为什么在serlvet层会提示queryMessageList(String,String)这个方法不存在呢？

//service层代码

package com.imooc.service;

import java.util.List;

import com.imooc.bean.Message;

import com.imooc.dao.MessageDao;

/*

 * 列表相关的业务功能,因为一般不直接调dao层的，都是要经过service来调用

 */

public class ListService {

public List<Message> queryMessageList(String command,String description){

MessageDao messageDao=new MessageDao();

return messageDao.queryMessageList(command, description);

}

}

//servlet层代码

package com.imooc.servlet;

import java.io.IOException;

import java.sql.SQLException;

import java.util.ArrayList;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.imooc.bean.Message;

import com.imooc.service.ListService;

/**

 *列表页面的初始化 

 *

 */

@SuppressWarnings("serial")

public class ListServlet extends HttpServlet {

@Override

protected void doGet(HttpServletRequest req, HttpServletResponse resp)

throws ServletException, IOException {

//设置编码

req.setCharacterEncoding("UTF-8");

//接受页面的值

String command=req.getParameter("command");

String description=req.getParameter("description");

//向页面传值

req.setAttribute("command", command);

req.setAttribute("description", description);

ListService listService=new ListService();

List<Message> messageList=new ArrayList<Message>();

messageList=listService.queryMessageList(command, description);

     req.setAttribute("messageList", messageList);

//在上面用jdbc把数据库取出来之后放在messageList里面，然后就可以转发了，转发到由req带到前台去使用请求转发，就是servlet那里传过来的请求会直接转给这个页面来处理

req.getRequestDispatcher("/WEB-INF/jsp/background/list.jsp").forward(req, resp);

}

@Override

protected void doPost(HttpServletRequest req, HttpServletResponse resp)

throws ServletException, IOException {

this.doGet(req, resp);

}

}