UPnP Port Forwarding for Java couldn't get any easier!
WaifUPnP is an extermely lightweight Java library that allows you to

• Open/Close TCP/UDP ports
• Check if there's an UPnP router available
• Check if a port is already mapped

using literally 1 line of code, as it should be!

It's as easy as

Java 6 or newer

Binaries, Source code, Javadoc and Examples
Browse GitHub repository

WaifUPnP is a very basic implementation of UPnP, that only scans for the default gateway, and can only open/close ports.
While this is enough for most people, if you need a full implementation of UPnP, you should take a look at Cling.

GNU LGPL Version 2.1 or newer