【@EnableWebMvc的原理】

用途

启用SpringMvc 的 Java 配置类，代替 xml 格式的配置文件。

一、查看运用（注解 `@EnableWebMvc` ，实现 `WebMvcConfigurer` ）

@Component("com.ibicd")
@EnableWebMvc
public class AppConfig implements WebMvcConfigurer {public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {converters.add(new MappingJackson2HttpMessageConverter());}// 添加拦截器public void addInterceptors(InterceptorRegistry registry) {registry.addInterceptor(new WebContentInterceptor());}
}

二、`@EnableWebMvc` 的注解定义

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

1、查看 `DelegatingWebMvcConfiguration` 的配置

@Configuration(proxyBeanMethods = false)
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
//具体的代码...
}

2、查看 `WebMvcConfigurationSupport`

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {//具体的代码...@Bean@Nullablepublic HandlerMapping viewControllerHandlerMapping(@Qualifier("mvcConversionService") FormattingConversionService conversionService,@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {ViewControllerRegistry registry = new ViewControllerRegistry(this.applicationContext);addViewControllers(registry);AbstractHandlerMapping handlerMapping = registry.buildHandlerMapping();if (handlerMapping == null) {return null;}PathMatchConfigurer pathConfig = getPathMatchConfigurer();if (pathConfig.getPatternParser() != null) {handlerMapping.setPatternParser(pathConfig.getPatternParser());}else {handlerMapping.setUrlPathHelper(pathConfig.getUrlPathHelperOrDefault());handlerMapping.setPathMatcher(pathConfig.getPathMatcherOrDefault());}//视图映射器设置拦截器handlerMapping.setInterceptors(getInterceptors(conversionService, resourceUrlProvider));handlerMapping.setCorsConfigurations(getCorsConfigurations());return handlerMapping;}/*** 提供对共享处理程序拦截器的访问，用于配置HandlerMapping实例。* * <p>This method cannot be overridden; use {@link #addInterceptors} instead.*/protected final Object[] getInterceptors(FormattingConversionService mvcConversionService,ResourceUrlProvider mvcResourceUrlProvider) {if (this.interceptors == null) {InterceptorRegistry registry = new InterceptorRegistry();addInterceptors(registry); // 添加自定义的拦截器registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService));registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider));this.interceptors = registry.getInterceptors();}return this.interceptors.toArray();}/*** 重写此方法以添加用于控制器调用的Spring MVC拦截器的前置和后置处理。* @see InterceptorRegistry*/protected void addInterceptors(InterceptorRegistry registry) {}
}

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3、查看 `addInterceptors` 的实现类情况（共2个）

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4、查看 `DelegatingWebMvcConfiguration` 中的方法

@Configuration(proxyBeanMethods = false)
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();@Overrideprotected void addInterceptors(InterceptorRegistry registry) {this.configurers.addInterceptors(registry); // 调用 WebMvcConfigurerComposite 的 addInterceptors 方法}}

5、查看 WebMvcConfigurerComposite

class WebMvcConfigurerComposite implements WebMvcConfigurer {private final List<WebMvcConfigurer> delegates = new ArrayList<>();// 查找调用此处的地方public void addWebMvcConfigurers(List<WebMvcConfigurer> configurers) {if (!CollectionUtils.isEmpty(configurers)) {this.delegates.addAll(configurers); // delegates 中添加所有的配置类}}@Overridepublic void addInterceptors(InterceptorRegistry registry) {for (WebMvcConfigurer delegate : this.delegates) { // this.delegates 为类中的 delegates delegate.addInterceptors(registry);  //调用每个配置类的 addInterceptors()}}}

6、在哪进行初始化 `WebMvcConfigurer` 的实现类？

目的就是查找调用 org.springframework.web.servlet.config.annotation.WebMvcConfigurerComposite#addWebMvcConfigurers 的地方：

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7、结论

根据上面的图可以知道：
我自己实现的AppConfig 配置类，使用了注解： @Component("com.ibicd") 和 @EnableWebMvc，而在进行配置类扫描的时候，依赖注入的属性会自动注入，也就是在启动容器时会自动执行： org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration#setConfigurers，然后再一次调用 addInterceptors()

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