Java Sample Code

import java.util.Base64;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
public static String calculateRFC2104HMAC(String stringToSign, String appSecret) {
    SecretKeySpec signingKey = new SecretKeySpec(appSecret.getBytes(),"HmacSHA1");
    Mac mac = Mac.getInstance("HmacSHA1");
    mac.init(signingKey);
    byte[] hashBytes = mac.doFinal(stringToSign.getBytes());
    return Base64.getEncoder().encodeToString(hashBytes);
}
String timestamp = Long.toString(System.currentTimeMillis() / 1000);
String stringToSign = "app_id=" + app_id + "&secret=" + secret + "&timestamp=" + timestamp;
String signature = calculateRFC2104HMAC(stringToSign, appSecret)