Signing API requests

import com.google.gson.Gson;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        String accessKey = "01h6tn69wfcpy5q5x3vpb3x9me"; // Your client key
        String secretKey = "your-client-secret";

        // Request parameters
        Map<String, Object> params = new HashMap<>();
        params.put("client_key", accessKey);
        params.put("amount", "50000.00");
        params.put("channel_id", "1001");
        params.put("out_trade_no", "20230101000000");
        params.put("notify_url", "https://your-domain.com/webhook");

        // Ensure 'extra' is always a JSON string
        //
        // Note: Not every request requires the "extra" parameter,
        // Please refer to the request examples for each country.
        // The following example is for a specific bank in Vietnam to receive money.
        Map<String, String> extra = new HashMap<>();
        extra.put("bank_code", "VCB");
        params.put("extra", new Gson().toJson(extra));

        params.put("empty_string", ""); // Empty string (excluded from signature)
        params.put("null_value", null); // NULL value (excluded from signature)
        params.put("should_not_include", "example"); // Parameter to be excluded from the signature

        // Parameters to exclude from the signature
        Set<String> except = new HashSet<>(List.of("should_not_include"));

        // Filter parameters (exclude null, empty, and specified keys)
        Map<String, Object> filteredParams = params.entrySet().stream()
                .filter(entry -> !except.contains(entry.getKey()) && entry.getValue() != null && !entry.getValue().toString().isEmpty())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        // Sort the parameters by key in ascending order
        TreeMap<String, Object> sortedParams = new TreeMap<>(filteredParams);

        // Build the string to sign
        String stringToSign = sortedParams.entrySet().stream()
                .map(entry -> entry.getKey() + "=" + entry.getValue())
                .collect(Collectors.joining("&"));

        System.out.println("String to Sign: " + stringToSign);
        // Output:
        // amount=50000.00&channel_id=1001&client_key=01h6tn69wfcpy5q5x3vpb3x9me&extra={"bank_code":"VCB"}&notify_url=https://your-domain.com/webhook&out_trade_no=20230101000000

        // Generate the HMAC-SHA256 signature
        byte[] hmacSha256 = calcHmacSha256(
                secretKey.getBytes(StandardCharsets.UTF_8),
                stringToSign.getBytes(StandardCharsets.UTF_8)
        );

        System.out.printf("Signature: %064x%n", new BigInteger(1, hmacSha256));
        // Output:
        // 32db0797717edf25775a95cbbf61c4f693b47604a309fb63d46e36faf75e58ce
    }

    // Calculate the HMAC-SHA256 signature
    public static byte[] calcHmacSha256(byte[] secretKey, byte[] stringToSign) {
        byte[] hmacSha256 = null;

        try {
            Mac mac = Mac.getInstance("HmacSHA256");
            SecretKeySpec secretKeySpec = new SecretKeySpec(secretKey, "HmacSHA256");
            mac.init(secretKeySpec);
            hmacSha256 = mac.doFinal(stringToSign);
        } catch (Exception e) {
            throw new RuntimeException("Failed to calculate HMAC-SHA256", e);
        }

        return hmacSha256;
    }
}