本文研究背景：项目是传统SSM项目，非SpringBoot项目。项目使用xml方式配置SpringMVC并且没有手动往IOC容器中注入<mvc:annotation-driven/>注解注入的bean。
【1】BeanDefinitionParser
该接口被DefaultBeanDefinitionDocumentReader使用以处理自定义和顶级(直属<beans/>)标签。
实现类可以自由地将标签里面的元数据转化成许多需要的BeanDefinition。
解析器从标签隶属的namespace对应的NamespaceHandler找到合适的BeanDefinitonParser。该解析器的实现类有：
我们看到有多个AnnotationDrivenBeanDefinitionParser
他们是用来处理不同命名空间下的标签的，我们今天研究<annotation-driven/>的是<mvc:annotation-driven/>标签，所以我们找到对应的实现类org.springframework.web.servlet.config.AnnotationDrivenBeanDefinitionParser。
【2】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。前面说过了，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。
AnnotationDrivenBeanDefinitionParser的文档说明该解析器想容器中注入了如下类：
HandlerMapping处理器映射器
RequestMappingHandlerMapping，ordered at 0 for mapping requests to annotated controller methods.
BeanNameUrlHandlerMapping，ordered at 2 to map URL paths to controller bean names.
也可以通过<mvc:view-controller>或<mvc:resources>注册其他的HandlerMappings
HandlerAdapter处理器适配器
RequestMappingHandlerAdapter,for processing requests with annotated controller methods.
HttpRequestHandlerAdapter,使用HttpRequestHandler处理请求
SimpleControllerHandlerAdapter,for processing requests with interface-based {@link Controller Controllers}.
HandlerExceptionResolver
ExceptionHandlerExceptionResolver,通过@ExceptionHandler注解声明的方法处理异常
ResponseStatusExceptionResolver，处理那些标注了@ResponseStatus注解
DefaultHandlerExceptionResolver，解析已知的Spring异常类型
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
【3】代码追溯
@Override @Nullable public BeanDefinition parse(Element element, ParserContext context) { Object source = context.extractSource(element); XmlReaderContext readerContext = context.getReaderContext(); CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source); context.pushContainingComponent(compDefinition); RuntimeBeanReference contentNegotiationManager = getContentNegotiationManager(element, source, context); // RequestMappingHandlerMapping RootBeanDefinition handlerMappingDef = new RootBeanDefinition(RequestMappingHandlerMapping.class); handlerMappingDef.setSource(source); handlerMappingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); handlerMappingDef.getPropertyValues().add("order", 0); handlerMappingDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager); if (element.hasAttribute("enable-matrix-variables")) { boolean enableMatrixVariables = Boolean.parseBoolean(element.getAttribute("enable-matrix-variables")); handlerMappingDef.getPropertyValues().add("removeSemicolonContent", !enableMatrixVariables); } configurePathMatchingProperties(handlerMappingDef, element, context); readerContext.getRegistry().registerBeanDefinition(HANDLER_MAPPING_BEAN_NAME, handlerMappingDef); // 跨域配置 RuntimeBeanReference corsRef = MvcNamespaceUtils.registerCorsConfigurations(null, context, source); handlerMappingDef.getPropertyValues().add("corsConfigurations", corsRef); //ConversionService RuntimeBeanReference conversionService = getConversionService(element, source, context); // Validator RuntimeBeanReference validator = getValidator(element, source, context); //MessageCodesResolver RuntimeBeanReference messageCodesResolver = getMessageCodesResolver(element); //ConfigurableWebBindingInitializer RootBeanDefinition bindingDef = new RootBeanDefinition(ConfigurableWebBindingInitializer.class); bindingDef.setSource(source); bindingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); bindingDef.getPropertyValues().add("conversionService", conversionService); bindingDef.getPropertyValues().add("validator", validator); bindingDef.getPropertyValues().add("messageCodesResolver", messageCodesResolver); ManagedList<?> messageConverters = getMessageConverters(element, source, context); ManagedList<?> argumentResolvers = getArgumentResolvers(element, context); ManagedList<?> returnValueHandlers = getReturnValueHandlers(element, context); String asyncTimeout = getAsyncTimeout(element); RuntimeBeanReference asyncExecutor = getAsyncExecutor(element); ManagedList<?> callableInterceptors = getInterceptors(element, source, context, "callable-interceptors"); ManagedList<?> deferredResultInterceptors = getInterceptors(element, source, context, "deferred-result-interceptors"); //RequestMappingHandlerAdapter RootBeanDefinition handlerAdapterDef = new RootBeanDefinition(RequestMappingHandlerAdapter.class); handlerAdapterDef.setSource(source); handlerAdapterDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); handlerAdapterDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager); handlerAdapterDef.getPropertyValues().add("webBindingInitializer", bindingDef); handlerAdapterDef.getPropertyValues().add("messageConverters", messageConverters); addRequestBodyAdvice(handlerAdapterDef); addResponseBodyAdvice(handlerAdapterDef); if (element.hasAttribute("ignore-default-model-on-redirect")) { Boolean ignoreDefaultModel = Boolean.valueOf(element.getAttribute("ignore-default-model-on-redirect")); handlerAdapterDef.getPropertyValues().add("ignoreDefaultModelOnRedirect", ignoreDefaultModel); } if (argumentResolvers != null) { handlerAdapterDef.getPropertyValues().add("customArgumentResolvers", argumentResolvers); } if (returnValueHandlers != null) { handlerAdapterDef.getPropertyValues().add("customReturnValueHandlers", returnValueHandlers); } if (asyncTimeout != null) { handlerAdapterDef.getPropertyValues().add("asyncRequestTimeout", asyncTimeout); } if (asyncExecutor != null) { handlerAdapterDef.getPropertyValues().add("taskExecutor", asyncExecutor); } handlerAdapterDef.getPropertyValues().add("callableInterceptors", callableInterceptors); handlerAdapterDef.getPropertyValues().add("deferredResultInterceptors", deferredResultInterceptors); readerContext.getRegistry().registerBeanDefinition(HANDLER_ADAPTER_BEAN_NAME, handlerAdapterDef); //CompositeUriComponentsContributorFactoryBean RootBeanDefinition uriContributorDef = new RootBeanDefinition(CompositeUriComponentsContributorFactoryBean.class); uriContributorDef.setSource(source); uriContributorDef.getPropertyValues().addPropertyValue("handlerAdapter", handlerAdapterDef); uriContributorDef.getPropertyValues().addPropertyValue("conversionService", conversionService); String uriContributorName = MvcUriComponentsBuilder.MVC_URI_COMPONENTS_CONTRIBUTOR_BEAN_NAME; readerContext.getRegistry().registerBeanDefinition(uriContributorName, uriContributorDef); // ConversionServiceExposingInterceptor RootBeanDefinition csInterceptorDef = new RootBeanDefinition(ConversionServiceExposingInterceptor.class); csInterceptorDef.setSource(source); csInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(0, conversionService); RootBeanDefinition mappedInterceptorDef = new RootBeanDefinition(MappedInterceptor.class); mappedInterceptorDef.setSource(source); mappedInterceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); mappedInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(0, (Object) null); mappedInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(1, csInterceptorDef); String mappedInterceptorName = readerContext.registerWithGeneratedName(mappedInterceptorDef); // ExceptionHandlerExceptionResolver RootBeanDefinition methodExceptionResolver = new RootBeanDefinition(ExceptionHandlerExceptionResolver.class); methodExceptionResolver.setSource(source); methodExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); methodExceptionResolver.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager); methodExceptionResolver.getPropertyValues().add("messageConverters", messageConverters); methodExceptionResolver.getPropertyValues().add("order", 0); addResponseBodyAdvice(methodExceptionResolver); if (argumentResolvers != null) { methodExceptionResolver.getPropertyValues().add("customArgumentResolvers", argumentResolvers); } if (returnValueHandlers != null) { methodExceptionResolver.getPropertyValues().add("customReturnValueHandlers", returnValueHandlers); } String methodExResolverName = readerContext.registerWithGeneratedName(methodExceptionResolver); // ResponseStatusExceptionResolver RootBeanDefinition statusExceptionResolver = new RootBeanDefinition(ResponseStatusExceptionResolver.class); statusExceptionResolver.setSource(source); statusExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); statusExceptionResolver.getPropertyValues().add("order", 1); String statusExResolverName = readerContext.registerWithGeneratedName(statusExceptionResolver); // DefaultHandlerExceptionResolver RootBeanDefinition defaultExceptionResolver = new RootBeanDefinition(DefaultHandlerExceptionResolver.class); defaultExceptionResolver.setSource(source); defaultExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); defaultExceptionResolver.getPropertyValues().add("order", 2); String defaultExResolverName = readerContext.registerWithGeneratedName(defaultExceptionResolver); context.registerComponent(new BeanComponentDefinition(handlerMappingDef, HANDLER_MAPPING_BEAN_NAME)); context.registerComponent(new BeanComponentDefinition(handlerAdapterDef, HANDLER_ADAPTER_BEAN_NAME)); context.registerComponent(new BeanComponentDefinition(uriContributorDef, uriContributorName)); context.registerComponent(new BeanComponentDefinition(mappedInterceptorDef, mappedInterceptorName)); context.registerComponent(new BeanComponentDefinition(methodExceptionResolver, methodExResolverName)); context.registerComponent(new BeanComponentDefinition(statusExceptionResolver, statusExResolverName)); context.registerComponent(new BeanComponentDefinition(defaultExceptionResolver, defaultExResolverName)); // BeanNameUrlHandlerMapping //HttpRequestHandlerAdapter //SimpleControllerHandlerAdapter // Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off" MvcNamespaceUtils.registerDefaultComponents(context, source); context.popAndRegisterContainingComponent(); return null; }
注册默认mapping和adapter方法如下：
public static void registerDefaultComponents(ParserContext parserContext, Object source) { registerBeanNameUrlHandlerMapping(parserContext, source); registerHttpRequestHandlerAdapter(parserContext, source); registerSimpleControllerHandlerAdapter(parserContext, source); }
【4】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？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<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（即，项目启动完成前这些被注解的组件就会被实例化bean放入容器上下文中）和注册AutowiredAnnotationBeanPostProcessor、 RequiredAnnotationBeanPostProcessor、 CommonAnnotationBeanPostProcessor以及 PersistenceAnnotationBeanPostProcessor。
而<mvc:annotation-driven/>是告知Spring，我们启用注解驱动，为WEB 应用服务(我们就可以使用该标签注册的几个bean的功能)。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。换句话说，<context:component-scan/>向容器中显式或隐式注册了一系列单个组件，但是项目要想将组件关联起来正常运转，则需要<mvc:annotation-driven/>注册的组件完成。
比如请求一个URL，我要知道这个URL匹配哪个controller中的哪个方法。哪个Controller就需要<context:component-scan/>注解，如何匹配，匹配哪个方法就需要<mvc:annotation-driven/>注解。
当然，你也可以不使用<context:component-scan/>而手动将所有需要的bean显示注册进applicationContext.xml中，这实在是太愚蠢！如果不想手动配置bean(xml)，使用注解获取bean就必须使用<context:component-scan/>。
如果想使web项目正常运转，一般情况下，<mvc:annotation-driven/>和<context:component-scan/>是必需的！项目中二者相辅相成，缺一不可!
当然随着版本迭代、尤其是现在SpringBoot主流为我们省去了很多配置。但是想往高级走，我们还是需要了解其底层原理细节。
