在Java Web中实现在线支付功能，通常需要集成第三方支付网关（如支付宝、微信支付、Stripe等）的API。以下是实现在线支付功能的一般步骤和关键点：

import com.alipay.api.AlipayClient;
import com.alipay.api.request.AlipayTradePagePayRequest;
import com.alipay.api.response.AlipayTradePagePayResponse;

public class PaymentService {
    private AlipayClient alipayClient;

    public String initiatePayment(String orderId, String totalAmount, String subject) {
        AlipayTradePagePayRequest request = new AlipayTradePagePayRequest();
        request.setReturnUrl("http://your-domain.com/return-url");
        request.setNotifyUrl("http://your-domain.com/notify-url");
        request.setBizContent("{"
                + ""out_trade_no": "" + orderId + "","
                + ""total_amount": "" + totalAmount + "","
                + ""subject": "" + subject + "","
                + ""product_code": "FAST_INSTANT_TRADE_PAY""
                + "}");
        try {
            AlipayTradePagePayResponse response = alipayClient.pageExecute(request);
            return response.getBody();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}

@WebServlet("/payOnline")
public class PayOnline extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String orderid = request.getParameter("orderid");
        String money = request.getParameter("money");
        String pd_FrpId = request.getParameter("pd_FrpId");
        // 构建支付请求参数
        String hmac = PaymentUtil.buildHmac("Buy", "YourMerchantId", orderid, money, "CNY", "", "", "", "http://your-domain.com/callback", "", "", pd_FrpId, "1", "YourKeyValue");
        request.setAttribute("pd_FrpId", pd_FrpId);
        request.setAttribute("hmac", hmac);
        request.getRequestDispatcher("/confirm.jsp").forward(request, response);
    }
}