[Java知识库] 3 SpringBoot静态资源配置原理

开发: C++知识库 Java知识库 JavaScript Python PHP知识库人工智能区块链大数据移动开发嵌入式开发工具数据结构与算法开发测试游戏开发网络协议系统运维
教程: HTML教程 CSS教程 JavaScript教程 Go语言教程 JQuery教程 VUE教程 VUE3教程 Bootstrap教程 SQL数据库教程 C语言教程 C++教程 Java教程 Python教程 Python3教程 C#教程
数码: 电脑笔记本显卡显示器固态硬盘硬盘耳机手机 iphone vivo oppo 小米华为单反装机图拉丁

-> Java知识库 -> 3 SpringBoot静态资源配置原理 -> 正文阅读

[Java知识库]3 SpringBoot静态资源配置原理

在 SpringBoot 中, 默认访问主页(index.html)可以配置在 resources/static or resources/templates 下, 容器启动后, 可以默认去访问 index.html 文件, 其中的原理是什么?

默认访问规则

SpringBoot启动时会加载 xxxAutoConfiguration 类 (自动配置类), 关于 SpringMvc 的自动配置类是 WebMvcAutoConfiguration

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

@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })

当容器中存在 Servlet, DispatcherServlet, WebMvcConfigurer 类时, WebMvcAutoConfiguration 才会生效, 显然是存在的

@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)

当容器中不存在 WebMvcConfigurationSupport 才会生效

那么WebMvcAutoConfiguration中干了什么, 容器启动时才可以默认访问 Index.html 静态资源

我们发现 WebMvcAutoConfiguration 类中有一个实现了 WebMvcConfigurer 的接口 -> WebMvcAutoConfigurationAdapter 静态内部类

@Configuration(proxyBeanMethods = false)
@Import(EnableWebMvcConfiguration.class)
@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
@Order(0)
public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {
}

@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class }): 注入了两个配置类

WebMvcProperties:
@ConfigurationProperties(prefix = "spring.mvc")
public class WebMvcProperties {
}
跟配置文件 spring.mvc 开头的属性绑定

ResourceProperties:
@ConfigurationProperties(prefix = "spring.resources", ignoreUnknownFields = false)
public class ResourceProperties {
}
跟配置文件 spring.resources 开头的属性绑定

当上述条件都生效, 则意味着WebMvcAutoConfigurationAdapter 也会去执行自己的逻辑

@Configuration(proxyBeanMethods = false)
@Import(EnableWebMvcConfiguration.class)
@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
@Order(0)
public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {
   
   // WebMvcAutoConfigurationAdapter 构造器, 参数详解见下段代码
   public WebMvcAutoConfigurationAdapter(ResourceProperties resourceProperties,
                                         WebMvcProperties mvcProperties,
										 ListableBeanFactory beanFactory, 
                                         ObjectProvider<HttpMessageConverters> messageConvertersProvider,
										 ObjectProvider<ResourceHandlerRegistrationCustomizer> r,
										 ObjectProvider<DispatcherServletPath> dispatcherServletPath,
										 ObjectProvider<ServletRegistrationBean<?>> servletRegistrations)
   {
       
       this.resourceProperties = resourceProperties;
       this.mvcProperties = mvcProperties;
       this.beanFactory = beanFactory;
       this.messageConvertersProvider = messageConvertersProvider;
       this.resourceHandlerRegistrationCustomizer = r.getIfAvailable();
       this.dispatcherServletPath = dispatcherServletPath;
       this.servletRegistrations = servletRegistrations;
   }
    
   // 添加资源处理器
   @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        // addMappings(是否默认的资源映射): 默认为true, 所以此if进不去
        // 如果不想访问静态资源, 则在配置文件中设置 add-mappings: false
        if (!this.resourceProperties.isAddMappings()) {
            logger.debug("Default resource handling disabled");
            return;
        }
        
        // -----静态资源默认配置规则-----
        
        // 获取静态资源缓存时间, 默认为0
        Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
        CacheControl cacheControl = this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl();
        
        /* 设置 /webjars/** 规则
         * 如果访问此路径, 则在 classpath:/META-INF/resources/webjars/ 路径下寻找对应的静态资源,
         * 找到后并缓存之前设置的缓存时间
         */
        if (!registry.hasMappingForPattern("/webjars/**")) {
            customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**")
                                                 .addResourceLocations("classpath:/META-INF/resources/webjars/")
                                                .setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
        }
        
        // 获取自动注入类 WebMvcProperties 的属性 staticPathPattern, 默认为 "/**", 可以通过配置文件修改
        String staticPathPattern = this.mvcProperties.getStaticPathPattern();
        // 是否给此路径配置了处理程序, 显然, "/**" 没有对应的处理程序
        if (!registry.hasMappingForPattern(staticPathPattern)) {
            // 和webjars处理规则一样, 去getResourceLocations(this.resourceProperties.getStaticLocations())获取资源路径
            // 详解见下段代码
            customizeResourceHandlerRegistration(registry.addResourceHandler(staticPathPattern)
            .addResourceLocations(getResourceLocations(this.resourceProperties.getStaticLocations()))
                         .setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
        }
    }
}

WebMvcAutoConfigurationAdapter 构造器参数:

ResourceProperties: 对应WebMvcAutoConfiguration 注入的 ResourceProperties, 绑定了 spring.resources 的值
WebMvcProperties: 对应WebMvcAutoConfiguration 注入的 WebMvcProperties, 绑定了 spring.mvc 的值
ListableBeanFactory: Bean 容器, 实现了 BeanFactory
ObjectProvider<HttpMessageConverters>: 消息转换器
ObjectProvider<ResourceHandlerRegistrationCustomizer>: 自定义资源处理器
ObjectProvider<DispatcherServletPath>: 请求分发处理器
ObjectProvider<ServletRegistrationBean<?>>: 给应用注册原生的 Servlet 等组件

/** 资源访问规则(getResourceLocations(this.resourceProperties.getStaticLocations())):

this.resourceProperties.getStaticLocations():

resourceProperties 对应上文所注入的 ResourceProperties, getStaticLocations
?????获取到对应的值为静态常量: CLASSPATH_RESOURCE_LOCATIONS

CLASSPATH_RESOURCE_LOCATIONS =
????????????????????????????????????????{
??????????????????????????????????????????“classpath:/META-INF/resources/”,
??????????????????????????????????????????“classpath:/resources/”,
??????????????????????????????????????????“classpath:/static/”,
??????????????????????????????????????????“classpath:/public/”
????????????????????????????????????????};

getResourceLocations(this.resourceProperties.getStaticLocations()):

那 getResourceLocations 又干了什么事呢?
static String[] getResourceLocations(String[] staticLocations) {
    String[] locations = new String[staticLocations.length + SERVLET_LOCATIONS.length];
    System.arraycopy(staticLocations, 0, locations, 0, staticLocations.length);
    System.arraycopy(SERVLET_LOCATIONS, 0, locations, staticLocations.length, SERVLET_LOCATIONS.length);
    return locations;
}

// 其中 SERVLET_LOCATIONS = { "/" };
这个时候我们明白了, "/**"的访问规则有 5 种

分别是: “/”, “classpath:/META-INF/resources/”, “classpath:/resources/”, “classpath:/static/”, “classpath:/public/”

注意: "/" 代表项目目录下的 webapp 文件夹

所以SpringBoot静态资源默认访问是 “/”, “classpath:/META-INF/resources/”, “classpath:/resources/”, “classpath:/static/”, “classpath:/public/”

欢迎页访问规则

讲完了SpringBoot默认静态资源访问规则, 再来看看欢迎页的规则

在 WebMvcAutoConfiguration 中注册了 WelcomePageHandlerMapping Bean

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext,
                                                           FormattingConversionService mvcConversionService, 
                                                           ResourceUrlProvider mvcResourceUrlProvider) {
    WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(
        new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),
        this.mvcProperties.getStaticPathPattern());
    welcomePageHandlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));
    welcomePageHandlerMapping.setCorsConfigurations(getCorsConfigurations());
    return welcomePageHandlerMapping;
}

打开 WelcomePageHandlerMapping 构造器

WelcomePageHandlerMapping(TemplateAvailabilityProviders templateAvailabilityProviders,
                          ApplicationContext applicationContext, Optional<Resource> welcomePage, String 	
                          staticPathPattern) {
    if (welcomePage.isPresent() && "/**".equals(staticPathPattern)) {
        logger.info("Adding welcome page: " + welcomePage.get());
        setRootViewName("forward:index.html");
    }
    else if (welcomeTemplateExists(templateAvailabilityProviders, applicationContext)) {
        logger.info("Adding welcome page template: index");
        setRootViewName("index");
    }
}