购物网站如何接入支付宝后台接口实现支付功能？

你可以去支付宝官方网站看到，支付宝提供开发者文档还有demo，很清楚的

//创建支付请求
	String result = null;
		try {
			Map<String, String> sParaTemp = new HashMap<String, String>();
			sParaTemp.put("service", "create_direct_pay_by_user");
	        sParaTemp.put("partner", partner);
	        sParaTemp.put("seller_email", seller_email);
	        sParaTemp.put("_input_charset", input_charset);
			sParaTemp.put("payment_type", payment_type);
			sParaTemp.put("notify_url", notify_url);
			sParaTemp.put("return_url", return_url);
			sParaTemp.put("out_trade_no", order.getOrderNumber());
			sParaTemp.put("subject", "订单号"+order.getOrderNumber());
			sParaTemp.put("total_fee", getOrderFee(order));
			sParaTemp.put("body", getOrderDetail(order));
			sParaTemp.put("anti_phishing_key", AlipayConfig.anti_phishing_key);
			sParaTemp.put("exter_invoke_ip", AlipayConfig.exter_invoke_ip);
			Map<String, String> sPara = AlipaySubmit.buildRequestPara(sParaTemp);
			StringBuffer sb = new StringBuffer();
			Set es = sPara.entrySet();
			Iterator it = es.iterator();
			while(it.hasNext()) {
				Map.Entry entry = (Map.Entry)it.next();
				String k = (String)entry.getKey();
				String v = (String)entry.getValue();
				sb.append(k + "=" + URLEncoder.encode(v, input_charset) + "&");
			}
			String reqPars = sb.substring(0, sb.lastIndexOf("&"));
			//将数组拼接成参数
			result = AlipaySubmit.ALIPAY_GATEWAY_NEW + reqPars;

return "redirect:" + result;

谢谢