Java4gba is an early port of KVM, Sun's Java Virtual Machine, part of CLDC 1.1 RI

From the README.txt :
KVM is the Java VM you may find in mobile phones for running "Java games". This small work is only about CLDC, which means that there is nothing about graphics, sound, or whatever. You can only do some plain console output. Sorry, guys :)
However, it brings to GBA almost all Java interesting features like Exceptions, Threading, Monitors... You will find some examples that illustrate these Java features.
Just rteun the examples.bat script to build some example ROMs (you don't necessarily need a Java compiler, but it is recommended, especially if you want to play with the VM :))

Download it here
Download the source code here