基础11-WebMVC自动化配置原理

打开Spring Boot官网，按照图片找到如下信息：

这里的关键就在于Spring MVC Auto-configuration

Spring MVC Auto-configuration

稍微翻译下图中信息：

Spring Boot为大多数应用提供了自动化配置的Spring MVC。自动化配置添加了如下内容：

①包含ContentNegotiatingViewResolver、BeanNameViewResolver；

②支持静态资源，包括对webjars的支持；

③自动注册Converter、GenericConverter、Formatter；

④支持HttpMessageConverters；

⑤自动注册MessageCodesResolver；

⑥静态index.html的支持；

⑦对favicon图标的支持；

⑧自动使用ConfigurableWebBindingInitializer；

如果你想保留Spring Boot MVC功能，并且想要添加自定义的MVC配置（拦截器，格式化器，视图控制器和其他功能），则可以在自己实现的WebMvcConfigurer接口子类添加@Configuration注解，但不可以添加@EnableWebMvc注解。如果你想用自定义类来取代RequestMappingHandlerMapping，RequestMappingHandlerAdapter或ExceptionHandlerExceptionResolver，则开发者就必须声明一个WebMvcRegistrationsAdapter实例来作为被取代的组件。

如果想完全控制Spring MVC，则可以添加自己的@EnableWebMvc注解的@Configuration。

ContentNegotiatingViewResolver

这2个类的作用就是：根据方法返回值，找到对应的View对象，然后进行解析，根据View内容来决定是展示，还是转发，重定向等操作；

打开WebMvcAutoConfiguration.viewResolver方法：

@Bean @ConditionalOnBean({ViewResolver.class}) @ConditionalOnMissingBean(     name = {"viewResolver"},     value = {ContentNegotiatingViewResolver.class} ) public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {     ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();     resolver.setContentNegotiationManager((ContentNegotiationManager)beanFactory.getBean(ContentNegotiationManager.class));     resolver.setOrder(-2147483648);     return resolver; }

这里是new ContentNegotiatingViewResolver();打开ContentNegotiatingViewResolver.resolveViewName方法，该方法是负责解析的核心方法：

@Nullable     public View resolveViewName(String viewName, Locale locale) throws Exception {         RequestAttributes attrs = RequestContextHolder.getRequestAttributes();         Assert.state(attrs instanceof ServletRequestAttributes, "No current ServletRequestAttributes");         List requestedMediaTypes =  this.getMediaTypes(((ServletRequestAttributes)attrs).getRequest());         if (requestedMediaTypes != null) {             List candidateViews = this.getCandidateViews(viewName, locale, requestedMediaTypes);             View bestView = this.getBestView(candidateViews, requestedMediaTypes, attrs);             if (bestView != null) {                 return bestView;             }         }           String mediaTypeInfo = this.logger.isDebugEnabled() && requestedMediaTypes != null ? " given " + requestedMediaTypes.toString() : "";         if (this.useNotAcceptableStatusCode) {             if (this.logger.isDebugEnabled()) {                 this.logger.debug("Using 406 NOT_ACCEPTABLE" + mediaTypeInfo);             }               return NOT_ACCEPTABLE_VIEW;         } else {             this.logger.debug("View remains unresolved" + mediaTypeInfo);             return null;         }     }

这里的核心方法就是：getCandidateViews、getBestView方法；下面是getCandidateViews方法源码：

private List getCandidateViews(String viewName, Locale locale, List requestedMediaTypes) throws Exception {     List candidateViews = new ArrayList();     if (this.viewResolvers != null) {         Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");         Iterator var5 = this.viewResolvers.iterator();         while(var5.hasNext()) {             ViewResolver viewResolver = (ViewResolver)var5.next();             View view = viewResolver.resolveViewName(viewName, locale);             if (view != null) {                 candidateViews.add(view);             }             Iterator var8 = requestedMediaTypes.iterator();             while(var8.hasNext()) {                 MediaType requestedMediaType = (MediaType)var8.next();                 List extensions = this.contentNegotiationManager.resolveFileExtensions(requestedMediaType);                 Iterator var11 = extensions.iterator();                 while(var11.hasNext()) {                     String extension = (String)var11.next();                     String viewNameWithExtension = viewName + '.' + extension;                     view = viewResolver.resolveViewName(viewNameWithExtension, locale);                     if (view != null) {                         candidateViews.add(view);                     }                 }             }         }     }     if (!CollectionUtils.isEmpty(this.defaultViews)) {         candidateViews.addAll(this.defaultViews);     }     return candidateViews; }

这里关键的代码是Iterator var5 = this.viewResolvers.iterator();这里看一下this.viewResolvers的初始化过程，这就需要看initServletContext方法：

protected void initServletContext(ServletContext servletContext) {     Collection matchingBeans =  BeanFactoryUtils.beansOfTypeIncludingAncestors(this.obtainApplicationContext(), ViewResolver.class).values();     ViewResolver viewResolver;     if (this.viewResolvers == null) {         this.viewResolvers = new ArrayList(matchingBeans.size());         Iterator var3 = matchingBeans.iterator();         while(var3.hasNext()) {             viewResolver = (ViewResolver)var3.next();             if (this != viewResolver) {                 this.viewResolvers.add(viewResolver);             }         }     } else {         for(int i = 0; i < this.viewResolvers.size(); ++i) {             viewResolver = (ViewResolver)this.viewResolvers.get(i);             if (!matchingBeans.contains(viewResolver)) {                 String name = viewResolver.getClass().getName() + i;                 this.obtainApplicationContext().getAutowireCapableBeanFactory().initializeBean(viewResolver, name);             }         }     }     AnnotationAwareOrderComparator.sort(this.viewResolvers);     this.cnmFactoryBean.setServletContext(servletContext); }

BeanFactoryUtils.beansOfTypeIncludingAncestors方法遍历整个Spring容器，来获取ViewResolver列表，然后赋值给viewResolvers；

因此，用户可以自定义一个ViewResolver接口的实现类并添加到容器中，最后Spring Boot会统一由ContentNegotiatingViewResolver添加到ViewResolver列表viewResolvers中，而用户只需提供实现类即可，不需要其他额外的配置；

Formatter

Formatter格式化器，是一个接口，下面是该接口的实现类：

在WebMvcAutoConfiguration.addFormatters、WebMvcAutoConfiguration.mvcConversionService等其他方法里面能看到添加格式化器的代码，下面是日期格式化器的源码：

@Bean public FormattingConversionService mvcConversionService() {     WebConversionService conversionService = new WebConversionService(this.mvcProperties.getDateFormat());     this.addFormatters(conversionService);     return conversionService; }

这里重点是this.mvcProperties.getDateFormat()，返回值String类型，然后我们跟踪WebMvcProperties的dateformat属性，其日期格式化是通过spring.mvc.date-format来定义的；

这里是addFormatters方法源码：

public void addFormatters(FormatterRegistry registry) {       ApplicationConversionService.addBeans(registry, this.beanFactory); } public static void addBeans(FormatterRegistry registry, ListableBeanFactory beanFactory) {         Set

Formatter接口继承自Printer接口，只要你实现了Formatter接口并添加到容器中，到了else if (bean instanceof Formatter) 这里，自然而然就会把你自定义的Formatter注册到容器中；用户只需提供实现类即可，不需要其他额外的配置；

HttpMessageConverters

public class HttpMessageConverters implements Iterable> {          }   public interface HttpMessageConverter {        }

下面是HttpMessageConverter接口的实现类：

HttpMessageConverter负责对Http进行各种转换，例如：StringHttpMessageConverter把请求内容换换成String、GsonHttpMessageConverter负责按照JSON格式进行转换Http请求，下面是源码：

这是StringHttpMessageConverter的： protected String readInternal(Class clazz, HttpInputMessage inputMessage) throws IOException {    Charset charset = this.getContentTypeCharset(inputMessage.getHeaders().getContentType());    return StreamUtils.copyToString(inputMessage.getBody(), charset); } 这是GsonHttpMessageConverter的： protected Object readInternal(Type resolvedType, Reader reader) throws Exception {     return this.getGson().fromJson(reader, resolvedType);  }

HttpMessageConverters则是作为HttpMessageConverter容器，来持有HttpMessageConverter众多的实例；

下面是WebMvcAutoConfiguration初始化HttpMessageConverters的代码：

public WebMvcAutoConfigurationAdapter(ResourceProperties resourceProperties, WebMvcProperties mvcProperties, ListableBeanFactory beanFactory, ObjectProvider messageConvertersProvider, ObjectProvider resourceHandlerRegistrationCustomizerProvider) {             this.resourceProperties = resourceProperties;             this.mvcProperties = mvcProperties;             this.beanFactory = beanFactory;             this.messageConvertersProvider = messageConvertersProvider;             this.resourceHandlerRegistrationCustomizer = (WebMvcAutoConfiguration.ResourceHandlerRegistrationCustomizer)resourceHandlerRegistrationCustomizerProvider.getIfAvailable();         }

也就是说，由外部初始化后，直接调用调用WebMvcAutoConfigurationAdapter方法来完成一部分注册；

只要你实现了HttpMessageConverter接口并添加到容器中，Spring Boot自然就会完成对你实现类的注册；用户只需提供实现类即可，不需要其他额外的配置；

MessageCodesResolver

定义错误代码的生成规则；

WebMvcAutoConfiguration中调用源码：

public MessageCodesResolver getMessageCodesResolver() {             if (this.mvcProperties.getMessageCodesResolverFormat() != null) {                 DefaultMessageCodesResolver resolver = new DefaultMessageCodesResolver();                 resolver.setMessageCodeFormatter(this.mvcProperties.getMessageCodesResolverFormat());                 return resolver;             } else {                 return null;             }  }

这里指向了WebMvcProperties的messageCodesResolverFormat，即spring.mvc.message-codes-resolver-format属性；

这个，个人只能定义格式，无法自定义实例了；

ConfigurableWebBindingInitializer

初始化WebBindingInitializer；

protected ConfigurableWebBindingInitializer getConfigurableWebBindingInitializer(FormattingConversionService mvcConversionService, Validator mvcValidator) {             try {                 return (ConfigurableWebBindingInitializer)this.beanFactory.getBean(ConfigurableWebBindingInitializer.class);             } catch (NoSuchBeanDefinitionException var4) {                 return super.getConfigurableWebBindingInitializer(mvcConversionService, mvcValidator);             }         }

@EnableWebMvc注解

首先看看@EnableWebMvc注解源代码：

@Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE}) @Documented @Import({DelegatingWebMvcConfiguration.class}) public @interface EnableWebMvc { }

这里通过Import导入了DelegatingWebMvcConfiguration类，下面看看这个类的类声明：

@Configuration(     proxyBeanMethods = false ) public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport { }

这里说明该类是WebMvcConfigurationSupport子类，下面我们再去看看WebMvcAutoConfiguration类声明：

@Configuration(     proxyBeanMethods = false ) @ConditionalOnWebApplication(     type = Type.SERVLET ) @ConditionalOnClass({Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class}) @ConditionalOnMissingBean({WebMvcConfigurationSupport.class}) @AutoConfigureOrder(-2147483638) @AutoConfigureAfter({DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class, ValidationAutoConfiguration.class}) public class WebMvcAutoConfiguration { }

当我们使用@EnableWebMvc的时候，通过@Import注解创建WebMvcConfigurationSupport实例（该实例对象被注入到Spring容器），鉴于WebMvcAutoConfiguration的@ConditionalOnMissingBean注解的作用从而导致了WebMvcAutoConfiguration实例无法创建，所以，此时，由开发者完全控制了Spring MVC；

WebMvcConfigurerComposite把所有WebMvcConfigurer实现类都放到List，等调用方法的时候，就遍历这个List里面的所有WebMvcConfigurer，去调用WebMvcConfigurer里的方法；

WebMvcConfigurer接口

WebMvcAutoConfiguration.WebMvcAutoConfigurationAdaptern内部类实现了WebMvcConfigurer接口，下面是内部类的类声明：

@Configuration( proxyBeanMethods = false ) @Import({WebMvcAutoConfiguration.EnableWebMvcConfiguration.class}) @EnableConfigurationProperties({WebMvcProperties.class, ResourceProperties.class}) @Order(0) public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {      }

这里Import了WebMvcAutoConfiguration.EnableWebMvcConfiguration类，下面看一下该类的类声明：

@Configuration( proxyBeanMethods = false ) public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {    }

该类继承了DelegatingWebMvcConfiguration类，而父类恰恰又是DelegatingWebMvcConfiguration，这里又跟@EnableWebMvc注解类似了，但与@EnableWebMvc注解有本质区别：

@EnableWebMvc注解通过@Import注解把DelegatingWebMvcConfiguration实例注入到Spring容器中，进而导致WebMvcAutoConfiguration找到该实例后，无法被创建；

而EnableWebMvcConfiguration 也是把DelegatingWebMvcConfiguration创建(该类没有声明@Bean/Component)，但是该实例是存在于JVM中的，并没有在Spring容器中进行注册，所以，WebMvcAutoConfiguration无法检测到该实例的存在；

总结：如何修改Spring Boot的默认配置

Spring Boot加载模式：

①Spring Boot在自动配置许多组件的时候，先看容器中有没有用户自己配置的组件（@Bean/@Component），如果有则就用用户配置，否则自动配置；

如果组件可以有多个，则将用户配置的、系统默认配置组合在一起，例如：ViewResolver；

②在Spring Boot中，会有很多的XXXConfigurer来帮助我们进行扩展配置；