Question

If entity is deleted or managed it is stored (or its hash) in current EntityManger’s persistence context, so JPA know about its state. But how JPA implementation can get to know that given entity is new or detached? Checking if @ID is null will not always work. Is it JPA provider specific?
In other words how JPA know that it need to throw javax.persistence.EntityExistsException during merging?

Answer 1

Here’s how Hibernate does it:

• if the identifier is generated, use the presence of the identifier
• if not, and the entity is versioned (for optimistic locking), use the timestamp or version
• if the above is not possible, query the second-level cache or the database to know if the identifier already exists or not.