Mybatis一对一延迟加载实现过程解析

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 + '\'' +
        '}';
  }
}

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 +
        '}';
  }
}

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);

}

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();
}

<?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>

<?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>

<?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>