Spring transaction management: org.hibernate.SessionException: Session is closed!

If you got this exception:

Exception in thread "main"
javax.persistence.PersistenceException:
org.hibernate.SessionException:
Session is closed!

... and you are using Spring transaction management, then the reason might be that your DAO method (the method that threw it and that uses persistence context) is not transactional and it should be:

import org.springframework.transaction.annotation.Transactional;
//...
    @Transactional(readOnly = true)
    public List<Dog> retrieveAllDogs() {
        return em.createQuery("from Dog").getResultList();
    }
//...

See my article on correct setup of Spring 3 & JPA 2.0.