Jenkins, specifying JAVA_HOME

Your JAVA_HOME variable must be set to /usr/lib/jvm/java-6-openjdk and it must be available for the user that starts Jenkins.

From Kyle Strand comment:

As of April 2015 (I think), Jenkins requires Java7. Also note that the java binary path (JAVA) must be set to the correct version if the system default is still Java 6. Finally, for anyone wondering where these variables are set, it's in a config file listed with the installation instructions on the Jenkins webpage (e.g. for Debian it's /etc/default/jenkins).

Using Jenkins 2 (2.3.2 in my case), the right way seems to insert the following into your pipeline file:

env.JAVA_HOME="${tool 'jdk1.8.0_111'}"
env.PATH="${env.JAVA_HOME}/bin:${env.PATH}"

"jdk1.8.0_111" beeing the name of the java configuration initially registered into Jenkins