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I understand that update is used to put a detached object into persistent state if no other object with the same id and type is attached to the session. Merge doesn't care about states. It just returns a persisted object of the same type if it doesn't exist in the session or it updates the old object with the values of the new object. My questions is regarding database hits. Does the method 'update' and 'merge' hit the database immediately? or changes are made apparent in the database when the session is closed.
Edit: What happens if we call the update method on a persisted instance by the save method?. I thought the update method was just used on detached instances.
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Both the MERGE and UPDATE statements are designed to modify data in one table based on data from another, but MERGE can do much more. Whereas UPDATE can only modify column values you can use the MERGE statement to synchronize all data changes such as removal and addition of row.
Hibernate merge can be used to update existing values, however this method create a copy from the passed entity object and return it. The returned object is part of persistent context and tracked for any changes, passed object is not tracked. This is the major difference with merge() from all other methods.
Persist should be called only on new entities, while merge is meant to reattach detached entities. If you're using the assigned generator, using merge instead of persist can cause a redundant SQL statement.
As with persist and save, the update method is an “original” Hibernate method. Its semantics differ in several key points: it acts upon a passed object (its return type is void). The update method transitions the passed object from a detached to persistent state.
Hibernate handles persisting any changes to objects in the session when the session is flushed. update can fail if an instance of the object is already in the session. Merge should be used in that case. It merges the changes of the detached object with an object in the session, if it exists.
Update: if you are sure that the session does not contains an already persistent instance with the same identifier,then use update to save the data in hibernate
Merge: if you want to save your modifications at any time with out knowing about the state of an session, then use merge() in hibernate.
When the entity instance is in the persistent state, all changes that you make to the mapped fields of this instance will be applied to the corresponding database records and fields upon flushing the Session. The persistent instance can be thought of as “online”, whereas the detached instance has gone “offline” and is not monitored for changes.
This means that when you change fields of a persistent object, you don’t have to call save, update or any of those methods to get these changes to the database: all you need is to commit the transaction, or flush or close the session, when you’re done with it. It is important to understand that all of the methods (persist, save, update, merge, saveOrUpdate) do not immediately result in the corresponding SQL UPDATE or INSERT statements. The actual saving of data to the database occurs on committing the transaction or flushing the Session.
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