Hi there,
i would like to ask you what would be the best approach to pass hibernate persistent objects, that might contain Collections, from the back to the front.
Considering that the front "does not know" hibernate, is there any other solution than converting the persistent objects to Dtos, creating then another layer of abstraction ?

Thank you in advance,
Thiago

Don't create another layer of abstraction, that's just a waste. Entity objects actually "don't know" about Hibernate either, so using them in the 'front' or presentation layer will not couple the presentation to the 'back' layer or persistence (Hibernate).

What about Collections such as Set ?
I've heard that is not a good idea to pass Hibernate Collections to the front because they are attached to a hibernate session.

What you think of that ?

You can transfer Domain Objects instead of DTO by network between layers, but in this case you must fully initialize properties of your Domain Objects, because if you try to acces proxy outside the Hibernete Session, you recive LazyInitializationException.

You can use fetch join in HQL and Criteria API or Hibernate.initialize() inside Hibernate Session for that.

I aggree with you.
My problem is that my hibernate objects have Sets that are quitte expensive in terms of data. If i have to full initialize them, i guess it might be a considerable cost to the memory and/or to the network.
Does it make sense ?

Yes, you're right. So you need to initialize only that associations, that you'll need on the other tier. Other associations must be leaved lazy.