当使用 spring root 和 mvc 时， mvc 一定要分开扫描组件，如果通过mvc 扫描所有组件会导致事务不起作用
一般 servlet 只扫描 web 的 组件，比如 Controller 等
其他组件如 Service 需要由 root 扫描
或者只使用 servlet
@EnableTransactionManagement and <tx:annotation-driven/> only looks for @Transactional on beans in the same application context they are defined in.
This means that,
if you put annotation driven configuration in a WebApplicationContext for a DispatcherServlet,
it only checks for @Transactional beans in your controllers,
and not your services.
See Section 22.2, “The DispatcherServlet” for more information.
