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Mybatis之一对多处理

发布时间 2023-10-13 10:42:42作者: nmmmmmmmm

一对多处理

运用场景: 一个老师有多名学生, 对于老师而言 就是一对多

  1. Teacher实体类

@Data
public class Teacher {
​
    private int id;
    private String name;
    List<Student> students;
}

 

  1. mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
​
    <properties resource="db.properties">
    </properties>
    
    <settings>
        <!--标准的日志工厂实现-->
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>
    
    <typeAliases>
<!--        <package name="com.kuang.pojo"/>-->
        <typeAlias type="com.kuang.pojo.Teacher" alias="Teacher"/>
    </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>
        <environment id="test">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false&amp;useUnicode=true&amp;characterEncoding=utf8&amp;zeroDateTimeBehavior=convertToNull&amp;serverTimezone=GMT%2B8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
​
    <mappers>
        <mapper class="com.kuang.mapper.TeacherMapper"/>
    </mappers>
</configuration>

 

  1. TeacherMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kuang.mapper.TeacherMapper">
​
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from mybatis.teacher t
        join mybatis.student s on s.tid = t.id
        where t.id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--复杂的属性,我们需要单独处理 对象:association 集合:collection
       javaType 指定属性类型!
       集合中的泛型信息,我们使用ofType获取
       -->
       <collection property="students" ofType="Student">
           <result property="id" column="sid"/>
           <result property="name" column="sname"/>
           <result property="tid" column="stid"/>
       </collection>
    </resultMap>
</mapper>

  1. 公共类

public class MybatisUtils {
​
    private static SqlSessionFactory sqlSessionFactory;
​
   static {
       try {
           String resource = "mybatis-config.xml";
           InputStream inputStream = Resources.getResourceAsStream(resource);
           sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
       } catch (IOException e) {
           e.printStackTrace();
       }
   }
   public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession(true);
   }
​
}

  1. 测试

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

  1. 输出结果

teacher = Teacher(id=1, name=秦老师, students=[Student(id=1, name=小明, tid=1), Student(id=2, name=小红, tid=1), Student(id=3, name=小张, tid=1), Student(id=4, name=小李, tid=1), Student(id=5, name=小王, tid=1)])