Unable to install openjdk-8-jre-headless on Debian Jessie

You can try below command. It worked for me:

apt install -t jessie-backports  openjdk-8-jre-headless ca-certificates-java

If above command doesn't work. Try Below Solution:

1. First, we will add jessie-backports repo.

   echo "deb http://http.debian.net/debian jessie-backports main" > /etc/apt/sources.list.d/jessie-backports.list

2. Install Jessie-backports command:

   sudo apt-get update
   sudo apt-get install -t jessie-backports openjdk-8-jre-headless

3. Then Switch to new Java: 

   sudo /usr/sbin/update-java-alternatives -s java-1.8.0-openjdk-amd64