Connection cannot be null when 'hibernate.dialect'

使用hibernate4时，配置文件里明明配了<property name="dialect">org.hibernate.dialect.MySQLDialect</property>，运行时就是报错Connection cannot be null when 'hibernate.dialect' not set；

后来在网上发现：

错误在于如下红色代码：
static{
        try{
            Configuration config = new Configuration();
            config.addClass(ProductInfoVo.class);
            sessionFactory = config.buildSessionFactory();
        }catch(MappingException e){
            e.printStackTrace();
        }catch(HibernateException e){
            e.printStackTrace();
        }
    }
以上代码只有按下面写才对：
static{
        try{
            Configuration config = new Configuration().configure();
            config.addClass(ProductInfoVo.class);
            sessionFactory = config.buildSessionFactory();
        }catch(MappingException e){
            e.printStackTrace();
        }catch(HibernateException e){
            e.printStackTrace();
        }
    }