首页 > 资讯 > 后端开发 > Python >Java开发实例之图书管理系统的实现

203

分享到

Java开发实例之图书管理系统的实现

2024-04-02 19:04:59 203人浏览独家记忆

Python 官方文档：入门教程 => 点击学习

摘要

目录一、项目分布二、代码展示1.sql语句2.Book类3.User类4.用户分为两种4.1NORMalUser类4.2Admin类5.DBUtil类6.BookDao类7.User

一、项目分布

在这里插入图片描述

db.sql类：SQL语句，在控制台执行，用于建立图书管理库。
Book类：定义了书的一些属性（书编号，书名，作者，价格，分类，状态）并且写了属性的get、set、toSting方法。
User类：定义了用户的一些属性（用户编号，用户名，密码和是否为管理员的标记），并且写了属性的get、set、toSting方法。
NormalUser类：描述普通用户，实现普通用户的构造方法，并打印普通用户的操作界面。
Admin类：描述管理员，实现管理员的构造方法并打印管理员的操作界面。
DBUtil类：封装与数据库建立连接的操作(主要用JDBC来编写)。
BookDao类：封装对表的增删查改操作。
UserDao类：封装识别用户的操作。

在这里插入图片描述

operations包中定义了IOperation（操作）接口，这个包中的下面这些类都实现了这个IOperation接口（完成增删查改等操作）：

AddOperation类：增加书籍
BorrowOperation类：借阅书籍
DeleteOperation类:删除书籍
DisplayOperation类：查看书籍列表
ExitOperation类：退出系统
FindOperation类：查找书籍
ReturnOperation类：返还书籍

二、代码展示

1.SQL语句

在控制台执行，用于建立图书管理库（设置“张三”为普通用户，密码为321；“admin”为管理员，密码为123）


create database if not exists bookmanager;

use bookmanager;

-- 创建一个书籍表
drop table if exists book;
create table book(
    bookId int primary key auto_increment,
    name varchar(20),
    author varchar(20),
    -- 为了准确，价格以 分 为单位
    price int,
    type varchar(20),
    -- 如果isBorrowed为1，表示已经借出，为0，表示未借出
    isBorrowed int
);

-- 创建一个用户表（用户有管理员和普通用户两种）
drop table if exists user;
create table user(
    userId int primary key auto_increment,
    username varchar(20) unique,
    passWord varchar(20),
    -- 如果isAdmin 为1，表示是管理员，为0，表示是普通用户
    isAdmin int
);

-- 插入一些数据
insert into book values(null,'西游记','吴承恩',10000,'古典小说',0);
insert into book values(null, '三国演义', '罗贯中', 10000, '古典小说', 0);
insert into book values(null, '水浒传', '施耐庵', 10000, '古典小说', 0);
insert into book values(null, '金瓶梅', '兰陵笑笑生', 10000, '古典小说', 0);
insert into book values(null,'三体','刘慈欣',10000,'科幻小说',0);

-- 插入一些用户
insert into user values(null,'admin','123',1);
insert into user values(null,'张三','321',0);

2.Book类

定义了书的一些属性（书编号，书名，作者，价格，分类，状态）并且写了属性的get、set、toSting方法


package Java100_1006;

//使用这个类来表示一本书
// +------------+-------------+------+-----+---------+----------------+
// | Field      | Type        | Null | Key | Default | Extra          |
// +------------+-------------+------+-----+---------+----------------+
// | bookId     | int(11)     | NO   | PRI | NULL    | auto_increment |
// | name       | varchar(20) | YES  |     | NULL    |                |
// | author     | varchar(20) | YES  |     | NULL    |                |
// | price      | int(11)     | YES  |     | NULL    |                |
// | type       | varchar(20) | YES  |     | NULL    |                |
// | isBorrowed | int(11)     | YES  |     | NULL    |                |
// +------------+-------------+------+-----+---------+----------------+

public class Book {
    private int bookId;
    private String name;
    private String author;
    private int price;
    private String type;
    private boolean isBorrowed;

    public int getBookId() {
        return bookId;
    }

    public void setBookId(int bookId) {
        this.bookId = bookId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public boolean isBorrowed() {
        return isBorrowed;
    }

    public void setBorrowed(boolean borrowed) {
        isBorrowed = borrowed;
    }

    @Override
    public String toString() {
        return "Book{" +
                "bookId=" + bookId +
                ", name='" + name + '\'' +
                ", author='" + author + '\'' +
                ", price=" + price +
                ", type='" + type + '\'' +
                ", isBorrowed=" + isBorrowed +
                '}';
    }
}

3.User类

定义了用户的一些属性（用户编号，用户名，密码和是否为管理员的标记），并且写了属性的get、set、toSting方法


package Java100_1006;

import Java100_1006.operations.IOperation;

abstract public class User {
    // 使用这个 User 类来表示一个用户
// +----------+-------------+------+-----+---------+----------------+
// | Field    | Type        | Null | Key | Default | Extra          |
// +----------+-------------+------+-----+---------+----------------+
// | userId   | int(11)     | NO   | PRI | NULL    | auto_increment |
// | username | varchar(20) | YES  |     | NULL    |                |
// | password | varchar(20) | YES  |     | NULL    |                |
// | isAdmin  | int(11)     | YES  |     | NULL    |                |
// +----------+-------------+------+-----+---------+----------------+
    // 针对此处的 isAdmin, 并不是直接使用一个 boolean 变量来区分, 而是使用两个不同的子类.
    // 原因是, 管理员和普通用户支持的方法, 是不一样的.
    private int userId;
    private String userName;
    private String passWord;

    // 包含了一个数组, 数组里面就是该用户支持的哪些操作.
    // 针对普通用户, 和管理员, 分别设置不同的操作.
    IOperation[] operations;

    //普通用户和管理员看到的菜单也不同
    abstract public int menu();

    public int getUserId() {
        return userId;
    }

    public void setUserId(int userId) {
        this.userId = userId;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getPassWord() {
        return passWord;
    }

    public void setPassWord(String passWord) {
        this.passWord = passWord;
    }

    @Override
    public String toString() {
        return "User{" +
                "userId=" + userId +
                ", userName='" + userName + '\'' +
                ", passWord='" + passWord + '\'' +
                '}';
    }
}

4.用户分为两种

普通用户和管理员，所以应该分别建立两个类来描述这两个用户。

4.1NormalUser类

**描述普通用户，实现普通用户的构造方法，并打印普通用户的操作界面。


package Java100_1006;

import Java100_1006.operations.*;

import java.util.Scanner;

//普通用户
public class NormalUser extends User{
    //普通用户的构造方法
    public NormalUser(){
        this.operations = new IOperation[]{
            	//0.退出
                new ExitOperation(),
            	//1.查看书籍列表
                new DisplayOperation(),
            	//2. 查找指定书籍
                new FindOperation(),
            	//3.借阅书籍
                new BorrowOperation(),
            	//4. 归还书籍
                new ReturnOperation()
        };
    }

    //打印普通用户的菜单
    @Override
    public int menu() {
        System.out.println("=============================");
        System.out.println("欢迎您"+this.getUserName()+"!");
        System.out.println("1. 查看书籍列表");
        System.out.println("2. 查找指定书籍");
        System.out.println("3. 借阅书籍");
        System.out.println("4. 归还书籍");
        System.out.println("0. 退出系统");
        System.out.println("=============================");
        System.out.println("请输入您的选项：");
        Scanner scanner = new Scanner(System.in);
        int choice = scanner.nextInt();
        return choice;
    }
}

4.2Admin类

描述管理员，实现管理员的构造方法并打印管理员的操作界面


package Java100_1006;

import Java100_1006.operations.*;

import java.util.Scanner;

//管理员
public class Admin extends User{
    //管理员的构造方法
    public Admin(){
        //初始化operation
        //继承与User类，所以可以使用父类的中的数组
        this.operations = new IOperation[]{
            	//0. 退出系统
                new ExitOperation(),
           	 	//1. 查看书籍列表
                new DisplayOperation(),
            	//2. 查找指定书籍
                new FindOperation(),
            	//3. 新增书籍
                new AddOperation(),
            	//4. 删除书籍
                new DeleteOperation()
        };
    }
    //打印管理员的菜单
    @Override
    public int menu() {
        System.out.println("================================");
        System.out.println("欢迎您, " + this.getUserName() + "! 您是管理员! ");
        System.out.println("1. 查看书籍列表");
        System.out.println("2. 查找指定书籍");
        System.out.println("3. 新增书籍");
        System.out.println("4. 删除书籍");
        System.out.println("0. 退出系统");
        System.out.println("================================");
        System.out.println("请输入选项: ");
        Scanner scanner = new Scanner(System.in);
        int choice = scanner.nextInt();
        return choice;
    }
}

5.DBUtil类

封装与数据库建立连接的操作(主要用JDBC来编写)


package Java100_1006;

import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

//先建立这个类来 封装 数据库建立连接的操作
//具体步骤：
//1.创建DataSouse实例
//2.根据DataSouse创建对应的Connection
public class DBUtil {
    private static final String URL = "jdbc:mysql://127.0.0.1:3306/bookmanager?characterEncoding=utf8&useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "1234";

    //懒汉模式：
    private static DataSource dataSource = null;

    //初始情况下，dataSource为null，没有被实例化
    //首次调用getDataSource()时才会被实例化
    private static DataSource getDataSource(){
        if(dataSource == null){
            dataSource = new MysqlDataSource();
            ((MysqlDataSource)dataSource).setUrl(URL);
            ((MysqlDataSource)dataSource).setUser(USERNAME);
            ((MysqlDataSource)dataSource).setPassword(PASSWORD);
        }
        return dataSource;
    }

    //提供一个方法来建立连接
    //用static来修饰，那么就无需实例化DBUtil实例就能调用这个方法，更加方便
    public static Connection getConnection() throws SQLException {
        return getDataSource().getConnection();
    }

    //释放资源
    public static void close(Connection connection, PreparedStatement statement, ResultSet resultSet){
        try{
            if(resultSet != null){
                resultSet.close();
            }
            if(statement != null){
                statement.close();
            }
            if(connection != null){
                connection.close();
            }
        }catch (SQLException e){
            e.printStackTrace();
        }

    }
}

6.BookDao类

封装对表的增删查改操作


package Java100_1006;

import Java100_1006.Book;
import Java100_1006.DBUtil;

import java.sql.*;
import java.util.ArrayList;
import java.util.List;

//封装 对Book表的增删查改操作
public class BookDao {

    //1.新增书籍（插入）
    public boolean add(Book book){
        //把book对象插入数据库的书籍表中
        Connection connection = null;
        PreparedStatement statement = null;
        try {
            //1.和数据库建立连接
            connection = DBUtil.getConnection();

            //2.拼装sql语句
            String sql = "insert into book values(null,?,?,?,?,?)";
            statement = connection.prepareStatement(sql);
            statement.setString(1,book.getName());
            statement.setString(2,book.getAuthor());
            statement.setInt(3,book.getPrice());
            statement.setString(4,book.getType());
            statement.setInt(5,book.isBorrowed()?1:0);

            //3.执行SQL
            int ret = statement.executeUpdate();
            if(ret == 1)
                return true;
            return false;

        }catch (SQLException e){
            e.printStackTrace();
        }finally {
            DBUtil.close(connection,statement,null);
        }
        return false;
    }
    //2.查找所有书籍（查找）
    public List<Book> selectAll() {
        List<Book> list = new ArrayList<>();
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;

        try {
            //1.与数据库建立连接
            connection = DBUtil.getConnection();
            //2.拼装SQL
            String sql = "select * from book";
            statement = connection.prepareStatement(sql);
            //3.执行SQL
            resultSet = statement.executeQuery();
            //4.遍历结果集合
            while(resultSet.next()){
                //取出当前行的这些列，构造一个Book对象来
                Book book = new Book();
                book.setBookId(resultSet.getInt("bookId"));
                book.setName(resultSet.getString("name"));
                book.setAuthor(resultSet.getString("author"));
                book.setPrice(resultSet.getInt("price"));
                book.setType(resultSet.getString("type"));
                book.setBorrowed(resultSet.getInt("isBorrowed")==1);
                //把这个book对象添加到链表中
                list.add(book);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            DBUtil.close(connection,statement,resultSet);
        }
        return list;
    }
    //3.根据名字查找书籍（查找）
    public List<Book> selectByName(String name){
        List<Book> list = new ArrayList<>();
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;

        try {
            //1.先与数据库建立连接
            connection = DBUtil.getConnection();
            //2.拼装SQL语句
            String sql = "select * from book where name=?";
            statement = connection.prepareStatement(sql);
            statement.setString(1,name);
            //3.执行SQL
            resultSet = statement.executeQuery();
            //4.遍历结果集合
            while(resultSet.next()){
                Book book = new Book();
                book.setBookId(resultSet.getInt("bookId"));
                book.setName(resultSet.getString("name"));
                book.setAuthor(resultSet.getString("author"));
                book.setPrice(resultSet.getInt("price"));
                book.setType(resultSet.getString("type"));
                book.setBorrowed(resultSet.getInt("isBorrowed") == 1);
                list.add(book);
            }

        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            DBUtil.close(connection,statement,resultSet);
        }
        return list;
    }
    //4.删除书籍（删除）
    public boolean delete(int bookId){
     Connection connection = null;
     PreparedStatement statement = null;
        try {
            //1.建立连接
            connection = DBUtil.getConnection();
            //2.拼装SQL
            String sql = "delete from book where bookId=?";
            statement = connection.prepareStatement(sql);
            statement.setInt(1,bookId);
            //3.执行SQL
            int ret = statement.executeUpdate();
            if(ret != 1){
                return  false;
            }
            return true;
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            DBUtil.close(connection,statement,null);
        }
        return false;
    }
    //5.借书（修改）
    public boolean borrowBook(int bookId){

        Connection connection = null;
        //statement1用于查找操作
        PreparedStatement statement1 = null;
        //statement2用于修改操作
        PreparedStatement statement2 = null;
        ResultSet resultSet = null;
        try {
            //1.建立连接
            connection = DBUtil.getConnection();
            //要借书，就必须知道这本书存不存在，所以我们先查找，再操作
            //2.拼装SQL
            String sql = "select * from book where bookId=?";
            statement1 = connection.prepareStatement(sql);
            statement1.setInt(1,bookId);
            //3.执行SQL
            resultSet = statement1.executeQuery();
            //4.遍历结果，预期结果要么是1条，要么是0条
            if(resultSet.next()){
                //进入if就说明书是存在的
                boolean isBorrowed = (resultSet.getInt("isBorrowed")==1);
                if(isBorrowed) {
                    //说明书已经被借出了，不能再次借阅
                    System.out.println("书已经借出! 无法再次借出! bookId = " + bookId);
                    return false;
                }
            }else{
                //进入else，那么书是不存在的
                System.out.println("书不存在! 无法借出! bookId = " + bookId);
                return false;
            }
            //[接下来是开始进行借书逻辑]
            //5.拼装SQL
            sql = "update book set isBorrowed=1 where bookId=?";
            statement2 = connection.prepareStatement(sql);
            statement2.setInt(1,bookId);

            //6.执行SQL
            int ret = statement2.executeUpdate();
            if(ret != 1){
                return false;
            }
            return true;

        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            if(resultSet != null){
                try {
                    resultSet.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if(statement2 != null){
                try {
                    statement2.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if(statement1 != null){
                try {
                    statement1.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if (connection != null){
                try {
                    connection.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
        return false;
    }
    //6.还书（修改）
    public boolean returnBook(int bookId){
        Connection connection = null;
        PreparedStatement statement1 = null;
        PreparedStatement statement2 = null;
        ResultSet resultSet = null;
        try {
            connection = DBUtil.getConnection();
            //查询书是否存在
            String sql = "select * from book where bookId=?";
            statement1 = connection.prepareStatement(sql);
            statement1.setInt(1,bookId);
            resultSet = statement1.executeQuery();
            if(resultSet.next()){
                //进入if语句就说明这本书存在于书籍名单中
                boolean isBorrowed = (resultSet.getInt("isBorrowed")==1);
                if(!isBorrowed){
                    System.out.println("书没有借出, 没必要还! bookId = " + bookId);
                    return false;
                }
            }else{
                //不是图书系统中的书，不需要还
                System.out.println("书不存在, 无法归还! bookId = " + bookId);
                return false;
            }
            //[还书]
            //修改书籍的借出状态就可
            sql = "update book set isBorrowed=0 where bookId=?";
            statement2 = connection.prepareStatement(sql);
            statement2.setInt(1,bookId);
            int ret = statement2.executeUpdate();
            if(ret != 1){
                return false;
            }
            return true;
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            if(resultSet != null){
                try {
                    resultSet.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if(statement2 != null){
                try {
                    statement2.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if(statement1 != null){
                try {
                    statement1.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            if (connection != null){
                try {
                    connection.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
        return false;
    }
}

7.UserDao类

封装识别用户的操作


package Java100_1006;


import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class UserDao {
    //实现根据用户名找密码这样一个过程（用户名 唯一）
    public User selectByName(String name){
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try {
            connection = DBUtil.getConnection();
            String sql = "select * from user where username=?";
            statement = connection.prepareStatement(sql);
            statement.setString(1,name);
            resultSet = statement.executeQuery();
            if(resultSet.next()){
                //存在这个用户，返回一个User对象
                //判断这个用户是普通用户还是管理员
                boolean isAdmin = (resultSet.getInt("isAdmin"))==1;
                User user = null;
                if (isAdmin){
                    user = new Admin();
                }else{
                    user = new NormalUser();
                }
                user.setUserId(resultSet.getInt("userId"));
                user.setUserName(resultSet.getString("username"));
                user.setPassWord(resultSet.getString("password"));
                return user;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            DBUtil.close(connection,statement,resultSet);
        }
        return null;
    }
}

8.operations包

创建IOperation接口和实现这个接口的其他类

接口：


package Java100_1006.operations;

//后续的增加 删除  借书 还书 等操作都要实现这个接口
public interface IOperation {
    void work();
}

8.1AddOperation类：增加书籍


package Java100_1006.operations;

import Java100_1006.Book;
import Java100_1006.BookDao;

import java.util.Scanner;

public class AddOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("新增书籍！");
        Scanner scanner  = new Scanner(System.in);
        System.out.println("请输入书名: ");
        String bookName = scanner.next();
        System.out.println("请输入作者: ");
        String author = scanner.next();
        System.out.println("请输入价格:");
        int price = scanner.nextInt();
        System.out.println("请输入类别:");
        String type = scanner.next();
        //假设新增书籍都是未借出的状态
        Book book = new Book();
        book.setName(bookName);
        book.setAuthor(author);
        book.setPrice(price);
        book.setType(type);
        BookDao bookDao = new BookDao();
        boolean ret = bookDao.add(book);
        if(ret){
            System.out.println("新增书籍成功");
        }else{
            System.out.println("新增书籍失败!");
        }
    }
}

8.2BorrowOperation类：借阅书籍


package Java100_1006.operations;

import Java100_1006.BookDao;

import java.util.Scanner;

public class BorrowOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("借阅书籍!");
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入要借阅的书籍id:");
        int bookId = scanner.nextInt();
        BookDao bookDao = new BookDao();
        boolean ret = bookDao.borrowBook(bookId);
        if (ret) {
            System.out.println("借阅书籍成功!");
        } else {
            System.out.println("借阅书籍失败!");
        }
    }
}

8.3DeleteOperation类:删除书籍


package Java100_1006.operations;

import Java100_1006.BookDao;

import java.util.Scanner;

public class DeleteOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("删除书籍!");
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入要删除的书籍id: ");
        int bookId = scanner.nextInt();

        BookDao bookDao = new BookDao();
        boolean ret = bookDao.delete(bookId);
        if (ret) {
            System.out.println("删除成功!");
        } else {
            System.out.println("删除失败!");
        }
    }
}

8.4DisplayOperation类：查看书籍列表


package Java100_1006.operations;

import Java100_1006.Book;
import Java100_1006.BookDao;

import java.util.List;

public class DisplayOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("展示所有书籍！");
        BookDao bookDao = new BookDao();
        List<Book> books = bookDao.selectAll();
        for(Book book:books){
            System.out.println(book);
        }
        System.out.println("展示所有书籍完毕！");
    }
}

8.5ExitOperation类：退出系统


package Java100_1006.operations;

//退出操作
public class ExitOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("退出程序");
        System.exit(0);
    }
}

8.6FindOperation类：查找书籍


package Java100_1006.operations;

import Java100_1006.Book;
import Java100_1006.BookDao;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class FindOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("根据名字查找书籍!");
        System.out.println("请输入要查找的书名: ");
        Scanner scanner = new Scanner(System.in);
        String bookName = scanner.next();
        BookDao bookDao = new BookDao();
        List<Book> books = bookDao.selectByName(bookName);
        for(Book book:books){
            System.out.println(book);
        }
        System.out.println("根据名字查找书籍完毕!");
    }
}

8.7ReturnOperation类：返还书籍


package Java100_1006.operations;

import Java100_1006.BookDao;

import java.util.Scanner;

public class ReturnOperation implements IOperation{
    @Override
    public void work() {
        System.out.println("归还书籍!");
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入要归还的书籍id:");
        int bookId = scanner.nextInt();
        BookDao bookDao = new BookDao();
        boolean ret = bookDao.returnBook(bookId);
        if (ret) {
            System.out.println("归还书籍成功!");
        } else {
            System.out.println("归还书籍失败!");
        }
    }
}

9.Main方法实现


package Java100_1006;

import java.util.Scanner;

//编写系统的主体逻辑
public class Main {
    public static void main(String[] args) {
        //通过login方法进行登录
        User user = login();
        while(true){
            int choice = user.menu();
            user.doOperation(choice);
        }
    }

    private static User login() {
        //1.实现用户登录逻辑
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入用户名：");
        String userName = scanner.next();
        System.out.println("请输入密码：");
        String passWord = scanner.next();
        //2.从数据库根据用户名查询密码
        UserDao userDao = new UserDao();
        User user = userDao.selectByName(userName);
        if(user == null){
            //不存在该用户
            System.out.println("登陆失败!");
            // 直接退出程序
            System.exit(0);
        }
        if(!user.getPassWord().equals(passWord)){
            //密码不正确
            System.out.println("登陆失败");
            System.exit(0);
        }
        return user;
    }
}