root@jenkins:/# cat /etc/apt/sources.list
deb http://ftp.debian.org/debian wheezy main contrib non-free
deb http://security.debian.org wheezy/updates main contrib non-free


root@jenkins:/# tac /path/to/jenkins.log | less
(...)
java.lang.UnsupportedClassVersionError: 50.0
Jenkins requires Java7 or later, but you are running 1.6.0_35-b35 from /usr/lib/jvm/java-6-openjdk-amd64/jre
        at Main.main(Main.java:90)
java.lang.UnsupportedClassVersionError: 50.0
Jenkins requires Java7 or later, but you are running 1.6.0_35-b35 from /usr/lib/jvm/java-6-openjdk-amd64/jre
(...)