Bozho 和 partenon 是正确的，但：

在JPA 2中，首选机制是 entityManager.unwrap（class）

  HibernateEntityManager hem = em.unwrap HibernateEntityManager.class）; 
 Session session = hem.getSession（）; 
 

我认为你的异常是因为你试图转换到实现类而导致的（也许你正在处理使用JDK代理）。投射到界面，一切都应该没问题（在JPA 2版本中，不需要投射）。

Is it possible to obtain the Hibernate Session object from the EntityManager? I want to access some hibernate specific API...

I already tried something like:

org.hibernate.Session hSession =
   ( (EntityManagerImpl) em.getDelegate() ).getSession();

but as soon as I invoke a method in the EJB I get "A system exception occurred during an invocation on EJB" with a NullPointerException

I use glassfish 3.0.1

Bozho and partenon are correct, but:

In JPA 2, the preferred mechanism is entityManager.unwrap(class)

HibernateEntityManager hem = em.unwrap(HibernateEntityManager.class);
Session session = hem.getSession();

I think your exception is caused because you are trying to cast to an implementation class (perhaps you were dealing with a JDK proxy). Cast to an interface, and everything should be fine (in the JPA 2 version, no casting is needed).