🐳 Mybatis 中的对一、对多关联

1. 数据库中实体之间的对应关系

一对一

例如：人和身份证、一个人只能拥有一张身份证

一对多

例如：学员拥有多本图书

多对多

例如：学员和老师，多对多会产生中间关系表、

2. 在mybatis中，上述三种关系分成了两大类

对一关联
对多关联

代码实现

2.1 环境搭建

1.新建数据库、四张表 (relation)

2.添加依赖 （mysql , mybatis , junit , lombok）

xml 复制代码

<dependencies>
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.5.11</version>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.32</version>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>5.10.1</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.24</version>
    </dependency>
    <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
         <version>1.2.3</version>
    </dependency>
</dependencies>

3.新建实体类、BookBean、Card、Person、Student

java 复制代码

@Data
@NoArgsConstructor
@AllArgsConstructor
public class BookBean {
    private Integer bid ;
    private String bname ;
}

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Card {
    private Integer cid;
    private String cno;
}

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Person {
    private Integer pid ;
    private String pname ;
    private String address ;
    //对一关联：一个人拥有一张身份证
    private Card card ; 
}

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {
    private Integer sid ;
    private String sname ;
    //一个学员拥有多本图书 ： 一对多
    private List<BookBean> bookBeanList;
}

4.新建mapper接口 PersonMapper、StudentMapper

java 复制代码

public interface PersonMapper {
    //查询所有的Person信息，每一个Person仅仅是自己的简单属性信息
    List<Person> getPersonList();
    //查询所有的Person信息，每一个Person不仅包含自己的简单属性信息也包含他拥有的身份证信息
    List<Person> getPersonListWithCardInfo();
    //查询所有的Person信息，每一个Person不仅包含自己的简单属性信息也包含他拥有的身份证信息
    List<Person> getPersonListWithCardInfo2();
}

public interface StudentMapper {
    List<Student> getStudentListWithBookBeanList();
}

5.在resources 目录下新建com.bottom.mybatis.mapper 目录，并新建两个mapper.xml文件

PersonMapper.xml

xml 复制代码

<?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="com.bottom.mybatis.mapper.PersonMapper">
    <select id="getPersonList" resultType="Person">
        select * from t_person
    </select>

    <!-- 这种方式发现person中的三个字段能成功映射，但是card仍然是null -->
    <!--
    <select id="getPersonListWithCardInfo" resultType="Person">
        select * from t_person t1 left join t_card t2 on t1.pid = t2.cid
    </select>
    -->
    <select id="getPersonListWithCardInfo" resultMap="PersonResultMap">
        select t1.* , t2.* from t_person t1 left join t_card t2 on t1.pid = t2.cid
    </select>

    <resultMap id="PersonResultMap" type="Person" autoMapping="true">
        <id property="pid" column="pid"/>
        <!-- 对一关联 -->
        <association property="card" javaType="Card" autoMapping="true">
            <id property="cid" column="cid"/>
        </association>
    </resultMap>


    <select id="getPersonListWithCardInfo2" resultType="Person">
        select t1.* , t2.cid as 'card.cid' , t2.cno as 'card.cno'
        from t_person t1 left join t_card t2 on t1.pid = t2.cid
    </select>


</mapper>

StudentMapper.xml

xml 复制代码

<?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="com.bottom.mybatis.mapper.StudentMapper">
    <select id="getStudentListWithBookBeanList" resultMap="StudentWithBookBeanListResultMap">
        select * from t_student t1 left join t_bookbean t2 on t1.sid = t2.stuid
    </select>

    <resultMap id="StudentWithBookBeanListResultMap" type="Student" autoMapping="true">
        <id property="sid" column="sid"/>
        <!-- 对多关联 collection -->
        <collection property="bookBeanList" ofType="BookBean" autoMapping="true">
            <id property="bid" column="bid"/>
        </collection>
    </resultMap>

</mapper>

5.在 resources 目录下新建 mybatis-config.xml、logback.xml

logback.xml

xml 复制代码

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="true">
    <appender name="STDOUT"
              class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>[%d{HH:mm:ss.SSS}] [%-5level] [%thread] [%logger] [%msg]%n</pattern>
            <charset>UTF-8</charset>
        </encoder>
    </appender>

    <root level="DEBUG">
        <appender-ref ref="STDOUT" />
    </root>

    <logger name="com.bottom.mybatis" level="DEBUG" />

</configuration>

mybatis-config.xml

xml 复制代码

<?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>
    <settings>
        <setting name="logImpl" value="SLF4J"/>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <typeAliases>
        <package name="com.bottom.mybatis.pojo"/>
    </typeAliases>
    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/relation"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.bottom.mybatis.mapper"/>
    </mappers>
</configuration>

6.测试类测试

java 复制代码

public class PersonMapperTest {
    private SqlSession sqlSession;
    private PersonMapper personMapper;
    private StudentMapper studentMapper ;

    @BeforeEach
    public void setup() throws IOException {
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
        sqlSession = sqlSessionFactory.openSession();
        personMapper = sqlSession.getMapper(PersonMapper.class);
        studentMapper = sqlSession.getMapper(StudentMapper.class);
    }

    @AfterEach
    public void teardown() throws SQLException {
        if(sqlSession!=null && !sqlSession.getConnection().isClosed()){
            sqlSession.commit();
            sqlSession.close();
        }
    }

    @Test
    public void test01(){
        personMapper.getPersonList().forEach(System.out::println);
    }

    //1.演示对一关联
    @Test
    public void test02(){
        personMapper.getPersonListWithCardInfo().forEach(System.out::println);
    }

    //2.演示对一关联的另一种解决方案
    @Test
    public void test03(){
        personMapper.getPersonListWithCardInfo2().forEach(System.out::println);
    }

    //3.演示对多关联
    @Test
    public void test04(){
        studentMapper.getStudentListWithBookBeanList().forEach(System.out::println);
    }
}