根据数据库中先前的下拉菜单选择显示第二个下拉菜单

Question

因此，我正在创建一个使用jsp和servlet的租车应用程序示例。 向用户显示所有汽车制造的下拉列表。 当用户从品牌中选择一个选项时，它将向您显示基于品牌的特定型号。 我从mysql数据库中获取所有的make和model值。 我能够同时显示所有品牌和型号的价值，但是我不确定如何根据品牌选项显示型号。 在线研究之后，我发现您必须使用javascript，但是我没有看到javascript从数据库中获取数据的示例。 到目前为止，这是我的代码：

JSP

租个车

请选择您的车辆类型

        <form name="rentForm" action="RentACar" method="post">
            Car Make: <select name="carMake">
                <%
                    ArrayList<String> getMake = (ArrayList) request.getAttribute("make");

                    for(int i = 0; i < getMake.size(); i++)
                    {
                        out.println("<option value=\"" + getMake.get(i) + "\">" + getMake.get(i) + "</option>");
                    }
                %>
            </select>

               Car Model: <select name="carModel">
                <%
                    ArrayList<String> getModel = (ArrayList) request.getAttribute("model");

                    for(int i = 0; i < getMake.size(); i++)
                    {
                        out.println("<option value=\"" + getModel.get(i) + "\">" + getModel.get(i) + "</option>");
                    }
                %>
            </select>

        </form>


    </body>
</html>

Servlet的

public class RentACar extends HttpServlet
{
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
        ArrayList<String> vehicleMake = RentService.getMake();
        request.setAttribute("make", vehicleMake);
        ArrayList<String> vehicleModel = RentService.getModel(model);
        request.setAttribute("model", vehicleModel);
        RequestDispatcher dispatcher = request.getRequestDispatcher("rent-car.jsp");
        dispatcher.forward(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
        String model = request.getParameter("carMake");
    }
}

数据库服务

public class RentService
{
    public static ArrayList<String> getMake()
    {
        ArrayList<String> vehicleList = new ArrayList<>();
        String make = "";

        try
        {
            Connection conn = DBConnection.getConnection();

            String query = "SELECT make FROM Vehicles";
            PreparedStatement stmt = conn.prepareStatement(query);
            ResultSet rs = stmt.executeQuery();

            while(rs.next())
            {            
                make = rs.getString("make");
                vehicleList.add(make);
            }


        } 
        catch (Exception e)
        {
            System.out.println(e);
        }
        return vehicleList;
    }

    public static ArrayList<String> getModel(String carMake)
    {
        ArrayList<String> modelList = new ArrayList<>();
        String model = "";

        try
        {
            Connection conn = DBConnection.getConnection();

            String query = "SELECT model FROM Vehicles WHERE make=? ";
            PreparedStatement stmt = conn.prepareStatement(query);
            stmt.setString(1, carMake);
            ResultSet rs = stmt.executeQuery();

            while(rs.next())
            {
                model = rs.getString("model");
                modelList.add(model);
            }
        } 
        catch (Exception e)
        {
            System.out.println(e);
        }
        return modelList;
    }
}