求助 Cannot forward after response has been committed 给个正确的修改方法

错误这样  
 HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception java.lang.IllegalStateException: Cannot forward after response has been committed
StudentLoginSvlt.doError(StudentLoginSvlt.java:107)
StudentLoginSvlt.doEnrol(StudentLoginSvlt.java:69)
StudentLoginSvlt.doGet(StudentLoginSvlt.java:47)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)jsp中这样
 <td><a href="StudentLoginSvlt?action=enrol&id=<%=id%>&cour_id=<%=cour_id%>&class_id=<%=class_id%>&prepare=<%=prepare%>  ">注册</a> studentologinsvlt.java代码
import java.io.*;
import java.sql.ResultSet;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
import com.sqlBean;
import com.checkEnrol;
public class StudentLoginSvlt extends HttpServlet{

public void doGet(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {
    
    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;
    String  action = req.getParameter("action");    ResultSet rs=null;       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("/test/student.jsp");
       }

   
    if ("check".equalsIgnoreCase(action)) {
     rs=getScore(stu_id)   ;
     sendResultSet(req,res,rs,"/check.jsp");
   }
    
    if("enrol".equalsIgnoreCase(action)){
    
    doEnrol(req,res,stu_id,cour_id,class_id,prepare);
    //res.sendRedirect("/test/DisplayCourse.jsp");
}    }
    
    
    public void doEnrol(HttpServletRequest req, HttpServletResponse res,String stu_id,String cour_id,String class_id,String prepare)  throws ServletException, IOException {
                          
        int num=0;                     
        checkEnrol check=new checkEnrol();  
              
        if(prepare.equals("0")  ) 
         {  num= check.enrol(class_id,stu_id);  }
        else   { 
             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！！");
         }
                          
                            }
   
    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");
     }
    
    
                             
  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;
                            }  
    
   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);
 
  }                       
                                                                           
  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;
 
  }
  
      public void doPost(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {    doGet(req, res);  } 
                         
    
     }