11、MyBatis一对多处理

比如：一个 老师拥有多个学生。

对于老师而言，就是一对多的关系

1.环境搭建，和mybatis-06一样

实体类：

package com.gongyi.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}

package com.gongyi.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;
    //一个老师拥有多个学生
    private List students;
}

    select s.id sid,s.name sname,t.name tname,t.id tid
    from student s,teacher t
    where s.tid = t.id and t.id = #{tid}


    
    
    
    
        
        
        
    

    select * from mybatis.teacher where id = #{tid}


    



    select * from mybatis.student where tid=#{tid}

1.关联-association【多对一】

2.集合-collection【一对多】

3.JavaType & ofType

• javaType 用来指定实体类中属性的类型
• ofType用来指定映射到List或者集合中的pojo类型，泛型中的约束类型

注意点：

• 保证SQL的可读性，尽量保证通俗易懂
• 注意一对多和多对一中，属性名和字段的问题
• 如果问题不好排查错误，可以使用日志，建议使用log4j

慢SQL 1s vs 1000s

面试高频

• MySQL引擎
• InnoDB底层原理
• 索引
• 索引优化

代码结构图：

核心代码：

pojo

package com.gongyi.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}

package com.gongyi.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;
    //一个老师拥有多个学生
    private List students;
}

dao

package com.gongyi.dao;


public interface StudentMapper {

}

package com.gongyi.dao;


import com.gongyi.pojo.Teacher;
import org.apache.ibatis.annotations.Param;

import java.util.List;

public interface TeacherMapper {
    //获取老师
    List getTeacher1();

    //获取指定老师下的所有学生及老师的信息
    Teacher getTeacher(@Param("tid") int id);

    Teacher getTeacher2(@Param("tid") int id);

}

utils

package com.gongyi.utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;


//sqlSessionFactory -->sqlSession
public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            //使用mybatis第一步：获取sqlSessionFactory对象
            String resource = "mybatis-config.xml";
            InputStream inputStream = null;
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession(true);
    }
}

resource

db.properties

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=UTF-8
username=root
password=123456

mybatis-config.xml

实体类映射文件

    
        select * from teacher;
    

    
    
        select s.id sid,s.name sname,t.name tname,t.id tid
        from student s,teacher t
        where s.tid = t.id and t.id = #{tid}
    
    
        
        
        
        
            
            
            
        
    

    
        select * from mybatis.teacher where id = #{tid}
    
    
        
    

    
        select * from mybatis.student where tid=#{tid}
    

测试类

import com.gongyi.dao.TeacherMapper;
import com.gongyi.pojo.Teacher;
import com.gongyi.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class MyTest {
    @Test
    public void testGetTeacher1() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        for (Teacher teacher : sqlSession.getMapper(TeacherMapper.class).getTeacher1()) {
            System.out.println(teacher);
        }

        sqlSession.close();
    }

    @Test
    public void testGetTeacher() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        
        sqlSession.close();
    }

    @Test
    public void testGetTeacher2() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher2(1);//teacher id为何变为了0
        
        System.out.println(teacher);
        sqlSession.close();
    }

}