I did something similar to this on a Raspberry Pi running Fedora. The application I needed to start was Python, not Java, but it's actually pretty easy.
It is detailed here http://makerbox.wordpress.com/2014/0...tric-painting/
However, I have not done exactly what you are looking to do; I did not need a GUI. So I'm thinking out loud here, with some ideas rather than exact instructions:
The way I think I'd go about what you are trying to do is to uninstall the desktop (LXDE) and the session manager (I think Lubuntu uses LightDM). You will probably want to do that from a text console (ctrl-alt-F3, for example).
Then find the systemd startup script that executes the graphical server (X). Modify it such that when the X environment starts, so does your java application.
I'm not sure what *buntu does when no session manager is found, so you might need to enable auto login to your text console. That's detailed in the post.
It will require some experimentation and testing.
If you try this sort of thing, let me know how it goes and what you did to make it happen, and I'll make my responses here less dependent on the external link.