One EntityManager doesn't see updates done by another EntityManager

Question

I'm using two EntityManager instances in standalone application with RESOURCE_LOCAL transaction type. I perform such operations:

• Save entity using first EntityManager (em1)
• Update entity using second EntityManager (em2)
• Read entity using first EntityManager (em1)

The problem is that em1 on the 3rd step doesn't see updates done by em2.

EntityManagerFactory emf = Persistence.createEntityManagerFactory("test");

// Step 1: create entity
EntityManager em1 = emf.createEntityManager();
em1.getTransaction().begin();
Article article = new Article("article_1");
em1.persist(article);
em1.getTransaction().commit();

// Step 2: update entity
EntityManager em2 = emf.createEntityManager();
em2.getTransaction().begin();
Article articleForUpdate = em2.find(Article.class, 1L);
articleForUpdate.setName("updated article_1");
em2.persist(articleForUpdate);
em2.getTransaction().commit();

// Step 3: read updated entity
em1.getTransaction().begin();
Article updatedArticle = em1.find(Article.class, 1L);
em1.getTransaction().commit();

log.info("updated entity: {}", updatedArticle); // logs stale data

em1.close();
em2.close();
emf.close();

Can anyone explain why em1 reads stale data?

Answer 1

The EntityManager looks first in its first level cache if a reference of the entity exists before requesting the database.
The EntityManager instance referenced by the em1 variable has the Article entity with the id 1 in cache as it has persisted it.
So this statement will retrieve the entity from the cache :

Article updatedArticle = em1.find(Article.class, 1L);

To prevent this behavior you have multiple ways :

• detach the entity from the EntityManager context by invoking EntityManager.detach().
• refresh the state of the entity from the database,by invoking EntityManager.refresh(). In this case, the the find() query is not any longer required.
• more radical : clear the EntityManager context by invoking : EntityManager.clear()