EntityManager的SessionFactory引发异常

我试图从JPA的EntityManager中获取Hibernate的SessionFactory，方法如下：

@PersistenceContext
EntityManager manager;

public SessionFactory getSessionFactory(){
    sessionFactory = manager.unwrap(SessionFactory.class);
}

但是它抛出了这个异常：

org.springframework.orm.jpa.JpaSystemException: Hibernate cannot unwrap interface org.hibernate.SessionFactory; nested exception is javax.persistence.PersistenceException: Hibernate cannot unwrap interface org.hibernate.SessionFactory
at org.springframework.orm.jpa.EntityManagerFactoryUtils.convertJpaAccessExceptionIfPossible(EntityManagerFactoryUtils.java:418)
...

Caused by: javax.persistence.PersistenceException: Hibernate cannot unwrap interface org.hibernate.SessionFactory
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.unwrap(AbstractEntityManagerImpl.java:1489)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...

这可能是什么原因？

提前致谢