【@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
	@Nullable
	public 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。
	 * 
	 * 
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) {
	
	}
}

3、查看 addInterceptors 的实现类情况（共2个）

4、查看 DelegatingWebMvcConfiguration 中的方法

@Configuration(proxyBeanMethods = false)
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
   
   	private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
	
	@Override
	protected 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
		}
	}

	@Override
	public 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：

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()