Session methods to use for object conversations
A new instance being attached to the session. An insert will be scheduled.
Call Update to make a transient object persistent again. It will force a SQL update on the transient object. This is because Hibernate does not know whether the object is dirty or not and to be safe by default schedules an update.
This method will throw an exception, if the entity is already registered with the session. - NonUniqueObjectException is thrown.
Either a save or an update will be called based on whether the identifier exists or not. No identifier - save is called, else update is called. Or for a better understanding, if the object is transient, then a save is called, if the object is persistent, then an update is called.
Reattaching an unmodified instance -
If you know for sure that an object is not modified and you just want to make it persistent again -
Changes made before lock is called is not propogated to the database. If there are any changes made after that call, then an update will be scheduled.
If an entity already exists in the session, the detached instance values are copied into the existing entity of the session.
If the entity does not exist in the session, a get() is called and the entity is loaded, then the detached instance values are copied.
If the entity does not exist in the DB, then a new instance is created.
The merged instance - the persistent instance is returned back, hence do not use the detached instance thereafter.
Hibernate will re-read the data from the database, if you believe something could have changed the state of the object in the DB.