Allow ETag generation configuration on ResourceHandlerRegistration

This commit surfaces the ETag generation feature for both
`ResourceHttpRequestHandler` and `ResourceWebHandler` on their
respective `ResourceHandlerRegistration` for easier configuration.

See gh-29031

spring-webflux/src/main/java/org/springframework/web/reactive/config/ResourceHandlerRegistration.java

@@ -21,6 +21,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Function;
 
 import org.springframework.cache.Cache;
 import org.springframework.core.io.Resource;
@@ -31,6 +32,7 @@
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.web.reactive.resource.ResourceWebHandler;
+import org.springframework.web.server.ServerWebExchange;
 
 /**
  * Assist with creating and configuring a static resources handler.
@@ -54,6 +56,9 @@ public class ResourceHandlerRegistration {
 
 	private boolean useLastModified = true;
 
+	@Nullable
+	private Function<Resource, String> etagGenerator;
+
 	private boolean optimizeLocations = false;
 
 	@Nullable
@@ -117,6 +122,22 @@ public ResourceHandlerRegistration setUseLastModified(boolean useLastModified) {
 		return this;
 	}
 
+
+	/**
+	 * Configure a generator function that will be used to create the ETag information,
+	 * given a {@link Resource} that is about to be written to the response.
+	 * <p>This function should return a String that will be used as an argument in
+	 * {@link ServerWebExchange#checkNotModified(String)}, or {@code null} if no value
+	 * can be generated for the given resource.
+	 * @param etagGenerator the HTTP ETag generator function to use.
+	 * @since 6.1
+	 * @see ResourceWebHandler#setEtagGenerator(Function)
+	 */
+	public ResourceHandlerRegistration setEtagGenerator(@Nullable Function<Resource, String> etagGenerator) {
+		this.etagGenerator = etagGenerator;
+		return this;
+	}
+
 	/**
 	 * Set whether to optimize the specified locations through an existence check on startup,
 	 * filtering non-existing directories upfront so that they do not have to be checked
@@ -211,6 +232,7 @@ protected ResourceWebHandler getRequestHandler() {
 			handler.setCacheControl(this.cacheControl);
 		}
 		handler.setUseLastModified(this.useLastModified);
+		handler.setEtagGenerator(this.etagGenerator);
 		handler.setOptimizeLocations(this.optimizeLocations);
 		if (this.mediaTypes != null) {
 			handler.setMediaTypes(this.mediaTypes);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -19,12 +19,14 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.function.Function;
 
 import org.springframework.cache.Cache;
 import org.springframework.core.io.Resource;
 import org.springframework.http.CacheControl;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.servlet.resource.PathResourceResolver;
 import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 
@@ -55,6 +57,9 @@ public class ResourceHandlerRegistration {
 
 	private boolean useLastModified = true;
 
+	@Nullable
+	private Function<Resource, String> etagGenerator;
+
 	private boolean optimizeLocations = false;
 
 
@@ -142,6 +147,21 @@ public ResourceHandlerRegistration setUseLastModified(boolean useLastModified) {
 		return this;
 	}
 
+	/**
+	 * Configure a generator function that will be used to create the ETag information,
+	 * given a {@link Resource} that is about to be written to the response.
+	 * <p>This function should return a String that will be used as an argument in
+	 * {@link ServerWebExchange#checkNotModified(String)}, or {@code null} if no value
+	 * can be generated for the given resource.
+	 * @param etagGenerator the HTTP ETag generator function to use.
+	 * @since 6.1
+	 * @see ResourceHttpRequestHandler#setEtagGenerator(Function)
+	 */
+	public ResourceHandlerRegistration setEtagGenerator(@Nullable Function<Resource, String> etagGenerator) {
+		this.etagGenerator = etagGenerator;
+		return this;
+	}
+
 	/**
 	 * Set whether to optimize the specified locations through an existence check on startup,
 	 * filtering non-existing directories upfront so that they do not have to be checked
@@ -224,6 +244,7 @@ else if (this.cachePeriod != null) {
 			handler.setCacheSeconds(this.cachePeriod);
 		}
 		handler.setUseLastModified(this.useLastModified);
+		handler.setEtagGenerator(this.etagGenerator);
 		handler.setOptimizeLocations(this.optimizeLocations);
 		return handler;
 	}