JPA: Detach a list of attached read-only entities

Question

Is it beneficial to detach entities of large lists after querying, i.e. use a default pattern for those entities like:

// Find all
for List<Device> devices = em.createNamedQuery("find.*.devices", Device.class).getResultList();

// Detach all
for (Device d : devices) {
  em.detach(d);
}

We could avoid some kinds of negative effects like erroneously update the database for entities considered as "read only" (which could be guaranteed) or save memory.

Answer 1

If you want to use a list without any updates in the database your can get the objects detached.

You have the following options:

1. Do not open a transaction if you are using programatic transaction. Notice that the transaction was not opened:

   EntityManager em = EntityManagerFactory.createEntityManager();

   List personList = em.query()

2. Mark your method as transaction not suported @TransactionAttribute(NOT_SUPPORTED)

3. If you clear your EntityManager you will detach all enties: em.clear()