What are the entity states in Hibernate?

In Hibernate, an entity can be in one of the following states:

• Transient: The object is created but not associated with a Hibernate session. It has no representation in the database. Operations on it are performed in memory but do not affect the database.

  // Student entity is created but not attached to a session
  Student student = new Student("Ivan", "Ivanov");
  

• Persistent: The object is associated with the current Hibernate session and managed by it. Any changes to the object will be synchronized with the database upon transaction commit or session flush. An object retrieved from the database using a session is in this state.

  // Student entity is retrieved from or saved to the database
  Session session = sessionFactory.openSession();
  session.beginTransaction();
  
  Student student = new Student("Petr", "Petrov");
  session.save(student); // student is now in Persistent state
  
  // Or
  // Student student = session.get(Student.class, 1L); // student in Persistent state
  
  session.getTransaction().commit();
  session.close();
  

• Detached: The object was previously associated with a session, but the session was closed or the object was explicitly detached (e.g., using session.evict(object)). The object still contains data from the database, but changes made to it will not be automatically synchronized with the database. To save changes, the object needs to be re-attached to another session.

  Session session1 = sessionFactory.openSession();
  session1.beginTransaction();
  
  Student student = session1.get(Student.class, 1L); // Persistent
  session1.getTransaction().commit();
  session1.close(); // student is now Detached
  
  // Modifying the object in Detached state does not affect the database
  student.setName("New Name"); 
  
  // To save changes, re-attach to a new session
  Session session2 = sessionFactory.openSession();
  session2.beginTransaction();
  session2.update(student); // student becomes Persistent again
  session2.getTransaction().commit();
  session2.close();
  

Transitions between states:

From/To State	Transient	Persistent	Detached
Transient	(remains)	save(), persist(), saveOrUpdate()	(not directly possible)
Persistent	(not possible)	(remains)	close(), evict(), clear()
Detached	(not possible)	update(), merge(), saveOrUpdate()	(remains)