private static String computeCheckSum(File appDir) {
        MessageDigest md5;
        try {
            md5 = MessageDigest.getInstance("MD5");
            for (File file : appDir.listFiles((dir, name) -> !name.equals(EXT_DIR) && !name.startsWith("."))) {
                addPathToDigest(file, "", md5, true, false);
            }
            return toLowerCase(HexDump.toHexString(md5.digest()));
        } catch (Exception e) {
            throw new RuntimeException(e);
        }

    }
Computes an md5 hash of the contents of the application package

@return an md5sum of the application package
config-application-package/src/main/java/com/yahoo/config/model/application/provider/FilesApplicationPackage.java