Add configuration key for GzipResourceResolver

This commit adds a new key that configures a GzipResourceResolver
in the resource handling chain.

Configuring an application with the following will add that resolver,
which checks for gzipped resources in the configured locations:

```
spring.resources.chain.gzipped=true
```

This means that if a resource "style.css" is requested, the
GzipResourceResolver will look for resources named "style.css.gz", which
should be a gzipped variant of the "style.css" file. Note that this
resolver only checks for variants if the client supports the "gzip"
encoding, as defined in the "Accept-Encoding" HTTP request headers.

Fixes #4683

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

@@ -169,6 +169,12 @@ public class ResourceProperties implements ResourceLoaderAware {
 		 */
 		private boolean htmlApplicationCache = false;
 
+		/**
+		 * Enable resolution of already gzipped resources. Checks for a resource
+		 * name variant with the *.gz extension.
+		 */
+		private boolean gzipped = false;
+
 		@NestedConfigurationProperty
 		private final Strategy strategy = new Strategy();
 
@@ -208,6 +214,13 @@ public class ResourceProperties implements ResourceLoaderAware {
 			this.htmlApplicationCache = htmlApplicationCache;
 		}
 
+		public boolean isGzipped() {
+			return this.gzipped;
+		}
+
+		public void setGzipped(boolean gzipped) {
+			this.gzipped = gzipped;
+		}
 	}
 
 	/**

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

@@ -85,6 +85,7 @@ import org.springframework.web.servlet.i18n.FixedLocaleResolver;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping;
 import org.springframework.web.servlet.resource.AppCacheManifestTransformer;
+import org.springframework.web.servlet.resource.GzipResourceResolver;
 import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 import org.springframework.web.servlet.resource.ResourceResolver;
 import org.springframework.web.servlet.resource.VersionResourceResolver;
@@ -396,6 +397,9 @@ public class WebMvcAutoConfiguration {
 			if (strategy.getFixed().isEnabled() || strategy.getContent().isEnabled()) {
 				chain.addResolver(getVersionResourceResolver(strategy));
 			}
+			if (properties.isGzipped()) {
+				chain.addResolver(new GzipResourceResolver());
+			}
 			if (properties.isHtmlApplicationCache()) {
 				chain.addTransformer(new AppCacheManifestTransformer());
 			}

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

@@ -71,6 +71,7 @@ import org.springframework.web.servlet.resource.CachingResourceTransformer;
 import org.springframework.web.servlet.resource.ContentVersionStrategy;
 import org.springframework.web.servlet.resource.CssLinkResourceTransformer;
 import org.springframework.web.servlet.resource.FixedVersionStrategy;
+import org.springframework.web.servlet.resource.GzipResourceResolver;
 import org.springframework.web.servlet.resource.PathResourceResolver;
 import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 import org.springframework.web.servlet.resource.ResourceResolver;
@@ -228,11 +229,13 @@ public class WebMvcAutoConfigurationTests {
 				"spring.resources.chain.strategy.fixed.enabled:true",
 				"spring.resources.chain.strategy.fixed.version:test",
 				"spring.resources.chain.strategy.fixed.paths:/**/*.js",
-				"spring.resources.chain.html-application-cache:true");
-		assertThat(getResourceResolvers("/webjars/**")).hasSize(2);
+				"spring.resources.chain.html-application-cache:true",
+				"spring.resources.chain.gzipped:true");
+		assertThat(getResourceResolvers("/webjars/**")).hasSize(3);
 		assertThat(getResourceTransformers("/webjars/**")).hasSize(2);
 		assertThat(getResourceResolvers("/**")).extractingResultOf("getClass")
-				.containsOnly(VersionResourceResolver.class, PathResourceResolver.class);
+				.containsOnly(VersionResourceResolver.class, GzipResourceResolver.class,
+						PathResourceResolver.class);
 		assertThat(getResourceTransformers("/**")).extractingResultOf("getClass")
 				.containsOnly(CssLinkResourceTransformer.class,
 						AppCacheManifestTransformer.class);