public void setup() throws Exception {
    PrivateKey merchantPrivateKey = PemUtil.loadPrivateKey(privateKey);
    // 获取证书管理器实例
    certificatesManager = CertificatesManager.getInstance();
    // 向证书管理器增加需要自动更新平台证书的商户信息
    certificatesManager.putMerchant(mchId, new WechatPay2Credentials(mchId,
            new PrivateKeySigner(mchSerialNo, merchantPrivateKey)), apiV3Key.getBytes(StandardCharsets.UTF_8));
    // 从证书管理器中获取verifier
    verifier = certificatesManager.getVerifier(mchId);
    // 构造httpclient
    httpClient = WechatPayHttpClientBuilder.create()
            .withMerchant(mchId, mchSerialNo, merchantPrivateKey)
            .withValidator(new WechatPay2Validator(verifier))
            .build();
}