SpringMVC中＜mvc:annotation-driven/＞标签原理与实践详解

阿里云周边

12 月 05, 2023

223 0

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)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，我们今天研究的是标签，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。

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.

也可以通过或注册其他的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!

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。