一、创建数据库表

CREATE DATABASE jdbcdatabase;


CREATE TABLE IF NOT EXISTS `user`(
`uid` INT PRIMARY KEY AUTO_INCREMENT,
`username` VARCHAR(10) UNIQUE NOT NULL,
`password` VARCHAR(10) NOT NULL,
`phone` VARCHAR(11) 
);

INSERT INTO `user`(`uid`,`username`,`password`,`phone`) VALUES(1001,'zhangsan','123456','13112345678');

INSERT INTO `user`(`uid`,`username`,`password`,`phone`) VALUES(1002,'lisi','123456','13844445555');

二、创建Users类

public class Users {
    // 存放从数据库取出的字段
    private int uid;
    private String username;
    private String password;
    private String phone;


    public Users() {
    }

    public Users(int uid, String username, String password, String phone) {
        this.uid = uid;
        this.username = username;
        this.password = password;
        this.phone = phone;
    }

    public Users(String username,String password){
        this.username = username;
        this.password = password;
    }

    /**
     * 获取
     * @return uid
     */
    public int getUid() {
        return uid;
    }

    /**
     * 设置
     * @param uid
     */
    public void setUid(int uid) {
        this.uid = uid;
    }

    /**
     * 获取
     * @return username
     */
    public String getUsername() {
        return username;
    }

    /**
     * 设置
     * @param username
     */
    public void setUsername(String username) {
        this.username = username;
    }

    /**
     * 获取
     * @return password
     */
    public String getPassword() {
        return password;
    }

    /**
     * 设置
     * @param password
     */
    public void setPassword(String password) {
        this.password = password;
    }

    /**
     * 获取
     * @return phone
     */
    public String getPhone() {
        return phone;
    }

    /**
     * 设置
     * @param phone
     */
    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String toString() {
        return "Users{uid = " + uid + ", username = " + username + ", password = " + password + ", phone = " + phone + "}";
    }
}

三、登录类

(1)代码

细节，流程都在注释里。

import java.sql.*;
import java.util.Scanner;

public class Login {
    public static void main(String[] args) {
        // 1.加载驱动
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            throw new RuntimeException(e);
        }

        // 2.准备所需数据
        String url = "jdbc:mysql://127.0.0.1:3306/jdbcdatabase";
        String user = "root";
        String password = "123456";
        // 2.1 提示用户输入
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入账号：");
        String un = scanner.nextLine();
        System.out.println("请输入密码：");
        String pw = scanner.nextLine();
        // 2.2 创建Users对象接受数据
        Users users = new Users(un, pw);
        // 2.3写入SQL语句
        String sql = "select password from user where username = '" + users.getUsername() + "'";

        try (
                // 3.获取连接对象
                Connection connection = DriverManager.getConnection(url, user, password);
                // 4.获取发送对象
                Statement statement = connection.createStatement()) {
            // 5.执行SQL
            ResultSet resultSet = statement.executeQuery(sql);
            // 6.判断是否存在该账号
            if (resultSet.next()){
                if (resultSet.getString(1).equals(pw)){
                    System.out.println("登录成功！");
                }else{
                    System.out.println("密码错误！");
                }
            }else {
                System.out.println("该账号不存在，请注册！");
            }
        } catch (SQLException e) {
            throw new RuntimeException(e);
        }

    }
}

（2）运行结果图

四、存在的漏洞

（1）SQL注入

即，当输入账号时，特意将SQL关键字输入，找到其漏洞，进行非法登录，如

当账号为任意字母' or 1=1;# ，密码为正确时，返回登录成功。

（2）分析

上述代码中的SQL语句为：

        "select password from user where username = '" + users.getUsername() + "'";

当输入 f' or 1=1;#时，我们做一个替换，得到

        "select password from user where username = 'f' or 1=1;#'";

解读为，从表user中查询password，过滤条件是当username为'f'，或着 1=1，

紧接着分号结束，#后面为注释则不执行，而1=1永远成立，所以查询结果返回所有密码

当输入的密码正确时，就登录成功了。

（3）补救

使用prepareStatement()方法改写代码

import java.sql.*;
import java.util.Scanner;

public class Login {
    public static void main(String[] args) {
        // 1.加载驱动
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            throw new RuntimeException(e);
        }

        // 2.准备所需数据
        String url = "jdbc:mysql://127.0.0.1:3306/jdbcdatabase";
        String user = "root";
        String password = "123456";
        // 2.1 提示用户输入
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入账号：");
        String un = scanner.nextLine();
        System.out.println("请输入密码：");
        String pw = scanner.nextLine();
        // 2.2 创建Users对象接受数据
        Users users = new Users(un, pw);
        // 2.3写入SQL语句 避免SQL注入 此处使用？占位符
        String sql = "select password from user where username = ?";

        try (
                // 3.获取连接对象
                Connection connection = DriverManager.getConnection(url, user, password);
                // 4.获取发送对象
                PreparedStatement preparedStatement = connection.prepareStatement(sql)) {
            // 5.给占位符赋值
            preparedStatement.setString(1,users.getUsername());
            // 5.执行SQL
            ResultSet resultSet = preparedStatement.executeQuery();
            // 6.判断是否存在该账号
            if (resultSet.next()){
                if (resultSet.getString(1).equals(pw)){
                    System.out.println("登录成功！");
                }else{
                    System.out.println("密码错误！");
                }
            }else {
                System.out.println("该账号不存在，请注册！");
            }
        } catch (SQLException e) {
            throw new RuntimeException(e);
        }

    }
}

运行结果：