Preserve URLStreamHandler in toRelativeURL and convertClassLoaderURL

Closes gh-33561
See gh-33199

spring-core/src/main/java/org/springframework/core/io/support/PathMatchingResourcePatternResolver.java

@@ -412,6 +412,7 @@ protected Set<Resource> doFindAllClassPathResources(String path) throws IOExcept
 	 * @see #doFindAllClassPathResources
 	 * @see #doFindPathMatchingFileResources
 	 */
+	@SuppressWarnings("deprecation")  // on JDK 20 (deprecated URL constructor)
 	protected Resource convertClassLoaderURL(URL url) {
 		if (ResourceUtils.URL_PROTOCOL_FILE.equals(url.getProtocol())) {
 			try {
@@ -429,14 +430,10 @@ protected Resource convertClassLoaderURL(URL url) {
 			if (!cleanedPath.equals(urlString)) {
 				// Prefer cleaned URL, aligned with UrlResource#createRelative(String)
 				try {
-					// Cannot test for URLStreamHandler directly: URL equality for same String
-					// in order to find out whether original URL uses default URLStreamHandler.
-					if (ResourceUtils.toURL(urlString).equals(url)) {
-						// Plain URL with default URLStreamHandler -> replace with cleaned path.
-						return new UrlResource(ResourceUtils.toURI(cleanedPath));
-					}
+					// Retain original URL instance, potentially including custom URLStreamHandler.
+					return new UrlResource(new URL(url, cleanedPath));
 				}
-				catch (URISyntaxException | MalformedURLException ex) {
+				catch (MalformedURLException ex) {
 					// Fallback to regular URL construction below...
 				}
 			}

spring-core/src/main/java/org/springframework/util/ResourceUtils.java

@@ -405,14 +405,14 @@ public static URI toURI(String location) throws URISyntaxException {
 	 * @see java.net.URI#toURL()
 	 * @see #toURI(String)
 	 */
-	@SuppressWarnings("deprecation")  // on JDK 20
+	@SuppressWarnings("deprecation")  // on JDK 20 (deprecated URL constructor)
 	public static URL toURL(String location) throws MalformedURLException {
 		try {
 			// Prefer URI construction with toURL conversion (as of 6.1)
 			return toURI(StringUtils.cleanPath(location)).toURL();
 		}
 		catch (URISyntaxException | IllegalArgumentException ex) {
-			// Lenient fallback to deprecated (on JDK 20) URL constructor,
+			// Lenient fallback to deprecated URL constructor,
 			// e.g. for decoded location Strings with percent characters.
 			return new URL(location);
 		}
@@ -429,11 +429,13 @@ public static URL toURL(String location) throws MalformedURLException {
 	 * @see #toURL(String)
 	 * @see StringUtils#applyRelativePath
 	 */
+	@SuppressWarnings("deprecation")  // on JDK 20 (deprecated URL constructor)
 	public static URL toRelativeURL(URL root, String relativePath) throws MalformedURLException {
 		// # can appear in filenames, java.net.URL should not treat it as a fragment
 		relativePath = StringUtils.replace(relativePath, "#", "%23");
 
-		return toURL(StringUtils.applyRelativePath(root.toString(), relativePath));
+		// Retain original URL instance, potentially including custom URLStreamHandler.
+		return new URL(root, StringUtils.cleanPath(StringUtils.applyRelativePath(root.toString(), relativePath)));
 	}
 
 	/**

spring-core/src/test/java/org/springframework/core/io/ResourceTests.java

@@ -377,6 +377,13 @@ void relativeResourcesAreEqual() throws Exception {
 			assertThat(relative).isEqualTo(new UrlResource("file:dir/subdir"));
 		}
 
+		@Test
+		void unusualRelativeResourcesAreEqual() throws Exception {
+			Resource resource = new UrlResource("file:dir/");
+			Resource relative = resource.createRelative("http://spring.io");
+			assertThat(relative).isEqualTo(new UrlResource("file:dir/http://spring.io"));
+		}
+
 		@Test
 		void missingRemoteResourceDoesNotExist() throws Exception {
 			String baseUrl = startServer();