Mybatis中一对一查询实现延迟加载

User类：

package com.qublog.domain;

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

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

    //一对多关系映射，主表实体应该包含从表实体的集合引用
    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 String getAddress() {
        return address;
    }

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

    public String getSex() {
        return sex;
    }

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

    public Date getBirthday() {
        return birthday;
    }

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

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

Account类：

package com.qublog.domain;

import java.io.Serializable;

public class Account implements Serializable {
    private Integer id;
    private Integer uid;
    private Double money;

    //从表实体应该包含一个主表实体的对象引用
    private User user;

    public User getUser() {
        return user;
    }

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

    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;
    }

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

UserDao接口：

package com.qublog.dao;


import com.qublog.domain.User;

import java.util.List;

//用户的持久层接口
public interface UserDao {

    //查询所有用户，同时获取到用户下所有账户的信息
    List<User> findAll();

    User findById();
}

AccountDao接口：

package com.qublog.dao;

import com.qublog.domain.Account;

import java.util.List;

public interface AccountDao {
    //查询所有账户，同时还要获取到当前账户的所属用户信息
    List<Account> findAll();
}

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.qublog.dao.UserDao">
    <!-- 定义user的resultMap -->
    <resultMap id="userAccountMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!-- 配置user对象中accounts集合的映射 -->
        <collection property="accounts" ofType="account">
            <id property="id" column="aid"></id>
            <result property="uid" column="uid"></result>
            <result property="money" column="money"></result>
        </collection>
    </resultMap>

    <!-- 配置查询所有 -->
    <select id="findAll" resultMap="userAccountMap">
        select u.*, a.id as aid, a.uid, a.money from user u left outer join account a on a.uid=u.id;
    </select>

    <!-- 根据id查询用户 -->
    <select id="findById" parameterType="Integer" resultType="user">
        select * from user where id=#{id};
    </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.qublog.dao.AccountDao">
    <!-- 定义封装account和user的resultMap -->
    <resultMap id="accountUserMap" type="account">
        <id property="id" column="id"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
        <!-- 一对一的关系映射，配置封装user的内容
             select属性指定的内容：查询用户的唯一标识
             column属性指定的内容：用户根据id查询时，所需要的参数的值
        -->
        <association property="user" column="uid" javaType="user" select="com.qublog.dao.UserDao.findById"></association>
    </resultMap>

    <!-- 配置查询所有 -->
    <select id="findAll" resultMap="accountUserMap">
        select * from account;
    </select>
</mapper>

AccountTest类：

package com.qublog.test;

import com.qublog.dao.AccountDao;
import com.qublog.domain.Account;
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.InputStream;
import java.util.List;

public class AccountTest {
    private InputStream in = null;
    private SqlSession sqlSession = null;
    AccountDao accountDao = null;

    @Before //用于在测试方法执行之前执行
    public void init() throws Exception {
        //读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //获取SqlSessionFactory对象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //获取SqlSession对象
        sqlSession = factory.openSession();
        //获取dao代理对象
        accountDao = sqlSession.getMapper(AccountDao.class);
    }

    @After //用于在测试方法执行之后执行
    public void destroy() throws Exception {
        //提交事务
        sqlSession.commit();
        //释放资源
        sqlSession.close();
        in.close();
    }

    @Test
    public void testFindAll() throws Exception {
        //执行查询所有方法
        List<Account> accounts = accountDao.findAll();
        for (Account account:accounts) {
            System.out.println("-----------每个account的信息-------------");
            System.out.println(account);
            System.out.println(account.getUser());
        }
    }

}

测试输出：

可以看出，此时还没有真正实现延迟查询。
进入mybatis官方文档查看。
在SqlMapConfig.xml中进行修改：

<?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">
<!-- mybatis的主配置文件 -->
<configuration>
    <!-- 配置properties -->
    <properties resource="jdbcConfig.properties"></properties>

    <!-- 配置参数 -->
    <settings>
        <!-- 开启延迟加载的全局开关 -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!---->
        <setting name="aggressiveLazyLoading" value="false"></setting>
    </settings>

    <!-- 使用typeAliases配置别名，它只能配置domain中类的别名 -->
    <typeAliases>
        <!-- typeAlias用于配置别名，type属性指定的是实体类全限定类名，alias指定别名，当指定了别名就不再区分大小写 -->
        <!-- 用于指定要配置别名的包，当指定之后，该包下的实体类都会注册别名，并且类名就是别名，不再区分大小写 -->
        <package name="com.qublog.domain"></package>
    </typeAliases>

    <!-- 配置环境 -->
    <environments default="mysql">
        <!-- 配置mysql的环境 -->
        <environment id="mysql">
            <!-- 配置事务类型 -->
            <transactionManager type="JDBC"></transactionManager>
            <!-- 配置连接池 -->
            <dataSource type="POOLED">
                <!-- 配置连接数据库的基本信息 -->
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 用注解时使用class属性指定被注解的dao全限定类名 -->
    <mappers>
        <!-- package标签是用于指定dao接口所在的包，当指定了之后就不需要再写mapper以及resource或者class了 -->
        <package name="com.qublog.dao"></package>
    </mappers>
</configuration>

再次运行测试，输出结果：

将AccountTest类中修改如下：

    @Test
    public void testFindAll() throws Exception {
        //执行查询所有方法
        List<Account> accounts = accountDao.findAll();
//        for (Account account:accounts) {
//            System.out.println("-----------每个account的信息-------------");
//            System.out.println(account);
//            System.out.println(account.getUser());
//        }
    }

查看输出：

对比上面结果可以看出，已经实现延迟加载。