从servlet中的方法加载选择框

Question

所以基本上我在index.jsp页面上有2个链接。当我单击addPet链接时，它会按预期执行。当我单击添加客户时，它的行为也符合预期，但问题在于加载选择框。我有一个我想使用的processRequest（）方法，但有没有办法加载框而不会抛出这样的错误

HTTP Status 500 - Cannot forward after response has been committed

我在下面包含了我的servlet代码....如果还有其他代码需要请告诉我。

SERVLET

package edu.witc.Assignment03.controller;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//import javax.servlet.annotation.WebServlet;
//import javax.servlet.http.HttpServlet;
//import javax.servlet.http.HttpServletRequest;
//import javax.servlet.http.HttpServletResponse;




import javax.servlet.http.HttpSession;

import edu.witc.Assignment03.model.Customer;
import edu.witc.Assignment03.model.Phone;
import edu.witc.Assignment03.model.States;


@WebServlet(description = "servlet to get act as controller between form and models", urlPatterns = { "/customerServlet","/addCustomer","/addPet" })
public class CustomerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public CustomerServlet() {
        super();
    }


    private void processRequest(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        HttpSession session = request.getSession();


        Phone phone = new Phone();
        States state = new States();


        Collection<Phone> phones = phone.getPhoneCollection();
        Collection<States> states = state.getStateCollection();

            session.setAttribute("phones", phones);
            session.setAttribute("states", states);
            request.getRequestDispatcher("/customerManagement.jsp").forward(request, response);
        //}

    }







    private List<edu.witc.Assignment03.model.Customer> customers = new ArrayList<Customer>();



 private void addCustomer(HttpServletResponse response, HttpServletRequest request)//redirect to index
           throws IOException, ServletException {
    String url = "/customerManagement.jsp";
    processRequest(request, response);
        request.getRequestDispatcher(url).forward(request,response);
    }

 private void addPet(HttpServletResponse response, HttpServletRequest request)//redirect to index
         throws IOException, ServletException {
    String url = "/pets.jsp";
    request.getRequestDispatcher(url).forward(request,response);
  }




    private Customer getCustomer(int customerId) {
        for (Customer customer : customers) {
            if (customer.getCustomerId() == customerId) {
                return customer;
            }
        }
        return null;
    }



    private void sendEditCustomerForm(HttpServletRequest request, 
            HttpServletResponse response) throws IOException, ServletException {

        String url = "/customerManagement.jsp";
        request.setAttribute("customers", customers);
        request.getRequestDispatcher(url).forward(request,response);
    }



    public void doGet(HttpServletRequest request, 
            HttpServletResponse response)
            throws ServletException, IOException {
        String action = request.getParameter("action");
        if("addCustomer".equals(action)) { 
            addCustomer(response, request);
            }
        else if("addPet".equals(action)) { 
            addPet(response, request);
        }

    }


    public void doPost(HttpServletRequest request, 
            HttpServletResponse response)
            throws ServletException, IOException {
        // processRequest(request, response);

        // update customer
        int customerId = 0;
        try {
            customerId = 
                    Integer.parseInt(request.getParameter("id"));
        } catch (NumberFormatException e) {
        }
        Customer customer = getCustomer(customerId);
        if (customer != null) {
            customer.setFirstName(request.getParameter("firstName"));
            customer.setLastName(request.getParameter("lastName"));
            customer.setEmail(request.getParameter("email"));
            customer.setPhone(request.getParameter("phone"));
            customer.setAddress(request.getParameter("address"));
            customer.setCity(request.getParameter("city"));

            customer.setZip(request.getParameter("zip"));
        }

    }
}