The MIT-licensed Rootbeer compiler weighs in at 20k lines of code for Windows and Linux. Rootbeer allows for all major Java code to be executed on the graphics processor in a seamless manner without explicitly targeting the graphics processor at development time. The code is written by a Phil Pratt-Szeliga of Syracuse University.
The goal of this work is to bring high-performance GPU computing to Java with minimal effort from the developer. More information on the Rootbeer Java project is available from its GitHub page.