MyBatis學習（六）MyBatis關聯映射之一對多映射

數據庫中一對多一般使用主外鍵關聯，外鍵應該在多方，即多方維護關係。

下面舉一個簡單實例來看看MyBatis怎麼處理一對多的關係。

1.建立一個項目，導入所需jar包，導入db.properties配置文件，導入log4j.properties配置文件

2.建立一個數據庫，在裏面建立兩張表

 

-- Table structure for `t_clazz`
-- ----------------------------
DROP TABLE IF EXISTS `t_clazz`;
CREATE TABLE `t_clazz` (
  `id` int(11) NOT NULL,
  `code` varchar(18) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_clazz
-- ----------------------------
INSERT INTO `t_clazz` VALUES ('1', '一班');
INSERT INTO `t_clazz` VALUES ('2', '二班');

 

 

 

-- Table structure for `t_student`
-- ----------------------------
DROP TABLE IF EXISTS `t_student`;
CREATE TABLE `t_student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(18) NOT NULL,
  `sex` varchar(3) NOT NULL,
  `age` int(11) NOT NULL,
  `cid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `cid` (`cid`),
  CONSTRAINT `cid` FOREIGN KEY (`cid`) REFERENCES `t_clazz` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_student
-- ----------------------------
INSERT INTO `t_student` VALUES ('1', '張三', '男', '11', '1');
INSERT INTO `t_student` VALUES ('2', '李四', '男', '12', '2');
INSERT INTO `t_student` VALUES ('3', '小紅', '女', '13', '1');

3.編寫對應的實體類

public class Student {

    private Integer id;
    private String name;
    private String sex;
    private Integer age;
    //關聯的clazz對象
    private Clazz clazz;

public class Clazz {

    private Integer id;
    private String code;
    //關聯的student集合
    private List<Student> students;

4.編寫對應的SQL映射文件

ClazzMapper.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="com.dj.mapper.ClazzMapper">

    <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
        select * from t_clazz where id =#{id}
    </select>
    <resultMap type="com.dj.domain.Clazz" id="clazzResultMap">
        <id property="id" column="id"/>
        <resultproperty="code" column="code"/>
        <!--property表示返回類型Clazz的屬性students
            column表示將id做爲參數進行以後的查詢
            fetchtype表示懶加載
            javaType表示屬性對應的類型 
            ofType表示集合當中的類型
        -->
        <collection property="students" column="id" fetchType="lazy" 
                javaType="ArrayList" ofType="com.dj.domain.Student" 
                select="com.dj.mapper.StudentMapper.selectStudentByClazzId">
                <id property="id" column="id"/>
                <resultproperty="name" column="name"/>
                <resultproperty="sex" column="sex"/>
                <resultproperty="age" column="age"/>
                </collection>
    </resultMap>
    
    
</mapper>

StudentMapper.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">

<!-- namespace指用戶自定義的命名空間 -->
<mapper namespace="com.dj.mapper.StudentMapper">

    <select id="selectStudentByClazzId" parameterType="int" resultType="com.dj.domain.Student">
        select * from t_student where cid=#{id}
    </select>
</mapper>

5.編寫mybatis-config.xml文件

<?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>
     <!-- 引入 外部db.properties文件-->
     <properties resource="db.properties"/>
     <!-- 指定 MyBatis 所用日誌的具體實現-->
     <settings>
         <setting name="logImpl" value="log4j"/>
     </settings>
     <!-- 環境配置 -->
     <environments default="mysql">
         <environment id="mysql">
             <!-- 指定事務類型 -->
             <transactionManager type="JDBC"/>
                 <!--  dataSource指數據源配置，POOLED是JDBC鏈接對象的數據源鏈接池的實現。 -->
             <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>
     <!-- SQL映射文件位置 -->
     <mappers>
         <mapper resource="com/dj/mapper/StudentMapper.xml"/>
         <mapper resource="com/dj/mapper/ClazzMapper.xml"/>
     </mappers>
 </configuration>

6.mybatis建議經過mapper接口的代理對象訪問mybatis，該對象關聯了一個sqlsession對象，開發者能夠經過該對象直接調用方法操做數據庫。

注意: mapper接口對象的類名必須和以前的mapper.xml的namespace一致，方法名和參數名及返回類型也要與mapper.xml的配置一致。

public interface ClazzMapper {
    //根據id查詢班級信息
    Clazz selectClazzById(int id);
    
}

7.測試

public class OneToManyTest {
    
    public static void main(String[] args) throws Exception {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //得到mapper接口的代理對象
        ClazzMapper mapper = sqlSession.getMapper(ClazzMapper.class);
        //調用接口中的方法
        Clazz clazz = mapper.selectClazzById(1);
        List<Student> students = clazz.getStudents();
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

}

在控制檯能夠看到以下結果：

 

測試成功。