Java实现小程序调起微信支付

要在Java中调用微信支付，需要使用微信开放平台提供的统一下单API。下面是一个简单的示例代码，展示了如何使用Java调用微信支付API：

public class WechatPayUtil {
    private static final String URL = "https://api.mch.weixin.qq.com/pay/unifiedorder"; // 统一下单API地址

    public static String createOrder(String appid, String mch_id, String out_trade_no, String body, int total_fee, String notify_url, String trade_type, String spbill_create_ip, String key) throws Exception {
        // 构造请求参数
        SortedMap<String, String> paramMap = new TreeMap<String, String>();
        paramMap.put("appid", appid);
        paramMap.put("mch_id", mch_id);
        paramMap.put("nonce_str", UUID.randomUUID().toString().replaceAll("-", ""));
        paramMap.put("body", body);
        paramMap.put("out_trade_no", out_trade_no);
        paramMap.put("total_fee", String.valueOf(total_fee));
        paramMap.put("spbill_create_ip", spbill_create_ip);
        paramMap.put("notify_url", notify_url);
        paramMap.put("trade_type", trade_type);

        // 生成签名
        String sign = createSign(paramMap, key);
        paramMap.put("sign", sign);

        // 将请求参数转换为XML格式
        String xml = mapToXml(paramMap);

        // 发起HTTP请求
        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(URL);
        httpPost.setEntity(new StringEntity(xml, "UTF-8"));
        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity httpEntity = httpResponse.getEntity();

        // 处理响应结果
        String responseXml = EntityUtils.toString(httpEntity, "UTF-8");
        SortedMap<String, String> responseMap = xmlToMap(responseXml);
        String prepayId = responseMap.get("prepay_id");
        return prepayId;
    }

    private static String createSign(SortedMap<String, String> paramMap, String key) {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<String, String> entry : paramMap.entrySet()) {
            if (entry.getValue() != null && !"".equals(entry.getValue()) && !"sign".equals(entry.getKey())) {
                sb.append(entry.getKey()).append("=").append(entry.getValue()).append("&");
            }
        }
        sb.append("key=").append(key);
        String str = sb.toString();
        String sign = MD5Util.MD5Encode(str, "UTF-8").toUpperCase();
        return sign;
    }

    private static String mapToXml(SortedMap<String, String> map) throws Exception {
        Document document = DocumentHelper.createDocument();
        Element root = document.addElement("xml");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            if (entry.getValue() != null) {
                Element element = root.addElement(entry.getKey());
                element.setText(entry.getValue());
            }
        }
        return document.asXML();
    }

    private static SortedMap<String, String> xmlToMap(String xml) throws Exception {
        SortedMap<String, String> map = new TreeMap<String, String>();
        Document document = DocumentHelper.parseText(xml);
        Element root = document.getRootElement();
        List<Element> elements = root.elements();
        for (Element element : elements) {
            map.put(element.getName(), element.getText());
        }
        return map;
    }
}

其中，createOrder方法用于调用统一下单API，传入的参数包括：

- appid：公众账号ID或应用ID
- mch_id：商户号
- out_trade_no：商户订单号
- body：商品描述
- total_fee：总金额，单位为分
- notify_url：接收微信支付异步通知回调地址
- trade_type：交易类型，JSAPI代表公众号支付
- spbill_create_ip：终端IP
- key：商户密钥，用于生成签名

调用成功后，会返回预支付ID（prepay_id），用于生成支付请求。