Symptoms:

Your build breaks with messages like:

$ /home/automan/jenkins/tools/jdk-6/jdk.sh -noregister
14:30:27 /home/automan/jenkins/tools/jdk-6/jdk.sh: line 1: jdk-6u16-oth-JPR@CDS-CDS_Developer: command not found
14:30:27 /home/automan/jenkins/tools/jdk-6/jdk.sh: line 2: jdk-6u22-oth-JPR@CDS-CDS_Developer: command not found

Or

FATAL: null
15:01:53 java.lang.NullPointerException
15:01:53 at hudson.tools.JDKInstaller.locateStage1(JDKInstaller.java:355)
15:01:53 at hudson.tools.JDKInstaller.locate(JDKInstaller.java:298)
15:01:53 at hudson.tools.JDKInstaller.performInstallation(JDKInstaller.java:121)
15:01:53 at hudson.tools.InstallerTranslator.getToolHome(InstallerTranslator.java:61)

Alternatively, when you configure Jenkins and scroll down to the JDKs section, you won't see any JDK installers.

The problem seems to be that this URL http://mirrors.jenkins-ci.org/updates/updates/hudson.tools.JDKInstaller.json contains no JDKs.