我参考微信官方提供的python文档，写了一份java的签名方法，但是一直提示签名错误，辛苦帮我看一下哪里写的有问题吗？以下是签名代码：

public String signature(String postBody, String sessionKey) throws NoSuchAlgorithmException, InvalidKeyException, UnsupportedEncodingException {
    String needSignMessage = postBody;
    Mac sha256Hmac = Mac.getInstance("HmacSHA256");
    SecretKeySpec secretKey = new SecretKeySpec(sessionKey.getBytes("UTF-8"), "HmacSHA256");
    sha256Hmac.init(secretKey);
    byte[] hashBytes = sha256Hmac.doFinal(needSignMessage.getBytes("UTF-8"));
    return Hex.encodeHexString(hashBytes);
}

@微信官方