Applogies if this question have already been asked. I've searched the Hibernate docs and forum for a while without finding an answer.

I have a Spring - Hibernate application.

When merge(Object o) is called and an error is encountered an exception is immediately thrown, but if I use for example update(Object o) the exception doesn't get thrown until a flush is executed(for example before a commit). In my case an exception related to optimistic locking is thrown(HibernateOptimisticLockingFailureException), this exception is expected because of 2 users updating the same entity. I'm Guessing other types of exceptions will produce the same behavior.

Can someone help me understand why merge is throwing the exception instantly whilst with the update method nothing happens until flushing?

This behaviour forces me to always use a merge when the entity is under version control by hibernate since I want the exception to be thrown at an expected point. Since I use Springs HibernateTransactionManager I do not have explicit control over when the commit of the transaction is performed.

Hi RobbieH,

calling merge() takes immediate effect whereas calling update() is postponed until the session is flushed. Maybe, reading Modifying detached objects and Automatic state detection of the Hibernate docs gives you some more insight.

CU
Froestel

Thanks for your reply Froestel.

I have previously read those parts of the doc but I don't really see where they explain the difference between the 2 i.e why merge takes immediate effect whilst update don't. But maybe I'm missing something.

merge explained in "Automatic state detection" part:


1. The loading from the database will perform a flush, right?
2. if there is an existing persistent instance the changes in the versioned property will be detected at copy time?
Is this correct? I'm just guessing. Or do you mean that merge is doing an instant flush no matter what?

By the way this is exactly what I'm doing:

The main reason for asking this queston is that I want to know if merge is the only reasonable option in my case, and if I can trust merge always to throw an exception instantly i.e not wait until a flush. (Actually I would rather not use merge at all in this case)

Hi RobbieH,

as I wrote before


This means anytime you call merge() the action will be executed in that very moment. So it is guaranteed that the exception will be thrown right away. You would have the same effect on but in this case the call to flush() will be the trigger for your exception. You cannot determine precisely when the exception is thrown with update() since Hibernate might do some statement re-ordering and stuff.

So if you need to have the exception thrown in that very moment when you call the method to be executed then use merge().

CU
Froestel

Ok. Thank you Froestel.
Along with your answer and some further reading I now have a better understanding.