<dependency>
    <groupId>com.auth0</groupId>
    <artifactId>java-jwt</artifactId>
    <version>3.8.3</version>
</dependency>

import com.auth0.jwt.JWT;
import com.auth0.jwt.algorithms.Algorithm;
import java.util.Calendar;
import java.util.Date;

public class SigUtils {
    /**
     * Create JWT Signature
     *
     * @param appId  Application ID
     * @param appKey Application Secret Key
     * @param sigExp Signature validity duration (seconds)
     * @return JWT signature string
     */
    public static String createSig(String appId, String appKey, int sigExp) {
        Calendar nowTime = Calendar.getInstance();
        nowTime.add(Calendar.SECOND, sigExp);
        Date expiresDate = nowTime.getTime();

        return JWT.create()
                // Issued time
                .withIssuedAt(new Date())
                // Expiration time
                .withExpiresAt(expiresDate)
                // Payload data
                .withClaim("appId", appId)
                // Sign using HMAC256 with appKey
                .sign(Algorithm.HMAC256(appKey));
    }
}