If you try opening http://jenkins-ci.org/jenkins.jnlp under Java7 update 40 you will get the exception mentioned in http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8022939

Here is what an Engineer over at Oracle had to say:

Tested this back to 7u13 and it still didn't launch due to some NPE somewhere, in every case because there is either no explicit codebase or no codebase can be derived. The only bug could see here is perhaps javaws should check for this case and throw some sort of Exception explaining that it cannot determine the codebase. For that reason, we leave it open.

As for Jenkins, they need to do one of the following for proper deployment:

• Use deployJava.js to launch the webstart applet instead of just using a <a href="xxx"> link
• Add codebase/href attributes to jenkins.jnlp
• Launch manually by using the URL instead of downloading: "javaws http://jenkins-ci.org/jenkins.jnlp"

Please use deployJava.js or update jenkins.jnlp.