Lazy loading not working with one to one mapping in Hibernate

Question

I am referring this link https://developer.jboss.org/wiki/SomeExplanationsOnLazyLoadingone-to-one to understand why lazy loading doesn't work with one to one but works with one to many.

I don't understand why SET object is never null in case of one to many as said in the above link.Can someone please explain?

I still don't understand why lazy loading works in one to many but not in one to one associations.Please explain.

Answer 1

The default fetch type of one to one relation is Eager, so to change this you have to annotate the field as

@OneToOne(fetch=FetchType.LAZY)

Answer 2

To be honest, I didn't even know about this until now, but this is how I understood that article.

For @OneToMany Hibernate creates a wrapper for the set, and that wrapper is never null. So, if there is no data on the other side, it will just be an empty set, while still being not null.

But for @OneToOne wrapper (proxy) can't be used because getCee() (from that article's example) would never return null, which isn't right. null for this property means there isn't a matching row in target table, not null means there is. Considering this, wrapper only makes sense if relation is mandatory (constrained=true).

Answer 3

The important point here is that in implementing a one-to-many relationship, the relationship is represented as a Set, which is an interface that Hibernate has provided an implementation for, and this implementation is what implements the lazy loading.

However, in the case of a one-to-one mapping, Hibernate has not at any point implemented a derived class for whatever arbitrary class the main class has a one-to-one relationship with.