I have a big problem in my diploma project and would be very glad if you guys could help me! I made a Maven Multi Module Project and have 3 \"Core-projects\"
Refering to Oliver Gierke's hint:
When the manipulation of the persistance.xml does the trick, then you created a normal java-class instead of a entity-class.
When creating a new entity-class then the entry in the persistance.xml should be set by Netbeans (in my case).
But as mentioned by Oliver Gierke you can add the entry later to the persistance.xml (if you created a normal java-class).