Wow, I broke my brains over this for the last three days.
I installed the apex listener as mentioned in the manual. It mentions you can use Java 6 Update 20 JDK or later.
However, what they mean is that you should use Java 6 JDK, and you can use update 20 or later.
What I did in my stupidity is to use Java 7. That’s later then Java 6 upd 20 right?
If you use Java 7 your listener might work (which was the case in my situation) but connecting to it using SQL Developer is a “no go” 🙁