深入Spring Boot：顯式配置 @EnableWebMvc 導致靜態資源訪問失敗

現象

當用戶在自己的spring boot main class上面顯式使用了 @EnableWebMvc，發現原來的放在 src/main/resources/static 目錄下面的靜態資源訪問不到了。

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

比如在用戶代碼目錄 src/main/resources裏有一個 hello.txt的資源。訪問 http://localhost:8080/hello.txt 返回的結果是404：

Whitelabel Error Page
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

靜態資源訪問失敗原因

@EnableWebMvc的實現

那麼爲什麼用戶顯式配置了 @EnableWebMvc，spring boot訪問靜態資源會失敗？

我們先來看下 @EnableWebMvc的實現：

@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裏引入的一個自動初始化配置的 @Configuration 類。

spring boot裏的靜態資源訪問的實現

再來看下spring boot裏是怎麼實現對 src/main/resources/static這些目錄的支持。

主要是通過 org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration來實現的。

@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) ，這個條件導致spring boot的 WebMvcAutoConfiguration不生效。

總結下具體的原因：

1. 用戶配置了 @EnableWebMvc

2. Spring掃描所有的註解，再從註解上掃描到 @Import，把這些 @Import引入的bean信息都緩存起來

3. 在掃描到 @EnableWebMvc時，通過 @Import加入了 DelegatingWebMvcConfiguration，也就是 WebMvcConfigurationSupport

4. spring再處理 @Conditional相關的註解，判斷髮現已有 WebMvcConfigurationSupport，就跳過了spring bootr的 WebMvcAutoConfiguration

所以spring boot自己的靜態資源配置不生效。

其實在spring boot的文檔裏也有提到這點： 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裏靜態資源目錄的配置是在 ResourceProperties裏。

@ConfigurationProperties(prefix = "spring.resources", ignoreUnknownFields = false)
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);
    }

然後在 WebMvcAutoConfigurationAdapter裏會初始始化相關的ResourceHandler。

//org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter
@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));
    }
  }

用戶可以自己修改這個默認的靜態資源目錄，但是不建議，因爲很容易引出奇怪的404問題。