org.hibernate.TransactionException: nested transactions not supported

贴主要部分代码

        public PlUser findByUsernameAndPassword(String username, String password) {
		Session sx = this.getHibernateTemplate().getSessionFactory().openSession();
		Transaction tx = sx.getTransaction();
		tx.begin();
		PlUser user = (PlUser) sx.createQuery("from PlUser u where u.username=? and u.password=?")
				.setString(0, username)
				.setString(1, password)
				.uniqueResult();
		tx.commit();
		sx.close();
		return user;
	}

这是修改后正确的代码。

原先的代码：

        public PlUser findByUsernameAndPassword(String username, String password) {
		Session sx = this.getHibernateTemplate().getSessionFactory().getCurrentSession();
		Transaction tx = sx.getTransaction();
		tx.begin();
		PlUser user = (PlUser) sx.createQuery("from PlUser u where u.username=? and u.password=?")
				.setString(0, username)
				.setString(1, password)
				.uniqueResult();
		tx.commit();
		sx.close();
		return user;
	}

提示：org.hibernate.TransactionException: nested transactions not supported

而后删除Transaction tx那一行，提示：org.hibernate.SessionException:
Session is closed

尽管在applicationContext.xml那里配置了连接池，初始化为20条连接，但似乎并没效果。

getHibernateTemplate()可参考http://blog.csdn.net/zmhinzaghi/article/details/7091629

个人案例，请谅解。