code: private String buildPayForm(Map<String, String> payParams) {
          StringBuilder form = new StringBuilder();
  
          form.append("<form id=\"pay_form\" name=\"pay_form\"").append(" action=\"" + Alipay.GATEWAY).append(AlipayField.INPUT_CHARSET+"=").append(alipay.inputCharset).append("\" method=\"POST\">");
          for (Map.Entry<String, String> param : payParams.entrySet()){
              form.append("<input type=\"hidden\" name=\"")
                      .append(param.getKey()).append("\" value=\"").append(param.getValue()).append("\" />");
          }
          form.append("<input type=\"submit\" value=\"去支付\" style=\"display:none;\" />");
          form.append("</form>");
          form.append("<script>document.forms['pay_form'].submit();</script>");
  
          return form.toString();
      }
docs: 构建支付表单
  @param payParams 支付参数
  @return 支付表单