Implementing Java functionality into 3dgs needs you to
either be a good c++ plugin programmer, where the dll is
then linked to java, or you need to make a workaround,
using a "communication" bridge of some kind to the acknex engine.

This could be realized by writing a dll, that communicated via sockets
with the Java Virtual Machine.

Or you do a very primitive approach by sharing
data, written into files between an instance of
the acknex engine, and a JVM, running at the same time.

You could also writa a Java-based server software, and an
internet communication protocol to 3dgs. (ask the maker
of GSTNet, for the communication protocol he uses, with this you can
establish a net communication to 3dgs.)
Using Java as Serversoftware is the only senseful use of Java+acknex I can think of.

If you simply look for a 3d Engine, that runs in an Java
environment, look for something else than 3dgs.