SpringBoot - WebJars

【1】是什么

WebJars是将web前端资源（js，css等）打成jar包文件，然后借助Maven工具，以jar包形式对web前端资源进行统一依赖管理，保证这些Web资源版本唯一性。WebJars的jar包部署在Maven中央仓库上。

官网地址 ： https://www.webjars.org/

Javaweb，

【2】WebJars使用

在官网首页，找到资源文件对应的maven依赖，写入项目pom.xml文件，然后在页面引用即可。

如下图所示，引入JQuery和Bootstrap：

maven java、pom.xml文件如下所示：

    <!--引入jquery-webjar--><dependency><groupId>org.webjars</groupId><artifactId>jquery</artifactId><version>3.3.1</version></dependency><!--引入bootstrap--><dependency><groupId>org.webjars</groupId><artifactId>bootstrap</artifactId><version>4.0.0</version></dependency>

引入的jar包如下：

【3】SpringBoot对WebJars支持

查看WebMWebMVCAutoConfiguration源码如下：

@Overridepublic void addResourceHandlers(ResourceHandlerRegistry registry) {if (!this.resourceProperties.isAddMappings()) {logger.debug("Default resource handling disabled");return;}Integer cachePeriod = this.resourceProperties.getCachePeriod();//注意这里，对webjars支持if (!registry.hasMappingForPattern("/webjars/**")) {customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**").addResourceLocations("classpath:/META-INF/resources/webjars/").setCachePeriod(cachePeriod));}//这里是对静态资源的映射支持String staticPathPattern = this.mvcProperties.getStaticPathPattern();if (!registry.hasMappingForPattern(staticPathPattern)) {customizeResourceHandlerRegistration(registry.addResourceHandler(staticPathPattern).addResourceLocations(this.resourceProperties.getStaticLocations()).setCachePeriod(cachePeriod));}}

java web start，其中如下代码说明SpringBoot如何对WebJars进行支持：

所有 /webjars/**都去 classpath:/META-INF/resources/webjars/ 找资源；

//注意这里，对webjars支持
if (!registry.hasMappingForPattern("/webjars/**")) {
customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**").addResourceLocations("classpath:/META-INF/resources/webjars/").setCachePeriod(cachePeriod));
}

【4】页面使用

页面引入如下：

<link rel="stylesheet" href="/webjars/bootstrap/4.0.0/css/bootstrap.css" />
<script src="/webjars/jquery/3.3.1/jquery.js"></script>
<script src="/webjars/bootstrap/4.0.0/js/bootstrap.js"></script>

如上所示，路径中是需要添加版本号的，这是很不友好的，下面去掉版本号。

springboot2、pom.xml添加依赖如下：

<dependency><groupId>org.webjars</groupId><artifactId>webjars-locator</artifactId><version>0.32</version>
</dependency>

对应的类-WebJarsResourceResolver如下：

• WebJarsResourceResolver 的作用是可以让页面引用省略 webjar 的版本。

package org.springframework.web.servlet.resource;import java.util.List;import javax.servlet.http.HttpServletRequest;import org.webjars.WebJarAssetLocator;import org.springframework.core.io.Resource;/*** A {@code ResourceResolver} that delegates to the chain to locate a resource and then* attempts to find a matching versioned resource contained in a WebJar JAR file.** <p>This allows WebJars.org users to write version agnostic paths in their templates,* like {@code <script src="/jquery/jquery.min.js"/>}.* This path will be resolved to the unique version {@code <script src="/jquery/1.2.0/jquery.min.js"/>},* which is a better fit for HTTP caching and version management in applications.** <p>This also resolves resources for version agnostic HTTP requests {@code "GET /jquery/jquery.min.js"}.** <p>This resolver requires the "org.webjars:webjars-locator" library on classpath,* and is automatically registered if that library is present.** @author Brian Clozel* @since 4.2* @see org.springframework.web.servlet.config.annotation.ResourceChainRegistration* @see <a href="http://www.webjars.org">webjars.org</a>*/
public class WebJarsResourceResolver extends AbstractResourceResolver {private final static String WEBJARS_LOCATION = "META-INF/resources/webjars/";//默认去该路径下寻找private final static int WEBJARS_LOCATION_LENGTH = WEBJARS_LOCATION.length();private final WebJarAssetLocator webJarAssetLocator;/*** Create a {@code WebJarsResourceResolver} with a default {@code WebJarAssetLocator} instance.*/public WebJarsResourceResolver() {this(new WebJarAssetLocator());}/*** Create a {@code WebJarsResourceResolver} with a custom {@code WebJarAssetLocator} instance,* e.g. with a custom index.* @since 4.3*/public WebJarsResourceResolver(WebJarAssetLocator webJarAssetLocator) {this.webJarAssetLocator = webJarAssetLocator;}@Overrideprotected Resource resolveResourceInternal(HttpServletRequest request, String requestPath,List<? extends Resource> locations, ResourceResolverChain chain) {Resource resolved = chain.resolveResource(request, requestPath, locations);if (resolved == null) {String webJarResourcePath = findWebJarResourcePath(requestPath);if (webJarResourcePath != null) {return chain.resolveResource(request, webJarResourcePath, locations);}}return resolved;}@Overrideprotected String resolveUrlPathInternal(String resourceUrlPath,List<? extends Resource> locations, ResourceResolverChain chain) {String path = chain.resolveUrlPath(resourceUrlPath, locations);if (path == null) {String webJarResourcePath = findWebJarResourcePath(resourceUrlPath);if (webJarResourcePath != null) {return chain.resolveUrlPath(webJarResourcePath, locations);}}return path;}// 这里获取请求对应的项目中实际资源路径，//返回结果如：bootstrap/4.0.0/css/bootstrap.cssprotected String findWebJarResourcePath(String path) {int startOffset = (path.startsWith("/") ? 1 : 0);int endOffset = path.indexOf("/", 1);if (endOffset != -1) {String webjar = path.substring(startOffset, endOffset);String partialPath = path.substring(endOffset + 1);String webJarPath = webJarAssetLocator.getFullPathExact(webjar, partialPath);if (webJarPath != null) {return webJarPath.substring(WEBJARS_LOCATION_LENGTH);}}return null;}}

页面引用修改如下：

<link rel="stylesheet" href="/webjars/bootstrap/css/bootstrap.css" />
<script src="/webjars/jquery/jquery.js"></script>
<script src="/webjars/bootstrap/js/bootstrap.js"></script>

此时，使用两种形式(带版本号和不带版本号)的请求都可以正确获取资源！