Mybatis一对一延迟加载实现过程解析

需求：用户和账户一对一关系，查询账户时实现用户的延迟加载

思路：根据id查询，需要延迟加载的一方

1、用户实体类

package com.yl.bean;

import java.io.Serializable;
import java.sql.Date;
import java.util.List;

/**
* 用户实体类
*/
public class User implements Serializable {
 private Integer id;//id
 private String username;//用户名
 private Date birthday;//生日
 private String sex;//性别
 private String address;//地址
 private Account account;//用户所拥有的账户

public User() {
 }

public Integer getId() {
   return id;
 }

public void setId(Integer id) {
   this.id = id;
 }

public String getUsername() {
   return username;
 }

public void setUsername(String username) {
   this.username = username;
 }

public Date getBirthday() {
   return birthday;
 }

public void setBirthday(Date birthday) {
   this.birthday = birthday;
 }

public String getSex() {
   return sex;
 }

public void setSex(String sex) {
   this.sex = sex;
 }

public String getAddress() {
   return address;
 }

public void setAddress(String address) {
   this.address = address;
 }

public Account getAccount() {
   return account;
 }

public void setAccount(Account account) {
   this.account = account;
 }

@Override
 public String toString() {
   return "User{" +
       "id=" + id +
       ", username='" + username + '\'' +
       ", birthday=" + birthday +
       ", sex='" + sex + '\'' +
       ", address='" + address + '\'' +
       '}';
 }
}

2、账户实体类

package com.yl.bean;

import java.io.Serializable;

/**
* 账户实体类
*/
public class Account implements Serializable {
 private Integer id;//账户id
 private Integer uid;//用户id
 private Double money;//余额
 private User user;//账户所属用户

public Account() {
 }

public Integer getId() {
   return id;
 }

public void setId(Integer id) {
   this.id = id;
 }

public Integer getUid() {
   return uid;
 }

public void setUid(Integer uid) {
   this.uid = uid;
 }

public Double getMoney() {
   return money;
 }

public void setMoney(Double money) {
   this.money = money;
 }

public User getUser() {
   return user;
 }

public void setUser(User user) {
   this.user = user;
 }

@Override
 public String toString() {
   return "Account{" +
       "id=" + id +
       ", uid=" + uid +
       ", money=" + money +
       '}';
 }
}

3、用户持久层接口

package com.yl.dao;

import com.yl.bean.User;

import java.util.List;

/**
* 用户持久层接口
*/
public interface IUserDao {

/**
  * 查询所有用户
  */
 List<User> queryAll();

/**
  * 根据id查询用户
  */
 User queryById(int id);

}

4、账户持久层接口

package com.yl.dao;

import com.yl.bean.Account;
import com.yl.bean.User;

import java.util.List;

/**
* 账户持久层接口
*/
public interface IAccountDao {

/**
  * 查询所有账户
  */
 List<Account> queryAll();
}

5、全局配置文件

<?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>

<settings>
   <!--开启延迟加载-->
   <setting name="lazyLoadingEnabled" value="true"/>
   <setting name="aggressiveLazyLoading" value="false"/>
 </settings>

<!--配置别名-->
 <typeAliases>
   <package name="com.yl.bean"/>
 </typeAliases>

<!--配置mybatis环境-->
 <environments default="mysql">
   <!--配置mysql环境-->
   <environment id="mysql">
     <!--事务类型-->
     <transactionManager type="JDBC"></transactionManager>
     <!--数据源-->
     <dataSource type="POOLED">
       <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
       <property name="url" value="jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf8&amp;serverTimezone=GMT％2B8"/>
       <property name="username" value="root"/>
       <property name="password" value="123456"/>
     </dataSource>
   </environment>
 </environments>

<!--指定映射配置文件位置-->
 <mappers>
   <mapper resource="IUserDao.xml"></mapper>
   <mapper resource="IAccountDao.xml"></mapper>
 </mappers>
</configuration>

6、用户映射配置文件

<?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.yl.dao.IUserDao">
 <!--配置用户实体类和数据库的对应信息-->
 <resultMap id="userMap" type="com.yl.bean.User">
   <id property="id" column="id"></id>
   <result property="username" column="username"></result>
   <result property="birthday" column="birthday"></result>
   <result property="sex" column="sex"></result>
   <result property="address" column="address"></result>
   <!--一对一映射关系-->
   <collection property="account" ofType="com.yl.bean.Account">
     <id property="id" column="id"></id>
     <result property="uid" column="uid"></result>
     <result property="money" column="money"></result>
   </collection>
 </resultMap>

<!--查询所有用户-->
 <select id="queryAll" resultMap="userMap">
   SELECT * FROM USER u LEFT OUTER JOIN account a ON u.`id`=a.`UID`
 </select>
 <!--根据id查询用户-->
 <select id="queryById" resultType="user" parameterType="int">
   select * from user where id=#{id}
 </select>
</mapper>

7、账户映射配置文件

<?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.yl.dao.IAccountDao">
 <!--配置Account属性对应数据库信息-->
 <resultMap id="accountUserMap" type="account">
   <id property="id" column="id"></id>
   <result property="uid" column="uid"></result>
   <result property="money" column="money"></result>
   <!--通过id查询用户-->
   <association property="user" column="uid" javaType="user" select="com.yl.dao.IUserDao.queryById"></association>
 </resultMap>

<!--查询所有账户-->
 <select id="queryAll" resultMap="accountUserMap">
   SELECT * FROM account
 </select>

</mapper>

来源：https://www.cnblogs.com/Y-wee/p/13851952.html