Support ETag generation on ResourceHttpRequestHandler

Prior to this commit, the `ResourceHttpRequestHandler` would support
HTTP caching when serving resources, but only driving it through the
`Resource#lastModified()` information.

This commit introduces an ETag generator function that can be configured
on the `ResourceHttpRequestHandler` to dynamically generate an ETag
value for the Resource that is going to be served.

Closes gh-29031

Author: bclozel

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

@@ -25,6 +25,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.function.Function;
 
 import jakarta.servlet.ServletException;
 import jakarta.servlet.http.HttpServletRequest;
@@ -141,6 +142,9 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 
 	private boolean useLastModified = true;
 
+	@Nullable
+	private Function<Resource, String> etagGenerator;
+
 	private boolean optimizeLocations = false;
 
 	@Nullable
@@ -383,6 +387,29 @@ public boolean isUseLastModified() {
 		return this.useLastModified;
 	}
 
+	/**
+	 * 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 ServletWebRequest#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
+	 */
+	public void setEtagGenerator(@Nullable Function<Resource, String> etagGenerator) {
+		this.etagGenerator = etagGenerator;
+	}
+
+	/**
+	 * Return the HTTP ETag generator function to be used when serving resources.
+	 * @return the HTTP ETag generator function
+	 * @since 6.1
+	 */
+	@Nullable
+	public Function<Resource, String> getEtagGenerator() {
+		return this.etagGenerator;
+	}
+
 	/**
 	 * Set whether to optimize the specified locations through an existence
 	 * check on startup, filtering non-existing directories upfront so that
@@ -567,7 +594,9 @@ public void handleRequest(HttpServletRequest request, HttpServletResponse respon
 		checkRequest(request);
 
 		// Header phase
-		if (isUseLastModified() && new ServletWebRequest(request, response).checkNotModified(resource.lastModified())) {
+		String eTagValue = (this.getEtagGenerator() != null) ? this.getEtagGenerator().apply(resource) : null;
+		long lastModified = (this.isUseLastModified()) ? resource.lastModified() : -1;
+		if (new ServletWebRequest(request, response).checkNotModified(eTagValue, lastModified)) {
 			logger.trace("Resource not modified");
 			return;
 		}

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

@@ -478,7 +478,7 @@ void configureVersionResourceResolver() throws Exception {
 		}
 
 		@Test
-		void shouldRespondWithNotModified() throws Exception {
+		void shouldRespondWithNotModifiedWhenModifiedSince() throws Exception {
 			this.handler.afterPropertiesSet();
 			this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "foo.css");
 			this.request.addHeader("If-Modified-Since", resourceLastModified("test/foo.css"));
@@ -496,6 +496,38 @@ void shouldRespondWithModifiedResource() throws Exception {
 			assertThat(this.response.getContentAsString()).isEqualTo("h1 { color:red; }");
 		}
 
+		@Test
+		void shouldRespondWithNotModifiedWhenEtag() throws Exception {
+			this.handler.setEtagGenerator(resource -> "testEtag");
+			this.handler.afterPropertiesSet();
+			this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "foo.css");
+			this.request.addHeader("If-None-Match", "\"testEtag\"");
+			this.handler.handleRequest(this.request, this.response);
+			assertThat(this.response.getStatus()).isEqualTo(HttpServletResponse.SC_NOT_MODIFIED);
+		}
+
+		@Test
+		void shouldRespondWithModifiedResourceWhenEtagNoMatch() throws Exception {
+			this.handler.setEtagGenerator(resource -> "noMatch");
+			this.handler.afterPropertiesSet();
+			this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "foo.css");
+			this.request.addHeader("If-None-Match", "\"testEtag\"");
+			this.handler.handleRequest(this.request, this.response);
+			assertThat(this.response.getStatus()).isEqualTo(HttpServletResponse.SC_OK);
+			assertThat(this.response.getContentAsString()).isEqualTo("h1 { color:red; }");
+		}
+
+		@Test
+		void shouldRespondWithNotModifiedWhenEtagAndLastModified() throws Exception {
+			this.handler.setEtagGenerator(resource -> "testEtag");
+			this.handler.afterPropertiesSet();
+			this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "foo.css");
+			this.request.addHeader("If-None-Match", "\"testEtag\"");
+			this.request.addHeader("If-Modified-Since", resourceLastModified("test/foo.css"));
+			this.handler.handleRequest(this.request, this.response);
+			assertThat(this.response.getStatus()).isEqualTo(HttpServletResponse.SC_NOT_MODIFIED);
+		}
+
 		@Test  // SPR-14005
 		void overwritesExistingCacheControlHeaders() throws Exception {
 			this.handler.setCacheSeconds(3600);