Open source 3D render engine for Java
I suggest jMonkeyEngine, it is great.
jME (jMonkeyEngine) and here are some game demos
Just for rendering, how about Sunflow ?
(source: sourceforge.net)
Also, it should be possible to run other renderers that have commandline interfaces, like Yafaray or Pixie from Java.
Also, Ogre3D is interesting for real time stuff.