Clean duplicate separators in resource URLs

Most Servlet containers do this anyway, but not all, and not
consistently for forward and backslashes.

Issue: SPR-16616

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

@@ -520,7 +520,15 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 		if (path.contains("%")) {
 			try {
 				// Use URLDecoder (vs UriUtils) to preserve potentially decoded UTF-8 chars
-				if (isInvalidPath(URLDecoder.decode(path, "UTF-8"))) {
+				String decodedPath = URLDecoder.decode(path, "UTF-8");
+				if (isInvalidPath(decodedPath)) {
+					if (logger.isTraceEnabled()) {
+						logger.trace("Ignoring invalid resource path with escape sequences [" + path + "].");
+					}
+					return null;
+				}
+				decodedPath = processPath(decodedPath);
+				if (isInvalidPath(decodedPath)) {
 					if (logger.isTraceEnabled()) {
 						logger.trace("Ignoring invalid resource path with escape sequences [" + path + "].");
 					}
@@ -543,13 +551,47 @@ public class ResourceHttpRequestHandler extends WebContentGenerator
 	}
 
 	/**
-	 * Process the given resource path to be used.
-	 * <p>The default implementation replaces any combination of leading '/' and
-	 * control characters (00-1F and 7F) with a single "/" or "". For example
-	 * {@code "  // /// ////  foo/bar"} becomes {@code "/foo/bar"}.
+	 * Process the given resource path.
+	 * <p>The default implementation replaces:
+	 * <ul>
+	 * <li>Backslash with forward slash.
+	 * <li>Duplicate occurrences of slash with a single slash.
+	 * <li>Any combination of leading slash and control characters (00-1F and 7F)
+	 * with a single "/" or "". For example {@code "  / // foo/bar"}
+	 * becomes {@code "/foo/bar"}.
+	 * </ul>
 	 * @since 3.2.12
 	 */
 	protected String processPath(String path) {
+		path = StringUtils.replace(path, "\\", "/");
+		path = cleanDuplicateSlashes(path);
+		return cleanLeadingSlash(path);
+	}
+
+	private String cleanDuplicateSlashes(String path) {
+		StringBuilder sb = null;
+		char prev = 0;
+		for (int i = 0; i < path.length(); i++) {
+			char curr = path.charAt(i);
+			try {
+				if ((curr == '/') && (prev == '/')) {
+					if (sb == null) {
+						sb = new StringBuilder(path.substring(0, i));
+					}
+					continue;
+				}
+				if (sb != null) {
+					sb.append(path.charAt(i));
+				}
+			}
+			finally {
+				prev = curr;
+			}
+		}
+		return sb != null ? sb.toString() : path;
+	}
+
+	private String cleanLeadingSlash(String path) {
 		boolean slash = false;
 		for (int i = 0; i < path.length(); i++) {
 			if (path.charAt(i) == '/') {