SpringBoot 读取Jar包中静态资源原理

## 原理

SpringBoot本身注入了一个 **Bean** `org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter`，其中的`addResourceHandlers`方法添加了资源路径映射。

源码：

```
@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
			if (!this.resourceProperties.isAddMappings()) {
				logger.debug("Default resource handling disabled");
				return;
			}
			Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
			CacheControl cacheControl = this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl();
			if (!registry.hasMappingForPattern("/webjars/**")) {
				customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**")
						.addResourceLocations("classpath:/META-INF/resources/webjars/")
						.setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
			}
			String staticPathPattern = this.mvcProperties.getStaticPathPattern();
			if (!registry.hasMappingForPattern(staticPathPattern)) {
				customizeResourceHandlerRegistration(registry.addResourceHandler(staticPathPattern)
						.addResourceLocations(getResourceLocations(this.resourceProperties.getStaticLocations()))
						.setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
			}
}
```

等价于下面代码：

```
registry.addResourceHandler("/webjars/**").addResourceLocations("classpath:/META-INF/resources/webjars/")

registry.addResourceHandler("/**").addResourceLocations("classpath:/META-INF/resources/",
			"classpath:/resources/", "classpath:/static/", "classpath:/public/")
```

也就是说，只要在jar包中的`"classpath:/META-INF/resources/","classpath:/resources/", "classpath:/static/", "classpath:/public/"`下放入静态资源，就可以正常访问

也就可以推断出，SpringBoot会读取所有Jar包下`"classpath:/META-INF/resources/","classpath:/resources/", "classpath:/static/", "classpath:/public/"`目录文件。

## 相关

- 参考[swagger-bootstrap-ui](https://mvnrepository.com/artifact/com.github.xiaoymin/knife4j-spring-ui)的实现
- 参考[springfox-swagger-ui](https://mvnrepository.com/artifact/io.springfox/springfox-swagger-ui)配合[springfox-boot-starter](https://github.com/springfox/springfox/tree/master/springfox-boot-starter)中的`SwaggerUiWebMvcConfigurer`类实现自定义的资源路径映射