Spring Boot：显式配置 @EnableWebMvc 导致静态资源访问失败
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：
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
静态资源访问失败原因
@EnableWebMvc的实现
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：
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 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。
@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);
}
然后在 WebMvcAutoConfigurationAdapter里会初始始化相关的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。
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版权声明：本文为CSDN博主「横云断岭」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/hengyunabc/java/article/details/103406178
