code: public static String id(Resource res) {
  	String str = ResourceUtil.getCanonicalPathEL(res) + "|" + res.length() + "|" + res.lastModified();
  	try {
  	    return Hash.md5(str);
  	}
  	catch (NoSuchAlgorithmException e) {
  	    return Caster.toString(HashUtil.create64BitHash(str));
  	}
      }
docs: does not involve the content to create an id, value returned is based on metadata of the file
  (lastmodified,size)
  
  @param res
  @return
  @throws NoSuchAlgorithmException