Append trailing slash to static location

Closes gh-33815

spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceHandlerUtils.java

@@ -65,20 +65,27 @@ else if (location instanceof ClassPathResource classPathResource) {
 			else {
 				path = location.getURL().getPath();
 			}
-			assertLocationPath(path);
+			Assert.isTrue(path.endsWith(FOLDER_SEPARATOR) || path.endsWith(WINDOWS_FOLDER_SEPARATOR),
+					"Resource location does not end with slash: " + path);
 		}
 		catch (IOException ex) {
 			// ignore
 		}
 	}
 
 	/**
-	 * Assert the given location path is a directory and ends on slash.
+	 * Check if the given static resource location path ends with a trailing
+	 * slash, and append it if necessary.
+	 * @param path the location path
+	 * @return the resulting path to use
 	 */
-	public static void assertLocationPath(@Nullable String path) {
-		Assert.notNull(path, "Resource location path must not be null");
-		Assert.isTrue(path.endsWith(FOLDER_SEPARATOR) || path.endsWith(WINDOWS_FOLDER_SEPARATOR),
-				"Resource location does not end with slash: " + path);
+	public static String initLocationPath(String path) {
+		String separator = (path.contains(FOLDER_SEPARATOR) ? FOLDER_SEPARATOR : WINDOWS_FOLDER_SEPARATOR);
+		if (!path.endsWith(separator)) {
+			path = path.concat(separator);
+			logger.warn("Appended trailing slash to static resource location: " + path);
+		}
+		return path;
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceWebHandler.java

@@ -379,7 +379,7 @@ private void resolveResourceLocations() {
 			Assert.isTrue(CollectionUtils.isEmpty(this.locationResources), "Please set " +
 					"either Resource-based \"locations\" or String-based \"locationValues\", but not both.");
 			for (String location : this.locationValues) {
-				ResourceHandlerUtils.assertLocationPath(location);
+				location = ResourceHandlerUtils.initLocationPath(location);
 				result.add(this.resourceLoader.getResource(location));
 			}
 		}

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

@@ -65,20 +65,27 @@ else if (location instanceof ClassPathResource classPathResource) {
 			else {
 				path = location.getURL().getPath();
 			}
-			assertLocationPath(path);
+			Assert.isTrue(path.endsWith(FOLDER_SEPARATOR) || path.endsWith(WINDOWS_FOLDER_SEPARATOR),
+					"Resource location does not end with slash: " + path);
 		}
 		catch (IOException ex) {
 			// ignore
 		}
 	}
 
 	/**
-	 * Assert the given location path is a directory and ends on slash.
+	 * Check if the given static resource location path ends with a trailing
+	 * slash, and append it if necessary.
+	 * @param path the location path
+	 * @return the resulting path to use
 	 */
-	public static void assertLocationPath(@Nullable String path) {
-		Assert.notNull(path, "Resource location path must not be null");
-		Assert.isTrue(path.endsWith(FOLDER_SEPARATOR) || path.endsWith(WINDOWS_FOLDER_SEPARATOR),
-				"Resource location does not end with slash: " + path);
+	public static String initLocationPath(String path) {
+		String separator = (path.contains(FOLDER_SEPARATOR) ? FOLDER_SEPARATOR : WINDOWS_FOLDER_SEPARATOR);
+		if (!path.endsWith(separator)) {
+			path = path.concat(separator);
+			logger.warn("Appended trailing slash to static resource location: " + path);
+		}
+		return path;
 	}
 
 	/**

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

@@ -496,7 +496,7 @@ private void resolveResourceLocations() {
 					charset = Charset.forName(value);
 					location = location.substring(endIndex + 1);
 				}
-				ResourceHandlerUtils.assertLocationPath(location);
+				location = ResourceHandlerUtils.initLocationPath(location);
 				Resource resource = applicationContext.getResource(location);
 				if (location.equals("/") && !(resource instanceof ServletContextResource)) {
 					throw new IllegalStateException(