老规矩，先看前端页面，把前端页面上没有的表达式都删掉：

 创建servlet接受和处理请求：

package com.atm.servlet;

import com.atm.pojo.User;
import com.atm.service.UserService;
import com.atm.service.impl.UserServiceImpl;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

//查询用户余额
@WebServlet("/UserSerch")
public class UserSerchServlet extends HttpServlet {

    private UserService userService=new UserServiceImpl();
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request,response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String icno=null;
        if(request.getSession().getAttribute("ua")!=null){
            User user=(User)(request.getSession().getAttribute("ua"));
            icno=user.getIcno();
        }
        try {
            double balance=userService.getBalance(icno);
//            request.setAttribute("balance",balance);
//            request.getRequestDispatcher("/changepage/SerchBalance.jsp").forward(request,response);
            request.getSession().setAttribute("balance",balance);
            response.sendRedirect("/changepage/SerchBalance.jsp");
        }catch (Exception e){
            e.printStackTrace();
        }

    }
}

接口：

    //查询用户余额
    public double getBalance(String icno)throws Exception;

实现类：

    //查询用户余额
    public double getBalance(String icno)throws Exception{
        return userDao.getBalance(icno);
    }

数据层：

    //查询用户余额
    public double getBalance(String icno) throws Exception{
        Connection connection= DBUtils.getConnection();
        PreparedStatement preparedStatement = connection.prepareStatement("select balance from user where icno=?");
        preparedStatement.setString(1,icno);
        ResultSet resultSet= preparedStatement.executeQuery();
        double balance=0;
        if(resultSet.next()){
            balance=resultSet.getDouble(1);
        }
        DBUtils.release(connection,preparedStatement,null);
        return balance;
    }

测试一下就发现余额查询成功啦~