Hibernate: How to fix “identifier of an instance altered from X to Y”?

Question

org.hibernate.HibernateException: identifier of an instance 
of org.cometd.hibernate.User altered from 12 to 3

in fact, my user table

Answer 1

Are you changing the primary key value of a User object somewhere? You shouldn't do that. Check that your mapping for the primary key is correct.

What does your mapping XML file or mapping annotations look like?