Delink BNPL Link & Pay API

package com.payu.apilayer.util;

import com.google.gson.Gson;
import com.google.gson.JsonObject;
import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;

import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class HmacAuth {

    public static JsonObject getRequestBody(){
        JsonObject requestJson = new JsonObject();
        requestJson.addProperty("udf1","123");
        return requestJson;
    }

    public static String getSha512Hash(String hashString) {
        {
            try {
                MessageDigest md = MessageDigest.getInstance("SHA-512");
                byte[] messageDigest = md.digest(hashString.getBytes());
                BigInteger signumBytes = new BigInteger(1, messageDigest);
                String hashtext = signumBytes.toString(16);
                while (hashtext.length() < 32) {
                    hashtext = "0" + hashtext;
                }
                return hashtext;
            }
            catch (NoSuchAlgorithmException e) {
                throw new RuntimeException(e);
            }
        }
    }

    public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeyException {
        String key = "smsplus";
        String secret = "admin";
        Gson gson = new Gson();
        String date = DateTimeFormat.forPattern("EEE, dd MMM yyyy HH:mm:ss 'GMT'").withZoneUTC().print(new DateTime());
        System.out.println(date);
        JsonObject requestJson = getRequestBody();
        String hashString = new StringBuilder()
                .append(gson.toJson(requestJson))
                .append("|")
                .append(date)
                .append("|")
                .append(secret).toString();
        System.out.println("Hash String is " + hashString);
        String hash = getSha512Hash(hashString);
        String authorization = new StringBuilder()
                .append("hmac username=\"")
                .append(key)
                .append("\", algorithm=\"sha512\", headers=\"date\", signature=\"")
                .append(hash)
                .append("\"").toString();
        System.out.println(authorization);
    }
}