Add caching headers to unmodified static resources

per https://www.rfc-editor.org/rfc/rfc7232#section-4.1

The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.

Closes gh-34614

Signed-off-by: James Yuzawa <jtyuzawa@gmail.com>

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

@@ -569,7 +569,7 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 	 * If the resource exists, the request will be checked for the presence of the
 	 * {@code Last-Modified} header, and its value will be compared against the last-modified
 	 * timestamp of the given resource, returning a {@code 304} status code if the
-	 * {@code Last-Modified} value  is greater. If the resource is newer than the
+	 * {@code Last-Modified} value is greater. If the resource is newer than the
 	 * {@code Last-Modified} value, or the header is not present, the content resource
 	 * of the resource will be written to the response with caching headers
 	 * set to expire one year in the future.
@@ -593,6 +593,9 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 		// Supported methods and required session
 		checkRequest(request);
 
+		// Apply cache settings, if any
+		prepareResponse(response);
+
 		// Header phase
 		String eTagValue = (this.getEtagGenerator() != null) ? this.getEtagGenerator().apply(resource) : null;
 		long lastModified = (this.isUseLastModified()) ? resource.lastModified() : -1;
@@ -601,9 +604,6 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 			return;
 		}
 
-		// Apply cache settings, if any
-		prepareResponse(response);
-
 		// Check the media type for the resource
 		MediaType mediaType = getMediaType(request, resource);
 		setHeaders(response, resource, mediaType);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -479,11 +479,13 @@ class ResourceHttpRequestHandlerTests {
 
 		@Test
 		void shouldRespondWithNotModifiedWhenModifiedSince() throws Exception {
+			this.handler.setCacheSeconds(3600);
 			this.handler.afterPropertiesSet();
 			this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "foo.css");
 			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);
+			assertThat(this.response.getHeader("Cache-Control")).isEqualTo("max-age=3600");
 		}
 
 		@Test
@@ -498,12 +500,14 @@ class ResourceHttpRequestHandlerTests {
 
 		@Test
 		void shouldRespondWithNotModifiedWhenEtag() throws Exception {
+			this.handler.setCacheSeconds(3600);
 			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);
+			assertThat(this.response.getHeader("Cache-Control")).isEqualTo("max-age=3600");
 		}
 
 		@Test