Thanks to Romain Cambier we now have an Application library available for Java. The library has been published to Maven and as you'll see when reading through the QuickStart it is really easy to set up.
Do you also want to help us bring The Things Network to all devices, gateways and applications out there? Drop us a mail if you'd like to become an ambassador like Romain!
The library wraps the MQTT protocol of our upcoming production environment, so be aware that you cannot use this in the current staging environment.