转自：Spring MVC 解读——<mvc:annotation-driven/>
摘自：使用@Controller注解为什么要配置<mvc:annotation-driven />
摘要: <mvc:annotation-driven/>是做什么的？它做了什么？它与<context:component-scan/>有什么区别？
Spring MVC 解读——<mvc:annotation-driven/>
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。第一篇文章中说过了，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。参看第一篇文章中的图片
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
RequestMappingHandlerMappingBeanNameUrlHandlerMappingRequestMappingHandlerAdapterHttpRequestHandlerAdapterSimpleControllerHandlerAdapterExceptionHandlerExceptionResolverResponseStatusExceptionResolverDefaultHandlerExceptionResolver
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) {
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);
}
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。（下片文章我们会详细介绍的）
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。（后面文章也会陆续详细介绍的）
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。
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This tag registers the DefaultAnnotationHandlerMapping and AnnotationMethodHandlerAdapter beans that are required for spring MVC to dispatch requests to Controllers.
这个标签注册了Spring MVC分发请求到控制器所必须的DefaultAnnotationHandlerMapping和AnnotationMethodHandlerAdapter实例
The tag configures those two beans with sensible defaults based on what is present in your classpath.
标签配置的这2个实例可以根据classpath中的内容默认提供以下功能：
The defaults are:
1. Support for Spring 3‘s Type ConversionService in addition to JavaBeans PropertyEditors during Data Binding.
A ConversionService instance produced by the
org.springframework.format.support.FormattingConversionServiceFactoryBean
is used by default.
This can be overriden by setting the conversion-service attribute.
支持spring3的javaBeans属性编辑器数据绑定时的类型转换服务。
类型转换服务实例默认为org.springframework.format.support.FormattingConversionServiceFactoryBean。
可以覆盖conversion-service属性来指定类型转换服务实例类。
2. Support for formatting Number fields using the @NumberFormat annotation
支持@NumberFormat 注解格式化数字类型字段。
3. Support for
formatting Date, Calendar, Long, and Joda Time fields using the
@DateTimeFormat annotation, if Joda Time 1.3 or higher is present on the
classpath.
@DateTimeFormat注解格式化 Date, Calendar, Long和 Joda Time（如classpath下存在Joda Time 1.3或更高版本）字段
4. Support for validating @Controller inputs with @Valid, if a JSR-303 Provider is present on the classpath.
The validation system can be explicitly configured by setting the validator attribute.
支持@Valid注解验证控制器数据，classpath中需JSR-303的**。
可以使用setting明确的配置
5. Support for reading and writing XML, if JAXB is present on the classpath.
支持读写xml，classpath中需JAXB 。
6. Support for reading and writing JSON, if Jackson is present on the classpath.
支持读写json，classpath中需Jackson 。
A typical usage is shown below:
下边是用法：
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> <!-- JSR-303 support will be detected on classpath and enabled automatically --> <mvc:annotation-driven/> </beans>