In the cases where you want to use your own Customization Class, make sure that this java class is available to jdeveloper when it restarts. Otherwise , when you use the "Customization Developer" role, you will see that in the Jdeveloper you will see messages like
MDS-00035: cannot load the class:xxxxx

and the customization context will be "none".

To prevent this one way is to create a jar file containing your Customization class and place it in
jdeveloper/jdev/lib/patches.
All the jar files placed in this directory are avaliable in the CLASSPATH when jdev restarts.