code: public static String makeAppSecretProof(String appSecret, String accessToken) {
  		try {
  			Mac mac = Mac.getInstance("HmacSHA256");
  			SecretKeySpec secretKey = new SecretKeySpec(appSecret.getBytes(), "HmacSHA256");
  			mac.init(secretKey);
  			byte[] bytes = mac.doFinal(accessToken.getBytes());
  
  			return Hex.encodeHexString(bytes);
  
  		} catch (NoSuchAlgorithmException | InvalidKeyException e) {
  			throw new RuntimeException(e);
  		}
  	}
docs: See https://developers.facebook.com/docs/graph-api/securing-requests