I'm using MapJoin like this:
The em.createQuery(cq) call throws a NullPointerException:
The same code works fine on EclipseLink.
Seems that the problem is in the MapKeyHelpers.MapKeySource constructor, which calls the super constructor from AbstractPathSource with a null pathSource argument:
This way, the pathSource is initialized to null. Then, AbstractPathSource's getPathIdentifier() will always throw a NPE.