For my Outside Engine I built a server-client communication system using Netty.IO for JME. Upon request, I have stripped it out of the Outside Engine, and created a standalone project for it.
You can find the source here: https://github.com/tlf30/monkey-netty/tree/master
You many notice that some things are structured odd, this is because this code came from the OutsideEngine, and had a supporting system underneath it that I had to strip out. I have tested this standalone system using the test I provided in the repo, but if you find any bugs please let me know so I can correct them. By adapting this for use without the Outside Engine I had to change some of the key network logic, so there may be bugs that I did not catch.
Please let me know if you have any questions, and feel free to use this in your projects.