1注册支付宝开放平台账号并创建app 建议全程用法人的 还需要icp备案 没备案就不需要看了
2申请支付宝商家 手机网站支付能力 如果登录过开放平台 那么商户应该也是直接登录的 指在一个浏览器
3申请填写各种资料
4

           <dependency>
                <groupId>com.aliyun</groupId>
                <artifactId>alibabacloud-dysmsapi20170525</artifactId>
                <version>3.0.1</version>
            </dependency>

public String pagePay(String outTradeNo, String totalAmount, String subject, String returnUrl1) {
        // 创建支付客户端
        AlipayClient alipayClient = new DefaultAlipayClient(
                gatewayHost,  //网关
                appId, //开放平台创建的app
                merchantPrivateKey, //开放平台申请的私钥
                "json",
                "UTF-8",
                alipayPublicKey,//支付宝公钥
                "RSA2"
        );

        // 创建支付请求
//        AlipayTradePagePayRequest request = new AlipayTradePagePayRequest();//PC端
        AlipayTradeWapPayRequest request = new AlipayTradeWapPayRequest();//手机端

        // 设置同步跳转地址
        request.setReturnUrl(returnUrl1);//支付完后跳转的地址

        // 设置异步通知地址
        request.setNotifyUrl(notifyUrl);//回调地址

        // 构建业务参数
        String bizContent = "{" +
                "\"out_trade_no\":\"" + outTradeNo + "\"," +//订单号
                "\"total_amount\":\"" + totalAmount + "\"," +//金额
                "\"subject\":\"" + subject + "\"," +//订单名称
                "\"product_code\":\"FAST_INSTANT_TRADE_PAY\"" +
                "}";

        request.setBizContent(bizContent);
      
        try {
            // 执行支付
            // 5. 执行支付请求，生成表单
            return alipayClient.pageExecute(request).getBody();

        } catch (Exception e) {
            log.error("支付异常",e);
            throw new ServiceException("支付异常");
        }
    }
        /**  
         * 创建订单
         */ 
        public Map createOrder(Order order){
                  HashMap<String, String> formMap = new HashMap<>();
                  String form = aliPayConfiguration.pagePay(
                  order.getOrderCode(),//订单编号
                  order.getOrderPrice(),//订单金额
                  order.getGoodsName(),//订单名称);
                  formMap.put("form",form);//返回表单
                  return formMap;
        }

/***
     * 支付宝支付回调
     * @param request
     * @return
     */
    @PostMapping("/aliPayh5Notify") 
    public String aliPayh5Notify(HttpServletRequest request){
        String success = "success";
        String failure = "failure";

        // https://opendocs.alipay.com/open/54/00y8k9 新老版本说明中有异步通知的新版说明
        // 获取支付宝异步回调信息, 将其转为 Map<String, String>
        Map<String, String> params = new HashMap<>();
        Map<String, String[]> requestParams = request.getParameterMap();
        for (Iterator iter = requestParams.keySet().iterator(); iter.hasNext();) {
            String name = (String) iter.next();
            String[] values = (String[]) requestParams.get(name);
            String valueStr = "";
            for (int i = 0; i < values.length; i++) {
                valueStr = (i == values.length - 1) ? valueStr + values[i]
                        : valueStr + values[i] + ",";
            }
            // 乱码解决，这段代码在出现乱码时使用。如果mysign和sign不相等也可以使用这段代码转化
            // valueStr = new String(valueStr.getBytes("ISO-8859-1"), "gbk");
            params.put(name, valueStr);
        }

        // 新版 SDK 不用移除 sign_type
        // params.remove("sign_type");

        // 验签
        boolean signVerified = false;
        try {
//            signVerified = Factory.Payment.Common().verifyNotify(params);
            // 2. 验签
            boolean verifyResult = AlipaySignature.rsaCheckV1(
                    params,
                    aliPayConfiguration.getAlipayPublicKey(),
                    "UTF-8",
                    "RSA2"
            );

            // 3. 验签成功后进行业务处理
            if (verifyResult) {
                // 获取通知参数 
                signVerified = verifyResult;

            }
        } catch (Exception e) {
            logger.error("alipay支付异常",e);
            throw new ServiceException("支付异常");
        }

        if(signVerified){ // 验签通过
            String outTradeNo = params.get("out_trade_no");
            String buyer_id = params.get("buyer_id");
            Order order = orderService.getOne(new LambdaQueryWrapper<Order>().eq(Order::getOrderCode, outTradeNo));
            if (order != null) {
                     //....其他处理 
            }else{
                return failure;
            }
            return success;
        }else{ // 验签失败
            return failure;
        }
    }

用的uniapp 封装了个页面 直接路由跳过来

<template>
    <view class="pay-page">
    <web-view  v-if="alipayForm" :src="'data:text/html;charset=utf-8,'+encodeURIComponent(alipayForm)"></web-view>
    </view>
</template>

<script>
    import {
        add1
    } from '@/api/system/order.js'
    export default {
        data(){
            return{
                order:{},
                alipayForm:null,
            }
        },
        onLoad(options) {
           //接收传递过来的参数
           this.pay()
        },
        methods:{
            pay(){
                add1(this.order).then(async res => {
                    // await alert(res.data.form)
                    if(this.isFormString(res.data.form)){
                        this.alipayForm = res.data.form 
                    }
                })
            },
            isFormString(str) {
              // 匹配是否包含 <form 开头和 </form> 结尾
              const formRegex = /<form[\s\S]*<\/form>/i;
              return formRegex.test(str);
            },
        }
    }
    
</script>

<style>
    .pay-page {
        min-height: 100vh;
        background-color: var(--background-color);
        padding-bottom: 40rpx;
    } 
</style>