现象

当用户在自己的spring boot main class上面显式使用了@EnableWebMvc，发现原来的放在 src/main/resources/static 目录下面的静态资源访问不到了。

@SpringBootApplication
@EnableWebMvc
public class DemoApplication {
 public static void main(String[] args) {
 SpringApplication.run(DemoApplication.class, args);
 }
}

比如在用户代码目录src/main/resources里有一个hello.txt的资源。访问 http://localhost:8080/hello.txt 返回的结果是404：

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Thu Jun 01 11:39:41 CST 2017
There was an unexpected error (type=Not Found, status=404).
No message available

那么为什么用户显式配置了@EnableWebMvc，spring boot访问静态资源会失败？
我们先来看下@EnableWebMvc的实现：

@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}

/**
 * A subclass of {@code WebMvcConfigurationSupport} that detects and delegates
 * to all beans of type {@link WebMvcConfigurer} allowing them to customize the
 * configuration provided by {@code WebMvcConfigurationSupport}. This is the
 * class actually imported by {@link EnableWebMvc @EnableWebMvc}.
 *
 * @author Rossen Stoyanchev
 * @since 3.1
 */
@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {

可以看到@EnableWebMvc 引入了 WebMvcConfigurationSupport，是spring mvc 3.1里引入的一个自动初始化配置的@Configuration 类。

spring boot里的静态资源访问的实现

再来看下spring boot里是怎么实现对src/main/resources/static这些目录的支持。
主要是通过org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration来实现的。

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

可以看到 @ConditionalOnMissingBean(WebMvcConfigurationSupport.class) ，这个条件导致spring boot的WebMvcAutoConfiguration不生效。

总结下具体的原因：

1. 用户配置了@EnableWebMvc
2. 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
3. 在扫描到@EnableWebMvc时，通过@Import加入了 DelegatingWebMvcConfiguration，也就是WebMvcConfigurationSupport
4. spring再处理@Conditional相关的注解，判断发现已有WebMvcConfigurationSupport，就跳过了spring bootr的WebMvcAutoConfiguration

所以spring boot自己的静态资源配置不生效。
其实在spring boot的文档里也有提到这点： http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-spring-mvc-auto-configuration

• If you want to keep Spring Boot MVC features, and you just want to add additional MVC configuration (interceptors, formatters, view controllers etc.) you can add your own @Configuration class of type WebMvcConfigurerAdapter, but without @EnableWebMvc. If you wish to provide custom instances of RequestMappingHandlerMapping, RequestMappingHandlerAdapter or ExceptionHandlerExceptionResolver you can declare a WebMvcRegistrationsAdapter instance providing such components.

Spring Boot ResourceProperties的配置

在spring boot里静态资源目录的配置是在ResourceProperties里。

public class ResourceProperties implements ResourceLoaderAware {

 private static final String[] SERVLET_RESOURCE_LOCATIONS = { "/" };

 private static final String[] CLASSPATH_RESOURCE_LOCATIONS = {
 "classpath:/META-INF/resources/", "classpath:/resources/",
 "classpath:/static/", "classpath:/public/" };

 private static final String[] RESOURCE_LOCATIONS;

 static {
 RESOURCE_LOCATIONS = new String[CLASSPATH_RESOURCE_LOCATIONS.length
 + SERVLET_RESOURCE_LOCATIONS.length];
 System.arraycopy(SERVLET_RESOURCE_LOCATIONS, 0, RESOURCE_LOCATIONS, 0,
 SERVLET_RESOURCE_LOCATIONS.length);
 System.arraycopy(CLASSPATH_RESOURCE_LOCATIONS, 0, RESOURCE_LOCATIONS,
 SERVLET_RESOURCE_LOCATIONS.length, CLASSPATH_RESOURCE_LOCATIONS.length);
 }

然后在 WebMvcAutoConfigurationAdapter里会初始始化相关的ResourceHandler。

@Configuration
@Import({ EnableWebMvcConfiguration.class, MvcValidatorRegistrar.class })
@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
public static class WebMvcAutoConfigurationAdapter extends WebMvcConfigurerAdapter {

  private static final Log logger = LogFactory
      .getLog(WebMvcConfigurerAdapter.class);

  private final ResourceProperties resourceProperties;

  @Override
  public void addResourceHandlers(ResourceHandlerRegistry registry) {
    if (!this.resourceProperties.isAddMappings()) {
      logger.debug("Default resource handling disabled");
      return;
    }
    Integer cachePeriod = this.resourceProperties.getCachePeriod();
    if (!registry.hasMappingForPattern("/webjars/**")) {
      customizeResourceHandlerRegistration(
          registry.addResourceHandler("/webjars/**")
              .addResourceLocations(
                  "classpath:/META-INF/resources/webjars/")
          .setCachePeriod(cachePeriod));
    }
    String staticPathPattern = this.mvcProperties.getStaticPathPattern();
    if (!registry.hasMappingForPattern(staticPathPattern)) {
      customizeResourceHandlerRegistration(
          registry.addResourceHandler(staticPathPattern)
              .addResourceLocations(
                  this.resourceProperties.getStaticLocations())
          .setCachePeriod(cachePeriod));
    }
  }

用户可以自己修改这个默认的静态资源目录，但是不建议，因为很容易引出奇怪的404问题。
文章转自: http://hengyunabc.github.io/spring-boot-enablewebmvc-static-404/