I'm a bit unsure of the discussion about object identity in the book Hibernate in Action (pg. 119).

While it is understood that hibernate guarantees that two references to the same in memory object for the same identifier value are equal (in terms of address equality), but if I have a reference to a detached object, say, to record A, and I call load(SomeClass.class, "A") in a new transaction, will the latter equals to the earlier if I reattach the earlier detached object to the transaction? Does hibernate guarantees reference equivalence in this case?

If you are doing the second transaction in the *same session*, then its guaranteed to be the same object. Otherwise it would be different

In that case, hibernate can no longer guarantee that there's only one instance for a particular row - unlike EJB, there could be more than one instances referring to the same row which are equivalent (by values) but not identical (address). Am I correct?

Thank you.

Yes, there could be multiple instances but in multiple sessions. Hibernate guarantees that there will be one object in a session. If you are worried about multiple sessions having records and trying to write over one-anohter, you probably need to look at one of the Optimistic concurrency strategies indicated at
http://www.hibernate.org/hib_docs/v3/re ... optimistic