springboot中使用@EnableWebMvc时导致静态资源失效

代码

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

如果把@EnableWebMvc注解去掉的话就可以访问静态资源。

首先看下springboot默认是如何访问静态资源的：
以下是实现了WebMvcConfigurer接口的类

WebMvcConfigurerAdapter 
WebConfig  //自定义的
WebMvcConfigurerComposite  
WebMvcAutoConfigurationAdapter in WebMvcAutoConfiguration 
Neo4jWebMvcConfiguration in Neo4jWebConfiguration in Neo4jDataAutoConfiguration 
JpaWebMvcConfiguration in JpaWebConfiguration in JpaBaseConfiguration

其中WebMvcConfigurerComposite 和 WebMvcAutoConfigurationAdapte中重写了addResourceHandlers方法

WebMvcConfigurerComposite ：

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

    public void addWebMvcConfigurers(List configurers) {
        if (!CollectionUtils.isEmpty(configurers)) {
            this.delegates.addAll(configurers);
        }
    }
    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        //遍历所有的WebMvcConfigurer(自定义的和下面的默认的一个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));
}

从上面的代码可以看出，WebMvcConfigurerComposite 中会遍历所有的WebMvcConfigurer，所以即使在自定义的配置类中重写了addResourceHandlers方法，在遍历的时候会被默认的覆盖，因为在WebMvcAutoConfigurationAdapter也有addResourceHandlers方法

webmvcconfig.png

但是加了@EnableWebMvc之后，为什么就不行了呢，先看下源码

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

这里有个@Import(DelegatingWebMvcConfiguration.class)，它是一个WebMvcConfigurationSupport ，并重写了addResourceHandlers方法，里面调用的还是之前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);
    }
}

那么此时，再调试一下。会发现此时只有自定义的一个配置类了，原本的WebMvcAutoConfiguration不在了

webmvcconfig2.png

此时再观察一下WebMvcAutoConfiguration中有一个注解：
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)

总结：

用户配置了@EnableWebMvc

Spring扫描所有的注解，再从注解上扫描到@Import，把这些@Import引入的bean信息都缓存起来 在扫描到@EnableWebMvc时，通过@Import加入了 DelegatingWebMvcConfiguration，也就是WebMvcConfigurationSupport

spring再处理@Conditional相关的注解，判断发现已有WebMvcConfigurationSupport，就跳过了spring boot的WebMvcAutoConfiguration，所以默认的也就失效了。