Java.Hibernate.Middle.How can you reattach a detached object to a session?

✅ Short Answer

You can reattach a detached object to a Hibernate session using either:

• session.update(detachedEntity) → reattaches directly; expects no conflicting persistent instance with the same ID in the session.
• session.merge(detachedEntity) → copies the detached entity’s state into a new or existing persistent instance; always returns the managed instance.

🔎 Detailed Explanation

🔹 Using session.update()

• Reattaches the exact detached entity instance to the session.
• Hibernate starts tracking this object again → changes to it will be persisted on flush/commit.
• ⚠️ Important: if there’s already a persistent instance with the same identifier in the session, update() will throw a NonUniqueObjectException.
• Best used when you know the session is clean (no existing persistent instance with same ID).

🔹 Using session.merge()

• Safer alternative: creates or fetches a persistent instance, then copies the state of your detached object into it.
• Returns the managed persistent instance — you should use this object going forward.
• Avoids NonUniqueObjectException because it reconciles state with the session automatically.
• Recommended when:
  • You’re unsure about session state.
  • Or working in high-concurrency environments.

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🧑‍💻 Code Example

// Detached object from a previous session
MyEntity detachedEntity = ...;

// Open a new session
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

// Option 1: Reattach directly
session.update(detachedEntity); // Now Hibernate tracks this instance again

// OR

// Option 2: Merge state into a managed instance
MyEntity managedEntity = (MyEntity) session.merge(detachedEntity); // Use managedEntity for further operations

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

📊 Quick Comparison Table

Method	Behavior	Returns	Notes
update()	Reattaches the same instance	void	Fails if persistent instance with same ID exists
merge()	Copies state into a new/existing instance	Managed object	Handles conflicts safely

📌 Key Takeaways

✅ Use update() if you know the session is clear of the same entity ID.
✅ Use merge() to safely reattach detached objects in any session state.
✅ Always use the returned object from merge() — it’s the one Hibernate tracks after merging.