在Spring Boot项目中集成支付宝的网站支付、APP支付、单笔转账给用户以及退款功能，可以通过以下步骤实现：

<dependency>
    <groupId>com.alipay.sdk</groupId>
    <artifactId>alipay-sdk-java</artifactId>
    <version>4.9.79.ALL</version>
</dependency>

alipay:
  app-id: your-app-id
  merchant-private-key: your-private-key
  alipay-public-key: alipay-public-key
  notify-url: http://your-domain/notify
  return-url: http://your-domain/return
  gateway-url: https://openapi.alipay.com/gateway.do
  format: json
  charset: utf-8
  sign-type: RSA2

import com.alipay.api.AlipayClient;
import com.alipay.api.DefaultAlipayClient;
import com.alipay.api.request.AlipayTradePagePayRequest;

@Service
public class AlipayService {
    @Autowired
    private Environment env;

    public String createWebPayment(String subject, String totalAmount) {
        AlipayClient alipayClient = new DefaultAlipayClient(
                env.getProperty("alipay.gateway-url"),
                env.getProperty("alipay.app-id"),
                env.getProperty("alipay.merchant-private-key"),
                "json",
                "UTF-8",
                env.getProperty("alipay.alipay-public-key"),
                "RSA2");

        AlipayTradePagePayRequest request = new AlipayTradePagePayRequest();
        request.setReturnUrl(env.getProperty("alipay.return-url"));
        request.setNotifyUrl(env.getProperty("alipay.notify-url"));
        request.setBizContent("{"
                + "\"out_trade_no\":\"" + UUID.randomUUID().toString() + "\","
                + "\"total_amount\":\"" + totalAmount + "\","
                + "\"subject\":\"" + subject + "\","
                + "\"product_code\":\"FAST_INSTANT_TRADE_PAY\""
                + "}");

        try {
            return alipayClient.pageExecute(request).getBody();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}

import com.alipay.api.request.AlipayFundTransUniTransferRequest;

public String transfer(String outBizNo, String payeeAccount, String amount) {
    AlipayClient alipayClient = new DefaultAlipayClient(
            env.getProperty("alipay.gateway-url"),
            env.getProperty("alipay.app-id"),
            env.getProperty("alipay.merchant-private-key"),
            "json",
            "UTF-8",
            env.getProperty("alipay.alipay-public-key"),
            "RSA2");

    AlipayFundTransUniTransferRequest request = new AlipayFundTransUniTransferRequest();
    request.setBizContent("{"
            + "\"out_biz_no\":\"" + outBizNo + "\","
            + "\"payee_type\":\"ALIPAY_LOGON_ID\","
            + "\"payee_account\":\"" + payeeAccount + "\","
            + "\"amount\":\"" + amount + "\","
            + "\"remark\":\"转账备注\""
            + "}");

    try {
        return alipayClient.execute(request).getBody();
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }
}

import com.alipay.api.request.AlipayTradeRefundRequest;

public String refund(String outTradeNo, String refundAmount) {
    AlipayClient alipayClient = new DefaultAlipayClient(
            env.getProperty("alipay.gateway-url"),
            env.getProperty("alipay.app-id"),
            env.getProperty("alipay.merchant-private-key"),
            "json",
            "UTF-8",
            env.getProperty("alipay.alipay-public-key"),
            "RSA2");

    AlipayTradeRefundRequest request = new AlipayTradeRefundRequest();
    request.setBizContent("{"
            + "\"out_trade_no\":\"" + outTradeNo + "\","
            + "\"refund_amount\":\"" + refundAmount + "\""
            + "}");

    try {
        return alipayClient.execute(request).getBody();
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }
}

@PostMapping("/notify")
public String handleNotify(HttpServletRequest request) {
    Map<String, String> params = request.getParameterMap().entrySet().stream()
            .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue()[0]));

    boolean verifyResult = AlipaySignature.rsaCheckV1(params, env.getProperty("alipay.alipay-public-key"), "UTF-8", "RSA2");
    if (verifyResult) {
        String outTradeNo = params.get("out_trade_no");
        String totalAmount = params.get("total_amount");
        // 更新订单状态等业务逻辑
        return "success";
    } else {
        return "fail";
    }
}