Summay on 20080820: Hibernate operation method comparation

Hibernate provide kinds of operation methods to save/update/delete object. here we just give one simple conclusion of those method and their difference:

 

save/update/saveOrUpdate

those three methods are used to save transient/detached object to database via session.

so, they are mainly used for transient/detached object.

 

For persistent object, there is no use to use these method, because when session is flushed, the persistent object will be updated to database automatically.

 

 

save/persist:

those two methods are both used for save, and they are used for those transient/detached object, however, there is some difference:

1.1. when use save, the object will be changed to persistent object, it will be associated with one session, the identifier will be retured and assign to the object immediately

 

1.2. when use persist, the object will be saved to database, however, persist does not guarantee that the identifier will be returned immediately, so maybe after persist, the identifier is still 'null'

 

update/merge

update will be used for transient/detached object, and after operation, object will be changed to persistent object

merge will be used for save/update the transient/detached object too, however, there is some differrence, here is the working process of merge:

   1.2.1: if object is new, save it to database directly

   1.2.2: if object is detached, find it in session, if found, override the object in session and save to database

   1.2.3: if object is detached, find it in session, if not found,  load it from database, and save/update the object

however, after use persist, the object is still transient/detached object, not persist 

 

About the exception of 'another object with the same identifier' when save/update

We have said that save/update is used for the detached/transient object, so the method will associate the object with the session, if in the session, there is any object that with the same identifier, session will not be able to manage the two object, because their identifier id is the same, session do not know how to save/update the information depends on which object.

 

solution:

for this case, it's better to use the merge method to do merge.