哪位大虾告诉我这个怎么解决啊：Cannot forward after response has been committed

我是个JSP的初学者，今天碰到个问题非常头疼。。页面不能跳转，数据提交到数据库正常，出个这样的错：java.lang.IllegalStateException: Cannot forward after response has been committed
它的意思好象是不能在响应提交之后跳转怎么都搞不好，望各位高手帮忙出出主义代码如下：package bookshop;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.sql.*;
public class submitServlet extends HttpServlet {
    private static final String CONTENT_TYPE = "text/html; charset=GBK";
    //Initialize global variables
    public void init() throws ServletException {
    }
    //Process the HTTP Get request
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws
            ServletException, IOException {            
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws
            ServletException, IOException {
        doGet(request, response);
        response.setContentType(CONTENT_TYPE);
        PrintWriter out = response.getWriter();        
        request.setCharacterEncoding("GBK");
        response.setCharacterEncoding("GBK");
        HttpSession session1 = request.getSession(false);
        orderBean order = (orderBean) session1.getAttribute("order");
        String LoginName = (String) session1.getAttribute("LoginName");
        Integer number1;
        String pay1 = request.getParameter("pay");
        String carry1 = request.getParameter("carry");
        String bz1 = request.getParameter("bz");
        order.setPay(pay1);
        order.setCarry(carry1);
        order.setBz(bz1);
        String username = order.getUserName();
        System.out.println(username);
        String address = order.getAddress();
        String postcode = order.getPostCode();
        System.out.println(postcode);
        String usertel = order.getUserTel();
        String mobiletel = order.getMobileTel();
        String email = order.getEmail();
        String pay = order.getPay();
        String carry = order.getCarry();
        String bz = order.getBz();
        System.out.println(bz);
        try {
            DBBean dbconn = new DBBean();
            String sql1 = "insert into orderinfo (LoginName,UserName,Address,PostCode,UserTel,MobileTel,Email,Pay,Carry,Bz) values ('" +
                          LoginName + "','" + username + "','" + address +
                          "','" + postcode + "','" + usertel + "','" +
                          mobiletel + "','" + email + "','" + pay + "','" +
                          carry + "','" + bz + "')";
            String sql2 =
                    "select top 1 orderid from orderinfo where loginname='" +
                    LoginName + "' order by orderdate desc";
            if (dbconn.execute(sql1)) {
                ResultSet rs = dbconn.executeQuery(sql2);
                rs.next();
                long orderid = rs.getLong("orderid");
                dbconn.close();
                HashMap itemMap = (HashMap) session1.getAttribute("itemMap");
                Iterator it = itemMap.keySet().iterator();
                while (it.hasNext()) {
                    String bookID = (String) it.next();
                    DBBean dbconn1 = new DBBean();
                    ResultSet rs2 = dbconn1.executeQuery(
                            "select * from bookInfo where BookID='" +
                            bookID + "'");
                    if (rs2.next()) {
                        float memberPrice = rs2.getFloat("MemberPrice");
                        number1 = (Integer) itemMap.get(bookID);
                        int number = number1.intValue();
                        String sql4 =
                                "insert into order_ditail (OrderID,BookID,Price,Number) values (" +
                                orderid + ",'" + bookID + "'," + memberPrice +
                                "," + number + ")";
                        dbconn1.close();
                        DBBean dbconn2 = new DBBean();
                        if (dbconn2.execute(sql4)) {
                            session1.removeAttribute("itemMap");
                            javax.servlet.RequestDispatcher sumbit = request.
                                    getRequestDispatcher("/sesses.jsp");
                            sumbit.forward(request, response);
                        } else {
                            javax.servlet.RequestDispatcher sumbit = request.
                                    getRequestDispatcher("/unsess.jsp");
                            sumbit.forward(request, response);
                        }
                    } else {
                        javax.servlet.RequestDispatcher sumbit = request.
                                getRequestDispatcher("/unsess.jsp");
                        sumbit.forward(request, response);
                    }
                }
            } else {
                javax.servlet.RequestDispatcher sumbit = request.
                        getRequestDispatcher("/unsess.jsp");
                sumbit.forward(request, response);
            }
        } catch (SQLException ex) {
            ex.printStackTrace();
            javax.servlet.RequestDispatcher sumbit = request.
                    getRequestDispatcher("/unsess.jsp");
            sumbit.forward(request, response);
        } catch (ClassNotFoundException ex) {
            ex.printStackTrace();
            javax.servlet.RequestDispatcher sumbit = request.
                    getRequestDispatcher("/unsess.jsp");
            sumbit.forward(request, response);
        }
    }
    //Clean up resources
    public void destroy() {
    }
}