代碼

@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new MyInterceptors()).addPathPatterns("/**").excludePathPatterns("/admin/**");
    }
}

如果把@EnableWebMvc注解去掉的話就可以訪問靜態(tài)資源。

首先看下springboot默認(rèn)是如何訪問靜態(tài)資源的：
以下是實(shí)現(xiàn)了WebMvcConfigurer接口的類

WebMvcConfigurerAdapter 
WebConfig  //自定義的
WebMvcConfigurerComposite  
WebMvcAutoConfigurationAdapter in WebMvcAutoConfiguration 
Neo4jWebMvcConfiguration in Neo4jWebConfiguration in Neo4jDataAutoConfiguration 
JpaWebMvcConfiguration in JpaWebConfiguration in JpaBaseConfiguration

其中WebMvcConfigurerComposite 和 WebMvcAutoConfigurationAdapte中重寫了addResourceHandlers方法

WebMvcConfigurerComposite ：

    private final List<WebMvcConfigurer> delegates = new ArrayList<>();

    public void addWebMvcConfigurers(List<WebMvcConfigurer> configurers) {
        if (!CollectionUtils.isEmpty(configurers)) {
            this.delegates.addAll(configurers);
        }
    }
    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        //遍歷所有的WebMvcConfigurer(自定義的和下面的默認(rèn)的一個(gè)WebMvcAutoConfigurationAdapter)
        for (WebMvcConfigurer delegate : this.delegates) {
            delegate.addResourceHandlers(registry);
        }
    }

WebMvcAutoConfigurationAdapter

@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class,
        ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {


    @Configuration
    @Import(EnableWebMvcConfiguration.class)
    @EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
    public static class WebMvcAutoConfigurationAdapter
            implements WebMvcConfigurer, ResourceLoaderAware {
        @Override
        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            if (!this.resourceProperties.isAddMappings()) {
                logger.debug("Default resource handling disabled");
                return;
            }
            Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
            CacheControl cacheControl = this.resourceProperties.getCache()
                    .getCachecontrol().toHttpCacheControl();
            if (!registry.hasMappingForPattern("/webjars/**")) {
                customizeResourceHandlerRegistration(registry
                        .addResourceHandler("/webjars/**")
                        .addResourceLocations("classpath:/META-INF/resources/webjars/")
                        .setCachePeriod(getSeconds(cachePeriod))
                        .setCacheControl(cacheControl));
            }
            String staticPathPattern = this.mvcProperties.getStaticPathPattern();
            if (!registry.hasMappingForPattern(staticPathPattern)) {
                customizeResourceHandlerRegistration(
                        registry.addResourceHandler(staticPathPattern)
                                .addResourceLocations(getResourceLocations(
                                        this.resourceProperties.getStaticLocations()))
                                .setCachePeriod(getSeconds(cachePeriod))
                                .setCacheControl(cacheControl));
}

從上面的代碼可以看出，WebMvcConfigurerComposite 中會(huì)遍歷所有的WebMvcConfigurer，所以即使在自定義的配置類中重寫了addResourceHandlers方法，在遍歷的時(shí)候會(huì)被默認(rèn)的覆蓋，因?yàn)樵赪ebMvcAutoConfigurationAdapter也有addResourceHandlers方法

webmvcconfig.png

但是加了@EnableWebMvc之后，為什么就不行了呢，先看下源碼

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

這里有個(gè)@Import(DelegatingWebMvcConfiguration.class)，它是一個(gè)WebMvcConfigurationSupport ，并重寫了addResourceHandlers方法，里面調(diào)用的還是之前WebMvcConfigurerComposite中的addResourceHandlers方法。

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {

    private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
    @Override
    protected void addResourceHandlers(ResourceHandlerRegistry registry) {
        this.configurers.addResourceHandlers(registry);
    }
}

webmvcconfig2.png

此時(shí)再觀察一下WebMvcAutoConfiguration中有一個(gè)注解：
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)

總結(jié)：

用戶配置了@EnableWebMvc

Spring掃描所有的注解，再從注解上掃描到@Import，把這些@Import引入的bean信息都緩存起來 在掃描到@EnableWebMvc時(shí)，通過@Import加入了 DelegatingWebMvcConfiguration，也就是WebMvcConfigurationSupport

spring再處理@Conditional相關(guān)的注解，判斷發(fā)現(xiàn)已有WebMvcConfigurationSupport，就跳過了spring boot的WebMvcAutoConfiguration，所以默認(rèn)的也就失效了。