MyBatis的延迟加载（一对多查询案例）

1.什么是延迟加载？

开启延迟加载后，在真正使用数据的时候才发起级联查询，不用的时候不查询。

2.pojo

User类：

package com.wt.pojo;

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

public class User implements Serializable {
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;

    //加入List<Account>存储用户所拥有的账户
    private List<Account> accounts;

    public List<Account> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<Account> accounts) {
        this.accounts = accounts;
    }

    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;
    }

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

Account类：略

3.mapper

UserDao:

public interface UserDao {
    public List<User> findAll();
}

AccountDao:

public interface AccountDao {
    Account findAccountById(Integer id);
}

UserDao.xml:

<?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.wt.dao.UserDao">
    <resultMap type="User" id="findAllResultMap">
        <id column="id" property="id"></id>
        <result column="username" property="username"/>
        <result column="address" property="address"/>
        <result column="sex" property="sex"/>
        <result column="birthday" property="birthday"/>
         <!--
          property：属性名
          ofType：泛型
          select： 要调用的 select 映射的 id
          column ： 传递给 select 映射的参数
          fetchType="lazy"：懒加载，默认情况下是没有开启延迟加载的
         -->
        <collection property="accounts" ofType="Account"
                    select="com.by.dao.AccountDao.findAccountById" column="id"
                    fetchType="lazy">	
        </collection>
    </resultMap>

    <select id="findAll" resultMap="findAllResultMap">
      select * from user
    </select>
</mapper>

AccountDao.xml:

<?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.wt.dao.AccountDao">
    <select id="findAccountById" parameterType="int" resultType="account">
        select * from account where uid = #{id}
    </select>
</mapper>

4.测试

    @Test
    public void testFindAll() {
        UserDao userDao = sqlSession.getMapper(UserDao.class);
        List<User> userList = userDao.findAll();
        for(User user : userList){
            System.out.println(user.getUsername());//不查询account
            System.out.println(user.getAccounts());//查询account
        }
    }

5.结果

不查询Account:

查询Account：