@Override
    protected File getFileForLastModifiedCheck() throws IOException {
        URL url = getURL();
        if (ResourceUtils.isJarURL(url)) {
            URL actualUrl = ResourceUtils.extractJarFileURL(url);
            return ResourceUtils.getFile(actualUrl, "Jar URL");
        } else {
            return getFile();
        }
    }
This implementation determines the underlying File
(or jar file, in case of a resource in a jar/zip).
src/main/java/spark/resource/AbstractFileResolvingResource.java