根据以前从数据库中选择的下拉列表显示第二个下拉列表

Question

所以我正在创建一个使用jsp和servlet租用汽车应用程序的示例。向用户显示所有汽车制造的下拉列表。当用户从make中选择一个选项时，它将根据make向用户显示特定的模型。我从mysql数据库获取所有的make和model值。我能够一起显示所有品牌和型号值，但我不知道如何根据make选项显示模型选项。在线研究后，我发现你必须使用javascript，但我没有看到javascript从数据库中获取数据的例子。 到目前为止，这是我的代码：

JSP                      

租一辆车

            

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>

的Servlet

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;
    }
}