1.创建实体类（entity下）

package com.imooc.oa.entity;

public class User {
    private Long userId; //user_id
    private String username;
    private String password;
    private Long employeeId;
    private Integer salt;

    @Override
    public String toString() {
        return "User{" +
                "userId=" + userId +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", employeeId=" + employeeId +
                ", salt=" + salt +
                '}';
    }

    public Long getUserId() {
        return userId;
    }

    public void setUserId(Long 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;
    }

    public Long getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Long employeeId) {
        this.employeeId = employeeId;
    }

    public Integer getSalt() {
        return salt;
    }

    public void setSalt(Integer salt) {
        this.salt = salt;
    }
}

2.创建user.xml（resources/user.xml）

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="usermapper">
    <select id="selectByUsername" parameterType="String" resultType="com.imooc.oa.entity.User">
        select * from sys_user where username = #{value}
    </select>
</mapper>

3.在mybatis-config.xml进行注册

 <mappers>
        <mapper resource="mappers/user.xml"/>
    </mappers>

4.创建mapper（com.imooc.oa.entity.User）zaimvc业务中，mapper不包含业务逻辑，只操作数据库

package com.imooc.oa.mapper;

import com.imooc.oa.entity.User;
import com.imooc.oa.utils.MybatisUtils;

public class UserMapper
{
    public User selectByUsername(String username)
    {
        User user = (User) MybatisUtils.executeQuery(sqlSession -> sqlSession.selectOne("usermapper.selectByUsername", username));
        return user;
    }
}

5.在service中进行业务逻辑（package com.imooc.oa.service）

package com.imooc.oa.service;

import com.imooc.oa.entity.User;
import com.imooc.oa.mapper.UserMapper;
import com.imooc.oa.service.exception.LoginException;

public class UserService {
    private UserMapper userMapper = new UserMapper();
    /**
     * 根据前台输入进行登录校验
     * @param username 前台输入的用户名
     * @param password 前台输入的密码
     * @return 校验通过后,包含对应用户数据的User实体类
     * @throws LoginException 用户登录异常
     */
    public User checkLogin(String username , String password){
        User user = userMapper.selectByUsername(username);
        if(user == null){
            throw new LoginException("用户名不存在");
        }
        if(!password.equals(user.getPassword())){
            throw new LoginException("密码错误");
        }
        return user;
    }
}

5.1在service包下创建自定义异常exception（com.imooc.oa.service.exception）

package com.imooc.oa.service.exception;

public class LoginException extends RuntimeException{
    public LoginException(String message){
        super(message);//使用父类的方法
    }
}