The persistence context

Key points

• The persistence context is the EntityManager workspace: every entity it loaded or saved, held for the length of the transaction.
• Four states: transient (a plain new object), managed (tracked), detached (was tracked, context is gone), removed (queued for DELETE).
• persist makes an object managed. merge copies a detached object back in and returns the managed copy - the original stays detached.
• Inside one context, one row means one object. Load the same id twice and you get the same instance with no second SELECT.
• That identity map is the first-level cache. It is always on, one per EntityManager, and cannot be turned off.

Example

@Service
public class BookService {

    @PersistenceContext
    private EntityManager em;

    @Transactional
    public void lifecycle() {
        Book book = new Book("Effective Java", LocalDate.of(2018, 1, 6)); // transient
        em.persist(book);                       // managed, id assigned

        Book again = em.find(Book.class, book.getId());
        System.out.println(again == book);       // true - no SELECT, same context

        em.detach(book);                         // detached: edits are ignored
        book.setTitle("Effective Java, 3rd ed");

        Book managed = em.merge(book);           // a managed COPY comes back
        managed.setTitle("Effective Java");      // this one will be saved

        em.remove(managed);                      // removed: DELETE at flush
    }
}

Managed entities live in the persistence context. Everything Hibernate does starts there.