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: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。
如何启动延迟加载 :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。
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.
age 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());
// CustomerRepository customerRepository = sqlSession.getMapper(CustomerRepository.class);
// Customer customer = customerRepository.findCustomerById(1L);
// System.out.println(customer);
}
}