Transient, Persistent and Detached Objects.

A new instance of a a persistent class which is not associated with a Session, has no representation in the database and no identifier value is considered transient by Hibernate:
Person person = new Person();
person.setName("Foobar");
// person is in a transient state
A persistent instance has a representation in the database, an identifier value and is associated with a session. You can make a transient instance persistent by associating it with a Session:
Long id = (Long) session.save(person);
// person is now in a persistent state
Now, if we close the Hibernate session, the persistent instance will become a detached instance: it isn't attached to a Session anymore (but can still be modified and reattached to a new Session later though).
All this is clearly explained is the whole Chapter 10. Working with objects of the Hibernate documentation . Definitely a must read.
Source: Stackoverflow

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