The semantics of the persist operation, applied to an entity X are as follows:
- If X is a new entity, it becomes managed. The entity X will be entered into the database at or before transaction commit or as a result of the flush operation.
- If X is a preexisting managed entity, it is ignored by the persist operation. However, the persist operation is cascaded to entities referenced by X, if the relationships from X to these other entities are annotated with the cascade=PERSIST or cascade=ALL annotation element value or specified with the equivalent XML descriptor element.
- If X is a removed entity, it becomes managed.
- If X is a detached object, the EntityExistsException may be thrown when the persist operation is invoked, or the EntityExistsException or another PersistenceExceptionmay be thrown at flush or commit time.
- For all entities Y referenced by a relationship from X, if the relationship to Y has been annotated with the cascade element value cascade=PERSIST or cascade=ALL, the persist operation is applied to Y.
The semantics of the merge operation applied to an entity X are as follows:
- If X is a detached entity, the state of X is copied onto a pre-existing managed entity instance X' of the same identity or a new managed copy X' of X is created.
- If X is a new entity instance, a new managed entity instance X' is created and the state of X is copied into the new managed entity instance X'.
- If X is a removed entity instance, an IllegalArgumentException will be thrown by the merge operation (or the transaction commit will fail).
- If X is a managed entity, it is ignored by the merge operation, however, the merge operation is cascaded to entities referenced by relationships from X if these relationships have been annotated with the cascade element value cascade=MERGE or cascade=ALL annotation.
- For all entities Y referenced by relationships from X having the cascade element value cascade=MERGE or cascade=ALL, Y is merged recursively as Y'. For all such Y referenced by X, X' is set to reference Y'. (Note that if X is managed then X is the same object as X'.)
- If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.