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MyBatis的⾼级映射及延迟加载过程详解

@每天都要敲代码 人气:0

一:MyBatis的⾼级映射及延迟加载

前期准备:

模块名:mybatis-010-advanced-mapping

打包⽅式:jar

引⼊依赖:mysql驱动依赖、mybatis依赖、junit依赖、logback依赖

pojo:com.powernode.mybatis.pojo.Student和Calzz

mapper接⼝:com.powernode.mybatis.mapper.StudentMapper和ClazzMapper

引⼊配置⽂件:mybatis-config.xml、jdbc.properties、logback.xml

mapper配置⽂件:com/powernode/mybatis/mapper/StudentMapper.xml和ClazzMapper.xml

编写测试类:com.powernode.mybatis.test.StudentMapperTest和ClazzMapperTest

拷⻉⼯具类:com.powernode.mybatis.utils.SqlSessionUtil

准备数据库表:⼀个班级对应多个学⽣:

班级表:t_clazz

学⽣表:t_student

clazz实例类

package com.bjpowernode.mybatis.pojo;
 
/**
 * @Author:朗朗乾坤
 * @Package:com.bjpowernode.mybatis.pojo
 * @Project:mybatis
 * @name:Clazz
 * @Date:2023/1/7 18:18
 */
public class Clazz {
    private Integer cid;
    private String cname;
 
    public Clazz() {
    }
 
    public Clazz(Integer cid, String cname) {
        this.cid = cid;
        this.cname = cname;
    }
 
    @Override
    public String toString() {
        return "Clazz{" +
                "cid=" + cid +
                ", cname='" + cname + '\'' +
                '}';
    }
 
    public Integer getCid() {
        return cid;
    }
 
    public void setCid(Integer cid) {
        this.cid = cid;
    }
 
    public String getCname() {
        return cname;
    }
 
    public void setCname(String cname) {
        this.cname = cname;
    }
}

Student实体类:只定义两个字段;对于cid属性,是为了维护两者关系的属性

package com.bjpowernode.mybatis.pojo;
 
/**
 * @Author:朗朗乾坤
 * @Package:com.bjpowernode.mybatis.pojo
 * @Project:mybatis
 * @name:Student
 * @Date:2023/1/7 18:18
 */
public class Student { 
    private Integer sid;
    private String sname;
   
    public Student() {
    }
    public Student(Integer sid, String sname) {
        this.sid = sid;
        this.sname = sname;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                '}';
    }
 
    public Integer getSid() {
        return sid;
    }
 
    public void setSid(Integer sid) {
        this.sid = sid;
    }
 
    public String getSname() {
        return sname;
    }
 
    public void setSname(String sname) {
        this.sname = sname;
    }
}

1. 多对⼀

pojo类Student中添加⼀个属性:Clazz clazz; 表示学⽣关联的班级对象,增加setter and getter方法,重写toString方法。

package com.bjpowernode.mybatis.pojo;
 
/**
 * @Author:朗朗乾坤
 * @Package:com.bjpowernode.mybatis.pojo
 * @Project:mybatis
 * @name:Student
 * @Date:2023/1/7 18:18
 */
public class Student { // Student是多的一方
    private Integer sid;
    private String sname;
    private Clazz clazz; // Clazz是一的一方
 
    public Student() {
    }
    public Student(Integer sid, String sname) {
        this.sid = sid;
        this.sname = sname;
    }
 
    @Override
    public String toString() {
        return "Student{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                ", clazz=" + clazz +
                '}';
    }
 
    public Clazz getClazz() {
        return clazz;
    }
 
    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }
 
    public Integer getSid() {
        return sid;
    }
 
    public void setSid(Integer sid) {
        this.sid = sid;
    }
 
    public String getSname() {
        return sname;
    }
 
    public void setSname(String sname) {
        this.sname = sname;
    }
}

多对一:多个学生Student对应一个班级Clazz

多种⽅式,常⻅的包括三种:

① 第⼀种⽅式:⼀条SQL语句,级联属性映射。

② 第⼆种⽅式:⼀条SQL语句,association。

③第三种⽅式:两条SQL语句,分步查询。

第⼀种⽅式:级联属性映射

三兄弟之一:StudentMapper接口,编写方法

根据id获取学生Student信息,同时获取学生关联的班级Clazz信息

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Student;
 
public interface StudentMapper {
    /*
     * @param id 学生的id
     * @return 学生对象,但是学生对象当中含有班级对象
     */
    Student selectById(Integer id);
    
}

三兄弟之二:StudentMapper.xml文件,编写sql语句

(1)使用resultMap来指定映射关系,结果映射resultMap有两个参数:

一个参数是id,指定resultMap的唯一标识,这个id将来在select标签中使用。

一个参数是type,用来指定POJO类的类名。

(2)在resultMap下还有一个子标签result;首先对于有主键的需要配一个id,不是必须的,但可以增加效率;下面使用result子标签的property属性和column属性分别指定POJO类的属性名和数据库表中的字段表之间的映射关系。

<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <!--多对一映射的第一种方式:一条SQL语句,级联属性映射-->
    <resultMap id="studentResultMap" type="Student">
        <id property="id" column="id"/>
        <result property="sid" column="sid"/>
        <result property="clazz.cid" column="cid"/>
        <result property="clazz.cname" column="cname" />
    </resultMap>
 
    <select id="selectById" resultMap="studentResultMap">
      select
        s.sid,s.sname,c.cid,c.cname
      from
        t_student s
      left join
        t_clazz c
      on s.cid = c.cid
      where
        s.sid = #{sid}
    </select>
</mapper>
<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <!--多对一映射的第一种方式:一条SQL语句,级联属性映射-->
    <resultMap id="studentResultMap" type="Student">
        <id property="id" column="id"/>
        <result property="sid" column="sid"/>
        <result property="clazz.cid" column="cid"/>
        <result property="clazz.cname" column="cname" />
    </resultMap>
 
    <select id="selectById" resultMap="studentResultMap">
      select
        s.sid,s.sname,c.cid,c.cname
      from
        t_student s
      left join
        t_clazz c
      on s.cid = c.cid
      where
        s.sid = #{sid}
    </select>
</mapper>

三兄弟之三:StudentMappeTest类,用来编写测试类

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.StudentMapper;
import com.bjpowernode.mybatis.pojo.Student;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
public class StudentMapperTest {
    @Test
    public void testSelectById(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        Student student = mapper.selectById(1);
        // 直接输出Student对象
        System.out.println(student);
        // 输出每个属性
        System.out.println(student.getSid());
        System.out.println(student.getSname());
        System.out.println(student.getClazz().getCid());
        System.out.println(student.getClazz().getCname());
 
        sqlSession.close();
    }
}

执行结果:

第⼆种⽅式:association标签

第二种方式,和第一种方式的代码很类似,就是多引入一个association标签,association翻译为关联的意思

三兄弟之一:StudentMapper接口,编写方法

根据id获取学生Student信息,同时获取学生关联的班级Clazz信息

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Student;
 
public interface StudentMapper {
     /**
     * 一条SQL语句,使用association标签的方式
     * @param id 学生的id
     * @return 学生对象,但是学生对象当中含有班级对象
     */
    Student selectByIdAssociation(Integer id);
    
}

三兄弟之二:StudentMapper.xml文件,编写sql语句

association:翻译为关联,一个Student对象关联一个Clazz对象

①property标签:提供要映射的POJO类的属性名,这里就是clazz

②javaType标签:用来指定要映射的java类型,这里就是com.bjpowernode.mybatis.Clazz

<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <!--一条SQL语句,引入association标签-->
    <resultMap id="studentResultMapAssociation" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <!--使用assocaition标签-->
        <association property="clazz" javaType="Clazz">
            <!--虽然字段是相同的,但是不能省略不写-->
            <id property="cid" column="cid"/>
            <result property="cname" column="cname" />
        </association>
    </resultMap>
    
    <select id="selectByIdAssociation" resultMap="studentResultMapAssociation">
        select
        s.sid,s.sname,c.cid,c.cname
      from
        t_student s
      left join
        t_clazz c
      on s.cid = c.cid
      where
        s.sid = #{sid}
    </select>
</mapper>

三兄弟之三:StudentMappeTest类,用来编写测试类

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.StudentMapper;
import com.bjpowernode.mybatis.pojo.Student;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
public class StudentMapperTest {
     @Test
    public void testSelectByIdAssociation(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        Student student = mapper.selectByIdAssociation(2);
        // 直接输出Student对象
        System.out.println(student);
        sqlSession.close();
    }
}

执行结果:

第三种⽅式:分步查询

分布查询,需要两条SQL语句,这种⽅式常⽤:

①优点⼀是可复⽤。

②优点⼆是⽀持懒加载(延迟加载)

(1)分布查询第一步:先根据学生的sid查询学生信息

①在StudentMapper中编写第一步的查询方法

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Student;
 
public interface StudentMapper {
    
    // 分布查询第一步:现根据学生的sid查询学生信息
    Student selectByIdStep1(Integer id);
 
}

②在StudentMapper.xml中编写SQL语句,并指明两者之间的关联关系

还是需要association标签:

①property属性:还是提供要映射的POJO类的属性名,这里就是clazz

②select属性:用来指定另外第二步SQL语句的id,这个id实际上就是namespace+id;通过第二步语句的查询结果,把值赋值给clazz

③column属性:是把第一条SQL语句查询出来的cid传给第第二条SQL语句

<?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.bjpowernode.mybatis.mapper.StudentMapper">   
    <!--这是第一步:根据学生的id查询学生的所有信息,包含cid-->
    <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname" />
        <!--使用association标签,指明关联关系-->
        <association property="clazz"
                     select="com.bjpowernode.mybatis.mapper.ClazzMapper.selectByIdStep2"
                     column="cid"/>
    </resultMap>
 
    <select id="selectByIdStep1" resultMap="studentResultMapByStep">
        select sid,sname,cid from t_student where sid = #{sid};
    </select>
 
</mapper>

(2)分布查询第二步:根据学生的cid查询班级信息

①在ClazzMapper中编写第二步的查询方法

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Clazz;
 
public interface ClazzMapper {
    // 分布查询第二步:根据cid获取查询信息
    Clazz selectByIdStep2(Integer cid);
}

②在ClazzMapper.xml中编写SQL语句

<?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.bjpowernode.mybatis.mapper.ClazzMapper">
    <!--分布查询第二步:根据cid获取班级信息-->
    <select id="selectByIdStep2" resultType="Clazz">
        select cid,cname from t_clazz where cid = #{id};
    </select>
</mapper>

(3)最终StudentMapperTest编写测试,因为Student是主表

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.StudentMapper;
import com.bjpowernode.mybatis.pojo.Student;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
public class StudentMapperTest {
    @Test
    public void testSelectByIdStep1(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        Student student = mapper.selectByIdStep1(3);
        System.out.println(student);
        sqlSession.close();
    }
}

执行结果:

2. 多对⼀延迟加载

(1)延迟加载的核心是:用到的在查询,暂时访问不到的数据可以先不查询。

(2)作用:提⾼程序的执⾏效率;不用的时候也查性能肯定低,例如笛卡尔积现象

(3)在MyBatis中如何开启延迟加载:asspciation标签当中添加fetchType="lazy"

例1:不开启延迟加载机制,假如只访问学生表t_student的sname属性,和t_clazz表实际上是没有任何关系的

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.StudentMapper;
import com.bjpowernode.mybatis.pojo.Student;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
 
public class StudentMapperTest {
    @Test
    public void testSelectByIdStep1(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        Student student = mapper.selectByIdStep1(3);
        // System.out.println(student);
        // 只访问sname属性
        System.out.println(student.getSname());
        sqlSession.close();
    }
}

执行结果:实际上执行了两个查询语句,效率变低

例2:在asspciation标签中开启延迟机制,就能做到只执行第一条SQL语句,第二条不执行;

<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname" />
        <!--使用association标签,并开启延迟加载机制-->
        <association property="clazz"
                     select="com.bjpowernode.mybatis.mapper.ClazzMapper.selectByIdStep2"
                     column="cid"
                     fetchType="lazy"/>
    </resultMap>
 
    <select id="selectByIdStep1" resultMap="studentResultMapByStep">
        select sid,sname,cid from t_student where sid = #{sid};
    </select>
</mapper>

执行结果:同样还是只访问学生表t_student的sname属性,此时就是只查询一张表即可

(4)在association标签中配置fetchType=“lazy”,实际上是局部的设置,只对当前的association关联的SQL语句起作用!

(5)那么怎样在mybatis中如何开启全局的延迟加载呢?需要setting配置,如下:

 (6)在核心配置文件mybatis-config.xml文件当中使用setting标签进行配置

    <!--启⽤全局延迟加载机制-->
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

(7)开启全局延迟加载之后,所有的SQL都会⽀持延迟加载,但是如果某个SQL你不希望它⽀持延迟加载怎么办呢? 将fetchType设置为eager

<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname" />
        <!--使用association标签,不开启延迟加载机制-->
        <association property="clazz"
                     select="com.bjpowernode.mybatis.mapper.ClazzMapper.selectByIdStep2"
                     column="cid"
                     fetchType="eager"/>
    </resultMap>
 
    <select id="selectByIdStep1" resultMap="studentResultMapByStep">
        select sid,sname,cid from t_student where sid = #{sid};
    </select>
</mapper>

3. ⼀对多

⼀对多的实现,通常是在⼀的⼀⽅中有List集合属性!

pojo类Clazz中添加⼀个属性:List<Student> stu; 表示班级关联的学生对象,增加setter and getter方法,重写toString方法。

package com.bjpowernode.mybatis.pojo;
 
import java.util.*;
 
public class Clazz {
    private Integer cid;
    private String cname;
    private List<Student> stus;
    
    public Clazz() {
    }
 
    public Clazz(Integer cid, String cname) {
        this.cid = cid;
        this.cname = cname;
    }
 
    @Override
    public String toString() {
        return "Clazz{" +
                "cid=" + cid +
                ", cname='" + cname + '\'' +
                ", stus=" + stus +
                '}';
    }
 
    public List<Student> getStus() {
        return stus;
    }
 
    public void setStus(List<Student> stus) {
        this.stus = stus;
    }
 
    public Integer getCid() {
        return cid;
    }
 
    public void setCid(Integer cid) {
        this.cid = cid;
    }
 
    public String getCname() {
        return cname;
    }
 
    public void setCname(String cname) {
        this.cname = cname;
    }
}

⼀对多的实现通常包括两种实现⽅式:

①第⼀种⽅式:collection

②第⼆种⽅式:分步查询

第⼀种⽅式:collection

注:这次t_calss是主表,所以是在ClazzMapper、ClazzMapper.xml、ClazzMapperTest当中完成一些列操作。

 三兄弟之一:ClazzMapper接口,编写方法

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Clazz;
 
public interface ClazzMapper {
    
   // 根据班级编号查询班级信息
    Clazz selectByIdCollection(Integer cid);
}

三兄弟之二:ClazzMapper.xml文件,编写sql语句

使用collection标签,和上面使用association标签的第二种方法是很相似的

①property属性:提供要映射的POJO类的属性名,这里就是stus

②ofType属性:用来指定集合当中的元素类型com.bjpowernode.mybatis.Student

<?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.bjpowernode.mybatis.mapper.ClazzMapper">
    <resultMap id="clazzResultMap" type="Clazz">
        <id property="cid" column="cid"/>
        <result property="cname" column="cname"/>
        <!--ofType用来指定集合当中的元素类型-->
        <collection property="stus" ofType="Student">
            <id property="sid" column="sid"/>
            <result property="sname" column="sname"/>
        </collection>
    </resultMap>
 
    <select id="selectByIdCollection" resultMap="clazzResultMap">
        select
          c.cid,c.cname,s.sid,s.sname
        from
          t_clazz c
        left join
          t_student s
        on c.cid = s.cid
        where c.cid = #{cid}
    </select>
</mapper>

三兄弟之三:ClazzMappeTest类,用来编写测试类

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.ClazzMapper;
import com.bjpowernode.mybatis.pojo.Clazz;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
public class ClazzMapperTest {
    @Test
    public void testSelectByIdCollection(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        ClazzMapper mapper = sqlSession.getMapper(ClazzMapper.class);
        Clazz clazz = mapper.selectByIdCollection(1001);
        System.out.println(clazz);
        sqlSession.close();
    }
}

执行结果:

 查询的结果是stus变量是一个有三个数据的List集合,其中clazz是null属于正常现象,如果clazz还有值,就会与前面的Student形成递归循环 

第⼆种⽅式:分步查询和延迟加载

(1)分布查询第一步:先根据班级编号获取班级信息

①在ClazzMapper中编写第一步的查询方法

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Clazz;
 
public interface ClazzMapper {
 
    // 分布查询第一步:根据班级编号,获取班级信息
    Clazz selectByStep1(Integer cid);
}

②在ClazzMapper.xml中编写SQL语句,并指明两者之间的关联关系

<?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.bjpowernode.mybatis.mapper.ClazzMapper">
   <!--分布查询第一步:根据班级的cid获取班级信息-->
    <resultMap id="clazzResultMapStep" type="Clazz">
        <id property="cid" column="cid"/>
        <result property="cname" column="cname"/>
        <collection property="stus"
                    select="com.bjpowernode.mybatis.mapper.StudentMapper.selectByCidStep2"
                    column="cid" />
    </resultMap>
    <select id="selectByStep1" resultMap="clazzResultMapStep">
        select cid,cname from t_clazz where cid = #{cid}
    </select>
</mapper>

(2)分布查询第二步:根据学生的cid查询班级信息

①在StudentMapper中编写第二步的查询方法

package com.bjpowernode.mybatis.mapper;
 
import com.bjpowernode.mybatis.pojo.Student;
import java.util.List;
 
public interface StudentMapper {
   
    // 分布查询第二步:根据班级编号查询学生信息
    List<Student> selectByCidStep2(Integer cid);
}

②在StudentMapper.xml中编写SQL语句

<?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.bjpowernode.mybatis.mapper.StudentMapper">
    <select id="selectByCidStep2" resultType="Student">
        select * from t_student where cid = #{cid}
    </select>
</mapper>

(3)最终ClazzMapperTest编写测试,因为Clazz是主表

package com.bjpowernode.mybatis.test;
 
import com.bjpowernode.mybatis.mapper.ClazzMapper;
import com.bjpowernode.mybatis.pojo.Clazz;
import com.bjpowernode.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
 
public class ClazzMapperTest {
    @Test
    public void testSelectByStep1(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        ClazzMapper mapper = sqlSession.getMapper(ClazzMapper.class);
        Clazz clazz = mapper.selectByStep1(1001);
        System.out.println(clazz);
        sqlSession.close();
    }
}

执行结果:

⼀对多延迟加载机制和多对⼀是⼀样的,同样是通过两种⽅式:

第⼀种:fetchType="lazy"

第⼆种:修改全局的配置setting,lazyLoadingEnabled=true,如果开启全局延迟加载,想让某个 sql不使⽤延迟加载:fetchType="eager"

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