Java.Hibernate.Beginner.When i will get NonUniqueObjectException ? Give example

You get a NonUniqueObjectException if you try to reattach a detached entity using update(), but the session already has another persistent instance with the same identifier (primary key).

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🔹 Why does it happen?

• Hibernate ensures that within a single session, there can be only one managed (persistent) object representing a given database row (primary key).
• If you try to update() a detached object with the same ID while another object with that ID is already loaded in the session → Hibernate throws NonUniqueObjectException.

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🔹 Example showing how it happens:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

// 1) Load entity → becomes persistent
User persistentUser = session.get(User.class, 1L); 

// 2) Later, you try to update a detached instance with same ID
User detachedUser = new User();
detachedUser.setId(1L); // same primary key as persistentUser
detachedUser.setUsername("new_username");

session.update(detachedUser); // 💥 Boom! NonUniqueObjectException

tx.commit();
session.close();

✅ Why? Because:

• The session already manages persistentUser with ID=1.
• You try to update() another object (detachedUser) with the same ID.
• Hibernate sees two objects representing the same row in one session → throws org.hibernate.NonUniqueObjectException.

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🔹 How to avoid it?
✅ Instead of update(), use session.merge(detachedUser):

User managedUser = (User) session.merge(detachedUser); // merges state into existing persistentUser

merge() copies the detached object’s state into the persistent one → no duplicate, no exception.

✅ Key takeaway:
You get NonUniqueObjectException if you call update() with a detached object whose ID matches an already-managed entity in the same session.
Use merge() instead when you can’t guarantee there’s no persistent instance with the same ID.

Would you like a full example comparing update() and merge() behavior side by side?