javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist


One of the issues to get your head around in both Hibernate and JPA is how to handle detached entities. In Hibernate one has to deal with the session object and in JPA it is called the persistence context.

An object when loaded in the persistence context is managed by JPA/Hibernate. You can force an object to be detached (ie. no longer managed by Hibernate) by closing the EntityManager or in a more fine-grained approach by calling the detach() method.

So it is very time consuming to debug when you face “Detached entity” exception being thrown by JPA/HIbernate. THere are few possible things you should look for.

1. See if you trying to persist or merge an entity which has the same id as another entity, and which is already present in the PersistenceContext.

2. See if you you’ve specified that @Id is GENERATED by Hibernate. Do not set an ID before you save/persist it. Hibernate looks at the Entity you’ve passed in and assumes that because it has its PK populated that it is already in the database.
save() and persist() do almost the same things with slightly different semantics . persist() is JPA compliant and save() is a carryover from the original Hibernate. Mainly, save() returns the PK and persist() does not. However, both will generate the PK before the actual SQL INSERT happens (if the PK is generated and not assigned).

One workaround I did to solve this was to first find and then save the entity. See below example.

@PersistenceContext(unitName = “JPAUnit”)
private EntityManager em;

public void saveDetails(EntityManager em, User user){
em.find(User.class, user.getId());


R Vashi

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