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/test/java/org/springframework/web/servlet/config/MvcNamespaceTests.java

@@ -119,6 +119,7 @@ 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;
 import org.springframework.web.servlet.theme.ThemeChangeInterceptor;
 import org.springframework.web.servlet.view.BeanNameViewResolver;
 import org.springframework.web.servlet.view.ContentNegotiatingViewResolver;
@@ -409,10 +410,11 @@ public class MvcNamespaceTests {
 		assertNotNull(handler);
 
 		List<ResourceResolver> resolvers = handler.getResourceResolvers();
-		assertThat(resolvers, Matchers.hasSize(3));
+		assertThat(resolvers, Matchers.hasSize(4));
 		assertThat(resolvers.get(0), Matchers.instanceOf(CachingResourceResolver.class));
 		assertThat(resolvers.get(1), Matchers.instanceOf(VersionResourceResolver.class));
-		assertThat(resolvers.get(2), Matchers.instanceOf(PathResourceResolver.class));
+		assertThat(resolvers.get(2), Matchers.instanceOf(WebJarsResourceResolver.class));
+		assertThat(resolvers.get(3), Matchers.instanceOf(PathResourceResolver.class));
 
 		CachingResourceResolver cachingResolver = (CachingResourceResolver) resolvers.get(0);
 		assertThat(cachingResolver.getCache(), Matchers.instanceOf(ConcurrentMapCache.class));

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/ResourceHandlerRegistryTests.java

@@ -40,6 +40,7 @@ import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 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;
 
 import static org.junit.Assert.*;
 
@@ -125,12 +126,13 @@ public class ResourceHandlerRegistryTests {
 
 		ResourceHttpRequestHandler handler = getHandler("/resources/**");
 		List<ResourceResolver> resolvers = handler.getResourceResolvers();
-		assertThat(resolvers.toString(), resolvers, Matchers.hasSize(3));
+		assertThat(resolvers.toString(), resolvers, Matchers.hasSize(4));
 		assertThat(resolvers.get(0), Matchers.instanceOf(CachingResourceResolver.class));
 		CachingResourceResolver cachingResolver = (CachingResourceResolver) resolvers.get(0);
 		assertThat(cachingResolver.getCache(), Matchers.instanceOf(ConcurrentMapCache.class));
 		assertThat(resolvers.get(1), Matchers.equalTo(mockResolver));
-		assertThat(resolvers.get(2), Matchers.instanceOf(PathResourceResolver.class));
+		assertThat(resolvers.get(2), Matchers.instanceOf(WebJarsResourceResolver.class));
+		assertThat(resolvers.get(3), Matchers.instanceOf(PathResourceResolver.class));
 
 		List<ResourceTransformer> transformers = handler.getResourceTransformers();
 		assertThat(transformers, Matchers.hasSize(2));
@@ -144,8 +146,9 @@ public class ResourceHandlerRegistryTests {
 
 		ResourceHttpRequestHandler handler = getHandler("/resources/**");
 		List<ResourceResolver> resolvers = handler.getResourceResolvers();
-		assertThat(resolvers, Matchers.hasSize(1));
-		assertThat(resolvers.get(0), Matchers.instanceOf(PathResourceResolver.class));
+		assertThat(resolvers, Matchers.hasSize(2));
+		assertThat(resolvers.get(0), Matchers.instanceOf(WebJarsResourceResolver.class));
+		assertThat(resolvers.get(1), Matchers.instanceOf(PathResourceResolver.class));
 
 		List<ResourceTransformer> transformers = handler.getResourceTransformers();
 		assertThat(transformers, Matchers.hasSize(0));
@@ -162,10 +165,11 @@ public class ResourceHandlerRegistryTests {
 
 		ResourceHttpRequestHandler handler = getHandler("/resources/**");
 		List<ResourceResolver> resolvers = handler.getResourceResolvers();
-		assertThat(resolvers.toString(), resolvers, Matchers.hasSize(3));
+		assertThat(resolvers.toString(), resolvers, Matchers.hasSize(4));
 		assertThat(resolvers.get(0), Matchers.instanceOf(CachingResourceResolver.class));
 		assertThat(resolvers.get(1), Matchers.sameInstance(versionResolver));
-		assertThat(resolvers.get(2), Matchers.instanceOf(PathResourceResolver.class));
+		assertThat(resolvers.get(2), Matchers.instanceOf(WebJarsResourceResolver.class));
+		assertThat(resolvers.get(3), Matchers.instanceOf(PathResourceResolver.class));
 
 		List<ResourceTransformer> transformers = handler.getResourceTransformers();
 		assertThat(transformers, Matchers.hasSize(3));

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/WebJarsResourceResolverTests.java

@@ -0,0 +1,107 @@
+/*
+ * 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 static org.junit.Assert.*;
+import static org.mockito.BDDMockito.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.core.io.Resource;
+
+/**
+ * Unit tests for
+ * {@link org.springframework.web.servlet.resource.WebJarsResourceResolver}.
+ *
+ * @author Brian Clozel
+ */
+public class WebJarsResourceResolverTests {
+
+	private List<Resource> locations;
+
+	private WebJarsResourceResolver resolver;
+
+	private ResourceResolverChain chain;
+
+	@Before
+	public void setup() {
+		// for this to work, an actual WebJar must be on the test classpath
+		this.locations = Collections.singletonList(new ClassPathResource("/META-INF/resources/webjars/"));
+		this.resolver = new WebJarsResourceResolver();
+		this.chain = mock(ResourceResolverChain.class);
+	}
+
+	@Test
+	public void resolveUrlExisting() {
+		this.locations = Collections.singletonList(new ClassPathResource("/META-INF/resources/webjars/", getClass()));
+		String file = "/foo/2.3/foo.txt";
+		given(this.chain.resolveUrlPath(file, this.locations)).willReturn(file);
+
+		String actual = this.resolver.resolveUrlPath(file, this.locations, this.chain);
+
+		assertEquals(file, actual);
+		verify(this.chain, times(1)).resolveUrlPath(file, this.locations);
+	}
+
+	@Test
+	public void resolveUrlExistingNotInJarFile() {
+		this.locations = Collections.singletonList(new ClassPathResource("/META-INF/resources/webjars/", getClass()));
+		String file = "/foo/foo.txt";
+		given(this.chain.resolveUrlPath(file, this.locations)).willReturn(null);
+
+		String actual = this.resolver.resolveUrlPath(file, this.locations, this.chain);
+
+		assertNull(actual);
+		verify(this.chain, times(1)).resolveUrlPath(file, this.locations);
+		verify(this.chain, never()).resolveUrlPath("/foo/2.3/foo.txt", this.locations);
+	}
+
+	@Test
+	public void resolveUrlWebJarResource() {
+		String file = "/underscorejs/underscore.js";
+		String expected = "/underscorejs/1.8.2/underscore.js";
+		given(this.chain.resolveUrlPath(file, this.locations)).willReturn(null);
+		given(this.chain.resolveUrlPath(expected, this.locations)).willReturn(expected);
+
+		String actual = this.resolver.resolveUrlPath(file, this.locations, this.chain);
+
+		assertEquals(expected, actual);
+		verify(this.chain, times(1)).resolveUrlPath(file, this.locations);
+		verify(this.chain, times(1)).resolveUrlPath(expected, this.locations);
+	}
+
+	@Test
+	public void resolverUrlWebJarResourceNotFound() {
+		String file = "/something/something.js";
+		given(this.chain.resolveUrlPath(file, this.locations)).willReturn(null);
+
+		String actual = this.resolver.resolveUrlPath(file, this.locations, this.chain);
+
+		assertNull(actual);
+		verify(this.chain, times(1)).resolveUrlPath(file, this.locations);
+	}
+
+}