Add trailing slash to static resource location if missing

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

@@ -38,6 +38,7 @@
  * {@link ResourceHttpRequestHandler} and {@link org.springframework.web.servlet.function}.
  *
  * @author Rossen Stoyanchev
+ * @author Edoardo Patti
  * @since 6.2
  */
 public abstract class ResourceHandlerUtils {
@@ -81,6 +82,23 @@ public static void assertLocationPath(@Nullable String path) {
 				"Resource location does not end with slash: " + path);
 	}
 
+	/**
+	 * Assert the given location path is not null and look for the trailing slash adding it when absent.
+	 * @return the input string if the trailing slash was already present
+	 * the input string plus the trailing slash otherwise.
+	 */
+	public static String addTrailingSlashIfAbsent(@Nullable String path) {
+		Assert.notNull(path, "Resource location path must not be null");
+		if(!path.endsWith(FOLDER_SEPARATOR) && !path.endsWith(WINDOWS_FOLDER_SEPARATOR)) {
+			logger.warn(
+					"Observed an attempting to register a resource with a location without trailing slash: '%s'. The framework will add it automatically."
+							.formatted(path));
+			var trailingSlash = path.contains(WINDOWS_FOLDER_SEPARATOR) ? WINDOWS_FOLDER_SEPARATOR : FOLDER_SEPARATOR;
+			return path.concat(trailingSlash);
+		}
+		return path;
+	}
+
 	/**
 	 * Normalize the given resource path replacing the following:
 	 * <ul>

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.addTrailingSlashIfAbsent(location);
 				Resource resource = applicationContext.getResource(location);
 				if (location.equals("/") && !(resource instanceof ServletContextResource)) {
 					throw new IllegalStateException(

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

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.servlet.resource;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ResourceHandlerUtils}.
+ *
+ * @author Edoardo Patti
+ */
+public class ResourceHandlerUtilsTests {
+
+	@Test
+	void testTrailingSlash() {
+		var windowsDirectory = "META-INF\\resources\\webjars";
+		var directory = "META-INF/resources/webjars";
+		var directoryWithoutSlash = "META-INF";
+		var correctWindowsDirectory = "META-INF\\resources\\webjars\\";
+		var correctDirectory = "META-INF/resources/webjars/";
+		assertThat(ResourceHandlerUtils.addTrailingSlashIfAbsent(windowsDirectory).endsWith("\\")).isTrue();
+		assertThat(ResourceHandlerUtils.addTrailingSlashIfAbsent(directory).endsWith("/")).isTrue();
+		assertThat(ResourceHandlerUtils.addTrailingSlashIfAbsent(directoryWithoutSlash).endsWith("/")).isTrue();
+		assertThat(ResourceHandlerUtils.addTrailingSlashIfAbsent(correctWindowsDirectory)).isEqualTo(correctWindowsDirectory);
+		assertThat(ResourceHandlerUtils.addTrailingSlashIfAbsent(correctDirectory)).isEqualTo(correctDirectory);
+	}
+}