[置顶] Spring MVC 解读——
一、AnnotationDrivenBeanDefinitionParser
通常如果我们希望通过注解的方式来进行Spring MVC开发，我们都会在***-servlet.xml中加入<mvc:annotation-driven/>标签来告诉Spring我们的目的。但是我们为什么这
么做呢？这个标签是什么意思呢？它做了什么呢？
同样为了弄清楚这些问题， 像<context:component-scan/>标签一样，我们先找到它的解析类。所有的自定义命名空间（像mvc，context等）下的标签解析都是由
BeanDefinitionParser接口的子类来完成的.
我们看到有多个AnnotationDrivenBeanDefinitionParser，他们是用来处理不同命名空间下的<annotation-driven/>标签的，我们今天研究的是<mvc:annotation-driven/>标
签，所以我们找到对应的实现类是org.springframework.web.servlet.config.AnnotationDrivenBeanDefinitionParser。
通过阅读类注释文档，我们发现这个类主要是用来向工厂中注册了RequestMappingHandlerMappingde、BeanNameUrlHandlerMappingde、RequestMappingHandlerAdapterde、HttpRequestHandlerAdapterde、SimpleControllerHandlerAdapterde、ExceptionHandlerExceptionResolverde、ResponseStatusExceptionResolverde、DefaultHandlerExceptionResolverde。上面几个Bean实例。这几个类都是用来做什么的呢？
前两个是HandlerMapping接口的实现类，用来处理请求映射的。其中第一个是处理@RequestMapping注解的。第二个会将controller类的名字映射为请求url。
中间三个是用来处理请求的。具体点说就是确定调用哪个controller的哪个方法来处理当前请求。第一个处理@Controller注解的处理器，支持自定义方法参数和返回值（很酷）。第二个是处理继承HttpRequestHandler的处理器。第三个处理继承自Controller接口的处理器。
后面三个是用来处理异常的解析器。
二、实现
光说无凭据，我们直接看代码：
public BeanDefinition parse(Element element, ParserContext parserContext) {
<span style="white-space:pre"> </span> Object source = parserContext.extractSource(element);
<span style="white-space:pre"> </span>CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);
<span style="white-space:pre"> </span>parserContext.pushContainingComponent(compDefinition);
<span style="white-space:pre"> </span>RuntimeBeanReference contentNegotiationManager = getContentNegotiationManager(element, source, parserContext);
<span style="white-space:pre"> </span>//第一个在这 RequestMappingHandlerMapping
<span style="white-space:pre"> </span>RootBeanDefinition handlerMappingDef = new RootBeanDefinition(RequestMappingHandlerMapping.class);
<span style="white-space:pre"> </span>handlerMappingDef.setSource(source);
<span style="white-space:pre"> </span>handlerMappingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
<span style="white-space:pre"> </span>handlerMappingDef.getPropertyValues().add("order", 0);
<span style="white-space:pre"> </span>handlerMappingDef.getPropertyValues().add("removeSemicolonContent", false);
<span style="white-space:pre"> </span>handlerMappingDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
<span style="white-space:pre"> </span>String methodMappingName = parserContext.getReaderContext().registerWithGeneratedName(handlerMappingDef);
<span style="white-space:pre"> </span>//第二个在这 RequestMappingHandlerAdapter
<span style="white-space:pre"> </span>RootBeanDefinition handlerAdapterDef = new RootBeanDefinition(RequestMappingHandlerAdapter.class);
<span style="white-space:pre"> </span>handlerAdapterDef.setSource(source);
<span style="white-space:pre"> </span>handlerAdapterDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("webBindingInitializer", bindingDef);
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("messageConverters", messageConverters);
<span style="white-space:pre"> </span>if (element.hasAttribute("ignoreDefaultModelOnRedirect")) {
<span style="white-space:pre"> </span>Boolean ignoreDefaultModel = Boolean.valueOf(element.getAttribute("ignoreDefaultModelOnRedirect"));
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("ignoreDefaultModelOnRedirect", ignoreDefaultModel);
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>if (argumentResolvers != null) {
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("customArgumentResolvers", argumentResolvers);
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>if (returnValueHandlers != null) {
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("customReturnValueHandlers", returnValueHandlers);
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>if (asyncTimeout != null) {
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("asyncRequestTimeout", asyncTimeout);
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>if (asyncExecutor != null) {
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("taskExecutor", asyncExecutor);
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("callableInterceptors", callableInterceptors);
<span style="white-space:pre"> </span>handlerAdapterDef.getPropertyValues().add("deferredResultInterceptors", deferredResultInterceptors);
<span style="white-space:pre"> </span>String handlerAdapterName = parserContext.getReaderContext().registerWithGeneratedName(handlerAdapterDef);
<span style="white-space:pre"> </span>//异常处理解析器
<span style="white-space:pre"> </span>RootBeanDefinition exceptionHandlerExceptionResolver = new RootBeanDefinition(ExceptionHandlerExceptionResolver.class);
<span style="white-space:pre"> </span>exceptionHandlerExceptionResolver.setSource(source);
<span style="white-space:pre"> </span>exceptionHandlerExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
<span style="white-space:pre"> </span>exceptionHandlerExceptionResolver.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
<span style="white-space:pre"> </span>exceptionHandlerExceptionResolver.getPropertyValues().add("messageConverters", messageConverters);
<span style="white-space:pre"> </span>exceptionHandlerExceptionResolver.getPropertyValues().add("order", 0);
<span style="white-space:pre"> </span>String methodExceptionResolverName =
<span style="white-space:pre"> </span>parserContext.getReaderContext().registerWithGeneratedName(exceptionHandlerExceptionResolver);
<span style="white-space:pre"> </span>//异常处理解析器
<span style="white-space:pre"> </span>RootBeanDefinition responseStatusExceptionResolver = new RootBeanDefinition(ResponseStatusExceptionResolver.class);
<span style="white-space:pre"> </span>responseStatusExceptionResolver.setSource(source);
<span style="white-space:pre"> </span>responseStatusExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
<span style="white-space:pre"> </span>responseStatusExceptionResolver.getPropertyValues().add("order", 1);
<span style="white-space:pre"> </span>String responseStatusExceptionResolverName =
<span style="white-space:pre"> </span>parserContext.getReaderContext().registerWithGeneratedName(responseStatusExceptionResolver);
<span style="white-space:pre"> </span>//异常处理解析器
<span style="white-space:pre"> </span>RootBeanDefinition defaultExceptionResolver = new RootBeanDefinition(DefaultHandlerExceptionResolver.class);
<span style="white-space:pre"> </span>defaultExceptionResolver.setSource(source);
<span style="white-space:pre"> </span>defaultExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
<span style="white-space:pre"> </span>defaultExceptionResolver.getPropertyValues().add("order", 2);
<span style="white-space:pre"> </span>String defaultExceptionResolverName =
<span style="white-space:pre"> </span>parserContext.getReaderContext().registerWithGeneratedName(defaultExceptionResolver);
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(handlerMappingDef, methodMappingName));
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(handlerAdapterDef, handlerAdapterName));
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(exceptionHandlerExceptionResolver, methodExceptionResolverName));
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(responseStatusExceptionResolver, responseStatusExceptionResolverName));
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(defaultExceptionResolver, defaultExceptionResolverName));
<span style="white-space:pre"> </span>parserContext.registerComponent(new BeanComponentDefinition(mappedCsInterceptorDef, mappedInterceptorName));
<span style="white-space:pre"> </span>//这里注册了BeanNameUrlHandlerMapping，SimpleControllerHandlerAdapter等
<span style="white-space:pre"> </span>// Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off"
<span style="white-space:pre"> </span>MvcNamespaceUtils.registerDefaultComponents(parserContext, source);
<span style="white-space:pre"> </span>parserContext.popAndRegisterContainingComponent();
<span style="white-space:pre"> </span>return null;
<span style="white-space:pre"> </span>}
<span style="white-space:pre"> </span>//在这啊。
public static void registerDefaultComponents(ParserContext parserContext, Object source) {
<span style="white-space:pre"> </span>registerBeanNameUrlHandlerMapping(parserContext, source);
<span style="white-space:pre"> </span>registerHttpRequestHandlerAdapter(parserContext, source);
<span style="white-space:pre"> </span>registerSimpleControllerHandlerAdapter(parserContext, source);
}
略长，但很容易看明白的代码。看注释我们发现，它的确注册了上面说的那几个类。
三、总结
我们知道了它们自动为我们注册了这么多的Bean，那这些Bean是做什么的呢？
我们主要说明里面的两个，RequestMappingHandlerMapping和RequestMappingHandlerAdapter。
第一个是HandlerMapping的实现类，它会处理@RequestMapping 注解，并将其注册到请求映射表中。
第二个是HandlerAdapter的实现类，它是处理请求的适配器，说白了，就是确定调用哪个类的哪个方法，并且构造方法参数，返回值。
那么它跟<context:component-scan/>有什么区别呢？其实想上篇文章中介绍的，<context:component-scan/>标签是告诉Spring 来扫描指定包下的类，并注册被@Component，@Controller，@Service，@Repository等注解标记的组件。
而<mvc:annotation-scan/>是告知Spring，我们启用注解驱动。然后Spring会自动为我们注册上面说到的几个Bean到工厂中，来处理我们的请求。
转载自 http://my.oschina.net/HeliosFly/blog/205343