Prerequisite

{
  "merchant_data": {
    "merchant_id": 12345678,
    "merchant_access_code": "4a8c422e-928d-4f84-bfe8-27a09af66647",
    "unique_merchant_txn_id": "XYZ123",
    "merchant_return_url": "https://www.pinelabs.com"
  },
  "payment_data": {
    "amount_in_paisa": 800
  },
  "txn_data": {
    "navigation_mode": "7",
    "payment_mode": "10",
    "transaction_type": "1",
    "time_stamp": 157588000000
  }
}

import java.util.Base64;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class SignatureGenerator {
  private static Map < String, Object > clean(Map < String, Object > arr) {
    return arr.entrySet().stream()
      .filter(entry -> entry.getValue() != null && !entry.getValue().toString().isEmpty())
      .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
  }
  public static void main(String[] args) {
    Map < String, Object > requestBody = new LinkedHashMap < > ();
    Map < String, Object > merchant_data = new LinkedHashMap < > ();
    merchant_data.put("merchant_id", "merchant_id");
    merchant_data.put("merchant_access_code", "merchant_access_code");
    merchant_data.put("unique_merchant_txn_id", "XYZ123");
    merchant_data.put("merchant_return_url", "whitelisted_merchant_return_url");
    requestBody.put("merchant_data", merchant_data);

    Map < String, Object > paymentDataObject = new LinkedHashMap < > ();
    paymentDataObject.put("amount_in_paisa", 100);
    requestBody.put("payment_data", paymentDataObject);
    Map < String, Object > txnObject = new LinkedHashMap < > ();
    txnObject.put("navigation_mode", "7");
    txnObject.put("payment_mode", "10");
    txnObject.put("transaction_type", "1");
    requestBody.put("txn_data", txnObject);

    Map < String, Object > requestBodyCleaned = clean(requestBody);
    Map < String, Object > mapStatus = new HashMap < > ();
    mapStatus.put("result", requestBodyCleaned);
    Gson gson = new GsonBuilder().disableHtmlEscaping().create();
    String jsonString2 = gson.toJson(requestBodyCleaned);
    jsonString2 = jsonString2.replace("\\/", "/");

    // Encoding JSON string to base64
    String base64Encoded = Base64.getEncoder().encodeToString(jsonString2.getBytes());

    System.out.println(base64Encoded);
  }
}

import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import javax.xml.bind.DatatypeConverter;

public class SignatureGenerator {

    public static String jsonHash(String request, String secret) throws NoSuchAlgorithmException, InvalidKeyException, IllegalStateException, UnsupportedEncodingException {
        SecretKeySpec secretKeySpec = new SecretKeySpec(DatatypeConverter.parseHexBinary(secret), "HmacSHA256");
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(secretKeySpec);
        byte[] hmacBytes = mac.doFinal(request.getBytes("UTF-8"));
        String ss = String.format("%02x", new BigInteger(1, hmacBytes));
        return bytesToHex(hmacBytes).toUpperCase();
    }
    private static String bytesToHex(byte[] bytes) {
        StringBuilder result = new StringBuilder();
        for (byte b : bytes) {
            result.append(String.format("%02X", b));
        }
        return result.toString();
    }

    public static void main(String[] args) throws UnsupportedEncodingException, NoSuchAlgorithmException, InvalidKeyException {
        String responseBody = "<Base64EncodedPayload>";
        String signingSecret = "<Secret>";

        String requestSignature = SignatureGenerator.jsonHash(responseBody,signingSecret);
        System.out.println(requestSignature);
    }
} 

ppc_MerchantAccessCode=cfd05c0c-39f1-4232-bd6f-6d3a8608e1be&ppc_MerchantID=279082&ppc_TransactionType=3&ppc_UniqueMerchantTxnID=testingedgeseamless1123145432

ppc_Amount=100&ppc_CurrencyCode=356&ppc_MerchantAccessCode=bcf441be-411b-46a1-aa88-c6e852a7d68c&ppc_MerchantID=106600&ppc_PinePGTransactionID=14709745&ppc_TransactionType=10&ppc_UniqueMerchantTxnID=refund%20test

import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import javax.xml.bind.DatatypeConverter;

public class SignatureGenerator {

    public static String jsonHash(String request, String secret) throws NoSuchAlgorithmException, InvalidKeyException, IllegalStateException, UnsupportedEncodingException {
        SecretKeySpec secretKeySpec = new SecretKeySpec(DatatypeConverter.parseHexBinary(secret), "HmacSHA256");
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(secretKeySpec);
        byte[] hmacBytes = mac.doFinal(request.getBytes("UTF-8"));
        String ss = String.format("%02x", new BigInteger(1, hmacBytes));
        return bytesToHex(hmacBytes).toUpperCase();
    }
    private static String bytesToHex(byte[] bytes) {
        StringBuilder result = new StringBuilder();
        for (byte b : bytes) {
            result.append(String.format("%02X", b));
        }
        return result.toString();
    }

    public static void main(String[] args) throws UnsupportedEncodingException, NoSuchAlgorithmException, InvalidKeyException {
        String responseBody = "<HashMap_payload>";
        String signingSecret = "<Secret_key>";

        String requestSignature = SignatureGenerator.jsonHash(responseBody,signingSecret);
        System.out.println(requestSignature);
    }
}