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2020
10-10

SpringBoot中的静态资源访问的实现

一、说在前面的话

我们之间介绍过SpringBoot自动配置的原理,基本上是如下:

xxxxAutoConfiguration:帮我们给容器中自动配置组件;
xxxxProperties:配置类来封装配置文件的内容;

二、静态资源映射规则

1、对哪些目录映射?

classpath:/META-INF/resources/ 
classpath:/resources/
classpath:/static/ 
classpath:/public/
/:当前项目的根路径

2、什么意思?

就我们在上面五个目录下放静态资源(比如:a.js等),可以直接访问(http://localhost:8080/a.js),类似于以前web项目的webapp下;放到其他目录下无法被访问。

3、为什么是那几个目录?

3.1、看源码

我们一起来读下源码,这个是SpringBoot自动配置的WebMvcAutoConfiguration.java类来帮我们干的。

@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));
  }
}

3.2、分析源码

我们重点分析后半截,前半截后面会介绍。

// staticPathPattern是/**
String staticPathPattern = this.mvcProperties.getStaticPathPattern();
if (!registry.hasMappingForPattern(staticPathPattern)) {
  customizeResourceHandlerRegistration(
      registry.addResourceHandler(staticPathPattern)
          .addResourceLocations(
              this.resourceProperties.getStaticLocations())
      .setCachePeriod(cachePeriod));
}
this.resourceProperties.getStaticLocations()
========>
ResourceProperties
public String[] getStaticLocations() {
  return this.staticLocations;
}
========>
private String[] staticLocations = RESOURCE_LOCATIONS;
========>
private static final String[] RESOURCE_LOCATIONS;
private static final String[] SERVLET_RESOURCE_LOCATIONS = { "/" };
private static final String[] CLASSPATH_RESOURCE_LOCATIONS = {
      "classpath:/META-INF/resources/", "classpath:/resources/",
      "classpath:/static/", "classpath:/public/" };
========>
static {
  // 可以看到如下是对上面两个数组进行复制操作到一个新数组上,也就是合并。
  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);
}
所以上述代码经过我的翻译后成为了如下样子:

registry.addResourceHandler("/**").addResourceLocations(
  "classpath:/META-INF/resources/", "classpath:/resources/",
      "classpath:/static/", "classpath:/public/", "/")
  // 设置缓存时间
  .setCachePeriod(cachePeriod));

3.3、一句话概括

WebMvcAutoConfiguration类自动为我们注册了如下目录为静态资源目录,也就是说直接可访问到资源的目录。

classpath:/META-INF/resources/ 
classpath:/resources/
classpath:/static/ 
classpath:/public/
/:当前项目的根路径

优先级从上到下。

所以,如果static里面有个index.html,public下面也有个index.html,则优先会加载static下面的index.html,因为优先级!

4、默认首页

PS:就是直接输入ip:port/项目名称默认进入的页面。

4.1、看源码

WebMvcAutoConfiguration.java

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(
   ResourceProperties resourceProperties) {
  return new WelcomePageHandlerMapping(resourceProperties.getWelcomePage(),
     this.mvcProperties.getStaticPathPattern());
}

4.2、分析源码

resourceProperties.getWelcomePage()
========>
public Resource getWelcomePage() {
  // 遍历默认静态资源目录后面拼接个index.html的数组
  // 比如:[/static/index.html, /public/index.html等等]
  for (String location : getStaticWelcomePageLocations()) {
    Resource resource = this.resourceLoader.getResource(location);
    try {
      if (resource.exists()) {
        resource.getURL();
        return resource;
      }
    }
    catch (Exception ex) {
      // Ignore
    }
  }
  return null;
}
========>
// 下面这段代码通俗易懂,就是给默认静态资源目录后面拼接个index.html并返回,比如:/static/index.html
private String[] getStaticWelcomePageLocations() {
  String[] result = new String[this.staticLocations.length];
  for (int i = 0; i < result.length; i++) {
    String location = this.staticLocations[i];
    if (!location.endsWith("/")) {
      location = location + "/";
    }
    result[i] = location + "index.html";
  }
  return result;
}

所以上述代码经过我的翻译后成为了如下样子:

return new WelcomePageHandlerMapping(
  "classpath:/META-INF/resources/index.html",
  "classpath:/resources/index.html",
  "classpath:/static/index.html",
  "classpath:/public/index.html",
  "/index.html"
  , "/**");

4.3、一句话概括

WebMvcAutoConfiguration类自动为我们注册了如下文件为默认首页。

classpath:/META-INF/resources/index.html
classpath:/resources/index.html
classpath:/static/index.html 
classpath:/public/index.html
/index.html

优先级从上到下。

所以,如果static里面有个index.html,public下面也有个index.html,则优先会加载static下面的index.html,因为优先级!

5、favicon.ico

PS:就是这个图标。

5.1、看源码

@Configuration
@ConditionalOnProperty(value = "spring.mvc.favicon.enabled", matchIfMissing = true)
public static class FaviconConfiguration {

  private final ResourceProperties resourceProperties;

  public FaviconConfiguration(ResourceProperties resourceProperties) {
   this.resourceProperties = resourceProperties;
  }

  @Bean
  public SimpleUrlHandlerMapping faviconHandlerMapping() {
   SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();
   mapping.setOrder(Ordered.HIGHEST_PRECEDENCE + 1);
   mapping.setUrlMap(Collections.singletonMap("**/favicon.ico",
      faviconRequestHandler()));
   return mapping;
  }

  @Bean
  public ResourceHttpRequestHandler faviconRequestHandler() {
   ResourceHttpRequestHandler requestHandler = new ResourceHttpRequestHandler();
   requestHandler
      .setLocations(this.resourceProperties.getFaviconLocations());
   return requestHandler;
  }

}

5.2、分析源码

// 首先可以看到的是可以设置是否生效,通过参数spring.mvc.favicon.enabled来配置,若无此参数,则默认是生效的。
@ConditionalOnProperty(value = "spring.mvc.favicon.enabled", matchIfMissing = true)
========》
// 可以看到所有的**/favicon.ico都是在faviconRequestHandler()这个方法里找。
mapping.setUrlMap(Collections.singletonMap("**/favicon.ico", faviconRequestHandler()));
========》
faviconRequestHandler().this.resourceProperties.getFaviconLocations()
// 就是之前的五个静态资源文件夹。  
List<Resource> getFaviconLocations() {
  List<Resource> locations = new ArrayList<Resource>(
      this.staticLocations.length + 1);
  if (this.resourceLoader != null) {
    for (String location : this.staticLocations) {
      locations.add(this.resourceLoader.getResource(location));
    }
  }
  locations.add(new ClassPathResource("/"));
  return Collections.unmodifiableList(locations);
}

5.3、一句话概括

只要把favicon.ico放到如下目录下,就会自动生效。

classpath:/META-INF/resources/ 
classpath:/resources/
classpath:/static/ 
classpath:/public/
/:当前项目的根路径

6、webjars

6.1、看源码

WebMvcAutoConfiguration

@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));
  }
}

6.2、分析源码

这次我们来分析前半截。

Integer cachePeriod = this.resourceProperties.getCachePeriod();
if (!registry.hasMappingForPattern("/webjars/**")) {
  customizeResourceHandlerRegistration(
      registry.addResourceHandler("/webjars/**")
          .addResourceLocations(
              "classpath:/META-INF/resources/webjars/")
      .setCachePeriod(cachePeriod));
}

6.3、一句话概括

所有/webjars/**都从classpath:/META-INF/resources/webjars/路径下去找对应的静态资源。

6.4、什么是webjars?

就是以jar包的方式引入静态资源。

官网地址:http://www.webjars.org/。类似于maven仓库。

我们可以做个例子,将jquery引入到项目中

<dependency>
  <groupId>org.webjars</groupId>
  <artifactId>jquery</artifactId>
  <version>3.3.1</version>
</dependency>

看项目依赖

会自动为我们引入jquery,要怎么使用呢?我们上面说过:

所有/webjars/*都从classpath:/META-INF/resources/webjars/路径下去找对应的静态资源。

所以我们启动项目,访问:http://localhost:8080/webjars/jquery/3.3.1/jquery.js即可。

必须在这几个路径下SpringBoot才会扫描到!

"classpath:/META-INF/resources/",
"classpath:/resources/",
"classpath:/static/",
"classpath:/public/"
"/":当前项目的根路径

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