String sessionKey = jsonObject.getString("session_key");
String nonceStr = WXPayUtil.generateNonceStr();
String timeStamp = String.valueOf(System.currentTimeMillis()/1000);
String signature = null;
//字典序排序
String str = this.sortStr(appId,groupId,timeStamp,nonceStr);
try {
    // Base64 解码 sessionKey
    byte[] keyBytes = Base64.getDecoder().decode(sessionKey);
    Mac sha256Hmac = Mac.getInstance("HmacSHA256");
    SecretKeySpec secretKey = new SecretKeySpec(keyBytes, "HmacSHA256");
    sha256Hmac.init(secretKey);

    // 计算签名
    byte[] signatureBytes = sha256Hmac.doFinal(str.getBytes(StandardCharsets.UTF_8));
    // 转换为十六进制
    signature = this.bytesToHex(signatureBytes);
} catch (Exception e) {
    e.printStackTrace();
}
private String bytesToHex(byte[] bytes) {
    StringBuilder result = new StringBuilder();
    for (byte b : bytes) {
        result.append(String.format("%02x", b));
    }
    return result.toString();
}

/**
 * 字典序排序
 *
 */
private String sortStr(String appId, String groupId, String timeStamp, String nonceStr) {
    // 1. 按字典序排序
    List<String> params = new ArrayList<>();
    params.add(appId);
    params.add(groupId);
    params.add(nonceStr);
    params.add(timeStamp);
    // 字典序排序
    Collections.sort(params);
    // 2. 拼接字符串
    return String.join("", params);
}