What will happen if you call save or saveOrUpdate on detached objects ?

NonUniqueObjectException

When you call saveOrUpdate() or just update() on a detached object, Hibernate throws NonUniqueObjectException.

There may be multiple copies modified and Hibernate doesn't know the latest one (as it doesn't have track of detached objects), so it throws NonUniqueObjectException.

We can avoid this exception, by calling merge() instead of saveOrUpdate() or save() method on detached objects.

merge() will merge all the changes in memory before the save.

Example

Session session = sessionFactory1.openSession();

Transaction tx = session.beginTransaction();

Item item = (Item) session.get(Item.class, new Long(1234));

tx.commit();

session.close(); // End of first session, "item" is detached

    

item.getId(); // The database identity is "1234"

item.setDescription("my new description");   // Update on detached object

Session session2 = sessionFactory.openSession();

Transaction tx2 = session2.beginTransaction();

Item item2 = (Item) session2.get(Item.class, new Long(1234));

// session2.update(item);   // throws NonUniqueObjectException

Item item3 = session2.merge(item);  // Success!

tx2.commit();

session2.close();