Probably very late, but as this page is pretty high ranked on google when searching "java psd library" I ll share my experience.
-As Boris said the java-psd-library is read only.
It has trouble reading recent CS4 format. If you ever find yourself with an error opening a psd file, opening it in Gimp 2.8 and saving it again will probably fix the issue.
-As I needed to work on layered graphic files, I took time to search for an alternative. The PSD format is proprietary and access to specs seems restricted.
I found that the OpenRaster format is a pretty good alternative : http://en.wikipedia.org/wiki/OpenRaster
It has open specs and it s really simple. It s a zipped file containing layers as png and a descriptor file. Gimp handles this format quite well and it s easy to export a psd file to an ora file.
You can find some Java tools to manipulate them. It s a part of a more vast project which is still in Alpha : https://code.google.com/p/mediaz/
Hope it ll help people.