SpringMVC - mvc:annotation-driven详解

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】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。

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查看BeanDefinitionParser说明：

该接口被默认bean定义文档阅读者使用以处理自定义和顶级(直属)标签。

实现者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，

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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，我们今天研究的是标签，所以我们找到对应的实现类是org.springframework.web.servlet.config.AnnotationDrivenBeanDefinitionParser。

AnnotationDrivenBeanDefinitionParser的文档说明如下：

 * A {@link BeanDefinitionParser} that provides the configuration for the
 * {@code } MVC namespace element.
 *
 * 
This class registers the following {@link HandlerMapping}s:
 * 

 * 
{@link RequestMappingHandlerMapping}
 * ordered at 0 for mapping requests to annotated controller methods.
 * 
{@link BeanNameUrlHandlerMapping}
 * ordered at 2 to map URL paths to controller bean names.
 * 
 *
 * 
Note: Additional HandlerMappings may be registered
 * as a result of using the {@code } or the
 * {@code } MVC namespace elements.
 *
 * 
This class registers the following {@link HandlerAdapter}s:
 * 

 * 
{@link RequestMappingHandlerAdapter}
 * for processing requests with annotated controller methods.
 * 
{@link HttpRequestHandlerAdapter}
 * for processing requests with {@link HttpRequestHandler}s.
 * 
{@link SimpleControllerHandlerAdapter}
 * for processing requests with interface-based {@link Controller}s.
 * 
 *
 * 
This class registers the following {@link HandlerExceptionResolver}s:
 * 

 * 
{@link ExceptionHandlerExceptionResolver} for handling exceptions
 * through @{@link ExceptionHandler} methods.
 * 
{@link ResponseStatusExceptionResolver} for exceptions annotated
 * with @{@link ResponseStatus}.
 * 
{@link DefaultHandlerExceptionResolver} for resolving known Spring
 * exception types
 * 
 *
 * 
This class registers an {@link org.springframework.util.AntPathMatcher}
 * and a {@link org.springframework.web.util.UrlPathHelper} to be used by:
 * 

 * 
the {@link RequestMappingHandlerMapping},
 * 
the {@link HandlerMapping} for ViewControllers
 * 
and the {@link HandlerMapping} for serving resources
 * 
 * Note that those beans can be configured by using the {@code path-matching} MVC namespace element.
 *
 * 
Both the {@link RequestMappingHandlerAdapter} and the
 * {@link ExceptionHandlerExceptionResolver} are configured with instances of
 * the following by default:
 * 

 * 
A {@link ContentNegotiationManager}
 * 
A {@link DefaultFormattingConversionService}
 * 
A {@link org.springframework.validation.beanvalidation.LocalValidatorFactoryBean}
 * if a JSR-303 implementation is available on the classpath
 * 
A range of {@link HttpMessageConverter}s depending on what 3rd party
 * libraries are available on the classpath.
 * 
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @author Arjen Poutsma
 * @author Rossen Stoyanchev
 * @author Brian Clozel
 * @author Agim Emruli
 * @since 3.0

该注释文档说明该解析器想容器中注入了如下类：

• RequestMappingHandlerMapping

• BeanNameUrlHandlerMapping

• RequestMappingHandlerAdapter

• HttpRequestHandlerAdapter

• SimpleControllerHandlerAdapter

• ExceptionHandlerExceptionResolver

• ResponseStatusExceptionResolver

• DefaultHandlerExceptionResolver

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。

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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。

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【2】代码追溯

源码如下：

public BeanDefinition parse(Element element, ParserContext parserContext) {
		Object source = parserContext.extractSource(element);
		XmlReaderContext readerContext = parserContext.getReaderContext();

		CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);
		parserContext.pushContainingComponent(compDefinition);
		RuntimeBeanReference contentNegotiationManager = getContentNegotiationManager(element, source, parserContext);
		
/*  RequestMappingHandlerMapping order:0*/
		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.valueOf(element.getAttribute("enable-matrix-variables"));
			handlerMappingDef.getPropertyValues().add("removeSemicolonContent", !enableMatrixVariables);
		}
		else if (element.hasAttribute("enableMatrixVariables")) {
			Boolean enableMatrixVariables = Boolean.valueOf(element.getAttribute("enableMatrixVariables"));
			handlerMappingDef.getPropertyValues().add("removeSemicolonContent", !enableMatrixVariables);
		}

		configurePathMatchingProperties(handlerMappingDef, element, parserContext);
		readerContext.getRegistry().registerBeanDefinition(HANDLER_MAPPING_BEAN_NAME , handlerMappingDef);

		RuntimeBeanReference corsConfigurationsRef = MvcNamespaceUtils.registerCorsConfigurations(null, parserContext, source);
		handlerMappingDef.getPropertyValues().add("corsConfigurations", corsConfigurationsRef);

		RuntimeBeanReference conversionService = getConversionService(element, source, parserContext);
		RuntimeBeanReference validator = getValidator(element, source, parserContext);
		RuntimeBeanReference messageCodesResolver = getMessageCodesResolver(element);

		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, parserContext);
		ManagedList argumentResolvers = getArgumentResolvers(element, parserContext);
		ManagedList returnValueHandlers = getReturnValueHandlers(element, parserContext);
		String asyncTimeout = getAsyncTimeout(element);
		RuntimeBeanReference asyncExecutor = getAsyncExecutor(element);
		ManagedList callableInterceptors = getCallableInterceptors(element, source, parserContext);
		ManagedList deferredResultInterceptors = getDeferredResultInterceptors(element, source, parserContext);

/* 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);
		}
		else if (element.hasAttribute("ignoreDefaultModelOnRedirect")) {
			// "ignoreDefaultModelOnRedirect" spelling is deprecated
			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);
		readerContext.getRegistry().registerBeanDefinition(HANDLER_ADAPTER_BEAN_NAME , handlerAdapterDef);

		String uriCompContribName = MvcUriComponentsBuilder.MVC_URI_COMPONENTS_CONTRIBUTOR_BEAN_NAME;
		RootBeanDefinition uriCompContribDef = new RootBeanDefinition(CompositeUriComponentsContributorFactoryBean.class);
		uriCompContribDef.setSource(source);
		uriCompContribDef.getPropertyValues().addPropertyValue("handlerAdapter", handlerAdapterDef);
		uriCompContribDef.getPropertyValues().addPropertyValue("conversionService", conversionService);
		readerContext.getRegistry().registerBeanDefinition(uriCompContribName, uriCompContribDef);

		RootBeanDefinition csInterceptorDef = new RootBeanDefinition(ConversionServiceExposingInterceptor.class);
		csInterceptorDef.setSource(source);
		csInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(0, conversionService);
		RootBeanDefinition mappedCsInterceptorDef = new RootBeanDefinition(MappedInterceptor.class);
		mappedCsInterceptorDef.setSource(source);
		mappedCsInterceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
		mappedCsInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(0, (Object) null);
		mappedCsInterceptorDef.getConstructorArgumentValues().addIndexedArgumentValue(1, csInterceptorDef);
		String mappedInterceptorName = readerContext.registerWithGeneratedName(mappedCsInterceptorDef);


/* ExceptionHandlerExceptionResolver */
		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);
		addResponseBodyAdvice(exceptionHandlerExceptionResolver);

		if (argumentResolvers != null) {
			exceptionHandlerExceptionResolver.getPropertyValues().add("customArgumentResolvers", argumentResolvers);
		}
		if (returnValueHandlers != null) {
			exceptionHandlerExceptionResolver.getPropertyValues().add("customReturnValueHandlers", returnValueHandlers);
		}

		String methodExceptionResolverName = readerContext.registerWithGeneratedName(exceptionHandlerExceptionResolver);

/* ResponseStatusExceptionResolver */
		RootBeanDefinition responseStatusExceptionResolver = new RootBeanDefinition(ResponseStatusExceptionResolver.class);
		responseStatusExceptionResolver.setSource(source);
		responseStatusExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
		responseStatusExceptionResolver.getPropertyValues().add("order", 1);
		String responseStatusExceptionResolverName =
				readerContext.registerWithGeneratedName(responseStatusExceptionResolver);

/* DefaultHandlerExceptionResolver */
		RootBeanDefinition defaultExceptionResolver = new RootBeanDefinition(DefaultHandlerExceptionResolver.class);
		defaultExceptionResolver.setSource(source);
		defaultExceptionResolver.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
		defaultExceptionResolver.getPropertyValues().add("order", 2);
		String defaultExceptionResolverName =
				readerContext.registerWithGeneratedName(defaultExceptionResolver);

		parserContext.registerComponent(new BeanComponentDefinition(handlerMappingDef, HANDLER_MAPPING_BEAN_NAME));
		parserContext.registerComponent(new BeanComponentDefinition(handlerAdapterDef, HANDLER_ADAPTER_BEAN_NAME));
		parserContext.registerComponent(new BeanComponentDefinition(uriCompContribDef, uriCompContribName));
		parserContext.registerComponent(new BeanComponentDefinition(exceptionHandlerExceptionResolver, methodExceptionResolverName));
		parserContext.registerComponent(new BeanComponentDefinition(responseStatusExceptionResolver, responseStatusExceptionResolverName));
		parserContext.registerComponent(new BeanComponentDefinition(defaultExceptionResolver, defaultExceptionResolverName));
		parserContext.registerComponent(new BeanComponentDefinition(mappedCsInterceptorDef, mappedInterceptorName));
		
/* BeanNameUrlHandlerMapping  HttpRequestHandlerAdapter 和 SimpleControllerHandlerAdapter*/
		// Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off"
		MvcNamespaceUtils.registerDefaultComponents(parserContext, source);

		parserContext.popAndRegisterContainingComponent();

		return null;
	}

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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);
	}

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【3】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。

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那么它跟有什么区别呢？

其实标签是告诉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，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就需要注解，如何匹配，匹配哪个方法就需要注解。

当然，你也可以不使用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。

如果想使web项目正常运转，一般情况下，和是必需的！项目中二者相辅相成，缺一不可！！！

参考博文： context:annotation-config与mvc:annotation-driven和context:component-scan详解。