spring MVC 中为什么要加一个HandlerAdapter

在学习 开涛的Spring MVC 有一处不明白,就是 spring MVC 中为什么要加一个HandlerAdapter?

<!-- HandlerMapping -->
    <bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping"/>
   
    <!-- HandlerAdapter -->
    <bean class="org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter"/>
   
    <!-- ViewResolver -->
    <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
        <property name="prefix" value="/WEB-INF/jsp/"/>
        <property name="suffix" value=".jsp"/>
    </bean>
   
    <!-- 处理器 -->
    <bean name="/hello" class="cn.javass.chapter2.web.controller.HelloWorldController"/>

上面的配置中,通过/hello这个url的映射已经可以找到对应的HelloWorldController,直接调用其方法不行吗?
为什么要加一层 HandlerAdapter,然后通过HandlerAdapter 来调用 Controller的控制方法?

请大家不吝赐教
谢谢

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找到答案，参考 http://stackoverflow.com/questions/23325111/spring-mvc-handlermapping-vs-handleradapter

Since introduction of RequestMappingHandlerMapping and RequestMappingHandlerAdapter in Spring 3.1 the distinction is even simpler: RequestMappingHandlerMapping finds the appropriate handler method for the given request. RequestMappingHandlerAdapter executes this method, providing it with all the arguments.