Spring MVC 解读——<mvc:annotatio

一、AnnotationDrivenBeanDefinitionParser

通过注解的方式开发Spring MVC，要在***-servlet.xml中加入<mvc:annotation-driven/>标签。
先找到它的解析类。所有自定义命名空间（像mvc，context等）标签解析都是由BeanDefinitionParser接口的子类来完成的。

<mvc:annotation-driven/>标签对应的解析类是`org.springframework.web.servlet.config.AnnotationDrivenBeanDefinitionParser·。这个类主要是向工厂中注册了

• RequestMappingHandlerMapping
• BeanNameUrlHandlerMapping
• RequestMappingHandlerAdapter
• HttpRequestHandlerAdapter
• SimpleControllerHandlerAdapter
• ExceptionHandlerExceptionResolver
• ResponseStatusExceptionResolver
• DefaultHandlerExceptionResolver

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。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。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

public BeanDefinition parse(Element element, ParserContext parserContext) {
        Object source = parserContext.extractSource(element);
 
        CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);
        parserContext.pushContainingComponent(compDefinition);
 
        RuntimeBeanReference contentNegotiationManager = getContentNegotiationManager(element, source, parserContext);
        //第一个在这 RequestMappingHandlerMapping
        RootBeanDefinition handlerMappingDef = new RootBeanDefinition(RequestMappingHandlerMapping.class);
        handlerMappingDef.setSource(source);
        handlerMappingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        handlerMappingDef.getPropertyValues().add("order", 0);
        handlerMappingDef.getPropertyValues().add("removeSemicolonContent", false);
        handlerMappingDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
        String methodMappingName = parserContext.getReaderContext().registerWithGeneratedName(handlerMappingDef);
        //第二个在这 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);
        if (element.hasAttribute("ignoreDefaultModelOnRedirect")) {
            Boolean ignoreDefaultModel = Boolean.valueOf(element.getAttribute("ignoreDefaultModelOnRedirect"));
            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);
        String handlerAdapterName = parserContext.getReaderContext().registerWithGeneratedName(handlerAdapterDef);
        //异常处理解析器
        RootBeanDefinition exceptionHandlerExceptionResolver = new RootBeanDefinition(ExceptionHandlerExceptionResolver.class);
        exceptionHandlerExceptionResolver.setSource(source);
        exceptionHandlerExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        exceptionHandlerExceptionResolver.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
        exceptionHandlerExceptionResolver.getPropertyValues().add("messageConverters", messageConverters);
        exceptionHandlerExceptionResolver.getPropertyValues().add("order", 0);
        String methodExceptionResolverName =
                parserContext.getReaderContext().registerWithGeneratedName(exceptionHandlerExceptionResolver);
        //异常处理解析器
        RootBeanDefinition responseStatusExceptionResolver = new RootBeanDefinition(ResponseStatusExceptionResolver.class);
        responseStatusExceptionResolver.setSource(source);
        responseStatusExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        responseStatusExceptionResolver.getPropertyValues().add("order", 1);
        String responseStatusExceptionResolverName =
                parserContext.getReaderContext().registerWithGeneratedName(responseStatusExceptionResolver);
        //异常处理解析器
        RootBeanDefinition defaultExceptionResolver = new RootBeanDefinition(DefaultHandlerExceptionResolver.class);
        defaultExceptionResolver.setSource(source);
        defaultExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        defaultExceptionResolver.getPropertyValues().add("order", 2);
        String defaultExceptionResolverName =
                parserContext.getReaderContext().registerWithGeneratedName(defaultExceptionResolver);
 
        parserContext.registerComponent(new BeanComponentDefinition(handlerMappingDef, methodMappingName));
        parserContext.registerComponent(new BeanComponentDefinition(handlerAdapterDef, handlerAdapterName));
        parserContext.registerComponent(new BeanComponentDefinition(exceptionHandlerExceptionResolver, methodExceptionResolverName));
        parserContext.registerComponent(new BeanComponentDefinition(responseStatusExceptionResolver, responseStatusExceptionResolverName));
        parserContext.registerComponent(new BeanComponentDefinition(defaultExceptionResolver, defaultExceptionResolverName));
        parserContext.registerComponent(new BeanComponentDefinition(mappedCsInterceptorDef, mappedInterceptorName));
        //这里注册了BeanNameUrlHandlerMapping，SimpleControllerHandlerAdapter等
        // Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off"
        MvcNamespaceUtils.registerDefaultComponents(parserContext, source);
 
        parserContext.popAndRegisterContainingComponent();
 
        return null;
    }
//在这啊。
public static void registerDefaultComponents(ParserContext parserContext, Object source) {
        registerBeanNameUrlHandlerMapping(parserContext, source);
        registerHttpRequestHandlerAdapter(parserContext, source);
        registerSimpleControllerHandlerAdapter(parserContext, source);
    }

三、总结

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。
<context:component-scan/>标签是告诉Spring扫描指定包下的类，并注册被@Component，@Controller，@Service，@Repository等注解标记的组件。
<mvc:annotation-scan/>是告知Spring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。