hibernate中: is not valid without active transaction

Session session= sf.getCurrentSession();

Teacher t = (Teacher)session.get(Teacher.class,2);

System.out.println(t.getId());

Teacher t2 = (Teacher)session.load(Teacher.class, 1);

System.out.println(t2.getName());

session.getTransaction().commit();

会报错，把sf.openSession()就不报错了，openSession()每一次都产生一个新的session