从父母DAO延伸的孩子

public abstract class ParentDAO<T> {
    @PersistenceContext
    private EntityManager entityManager;

    public EntityManager getEntityManager() {
        return entityManager;
    }

    public void setEntityManager(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    private EntityManager em() {
        if (entityManager == null)
            throw new IllegalStateException("The entity manager is not set");

        return entityManager;
    }
}