After having googled around and found this and this, I have decided for the simplest possible solution: use a char in the Entity and expose it with a boolean getter/setter - anyway JPA uses "field" access to the Entity, so she is not disturbed by my setters and getters.
Incidentally all my boolean fields are NOT NULL, so I use directly a boolean rather than a Boolean.
It's a shame that JPA doesn't cover something so basic. Everybody knows that Oracle doesn't support a BOOLEAN data type; so I expect everyone is having the same problem.
Thanks for the simple example. I'm encountering the same problem.