Java.Hibernate.Beginner.How do you update an object in Hibernate?

There are two main ways to update an object in Hibernate, depending on whether the object is still persistent or has become detached:

🔹 1) Updating a persistent object (easiest case)

• If the entity is still attached to an open session (persistent), simply modify its fields — Hibernate’s dirty checking will detect the changes and automatically generate the SQL UPDATE on flush() or commit().

Example:

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

User user = session.get(User.class, 1L); // user is now persistent
user.setEmail("newemail@example.com");   // modify persistent object

tx.commit();                            // flushes changes → SQL UPDATE
session.close();

✅ Here you don’t need to call update() — Hibernate tracks changes automatically!

🔹 2) Updating a detached object

• If the entity was loaded in a previous session but is now detached (e.g., session was closed), you must reattach it to a new session using either:
  • session.update(entity) → reattaches the entity → Hibernate will update its state in the database.
  • session.merge(entity) → copies the state of the detached object into a managed persistent instance, and returns it.

Example with update():

// Detached object (e.g., loaded before, session closed)
User detachedUser = new User();
detachedUser.setId(1L);
detachedUser.setEmail("updatedemail@example.com");

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

session.update(detachedUser);           // reattaches → schedules UPDATE

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

Example with merge():

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

User managedUser = (User) session.merge(detachedUser); // returns persistent copy

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

🔹 Which to use — update vs. merge?
✅ update() → works only if there’s no persistent instance with the same ID already in the session (else throws NonUniqueObjectException).
✅ merge() → safer in complex scenarios because it copies the state and returns a new managed object → avoids duplicate issues.

✅ Key takeaway:

• Persistent object? Modify fields directly → Hibernate auto-updates.
• Detached object? Reattach with update() or merge() → then commit the transaction.