JPA - saving changes without persist() invoked

Question

We are using Toplink implementation of JPA + Spring + EJB. In one of our EJBs we have something like this:

public void updateUser(long userId, String newName){
    User u = em.get(User.class, userId);
    u.setName(newName);
    // no persist is invoked here
}

So, basically this updateUser() method is supposed to update the name of a user with the given userId. But the author of this method forgot to invoke em.persist(u).

And the strangest thing is that it works fine. How can it be? I was 100% sure that without invoking em.persist() or em.merge() there is no way that changes could have been saved into database. Could they? Is there any scenario when this could happen?

Freetown

Answer 1

You're working with a managed entity. If the entity does not become detached because its entity manager is closed, all changes done to the entity are reflected to the database when the session is flushed/closed and the transaction commited.

From the Java EE tutorial:

The state of persistent entities is synchronized to the database when the transaction with which the entity is associated commits.

Edit for clarity and explanation: So there are three distinct modes that an entity could be in during its lifecycle:

• Unsaved: The entity has been instantiated, but persist() has not been called yet.
• Managed: The entity has been persisted using persist(), or loaded from the database, and is associated with an entity manager session. All changes to the entity are reflected to the database when the entity manager session is flushed.
• Detached: The entity's entity manager session was closed. Changes to the entity will not be reflected to the database automatically, but can be merged explicitly using the merge() command.

Borowski

Thanks. This helped a lot! I always thought that invoking em.persist() is required, even if object is associated with current EntityManager. – Freetown

Weird: In my setup, I have a reference to a managed entity. Then I change this entity's state. Then I call entityManager.flush(). In case this is running within a transaction, then the entity's changed state is written to database. In case this is running without a transaction, then the entity's changed state is not written to database. So is the following statement true?: "There always has to be a transaction commiting or an entityManager.persist(obj) call in order to write the latest state of obj to database.".... (I'm using JPA 2.0 with Hibernate (4.1.7.Final)) – Bomar

@Abdull, yes, the JPA spec does not define any autocommit behavior, so you need an ongoing transaction or nothing gets written to the database at all. – Borowski

@Henning, okay thanks! Currently, your answer states: "[...] All changes to the entity are reflected to the database when the entity manager session is flushed."...... What about rewriting it like: "[...] If there is an ongoing transaction, all changes to the entity are reflected to the database when the entity manager session is flushed." .... (or are there even more subtleties involved?) – Bomar

@Abdull, it really says "when the session is flushed/closed and the transaction commited", so I think we have the transaction part covered, right? – Borowski