為什么在serlvet層會提示queryMessageList(String,String)這個方法不存在呢？

//service層代碼

package com.imooc.service;

import java.util.List;

import com.imooc.bean.Message;

import com.imooc.dao.MessageDao;

/*

?* 列表相關的業務功能,因為一般不直接調dao層的，都是要經過service來調用

?*/

public class ListService {

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

MessageDao messageDao=new MessageDao();

return messageDao.queryMessageList(command, description);

}

}

//servlet層代碼

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;

/**

?*列表頁面的初始化?

?*

?*/

@SuppressWarnings("serial")

public class ListServlet extends HttpServlet {

@Override

protected void doGet(HttpServletRequest req, HttpServletResponse resp)

throws ServletException, IOException {

//設置編碼

req.setCharacterEncoding("UTF-8");

//接受頁面的值

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

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

//向頁面傳值

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把數據庫取出來之后放在messageList里面，然后就可以轉發了，轉發到由req帶到前臺去使用請求轉發，就是servlet那里傳過來的請求會直接轉給這個頁面來處理

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);

}

}