Enable resource chain if webjars locator is present

Webjars locator is a good hint that the resource chain should be
enabled. The sole presence of the library now enables the resource chain
unless the configuration states otherwise.

Closes gh-4403

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ConditionalOnEnabledResourceChain.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -26,7 +26,8 @@ import org.springframework.context.annotation.Conditional;
 
 /**
  * {@link Conditional} that checks whether or not the Spring resource handling chain is
- * enabled. Matches if {@link ResourceProperties.Chain#getEnabled()} is {@code true}.
+ * enabled. Matches if {@link ResourceProperties.Chain#getEnabled()} is {@code true} or
+ * if {@code webjars-locator} is on the classpath.
  *
  * @author Stephane Nicoll
  * @since 1.3.0

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/OnEnabledResourceChainCondition.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -24,6 +24,7 @@ import org.springframework.context.annotation.Condition;
 import org.springframework.context.annotation.ConditionContext;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.type.AnnotatedTypeMetadata;
+import org.springframework.util.ClassUtils;
 
 /**
  * {@link Condition} that checks whether or not the Spring resource handling chain is
@@ -33,6 +34,8 @@ import org.springframework.core.type.AnnotatedTypeMetadata;
  */
 class OnEnabledResourceChainCondition extends SpringBootCondition {
 
+	public static final String WEBJAR_ASSERT_LOCATOR = "org.webjars.WebJarAssetLocator";
+
 	@Override
 	public ConditionOutcome getMatchOutcome(ConditionContext context,
 			AnnotatedTypeMetadata metadata) {
@@ -42,6 +45,13 @@ class OnEnabledResourceChainCondition extends SpringBootCondition {
 		RelaxedDataBinder binder = new RelaxedDataBinder(properties, "spring.resources");
 		binder.bind(new PropertySourcesPropertyValues(environment.getPropertySources()));
 		Boolean match = properties.getChain().getEnabled();
+		if (match == null) {
+			boolean webJarsLocatorPresent = ClassUtils.isPresent(
+					WEBJAR_ASSERT_LOCATOR, getClass().getClassLoader());
+			return new ConditionOutcome(webJarsLocatorPresent,
+					"Webjars locator (" + WEBJAR_ASSERT_LOCATOR + ") is "
+							+ (webJarsLocatorPresent ? "present" : "absent"));
+		}
 		return new ConditionOutcome(match,
 				"Resource chain is " + (match ? "enabled" : "disabled"));
 	}

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ResourceProperties.java

@@ -172,10 +172,15 @@ public class ResourceProperties implements ResourceLoaderAware {
 		@NestedConfigurationProperty
 		private final Strategy strategy = new Strategy();
 
+		/**
+		 * Return whether the resource chain is enabled. Return {@code null} if no
+		 * specific settings are present.
+		 */
 		public Boolean getEnabled() {
-			return Boolean.TRUE.equals(this.enabled)
-					|| getStrategy().getFixed().isEnabled()
+			// Check if at least one of the available strategy has been enabled
+			Boolean strategyEnabled = getStrategy().getFixed().isEnabled()
 					|| getStrategy().getContent().isEnabled();
+			return (strategyEnabled ? Boolean.TRUE : this.enabled);
 		}
 
 		public void setEnabled(boolean enabled) {

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java

@@ -148,6 +148,9 @@ public class WebMvcAutoConfiguration {
 		@Autowired
 		private HttpMessageConverters messageConverters;
 
+		@Autowired(required = false)
+		ResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer;
+
 		@Override
 		public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
 			converters.addAll(this.messageConverters.getConverters());
@@ -255,51 +258,24 @@ public class WebMvcAutoConfiguration {
 			}
 			Integer cachePeriod = this.resourceProperties.getCachePeriod();
 			if (!registry.hasMappingForPattern("/webjars/**")) {
-				registerResourceChain(registry.addResourceHandler("/webjars/**")
+				customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**")
 						.addResourceLocations("classpath:/META-INF/resources/webjars/")
 						.setCachePeriod(cachePeriod));
 			}
 			String staticPathPattern = this.mvcProperties.getStaticPathPattern();
 			if (!registry.hasMappingForPattern(staticPathPattern)) {
-				registerResourceChain(registry.addResourceHandler(staticPathPattern)
+				customizeResourceHandlerRegistration(registry.addResourceHandler(staticPathPattern)
 						.addResourceLocations(
 								this.resourceProperties.getStaticLocations())
 						.setCachePeriod(cachePeriod));
 			}
 		}
 
-		private void registerResourceChain(ResourceHandlerRegistration registration) {
-			ResourceProperties.Chain properties = this.resourceProperties.getChain();
-			if (properties.getEnabled()) {
-				configureResourceChain(properties,
-						registration.resourceChain(properties.isCache()));
+		private void customizeResourceHandlerRegistration(ResourceHandlerRegistration registration) {
+			if (this.resourceHandlerRegistrationCustomizer != null) {
+				this.resourceHandlerRegistrationCustomizer.customize(registration);
 			}
-		}
 
-		private void configureResourceChain(ResourceProperties.Chain properties,
-				ResourceChainRegistration chain) {
-			Strategy strategy = properties.getStrategy();
-			if (strategy.getFixed().isEnabled() || strategy.getContent().isEnabled()) {
-				chain.addResolver(getVersionResourceResolver(strategy));
-			}
-			if (properties.isHtmlApplicationCache()) {
-				chain.addTransformer(new AppCacheManifestTransformer());
-			}
-		}
-
-		private ResourceResolver getVersionResourceResolver(
-				ResourceProperties.Strategy properties) {
-			VersionResourceResolver resolver = new VersionResourceResolver();
-			if (properties.getFixed().isEnabled()) {
-				String version = properties.getFixed().getVersion();
-				String[] paths = properties.getFixed().getPaths();
-				resolver.addFixedVersionStrategy(version, paths);
-			}
-			if (properties.getContent().isEnabled()) {
-				String[] paths = properties.getContent().getPaths();
-				resolver.addContentVersionStrategy(paths);
-			}
-			return resolver;
 		}
 
 		@Override
@@ -380,4 +356,61 @@ public class WebMvcAutoConfiguration {
 
 	}
 
+	@Configuration
+	@ConditionalOnEnabledResourceChain
+	static class ResourceChainCustomizerConfiguration {
+
+		@Bean
+		public ResourceChainResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer() {
+			return new ResourceChainResourceHandlerRegistrationCustomizer();
+		}
+
+	}
+
+	interface ResourceHandlerRegistrationCustomizer {
+
+		void customize(ResourceHandlerRegistration registration);
+
+	}
+
+	private static class ResourceChainResourceHandlerRegistrationCustomizer
+			implements ResourceHandlerRegistrationCustomizer {
+
+		@Autowired
+		private ResourceProperties resourceProperties = new ResourceProperties();
+
+		@Override
+		public void customize(ResourceHandlerRegistration registration) {
+			ResourceProperties.Chain properties = this.resourceProperties.getChain();
+			configureResourceChain(properties,
+					registration.resourceChain(properties.isCache()));
+		}
+
+		private void configureResourceChain(ResourceProperties.Chain properties,
+				ResourceChainRegistration chain) {
+			Strategy strategy = properties.getStrategy();
+			if (strategy.getFixed().isEnabled() || strategy.getContent().isEnabled()) {
+				chain.addResolver(getVersionResourceResolver(strategy));
+			}
+			if (properties.isHtmlApplicationCache()) {
+				chain.addTransformer(new AppCacheManifestTransformer());
+			}
+		}
+
+		private ResourceResolver getVersionResourceResolver(
+				ResourceProperties.Strategy properties) {
+			VersionResourceResolver resolver = new VersionResourceResolver();
+			if (properties.getFixed().isEnabled()) {
+				String version = properties.getFixed().getVersion();
+				String[] paths = properties.getFixed().getPaths();
+				resolver.addFixedVersionStrategy(version, paths);
+			}
+			if (properties.getContent().isEnabled()) {
+				String[] paths = properties.getContent().getPaths();
+				resolver.addContentVersionStrategy(paths);
+			}
+			return resolver;
+		}
+	}
+
 }

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/ResourcePropertiesTest.java

+/*
+ * Copyright 2012-2016 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.boot.autoconfigure.web;
+
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.CoreMatchers.nullValue;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Tests for {@link ResourceProperties}.
+ *
+ * @author Stephane Nicoll
+ */
+public class ResourcePropertiesTest {
+
+	private final ResourceProperties properties = new ResourceProperties();
+
+	@Test
+	public void resourceChainNoCustomization() {
+		System.out.println(this.properties.getChain().getEnabled());
+		assertThat(this.properties.getChain().getEnabled(), is(nullValue()));
+	}
+
+	@Test
+	public void resourceChainStrategyEnabled() {
+		this.properties.getChain().getStrategy().getFixed().setEnabled(true);
+		assertThat(this.properties.getChain().getEnabled(), is(true));
+	}
+
+	@Test
+	public void resourceChainEnabled() {
+		this.properties.getChain().setEnabled(true);
+		assertThat(this.properties.getChain().getEnabled(), is(true));
+	}
+
+	@Test
+	public void resourceChainDisabled() {
+		this.properties.getChain().setEnabled(false);
+		assertThat(this.properties.getChain().getEnabled(), is(false));
+	}
+
+}