初学Mybatis之多对一查询 association 和一对多查询 collection

XML 映射器

多对一：关联（association）

一对多：集合（collection）

mysql 创建教师、学生表，插入数据

create table `teacher`(
	id int(10) primary key,
    `name` varchar(30) default null
) engine=InnoDB default charset=utf8;

insert into teacher (id,`name`) values (1,'教师');

create table `student`(
	id int(10) not null,
    `name` varchar(30) default null,
    tid int(10) default null,
    primary key (id),
    key fktid (tid),
    constraint fktid foreign key (tid) references `teacher` (id)
) engine=InnoDB default charset=utf8;

insert into student (id,`name`,tid) values (1,'学生1',1);
insert into student (id,`name`,tid) values (2,'学生2',1);
insert into student (id,`name`,tid) values (3,'学生3',1);

pom.xml 导入相应 jar 包

  <dependencies>
    <!-- mysql驱动 -->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.33</version>
    </dependency>

    <!-- mybatis驱动 -->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.16</version>
    </dependency>

    <!-- junit驱动 -->
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.13.2</version>
    </dependency>

    <!-- lombok驱动 -->
    <dependency>
      <groupId>org.projectlombok</groupId>
      <artifactId>lombok</artifactId>
      <version>1.18.34</version>
    </dependency>
  </dependencies>

  <!-- 在build中配置resources，防止资源导出失败 -->
  <build>
    <resources>
      <resource>
        <directory>src/main/java</directory>
        <includes>
          <include>**/*.xml</include>
          <include>**/*.properties</include>
        </includes>
      </resource>

      <resource>
        <directory>src/main/resources</directory>
        <includes>
          <include>**/*.xml</include>
          <include>**/*.properties</include>
        </includes>
      </resource>
    </resources>
  </build>

resources 目录下创建 db.properties

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

resources 目录下创建 mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">

<!-- configuration核心配置文件 -->
<configuration>

    <!-- 引入外部配置文件 -->
    <properties resource="db.properties"/>

    <settings>
        <!-- 日志工厂实现 -->
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

    <!-- 扫描实体类的包 -->
    <typeAliases>
        <package name="com.demo.pojo"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper class="com.demo.dao.TeacherMapper"/>
        <mapper class="com.demo.dao.StudentMapper"/>
    </mappers>

</configuration>

MybatisUtils 工具类：

package com.demo.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;

public class MybatisUtils {

    private static SqlSessionFactory sqlSessionFactory;

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

        /*
        有了SqlSessionFactory，就可以获得SqlSession的实例
        SqlSession提供了在数据库执行SQL命令所需的所有方法
        可以通过SqlSession实例来执行已映射的SQL语句
         */
        public static SqlSession getSqlSession(){
            return sqlSessionFactory.openSession(true);
        }
}

实体类与表字段一致

Teacher 类：

package com.demo.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;
}

Student 类：

（因为 tid 设置了外键，直接写 int tid 则无法关联，所以通过组合的方式关联进来）

package com.demo.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int id;
    private String name;
    //多个学生关联一个老师
    private Teacher teacher;
}

TeacherMapper 接口：

import com.demo.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

public interface TeacherMapper {
    @Select("select * from teacher where id = #{tid}")
    Teacher getTeacher(@Param("tid") int id);
}

同理，写个 StudentMapper 接口

resources 目录创建同包名

TeacherMapper.xml：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.demo.dao.TeacherMapper">

</mapper>

同理，创建 StudentMapper.xml

在核心配置文件 mybatis-config.xml 绑定注册：

    <mappers>
        <mapper class="com.demo.dao.TeacherMapper"/>
        <mapper class="com.demo.dao.StudentMapper"/>
    </mappers>

测试类：

package com.demo.dao;

import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

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

输出结果：

测试环境搭建步骤：

1.导入 Lombok 依赖

2.新建实体类 Teacher、Student

3.建立 Mapper 接口

4.建立 Mapper.xml 文件

5.在核心配置文件中绑定注册 Mapper 接口或者文件

6.测试查询是否成功

多对一处理：

StudentMapper 实体类：

package com.demo.dao;

import com.demo.pojo.Student;

import java.util.List;

public interface StudentMapper {
    //查询所有学生信息，以及对应教师信息
    List<Student> getStudent();
}

方法一：查找嵌套，根据查询出来的学生 tid，寻找对应的教师

StudentMapper.xml：

关联：association

集合：collection

javaType：对象类型

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.demo.dao.StudentMapper">
    <select id="getStudent" resultMap="tid">
        select * from student
    </select>
    <resultMap id="tid" type="student">
        <result column="id" property="id"/>
        <result column="name" property="name"/>
        <!-- 关联：association -->
        <!-- 集合：collection -->
        <!-- javaType：对象类型 -->
        <association property="teacher" column="tid" javaType="teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="teacher">
        select * from teacher where id = #{tid}
    </select>
</mapper>

association 关联的 property 是 Student 类的组合

测试类：遍历数组

package com.demo.dao;

import com.demo.pojo.Student;
import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class MyTest {
    @Test
    public void getStudent(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentList = mapper.getStudent();
        for(Student student : studentList){
            System.out.println(student);
        }
        sqlSession.close();
    }
}

运行结果如下：

方法二：结果嵌套

StudentMapper.xml：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.demo.dao.StudentMapper">
    <select id="getStudent" resultMap="tid">
        select s.id sid,s.name sname,s.tid stid,t.name tname from student s,teacher t where s.tid = t.id;
    </select>
    <resultMap id="tid" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
            <result property="id" column="stid"/>
        </association>
    </resultMap>
</mapper>

结果如下：

Mybatis 多对一查询方式：

1.子查询

2.联表查询

一对多处理：

Student 类：

package com.demo.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

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

Teacher 类：

package com.demo.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;
    //一名教师教导多名学生
    private List<Student> students;
}

TeacherMapper 接口：

package com.demo.dao;

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

import java.util.List;

public interface TeacherMapper {
    List<Teacher> getTeacher();

    //获取指定教师下所有学生及教师的信息
    Teacher getTeacherById(@Param("tid") int id);
}

TeacherMapper.xml：

方法一：结果嵌套查询

javaType 指定属性的类型，ofType 应用于集合中的泛型信息

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.demo.dao.TeacherMapper">

    <select id="getTeacher" resultType="teacher">
        select * from teacher
    </select>

    <!-- 结果嵌套查询 -->
    <!-- resultMap 结果集映射 -->
    <select id="getTeacherById" resultMap="TeacherStudent">
        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>
    <resultMap id="TeacherStudent" type="teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!-- 集合：collection -->
        <!-- javaType指定属性的类型，ofType应用于集合中的泛型信息 -->
        <collection property="students" ofType="student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

测试类：

package com.demo.dao;

import com.demo.pojo.Teacher;
import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

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

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

运行结果：

方法二：查询嵌套

    <select id="getTeacherById" resultMap="TeacherStudent">
        select * from teacher where id = #{tid}
    </select>

    <resultMap id="TeacherStudent" type="teacher">
        <collection property="students" javaType="ArrayList" ofType="student" select="tid" column="id"/>
    </resultMap>

    <select id="tid" resultMap="student">
        select * from student where tid = #{tid}
    </select>

总结：

1.关联：association（多对一）

2.集合：collection（一对多）

3. javaType 用来指定实体类中属性的类型

4. ofType 用来指定映射到 List 或集合中的 pojo 类型，泛型中的约束类型

注意点：

1.保证 SQL 可读性

2.注意一对多或多对一属性名和字段

3.使用 Log4j 日志排查错误

综上，还需了解 Mysql 引擎、InnoDB 底层原理、索引、索引优化等