Java.Hibernate.Medium.Describe the difference between transient, persistent, and detached object states.

✅ Short Answer

• Transient: object exists in Java memory only, not yet saved to the database or associated with a session.
• Persistent: object is associated with an open Hibernate session, and changes are automatically tracked and saved.
• Detached: object was persistent, but the session was closed or cleared, so it’s no longer tracked by Hibernate.

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🔎 Detailed Explanation

🔹 Transient State

• Object is created with new, but:
  • Has no identifier assigned by Hibernate.
  • Not associated with any session.
  • No corresponding row in the database yet.
• Changes to the object are not tracked by Hibernate.
• Example:

MyEntity entity = new MyEntity(); // Transient
entity.setName("New");

🔹 Persistent State

• Object is associated with an open Hibernate session:
  • Loaded via session.get() / session.load().
  • Or saved via session.save(), session.persist().
• Hibernate tracks changes automatically and will propagate them to the database when the session is flushed or transaction committed.
• Example:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
MyEntity entity = session.get(MyEntity.class, 1L); // Persistent
entity.setName("Updated"); // Hibernate tracks this change
tx.commit();
session.close();

🔹 Detached State

• Object was previously persistent, but now:
  • The session was closed or session.clear() was called.
  • The object still has its identifier (primary key), but Hibernate no longer tracks changes.
  • Changes made to detached objects are not saved automatically.
• To persist changes, you must reattach the object using session.update(), session.merge(), or similar methods.
• Example:

Session oldSession = sessionFactory.openSession();
MyEntity entity = oldSession.get(MyEntity.class, 1L); // Persistent
oldSession.close(); // entity becomes Detached
entity.setName("Detached Change"); // Change is not tracked!

🧑‍💻 Code Walkthrough

// Transient
MyEntity entity = new MyEntity();
entity.setName("Transient");

// Persistent
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
session.save(entity); // Now persistent
entity.setName("Persistent Change"); // Hibernate tracks it
tx.commit();
session.close(); // Entity becomes detached here

// Detached
entity.setName("Detached Change"); // Hibernate doesn't track this anymore

📊 Quick Comparison Table

State	Associated with Session?	In Database?	Changes auto-tracked?
Transient	❌ No	❌ No	❌ No
Persistent	✅ Yes	✅ Yes/Will be	✅ Yes
Detached	❌ No (was associated)	✅ Yes	❌ No

📌 Key Takeaways

✅ Transient: brand new objects — no session, no DB row.
✅ Persistent: session-managed objects — changes auto-saved.
✅ Detached: previously persistent, but session is gone — changes ignored unless reattached.
✅ Understanding these states helps you avoid subtle bugs, like missing updates or unexpected inserts.