Support multiple nesting of JAR files

* Make sure that `jar:file:///...` are properly formed (the sub-URL must have a scheme of `file`)
* When there is no `!` in the JAR URL, treat the file as a JAR and process its root path
* Otherwise recursively resolve each segment which is `!`-terminated

classloader/src/main/java/io/smallrye/common/classloader/ClassPathUtils.java

@@ -129,22 +129,27 @@ public static <R> R processAsPath(URL url, Function<Path, R> function) {
             }
 
             final String file = url.getFile();
-            final int exclam = file.lastIndexOf('!');
-            final Path jar;
+            final int exclam = file.indexOf('!');
             try {
-                jar = toLocalPath(exclam >= 0 ? new URL(file.substring(0, exclam)) : url);
+                URL fileUrl;
+                Path jarPath;
+                String subPath;
+                if (exclam == -1) {
+                    // assume the first element is a JAR file, not a plain file, since it was a `jar:` URL
+                    fileUrl = new URL(file);
+                    subPath = "/";
+                } else {
+                    fileUrl = new URL(file.substring(0, exclam));
+                    subPath = file.substring(exclam + 1);
+                }
+                if (!fileUrl.getProtocol().equals("file")) {
+                    throw new IllegalArgumentException("Sub-URL of JAR URL is expected to have a scheme of `file`");
+                }
+                jarPath = toLocalPath(fileUrl);
+                return processAsJarPath(jarPath, subPath, function);
             } catch (MalformedURLException e) {
                 throw new RuntimeException("Failed to create a URL for '" + file.substring(0, exclam) + "'", e);
             }
-            try (FileSystem jarFs = FileSystems.newFileSystem(jar, (ClassLoader) null)) {
-                Path localPath = jarFs.getPath("/");
-                if (exclam >= 0) {
-                    localPath = localPath.resolve(file.substring(exclam + 1));
-                }
-                return function.apply(localPath);
-            } catch (IOException e) {
-                throw new UncheckedIOException("Failed to read " + jar, e);
-            }
         }
 
         if (FILE.equals(url.getProtocol())) {
@@ -154,6 +159,20 @@ public static <R> R processAsPath(URL url, Function<Path, R> function) {
         throw new IllegalArgumentException("Unexpected protocol " + url.getProtocol() + " for URL " + url);
     }
 
+    private static <R> R processAsJarPath(Path jarPath, String path, Function<Path, R> function) {
+        try (FileSystem jarFs = FileSystems.newFileSystem(jarPath, (ClassLoader) null)) {
+            Path localPath = jarFs.getPath("/");
+            int idx = path.indexOf('!');
+            if (idx == -1) {
+                return function.apply(localPath.resolve(path));
+            } else {
+                return processAsJarPath(localPath.resolve(path.substring(0, idx)), path.substring(idx + 1), function);
+            }
+        } catch (IOException e) {
+            throw new UncheckedIOException("Failed to read " + jarPath, e);
+        }
+    }
+
     /**
      * Invokes a consumer providing the input streams to read the content of the URL.
      * The consumer does not have to close the provided input stream.