Question

I’ve a misunderstanding of Spring @Transactional annotation and persist. I am using Spring 3.1, with JPA and Hibernate. I thought that persist meant, add the entity to the persistence context (but don’t execute any query until commit or flush), and that the @Transactional annotation meant, wrap the method with a transaction.

However, in this short example, when the execution pointer reaches persist, it fails with an exception, since name can’t be null (db constraint).

import javax.persistence.EntityManager;

@PersistenceContext
private EntityManager entityManager;

@Transactional
public void test() {
    Brand brand = new Brand();
    entityManager.persist(brand);
    brand.setName("test");
}

If I swap setName() and persist(), everything works. However, I don’t understand why the other way around doesn’t since I thought that any query would be built and executed at the end of the method.

Can someone please explain?

Answer 1

In JPA, once an object passed to persist() it becomes “managed”, as part of becoming managed JPA implementation must generate an id for the persistent object.

If id generation is based on auto-increment (GenerationType.IDENTITY), then an insert statement needs to be issued to the db to get and assign the key. When the id generation is based on sequence / table then ids are managed and assigne by the JPA Implementation managed id pools, in which case a straight insert is not a requirement.

Having an object is passed to persist() and has become managed, any changes to it is persistent fields must be flushed to the database at the and of the transaction. In your case if the id generation is Identity then an insert must be followed an update. If the id generation is some other method then, a single insert statement is sufficient. If the transaction is rolled back, no SQL should be get sent to database at all.

This is the implementation in Batoo JPA.

Hope this makes sense.