4.4.3. Detached objects and automatic versioning

With this paradigm, each interaction with the data store occurs in a new persistence context. However, the same persistent instances are reused for each interaction with the database. The application manipulates the state of detached instances originally loaded in another persistence context and then merges the changes using EntityManager.merge():

// foo is an instance loaded by a non-extended entity manager
foo.setProperty("bar");
entityManager = factory.createEntityManager();
entityManager.getTransaction().begin();
managedFoo = entityManager.merge(foo);  // discard foo and from now on use managedFoo
entityManager.getTransaction().commit();
entityManager.close();

Again, the entity manager will check instance versions during flush, throwing an exception if conflicting updates occurred.