fix(webmvc): fixes the trailing slash handling during resource locati…

…on resolution.

closes gh-33815

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 {
@@ -65,20 +66,27 @@ else if (location instanceof ClassPathResource classPathResource) {
 			else {
 				path = location.getURL().getPath();
 			}
-			assertLocationPath(path);
+			addTrailingSlashIfAbsent(path);
 		}
 		catch (IOException ex) {
 			// ignore
 		}
 	}
 
 	/**
-	 * Assert the given location path is a directory and ends on slash.
+	 * 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 void assertLocationPath(@Nullable String path) {
+	public static String addTrailingSlashIfAbsent(@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);
+		if(!path.endsWith(FOLDER_SEPARATOR) && !path.endsWith(WINDOWS_FOLDER_SEPARATOR)) {
+			logger.warn(("Observed an attempting to register a resource with a path without trailing slash: '%s' " +
+					"the framework will add it automatically. Consider to add it on your own.").formatted(path));
+			var trailingSlash = path.contains(WINDOWS_FOLDER_SEPARATOR) ? WINDOWS_FOLDER_SEPARATOR : FOLDER_SEPARATOR;
+			return path += trailingSlash;
+		}
+		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.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 +1,26 @@
+package org.springframework.web.servlet.resource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.jupiter.api.Test;
+
+/**
+ * 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);
+	}
+}