用java结合接口写个微信支付小程序的案例

好的，下面是一个简单的微信支付小程序的Java代码示例：

首先，我们需要创建一个接口，定义微信支付相关的方法：

public interface WeChatPayService {
    String unifiedOrder(String orderId, BigDecimal amount, String openid);
    boolean verifyNotify(String notifyData);
}

接着，我们实现这个接口：

public class WeChatPayServiceImpl implements WeChatPayService {
    private static final String APP_ID = "your_app_id";
    private static final String MCH_ID = "your_mch_id";
    private static final String KEY = "your_key";
    private static final String UNIFIED_ORDER_URL = "https://api.mch.weixin.qq.com/pay/unifiedorder";
    private static final String NOTIFY_URL = "your_notify_url";

    @Override
    public String unifiedOrder(String orderId, BigDecimal amount, String openid) {
        SortedMap<String, String> parameters = new TreeMap<>();
        parameters.put("appid", APP_ID);
        parameters.put("mch_id", MCH_ID);
        parameters.put("nonce_str", UUID.randomUUID().toString().replaceAll("-", ""));
        parameters.put("body", "商品描述");
        parameters.put("out_trade_no", orderId);
        parameters.put("total_fee", amount.multiply(new BigDecimal(100)).intValue() + "");
        parameters.put("spbill_create_ip", "127.0.0.1");
        parameters.put("notify_url", NOTIFY_URL);
        parameters.put("trade_type", "JSAPI");
        parameters.put("openid", openid);

        String sign = createSign(parameters, KEY);
        parameters.put("sign", sign);

        String requestXml = mapToXml(parameters);
        String responseXml = HttpUtil.post(UNIFIED_ORDER_URL, requestXml);

        Map<String, String> responseMap = xmlToMap(responseXml);
        String prepayId = responseMap.get("prepay_id");

        SortedMap<String, String> resultMap = new TreeMap<>();
        resultMap.put("appId", APP_ID);
        resultMap.put("timeStamp", String.valueOf(System.currentTimeMillis() / 1000));
        resultMap.put("nonceStr", UUID.randomUUID().toString().replaceAll("-", ""));
        resultMap.put("package", "prepay_id=" + prepayId);
        resultMap.put("signType", "MD5");

        String resultSign = createSign(resultMap, KEY);
        resultMap.put("paySign", resultSign);

        return mapToJson(resultMap);
    }

    @Override
    public boolean verifyNotify(String notifyData) {
        Map<String, String> map = xmlToMap(notifyData);
        if (!map.containsKey("sign")) {
            return false;
        }
        String sign = map.get("sign");
        map.remove("sign");
        String sign2 = createSign(map, KEY);
        return sign.equals(sign2);
    }

    private String createSign(SortedMap<String, String> parameters, String key) {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<String, String> entry : parameters.entrySet()) {
            if (StringUtils.isNotBlank(entry.getValue())) {
                sb.append(entry.getKey())
                        .append("=")
                        .append(entry.getValue())
                        .append("&");
            }
        }
        sb.append("key=").append(key);
        return DigestUtils.md5Hex(sb.toString()).toUpperCase();
    }

    private String mapToXml(Map<String, String> map) {
        StringBuilder sb = new StringBuilder("<xml>");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            sb.append("<").append(entry.getKey()).append(">")
                    .append(entry.getValue())
                    .append("</").append(entry.getKey()).append(">");
        }
        sb.append("</xml>");
        return sb.toString();
    }

    private Map<String, String> xmlToMap(String xml) {
        Map<String, String> map = new HashMap<>();
        try {
            Document doc = DocumentHelper.parseText(xml);
            Element root = doc.getRootElement();
            List<Element> elements = root.elements();
            for (Element element : elements) {
                map.put(element.getName(), element.getTextTrim());
            }
        } catch (DocumentException e) {
            e.printStackTrace();
        }
        return map;
    }

    private String mapToJson(Map<String, String> map) {
        ObjectMapper mapper = new ObjectMapper();
        try {
            return mapper.writeValueAsString(map);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
        return null;
    }
}

其中，`unifiedOrder` 方法用于生成预支付订单信息，并返回给前端；`verifyNotify` 方法用于验证微信支付异步通知的签名是否正确。

接着，我们可以在Spring Boot中使用这个接口：

@RestController
public class WeChatPayController {
    @Autowired
    private WeChatPayService weChatPayService;

    @PostMapping("/unifiedOrder")
    public String unifiedOrder(@RequestParam String orderId,
                               @RequestParam BigDecimal amount,
                               @RequestParam String openid) {
        return weChatPayService.unifiedOrder(orderId, amount, openid);
    }

    @PostMapping("/notify")
    public String notify(@RequestBody String notifyData) {
        if (weChatPayService.verifyNotify(notifyData)) {
            // 处理支付成功逻辑
            return "<xml><return_code><![CDATA[SUCCESS]]></return_code><return_msg><![CDATA[OK]]></return_msg></xml>";
        } else {
            return "<xml><return_code><![CDATA[FAIL]]></return_code><return_msg><![CDATA[签名失败]]></return_msg></xml>";
        }
    }
}

其中，`/unifiedOrder` 接口用于生成预支付订单信息，并返回给前端；`/notify` 接口用于接收微信支付的异步通知。如果签名验证成功，我们可以在这个接口中处理支付成功的逻辑。

以上就是一个简单的微信支付小程序的Java代码示例。