public static String hmac_sha256(String sessionKey) throws NoSuchAlgorithmException, InvalidKeyException, UnsupportedEncodingException {
    if (!StringUtils.hasText(sessionKey)) {
        return null;
    }
    SecretKeySpec secretKeySpec = new SecretKeySpec(sessionKey.getBytes(), "HmacSHA256");
    Mac mac = Mac.getInstance("HmacSHA256");
    mac.init(secretKeySpec);

    byte[] hmac = mac.doFinal("".getBytes("utf-8"));

    return Base64.getEncoder().encodeToString(hmac);
}