MyBatis框架——延迟加载

　　延迟加载也叫惰性加载或者懒加载，使⽤延迟是为了提⾼程序的运⾏效率，具体是通过尽量少执⾏ SQL 语句来提升效率。Java 程序与数据库的交互频率越低越好，MyBatis 提供的延迟加载功能就可以做到这 ⼀点。

　　延迟加载是作⽤于级联查询的场景下：

　　　　查询⽬标表时，如果仅查询⽬标表就可以获取相应的字段，则不需要查询级联表；

　　　　除⾮必须查询级联表才能获取的字段，则查询级联表，通过延迟加载可以有效减少 SQL 语句的执⾏次数。

　　延迟加载的思路：

　　　　将级联查询的 SQL 语句进⾏拆分，不要⽤⼀条 SQL 语句进⾏级联查询，改为两条 SQL 的单表查询。

　　　　当我们查询 Order 的时候，如果没有访问 customer 的属性，则只发送⼀条 SQL 语句查询 Order；

　　　　如果需要访问 customer 的属性，则发送两条 SQL 语句查询 Order 和 Customer。

示例：

1、创建数据库

2、创建实体类

package com.sunjian.entity;

import java.util.List;

/**
 * @author sunjian
 * @date 2020/3/23 14:37
 */
public class Customer {
    private Long id;
    private String name;
    private List<Order> orders;

    public List<Order> getOrders() {
        return orders;
    }

    public void setOrders(List<Order> orders) {
        this.orders = orders;
    }

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", orders=" + orders +
                '}';
    }
}

package com.sunjian.entity;

/**
 * @author sunjian
 * @date 2020/3/23 14:38
 */
public class Order {
    public Long getId() {
        return id;
    }

    @Override
    public String toString() {
        return "Order{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", customer=" + customer +
                '}';
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }

    private Long id;
    private String name;
    private Customer customer;

}

3、创建repository接口

package com.sunjian.repository;

import com.sunjian.entity.Customer;
import com.sunjian.entity.Order;

/**
 * @author sunjian
 * @date 2020/3/23 14:41
 */
public interface CustomerRepository {
    public Customer findCustomerById(Long id);
}

package com.sunjian.repository;

import com.sunjian.entity.Order;

/**
 * @author sunjian
 * @date 2020/3/23 14:41
 */
public interface OrderRepository {
    public Order findOrderById(Long id);
}

4、创建repository.xml编写SQL（单独查询）

<?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.sunjian.repository.CustomerRepository">

    <select id="findCustomerById" parameterType="java.lang.Long" resultType="com.sunjian.entity.Customer">
            select * from t_customer 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.sunjian.repository.OrderRepository">

    <resultMap id="orderMap" type="com.sunjian.entity.Order">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <!-- 将查询结果中的cid字段，映射为另一个接口中的参数；达到级联查询-->
        <association property="customer" javaType="com.sunjian.entity.Customer" column="cid" select="com.sunjian.repository.CustomerRepository.findCustomerById"></association>
    </resultMap>

    <select id="findOrderById" parameterType="java.lang.Long" resultMap="orderMap">
            select * from t_order where id = #{id}
    </select>
</mapper>

5、创建config.xml全局配置文件（开启延迟加载配置），将repository.xml注册到config.xml中

<?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>
    <!-- 打印SQL语句 -->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
        <!-- 开启延迟加载 -->
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

    <!-- 配置 MyBatis 数据源 -->
    <environments default="development">
        <environment id="development">
            <!-- JDBC事务管理 -->
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 注册 -->
    <mappers>
        <!-- 单表 -->
        <mapper resource="com/sunjian/mapper/UserMapper.xml"></mapper>
        <mapper resource="com/sunjian/repository/UserRepository.xml"></mapper>

        <!-- 多表 一对一 -->
        <mapper resource="com/sunjian/repository/StudentRepository.xml"></mapper>
        <!-- 多表 一对多 -->
        <mapper resource="com/sunjian/repository/ClassesRepository.xml"></mapper>
        <!-- 多表 多对多 -->
        <mapper resource="com/sunjian/repository/CourseRepository.xml"></mapper>

        <mapper resource="com/sunjian/repository/OrderRepository.xml"></mapper>
        <mapper resource="com/sunjian/repository/CustomerRepository.xml"></mapper>

    </mappers>

</configuration>

6、创建测试类

package com.sunjian.test;

import com.sunjian.entity.Customer;
import com.sunjian.entity.Order;
import com.sunjian.entity.User;
import com.sunjian.repository.CustomerRepository;
import com.sunjian.repository.OrderRepository;
import com.sunjian.repository.UserRepository;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;

/**
 * @author sunjian
 * @date 2020/3/22 10:20
 */
public class Test {
    public static void main(String[] args) {

        // Mapper 代理实现自定义接口
        InputStream inputStream = Order.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 获取接口的处理对象
        OrderRepository orderRepository = sqlSession.getMapper(OrderRepository.class);
        Order order = orderRepository.findOrderById(1L);
        System.out.println(order.getName());

    }

}

开启延迟加载

Opening JDBC Connection
Created connection 793315160.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2f490758]
==>  Preparing: select * from t_order where id = ?
==> Parameters: 1(Long)
<==    Columns: id, name, cid
<==        Row: 1, 订单1, 1
<==      Total: 1
订单1

未开启延迟加载

Opening JDBC Connection
Created connection 793315160.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2f490758]
==>  Preparing: select * from t_order where id = ?
==> Parameters: 1(Long)
<==    Columns: id, name, cid
<==        Row: 1, 订单1, 1
====>  Preparing: select * from t_customer where id = ?
====> Parameters: 1(Long)
<====    Columns: id, name
<====        Row: 1, 张三
<====      Total: 1
<==      Total: 1
订单1

OK.