Cannot forward after response has been committed(着急解决）

Question

package com.course;
import java.io.*;
import java.sql.ResultSet;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;public class StudentLoginSvlt extends HttpServlet{
 //响应get请求
public void doGet(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {
    //接收提交的跟选课相关的各种信息
    String stu_id =req.getParameter("id");
    String cour_id=req.getParameter("cour_id");
    String class_id=req.getParameter("class_id");
    String prepare=req.getParameter("prepare");
    String pw1=null;
    String pw2=null;
    String e_mail=null;
    String tel=null;
    //获得请求中的参数
    String  action = req.getParameter("action");    ResultSet rs=null;      
    //如果请求中是更新参数
if ("update".equalsIgnoreCase(action)) {  stu_id =req.getParameter("id");
     pw1=req.getParameter("password1");
     pw2=req.getParameter("password2");
     if(pw1.equals("") || pw2.equals("") || pw1==null || pw2==null)
     doError(req,res,"密码不能为空！");
     e_mail=req.getParameter("e_mail");
     tel=req.getParameter("tel");
     doUpdate(req,res,pw1,pw2,e_mail,tel,stu_id);
     res.sendRedirect("student.jsp");
       }

//如果请求中是更改成绩参数    
    if ("check".equalsIgnoreCase(action)) {
     rs=getScore(stu_id)   ;
     sendResultSet(req,res,rs,"/check.jsp");
   }
    //如果参数中是注册课程参数
    if("enrol".equalsIgnoreCase(action)){
    
    doEnrol(req,res,stu_id,cour_id,class_id,prepare);
    res.sendRedirect("DisplayCourse.jsp");}    }
    
    //注册课程课方法
    public void doEnrol(HttpServletRequest req, HttpServletResponse res,
    String stu_id,String cour_id,String class_id,String prepare)
                           
                           throws ServletException, IOException {
     //System.out.println("test doEnrol");                 
        int num=0;  
        //声明JavaBean对象
        checkEnrol check=new checkEnrol();
        //System.out.println(check);
         //如果预修课学分为0，则注册     
        if(prepare.equals("0")  ) 
         {  num= check.enrol(class_id,stu_id);  }
        else   {
              //判断预修课是否符合要求   
             if( check.hasPassPrepare(prepare))                              
               {num= check.enrol(class_id,stu_id);}
                else doError(req,res,"清先完成预修课");                                                        
                }
        
         if(num==0){
         doError(req,res,"注册课程失败！！");
         }
    }                      
                                //修改学生信息
    public void doUpdate(HttpServletRequest req, HttpServletResponse res,
    String pw1,String pw2,String e_mail,String tel,String id)
    throws ServletException, IOException{
     int num=0;
     if(!pw1.equals(pw2))
     doError(req,res,"密码不一致，请重输！");
     checkEnrol check=new checkEnrol(); 
     sqlBean db = new sqlBean();
        num= check.updatestu(pw1,id,e_mail,tel);
       if(num==0) doError(req,res,"更新失败");
     }
    
    
  //获得课程成绩                           
  public ResultSet getScore(String stu_id){
     String sql="select enrol.score , course.name ,course. "+
           "from enrol ,course ,classes "+
     "where stu_id='"+stu_id+"' "+
     "and enrol.class_id=classes.id "+
 "and classes.cour_id=course.id ";
sqlBean db = new sqlBean();
ResultSet rs= db.executeQuery(sql);  
return rs;
                            }  
     //错误处理方法
      public void doError(HttpServletRequest req,
                      HttpServletResponse res,
                      String str)
                      throws ServletException, IOException {
    req.setAttribute("problem", str);
    RequestDispatcher rd = getServletContext().getRequestDispatcher("/errorpage.jsp");
    rd.forward(req, res);
    return;
  }                        
  //将查询后的结果集存储在session中                                                                        
  public void sendResultSet(HttpServletRequest req, HttpServletResponse res,
                      java.sql.ResultSet rs, String target)
                       throws ServletException, IOException {              
    req.setAttribute("rs", rs);
    RequestDispatcher rd = getServletContext().getRequestDispatcher(target);
    rd.forward(req, res);
    return;
  }
//响应post请求
      public void doPost(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {       doGet(req, res);  } 
                         
    
     }