Jenkins requires Java > 1.7 that ships by default. Install java from backports:

echo "deb http://http.debian.net/debian jessie-backports main" | tee --append /etc/apt/sources.list.d/jessie-backports.list > /dev/null

Add jenkins repo:

wget -q -O - https://pkg.jenkins.io/debian/jenkins.io.key | apt-key add -
sh -c 'echo deb http://pkg.jenkins.io/debian-stable binary/ > /etc/apt/sources.list.d/jenkins.list'
apt-get update
apt-get install jenkins

• Debian 8

• https://jenkins.io/doc/book/installing/#linux
• https://linux-tips.com/t/how-to-install-java-8-on-debian-jessie/349/2