JPA双向@OneToOne和级联合并

Question

If I have those two entities:

@Entity class A { 
    @OneToOne(cascade = CascadeType.MERGE, mappedBy = "a") B b; 
    //getters+setters
}
@Entity class B { 
    @OneToOne(cascade = CascadeType.MERGE) A a; 
    //getters+setters
}

Does the JPA specification guarantee this?

A a = new A();
B b = new B();
a.setB(b);
b.setA(a);
A managedA = entityManager.merge(a);
// after commit, managedA will have a reference to a managed B which JPA implementation will link to managedA instead of original unmanaged A
Assert.assertTrue(managedA.getB().getA() == managedA);

My understanding is that it works (at least with latest EclipseLink, at least sometimes), but that the spec doesn't guarantee that.

Counter-examples / spec excerpts welcome :-)

Answer 1

You are right, the JPA specification does not guarantee that. In fact in most cases the semantics for merge() is that it returns a new instance - a managed instance created by the entity manager, into which the state of the given object is merged - but this is just a simplification. If it attached the very object that you put in then the method could actually return void (like the persist() method does). I have no clue why it was designed like that - it was actually a bit of a pain in the ass to have this different semantics and it took us some time to encapsulate the desired behavior in our application code.

The actual semantics of the merge operation can be found in 3.2.7.1 Merging Detached Entity State of the JPA specifications final release. It does behave differently depending on the object that you put in and on the current persistence context.

Side note: An entity does become detached as soon as the transaction is over as long as you don't use an extended persistence context.

Please also note that you should usually not compare objects using the == operator since this is actually not the semantics you desire for your object comparison (jvm identity is not equal to business object identity)