Hi and it's good news that you are packaging libresonic.
Yes, you can install openjdk8 from backport. You must just create a
backports.list with the URL of the Jessie-Backport Repos and then you move this list to the
You update then and you do an
sudo apt-get install -t jessie-backport -y openjdk-8
(like in this package : https://github.com/YunoHost-Apps/minidlna_ynh/blob/master/scripts/install#L65)
I hope that will help you,
PS: Don't hesitate to ask questions if you don't understand.