As far as I'm aware, Java interpreters on Amiga platforms are currently too limited. It would require full JRE graphical support as well as support for Swing API. It may be possible, however, to run some non-graphical jAMAL code modules on e.g. Kaffe or Jamiga, albeit slothfully. (A debug module is provided for such non-graphical testing.)
Java is too slow for vintage hardware. A decent Java VM for AROS would be a better idea for this sort of project. jAMAL runs fine on an Intel Atom netbook (1.6GHz), which is the very lowest-end modern PC available, however it would choke on a 50MHz Amiga. (Besides, François Lionet already wrote a fast AMAL compiler for Motorola 68k-based Amiga platforms.)
You'll also find this alpha build of jAMAL is currently very memory hungry, although will run fine on a 512MB PC.
jAMAL source is now available:
https://sourceforge.net/projects/javaamal/