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*仓库上。

官网地址 ： https://www.webjars.org/

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【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：

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：

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【3】SpringBoot对WebJars支持

查看WebMWebMVCAutoConfiguration源码如下：

@Override
        public 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));
            }
        }

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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));
}

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【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>

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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。

pom.xml添加依赖如下：

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

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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;
    }


    @Override
    protected 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;
    }

    @Override
    protected 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.css
    protected 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;
    }

}

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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！