copy a desktopfile from /usr/share/applications/ to your Desktopdirectory.
change the Exec= line to "java -jar yourjarfile.jar"
Test it by double clicking the desktop icon resulting from your copy.
After this is working, your the next step depends on your desktop environment:
Gnome: use the gnome tweak tool to add your "APP" to the autostart sequence.
Cinnamon: there is a seperate Start app to manage the autostartup. Use it to add your app to the list.
The Others: i have no idea, but it will be similar to the above :)