev caches a lot so maybe clearing the cache would solve the problem. Someone also mentioned you could use -clean as a parameter when starting JDev but to be quite thorough I just renamed the cache folder which is default under C:\Users\