Question

Why does hibernate require it’s entities to be written to an interface in order for them to work correctly with lazy loading?

Answer 1

The only scenario where hibernate requires interfaces is collections. It’s because hibernate uses its own collection implementations for lazy loading – PersistentBag, PersistentSet, etc. and assigns them to your fields. The implementations hold a reference to the session so that they can fill their data whenever required.

Hibernate can assign PersistentSet to private Set<Foo> set; (they do it with reflection), but it is not possible to do so for private HashSet<Foo> set;, because PersistentSet does not extend HashSet

As for lazy @*ToOne associations – hibernate creates a proxy object, using javassist (or cglib), and it does not require an interface. This is a rarely used feature anyway, and the proxy is a subclass of the real object, so unless you use getclass() (which you should not do), it works fine.