Add a ResourceResolver implementation for WebJars

Prior to this commit, WebJars users needed to use versioned links within
templates for WebJars resources, such as `/jquery/1.2.0/jquery.js`.
This can be rather cumbersome when updating libraries - all references
in templates need to be updated.

One could use version-less links in templates, but needed to add a
specific MVC Handler that uses webjars.org's webjar-locator library.
While this approach makes maintaing templates easier, this makes HTTP
caching strategies less optimal.

This commit adds a new WebJarsResourceResolver that search for resources
located in WebJar locations. This ResourceResolver is automatically
registered if the "org.webjars:webjars-locator" dependency is present.

Registering WebJars resource handling can be done like this:

```java
@Override
protected void addResourceHandlers(ResourceHandlerRegistry registry) {
  registry.addResourceHandler("/webjars/**")
          .addResourceLocations("classpath:META-INF/resources/webjars")
          .resourceChain(true)
              .addResolver(new WebJarsResourceResolver());
}
```

Issue: SPR-12323

polish

spring-webmvc/src/main/java/org/springframework/web/servlet/config/ResourcesBeanDefinitionParser.java

@@ -33,6 +33,7 @@ import org.springframework.beans.factory.xml.BeanDefinitionParser;
 import org.springframework.beans.factory.xml.ParserContext;
 import org.springframework.cache.concurrent.ConcurrentMapCache;
 import org.springframework.core.Ordered;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.util.xml.DomUtils;
 import org.springframework.http.CacheControl;
@@ -51,6 +52,7 @@ import org.springframework.web.servlet.resource.ResourceTransformer;
 import org.springframework.web.servlet.resource.ResourceUrlProvider;
 import org.springframework.web.servlet.resource.ResourceUrlProviderExposingInterceptor;
 import org.springframework.web.servlet.resource.VersionResourceResolver;
+import org.springframework.web.servlet.resource.WebJarsResourceResolver;
 
 /**
  * {@link org.springframework.beans.factory.xml.BeanDefinitionParser} that parses a
@@ -78,6 +80,9 @@ class ResourcesBeanDefinitionParser implements BeanDefinitionParser {
 
 	private static final String RESOURCE_URL_PROVIDER = "mvcResourceUrlProvider";
 
+	private static final boolean isWebJarsAssetLocatorPresent = ClassUtils.isPresent(
+			"org.webjars.WebJarAssetLocator", ResourcesBeanDefinitionParser.class.getClassLoader());
+
 
 	@Override
 	public BeanDefinition parse(Element element, ParserContext parserContext) {
@@ -302,6 +307,12 @@ class ResourcesBeanDefinitionParser implements BeanDefinitionParser {
 		}
 
 		if (isAutoRegistration) {
+			if(isWebJarsAssetLocatorPresent) {
+				RootBeanDefinition webJarsResolverDef = new RootBeanDefinition(WebJarsResourceResolver.class);
+				webJarsResolverDef.setSource(source);
+				webJarsResolverDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
+				resourceResolvers.add(webJarsResolverDef);
+			}
 			RootBeanDefinition pathResolverDef = new RootBeanDefinition(PathResourceResolver.class);
 			pathResolverDef.setSource(source);
 			pathResolverDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/ResourceChainRegistration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@ import java.util.List;
 import org.springframework.cache.Cache;
 import org.springframework.cache.concurrent.ConcurrentMapCache;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.web.servlet.resource.CachingResourceResolver;
 import org.springframework.web.servlet.resource.CachingResourceTransformer;
 import org.springframework.web.servlet.resource.CssLinkResourceTransformer;
@@ -29,6 +30,7 @@ import org.springframework.web.servlet.resource.PathResourceResolver;
 import org.springframework.web.servlet.resource.ResourceResolver;
 import org.springframework.web.servlet.resource.ResourceTransformer;
 import org.springframework.web.servlet.resource.VersionResourceResolver;
+import org.springframework.web.servlet.resource.WebJarsResourceResolver;
 
 /**
  * Assists with the registration of resource resolvers and transformers.
@@ -40,6 +42,9 @@ public class ResourceChainRegistration {
 
 	private static final String DEFAULT_CACHE_NAME = "spring-resource-chain-cache";
 
+	private static final boolean isWebJarsAssetLocatorPresent = ClassUtils.isPresent(
+			"org.webjars.WebJarAssetLocator", ResourceChainRegistration.class.getClassLoader());
+
 	private final List<ResourceResolver> resolvers = new ArrayList<ResourceResolver>(4);
 
 	private final List<ResourceTransformer> transformers = new ArrayList<ResourceTransformer>(4);
@@ -98,6 +103,9 @@ public class ResourceChainRegistration {
 	protected List<ResourceResolver> getResourceResolvers() {
 		if (!this.hasPathResolver) {
 			List<ResourceResolver> result = new ArrayList<ResourceResolver>(this.resolvers);
+			if(isWebJarsAssetLocatorPresent) {
+				result.add(new WebJarsResourceResolver());
+			}
 			result.add(new PathResourceResolver());
 			return result;
 		}

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/WebJarsResourceResolver.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.servlet.resource;
+
+import java.util.List;
+
+import javax.servlet.http.HttpServletRequest;
+
+import org.webjars.MultipleMatchesException;
+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 WabJar users to use version-less paths in their templates, like {@code "/jquery/jquery.min.js"}
+ * while this path is resolved to the unique version {@code "/jquery/1.2.0/jquery.min.js"}, which is a better fit
+ * for HTTP caching and version management in applications.
+ *
+ * <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 <a href="http://www.webjars.org">webjars.org</a>
+ */
+public class WebJarsResourceResolver extends AbstractResourceResolver {
+
+	private final static String WEBJARS_LOCATION = "META-INF/resources/webjars";
+
+	private final static int WEBJARS_LOCATION_LENGTH = WEBJARS_LOCATION.length();
+
+	private final WebJarAssetLocator webJarAssetLocator;
+
+	public WebJarsResourceResolver() {
+		this.webJarAssetLocator = new WebJarAssetLocator();
+	}
+
+	@Override
+	protected Resource resolveResourceInternal(HttpServletRequest request, String requestPath,
+			List<? extends Resource> locations, ResourceResolverChain chain) {
+
+		return chain.resolveResource(request, requestPath, locations);
+	}
+
+	@Override
+	protected String resolveUrlPathInternal(String resourceUrlPath,
+			List<? extends Resource> locations, ResourceResolverChain chain) {
+
+		String path = chain.resolveUrlPath(resourceUrlPath, locations);
+		if (path == null) {
+			try {
+				int startOffset = resourceUrlPath.startsWith("/") ? 1 : 0;
+				int endOffset = resourceUrlPath.indexOf("/", 1);
+				if (endOffset != -1) {
+					String webjar = resourceUrlPath.substring(startOffset, endOffset);
+					String partialPath = resourceUrlPath.substring(endOffset);
+					String webJarPath = webJarAssetLocator.getFullPath(webjar, partialPath);
+					return chain.resolveUrlPath(webJarPath.substring(WEBJARS_LOCATION_LENGTH), locations);
+				}
+			}
+			catch (MultipleMatchesException ex) {
+				logger.warn("WebJar version conflict for \"" + resourceUrlPath + "\"", ex);
+			}
+			catch (IllegalArgumentException ex) {
+				if (logger.isTraceEnabled()) {
+					logger.trace("No WebJar resource found for \"" + resourceUrlPath + "\"");
+				}
+			}
+		}
+		return path;
+	}
+
+}