after performing an merge on the detached object in hibernate in the current session will the changes on the object be tracked?

Question

In a container managed transaction i get a detached object and merge it so that the detached object is brought to managed state.My initial question is by caching the Pojo java objects and merging is a better idea to get the object into session or performing the get of the data from the DB to get in to session context a better idea in terms of cost of operation/time involved in getting the data from the DB?If i am performing an merge at start to get the object into the session context and doing the modification on this merged object will the hibernate take care of generating all the required sql statements and at the end will it be taken care ?

Please comment back which is better approach to get the entity to session , using a merge of the cached detached object or fetching the data from the DB is lesser time consumption?

Answer 1

Copy the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance. The given instance does not become associated with the session. This operation cascades to associated instances if the association is mapped with cascade="merge".

According to the API it saves a copy when you perform the merge and then returns a new instance. Based on my experience its always better to merge at the end after you have performed all the updates on the objects in detached state. Its better because you will call merge operation only at the end when the object state is ready to be persisted.

Also this will perform better because the object is moved to persistent context at the end and hence Hibernate will not have to come into picture till the end.

Answer 2

when you call detach and then merge, merge returns you the attached entity in the context. it's a common mistake that users would use the passed entity after merge operation hoping that would be managed but this is not the case. you have to use the returned entity from merge which will be managed by hibernate and subsequent changes will be flushed at transaction end automatically. it doesnt matter much when u load your entity coz hibernate will anyways fire a select if it is already not loaded in the context. also even if you keep on doing changes to your managed entity, hibernate will fire update only when you exit your transaction or call flush() explicitly.