103、【JavaEE】【Mybatis】延迟加载

作者: yscyber | 来源:发表于2021-10-18 22:22 被阅读0次

1、概述

延迟加载，就是在需要用到数据时才进行加载，不需要用到数据时就不加载数据。延迟加载也称懒加载。
一般，“一对多”、“多对多”的情况下，会采用延迟加载，即先从一个表中查询，再去从关联的表中查询数据。
只有嵌套查询，才能实现延迟加载。

2、全局延迟加载

全局延迟加载，在 MyBatis 的核心配置文件中进行相关配置，与延迟加载有关的配置有：

<!-- 下面均是默认值 -->

<?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>
        ······
        <setting name="aggressiveLazyLoading" value="false"/>
        <setting name="lazyLoadingEnabled" value="false"/>
        <setting name="lazyLoadTriggerMethods" value="equals,clone,hashCode,toString"/>
        ······
    </settings>

     ······   

</configuration>

lazyLoadingEnabled：“开启延迟加载”。有效值true或false，默认值为false。为true时，说明开启了全局的延迟加载。

aggressiveLazyLoading：“激进的延迟加载”。有效值true或false，默认值为false（在3.4.1及之前的版本中默认为true）。一旦配置为true，只要对这个对象（查询出的对象）的任意操作，都将完整加载整个类的所有属性即执行级联的SQL语句。

lazyLoadTriggerMethods：“触发加载的方法”。有效值为用逗号分隔的方法列表，默认值为equals,clone,hashCode,toString。这个配置的含义是“除了直接使用的时候进行加载以外，哪些方法还会触发加载”。

案例一：

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

// 这里不用 @ToString，因为这个注解生成的 toString() 方法，会使用 for 循环去遍历 List

@NoArgsConstructor
@AllArgsConstructor
@Data
public class User {

    private Integer id;

    private String nickname;

    private String phoneNumber;

    private List<Order> orderList;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", nickname='" + nickname + '\'' +
                ", phoneNumber='" + phoneNumber + '\'' +
                ", orderList=" + orderList +
                '}';
    }

}

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.math.BigDecimal;

@NoArgsConstructor
@AllArgsConstructor
@Data
@ToString
public class Order {

    private Integer id;

    private BigDecimal account;

}

import com.yscyber.mybatis.five.pojo.Order;

import java.util.List;

public interface OrderRepo {

    List<Order> listOrdersByUserId(Integer userId);

}

import com.yscyber.mybatis.five.pojo.User;

public interface UserRepo {

    User getUserById(Integer id);

}

<?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.yscyber.mybatis.five.repo.OrderRepo">

    <resultMap id="orderResultMap" type="Order">
        <id property="id" column="order_id"/>
        <result property="account" column="order_account"/>
    </resultMap>

    <select id="listOrdersByUserId" parameterType="int" resultMap="orderResultMap">
        SELECT order_id, order_account
        FROM lg_order
        WHERE order_user_id=#{userId}
    </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.yscyber.mybatis.five.repo.UserRepo">

    <resultMap id="userResultMap" type="User">
        <id property="id" column="user_id"/>
        <result property="nickname" column="user_nickname"/>
        <result property="phoneNumber" column="user_phone_number"/>

        <!--
            将查询出的 column="user_id" 作为 listOrdersByUserId 参数
         -->
        <collection property="orderList" ofType="Order" select="com.yscyber.mybatis.five.repo.OrderRepo.listOrdersByUserId" column="user_id">
            <id property="id" column="order_id"/>
            <result property="account" column="order_account"/>
        </collection>
    </resultMap>
    
    <select id="getUserById" parameterType="int" resultMap="userResultMap">
        SELECT user_id, user_nickname, user_phone_number
        FROM lg_user
        WHERE user_id=#{id}
    </select>

</mapper>

<?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>
    
    <properties resource="jdbc.properties"/>
    
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>

        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
<!-- 为了能够更直观的看到“延迟加载”，将“触发的方法”设置为空，否则打印输出也触发加载 -->
        <setting name="lazyLoadTriggerMethods" value=""/>
    </settings>

    <typeAliases>
        <package name="com.yscyber.mybatis.five.pojo"/>
    </typeAliases>
    
    <plugins>
        <plugin interceptor="com.github.pagehelper.PageInterceptor">
            <property name="helperDialect" value="mysql"/>
        </plugin>
    </plugins>

    <environments default="dev">
        <!-- 配置数据源 -->
        <environment id="dev">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <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>

    <mappers>
        <package name="com.yscyber.mybatis.five.repo"/>
    </mappers>
    
</configuration>

    @Test
    public void test1() throws IOException {
        InputStream ris = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(ris);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserRepo userRepo = sqlSession.getMapper(UserRepo.class);
        User user = userRepo.getUserById(1);
        System.out.println(user); // User{id=1, nickname='张三', phoneNumber='1820000000', orderList=null}

        List<Order> orderList = user.getOrderList(); // 执行此语句时，才执行子查询
        for (Order order : orderList) {
            System.out.println(order);
        }

        sqlSession.close();
    }

案例二：aggressiveLazyLoading为true时，查询出的对象只要进行任何操作，立即触发加载

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@NoArgsConstructor
@AllArgsConstructor
@Data
public class User {

    private Integer id;

    private String nickname;

    private String phoneNumber;

    private List<Order> orderList;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", nickname='" + nickname + '\'' +
                ", phoneNumber='" + phoneNumber + '\'' +
                ", orderList=" + orderList +
                '}';
    }

}

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.math.BigDecimal;

@NoArgsConstructor
@AllArgsConstructor
@Data
@ToString
public class Order {

    private Integer id;

    private BigDecimal account;

}

import com.yscyber.mybatis.five.pojo.Order;

import java.util.List;

public interface OrderRepo {

    List<Order> listOrdersByUserId(Integer userId);

}

import com.yscyber.mybatis.five.pojo.User;

public interface UserRepo {

    User getUserById(Integer id);

}

<?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.yscyber.mybatis.five.repo.OrderRepo">

    <resultMap id="orderResultMap" type="Order">
        <id property="id" column="order_id"/>
        <result property="account" column="order_account"/>
    </resultMap>

    <select id="listOrdersByUserId" parameterType="int" resultMap="orderResultMap">
        SELECT order_id, order_account
        FROM lg_order
        WHERE order_user_id=#{userId}
    </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.yscyber.mybatis.five.repo.UserRepo">

    <resultMap id="userResultMap" type="User">
        <id property="id" column="user_id"/>
        <result property="nickname" column="user_nickname"/>
        <result property="phoneNumber" column="user_phone_number"/>

        <!--
            将查询出的 column="user_id" 作为 listOrdersByUserId 参数
         -->
        <collection property="orderList" ofType="Order" select="com.yscyber.mybatis.five.repo.OrderRepo.listOrdersByUserId" column="user_id">
            <id property="id" column="order_id"/>
            <result property="account" column="order_account"/>
        </collection>
    </resultMap>
    
    <select id="getUserById" parameterType="int" resultMap="userResultMap">
        SELECT user_id, user_nickname, user_phone_number
        FROM lg_user
        WHERE user_id=#{id}
    </select>

</mapper>

<?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>
    
    <properties resource="jdbc.properties"/>
    
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>

        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="true"/>
        <setting name="lazyLoadTriggerMethods" value=""/>
    </settings>

    <typeAliases>
        <package name="com.yscyber.mybatis.five.pojo"/>
    </typeAliases>
    
    <plugins>
        <plugin interceptor="com.github.pagehelper.PageInterceptor">
            <property name="helperDialect" value="mysql"/>
        </plugin>
    </plugins>

    <environments default="dev">
        <!-- 配置数据源 -->
        <environment id="dev">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <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>

    <mappers>
        <package name="com.yscyber.mybatis.five.repo"/>
    </mappers>
    
</configuration>

    @Test
    public void test1() throws IOException {
        InputStream ris = Resources.getResourceAsStream("mybatis.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(ris);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserRepo userRepo = sqlSession.getMapper(UserRepo.class);
        User user = userRepo.getUserById(1);
        System.out.println(user); // 由于“打印输出”相当于调用了对象的 toString() 方法，所以在 aggressiveLazyLoading 为 true 的情况下，触发加载

        sqlSession.close();
    }

3、局部延迟加载

局部配置的优先级高于全局配置，如果局部要求非延迟（映射配置文件中配置），全局要求延迟（核心配置文件中配置），以局部为准。
局部延迟加载，配置在映射文件中。fetchType="lazy"表示延迟加载，fetchType="eager"表示非延迟加载

<?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.yscyber.mybatis.five.repo.UserRepo">

    <resultMap id="userResultMap" type="User">
        <id property="id" column="user_id"/>
        <result property="nickname" column="user_nickname"/>
        <result property="phoneNumber" column="user_phone_number"/>

        <!--
            将查询出的 column="user_id" 作为 listOrdersByUserId 参数
         -->
        <collection property="orderList" ofType="Order" select="com.yscyber.mybatis.five.repo.OrderRepo.listOrdersByUserId" column="user_id" fetchType="lazy">
            <id property="id" column="order_id"/>
            <result property="account" column="order_account"/>
        </collection>
    </resultMap>
    
    <select id="getUserById" parameterType="int" resultMap="userResultMap">
        SELECT user_id, user_nickname, user_phone_number
        FROM lg_user
        WHERE user_id=#{id}
    </select>

</mapper>

103、【JavaEE】【Mybatis】延迟加载

1、概述

2、全局延迟加载

3、局部延迟加载

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