//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 {
                          
        int num=0;  
        //声明JavaBean对象
        checkEnrol check=new checkEnrol();  
         //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");
                        return;                                       
                }
        
         if(num==0){
         doError(req,res,"注册课程失败!!");
         return;
         }
                          
                            }
就是跳转不到DisplayCourse.jsp这个页面!!!!!