private Collection<URL> locateDescriptors(String descriptorPath) {
    try {
      return ReflectionUtil.getResourcesFromClassLoader(descriptorPath);
    } catch (Exception e) {
      throw new ExtensionException("Failed to locate extension descriptors", e);
    }
  }
Scans class path for extension descriptors and loads the extension classes specified therein.
spock-core/src/main/java/org/spockframework/runtime/ExtensionClassesLoader.java