How to know if a detached JPA entity has already been persisted or not?

问题

I have a JPA entity instance in the web UI layer of my application. I'd like to know at anytime if this entity has been already persisted in database or if it is only present in the user session.

It would be in the business layer, I would use entitymanager.contains(Entity) method, but in my UI layer I think I need an extra attribute indicating whether the entity has been saved or not. How implement that ? I'm considering following option for the moment:

a JPA attribute with a default value set by the database, but would force a new read after each update ?
a non JPA attribute manually set in my code or automatically set by JPA?

Any advice / other suggestions ?

I'm using JPA 1 with Hibernate 3.2 implementation and would prefer stick to the standard.

回答1:

First, let's remind the various states of an entity. From the JPA 1.0 specification (in section 3.2 Entity Instance’s Life Cycle):

This section describes the EntityManager operations for managing an entity instance’s lifecycle. An entity instance may be characterized as being new, managed, detached, or removed.

A new entity instance has no persistent identity, and is not yet associated with a persistence context.

A managed entity instance is an instance with a persistent identity that is currently associated with a persistence context.

A detached entity instance is an instance with a persistent identity that is not (or no longer) associated with a persistence context.

A removed entity instance is an instance with a persistent identity, associated with a persistence context, that is scheduled for removal from the database.

And a graphical illustration:

So, by definition, a detached entity has already been persisted, and I actually don't think that this is your real question. Now, if you want to know if an entity is new (i.e. doesn't have any persistent identity), what about this:

@Transient
public boolean isNew() {
    return (this.id == null);
}

来源：https://stackoverflow.com/questions/2779857/how-to-know-if-a-detached-jpa-entity-has-already-been-persisted-or-not

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