do you know if it is possible to map a xor contraint with Hibernate?
if yes can you explain how?

What is an XOR constraint?

it is an association with two Class with a condition so if the condition is true the association is with the first class and if the condition is false with the second one.

So its at the normal Java level, not a database constraint? Or do you not mean "constraint", but "view"? If Java can do it, Hibernate can do it. If its a view, your database can do it. There are several extension points in Hibernate for custom behavior as well.

I think that it is at uml level. you can see an example at
http://www.essentialstrategies.com/publ ... ng/uml.htm

Uhm, so you didn't think about the physical implementation of nice pictures so far? :)