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