jsp显示数据问题

页面代码
  <% 
  ArrayList carList=(ArrayList)request.getAttribute("carList");
  if(carList==null||carList.size()==0){
  %> <tr height="80"><td colspan="9" align="center">没有数据可显示</td></tr> 
  <% }
  //out.print("<tr><td align='center'>暂无数据可显示</td></tr>");
  else
  {  
  int i=0;
  while(i<carList.size())
  {
  JBCar carSingle=(JBCar)carList.get(i);
  %>
servlet 代码：
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String action=request.getParameter("action");
if(action==null)
action="";
/*查询车位信息*/
if(action.equals("single"))
this.ListCar(request,response);
}



  public void ListCar(HttpServletRequest request, HttpServletResponse response)throws IOException,
  ServletException
  { cardao car=new cardao();
  List carList=car.queryCar("all");
  request.setAttribute("carList", carList);
  request.getRequestDispatcher("/admin/carView.jsp").forward(request,response);
  }



  数据库中有数据 为什么carList总是执行为空。

断点 debug 看看不就知道了！