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Hibernate has a handful of methods that, one way or another, takes your object and puts it into the database. What are the differences between them, when to use which, and why isn't there just one intelligent method that knows when to use what?
The methods that I have identified thus far are:
save()update()saveOrUpdate()saveOrUpdateCopy()merge()persist()
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Difference between saving and persist method in Hibernate 1)The first difference between save and persist is there return type. Similar to save method, persist also INSERT records into the database, but return type of persist is void while return type of save is Serializable Object.
The important difference between the org. hibernate. Session class methods, save & saveOrUpdate is, save generates a new identifier and results in an INSERT query, whereas saveOrUpdate does an INSERT or an UPDATE. Save method stores an object into the database.
save() : It returns an entity's identifier immediately in addition to making the object persistent. So an insert query is fired immediately. persist() : It returns the persistent object.
The save() method INSERTs an object in the database. It will persist the given transient instance, first assigning a generated identifier. It returns the id of the entity created. The saveOrUpdate() calls either save() or update() on the basis of identifier exists or not.
Here's my understanding of the methods. Mainly these are based on the API though as I don't use all of these in practice.
saveOrUpdate Calls either save or update depending on some checks. E.g. if no identifier exists, save is called. Otherwise update is called.
save Persists an entity. Will assign an identifier if one doesn't exist. If one does, it's essentially doing an update. Returns the generated ID of the entity.
update Attempts to persist the entity using an existing identifier. If no identifier exists, I believe an exception is thrown.
saveOrUpdateCopy This is deprecated and should no longer be used. Instead there is...
merge Now this is where my knowledge starts to falter. The important thing here is the difference between transient, detached and persistent entities. For more info on the object states, take a look here. With save & update, you are dealing with persistent objects. They are linked to a Session so Hibernate knows what has changed. But when you have a transient object, there is no session involved. In these cases you need to use merge for updates and persist for saving.
persist As mentioned above, this is used on transient objects. It does not return the generated ID.
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