首先：MyBatis的关联查询！！！（一对一、一对多、多对多）-CSDN博客以这个项目为基础。

1.在UserMapper接口中创建一个方法：

package com.by.mapper;import com.by.pojo.User;import java.util.List;/*** <p>Project: mybatis - UserMapper</p>* <p>Powered by scl On 2023-12-22 15:52:05</p>* <p>描述：<p>** @author 孙臣龙 [1846080280@qq.com]* @version 1.0* @since 17*/
public interface UserMapper {/*** 一对多的延迟加载*/User findUserById(Integer id);
}

2.在UserMapper.xml文件在实现这个方法：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.by.mapper.UserMapper"><resultMap id="findAllResultMap" type="user"><id column="id" property="id"></id><result column="username" property="username"></result><result column="birthday" property="birthday"></result><result column="sex" property="sex"></result><result column="address" property="address"></result><collection property="accountList"ofType="account"select="com.by.mapper.AccountMapper.selectAccountByUid"column="id"fetchType="lazy"><id column="aid" property="id"></id><result column="uid" property="uid"></result><result column="money" property="money"></result></collection></resultMap><!--一对多--><select id="findAllUserAccount" parameterType="int" resultMap="findAllResultMap">select a.id aid, a.uid uid, a.money money, u.*from user uleft join account a on u.id = a.uidwhere u.id = #{id}</select><!--一对多延迟加载--><select id="findUserById" parameterType="int" resultMap="findAllResultMap">select * from user where id=#{id}</select>
</mapper>

3.在AccountMapper.xml文件中实现这个方法：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.by.mapper.AccountMapper"><!--一对多延迟加载--><select id="selectAccountByUid" resultType="account">select * from account where uid=#{uid}</select>
</mapper>

 4.测试类：

/** Copyright (c) 2020, 2023,  All rights reserved.**/
package com.by;import com.by.mapper.AccountMapper;
import com.by.mapper.RoleMapper;
import com.by.mapper.UserMapper;
import com.by.pojo.Account;
import com.by.pojo.Role;
import com.by.pojo.User;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;import java.io.IOException;
import java.io.InputStream;
import java.util.List;/*** <p>Project: mybatis - MyBatisTest</p>* <p>Powered by scl On 2023-12-18 11:44:53</p>* <p>描述：<p>** @author 孙臣龙 [1846080280@qq.com]* @version 1.0* @since 17*/
public class MyBatisTestRole {private InputStream inputStream;private SqlSession sqlSession;@Beforepublic void init() throws IOException {// 加载配置文件String resource = "mybatis-config.xml";inputStream = Resources.getResourceAsStream(resource);// 创建sqlSessionFActorySqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);// 获得数据的绘画实例sqlSession = sessionFactory.openSession();}/*** 一对多延迟加载** @throws IOException*/@Testpublic void findUserById() {UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user = userMapper.findUserById(41);System.out.println(user.getUsername());//将注释接触就可以触发懒加载//List<Account> accountList = user.getAccountList();//for (Account account : accountList) {//    System.out.println(account);//}}@Afterpublic void close() throws IOException {inputStream.close();sqlSession.close();}}

5.结果展示：

总结：懒加载的触发条件是执行"equals", “clone”, “hashCode”, "toString"这4个方法。或者是执行getComments，获取懒加载的对象时。

注意事项1：如果你报了这样的错不要着急，看我表演：

 解决方法：

 注意事项2：如果你报了这个错，你就要看看你的返回值类型是否写错了。

解决方法：仔细查看这两个地方

 最后再补充一下开启懒加载的配置：

（1）局部开启懒加载：

（2）全局懒加载：在mybaits-config配置文件中加上配置信息

<settings>
<!--        &lt;!&ndash;        开启懒加载(开启延迟加载)&ndash;&gt;-->
<setting name="lazyLoadingEnabled" value="true"/>
</settings>