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前言：
什么是延迟加载？
延迟加载或者也叫惰性加载，懒加载。使用延迟加载可以提高程序的运行效率。Java程序与数据库交互的频次越低，程序运行效率越高，所以我们应该尽量减少Java程序与数据库的交互次数，MyBatis延迟加载就很好的做到了这一点。
通过一个具体的业务场景来理解延迟加载： 班级(Classes)和学生(Student)，当我们查询Student对象时，因为有级联关系，所以会将对应的Classes对象一并查询出来，这样就需要发送两条SQL语句，分别查询classes表和student表中的数据。
延迟加载的思路是：当我们查询Student的时候，如果没有调用classes属性，则只发送了一条SQL语句查询Student；如果需要调用classes属性，则发送两条SQL语句查询Student和Classes。所以延迟加载可以看做是一种优化机制，根据具体的代码，自动选择发送的SQL语句条数。
代码：
接下来我们通过代码来实现延迟加载。
1.数据库建表。
classes表：
DROP TABLE IF EXISTS `classes`;
CREATE TABLE `classes` (
`c_id` int(11) NOT NULL AUTO_INCREMENT,
`c_name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`c_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
student表：
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(11) DEFAULT NULL,
`address` varchar(11) DEFAULT NULL,
`tel` varchar(255) DEFAULT NULL,
`score` double(11,1) DEFAULT NULL,
`cid` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `tid` (`address`),
KEY `cid` (`cid`),
CONSTRAINT `student_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `classes` (`c_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
2.创建Classes实体类
package com.southwind.entity;
public class Classes {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return 'Classes [id=' + id + ', name=' + name + ']';
}
}
3.创建Student实体类
package com.southwind.entity;
public class Student {
private int id;
private String name;
private String address;
private String tel;
private int score;
private Classes classes;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getTel() {
return tel;
}
public void setTel(String tel) {
this.tel = tel;
}
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
@Override
public String toString() {
return 'Student [id=' + id + ', name=' + name + ', address=' + address
+ ', tel=' + tel + ', score=' + score + ', classes=' + classes
+ ']';
}
}
4.创建ClassesDAO
package com.southwind.dao;
import com.southwind.entity.Classes;
public interface ClassesDAO {
public Classes getById(int id);
}
5.创建StudentDAO
package com.southwind.dao;
import com.southwind.entity.Student;
public interface StudentDAO {
public Student getById(int id);
}
6.创建ClassesDAO.xml
mapper namespace='com.southwind.dao.ClassesDAO'>
resultMap type='classes' id='classesMap'>
id property='id' column='c_id'/>
result property='name' column='c_name'/>
resultMap>
select id='getById' parameterType='int' resultMap='classesMap'>
select * from classes where c_id = #{id}
select>
mapper>
7.创建StudentDAO.xml
mapper namespace='com.southwind.dao.StudentDAO'>
resultMap type='student' id='studentMap'>
id property='id' column='id'/>
result property='name' column='name'/>
result property='address' column='address'/>
result property='tel' column='tel'/>
result property='score' column='score'/>
association property='classes' javaType='classes' select='com.southwind.dao.ClassesDAO.getById' column='cid'>association>
resultMap>
select id='getById' parameterType='int' resultMap='studentMap'>
select * from student where id = #{id}
select>
mapper>
8.查询Student，输出name。
package com.southwind.test;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.StudentDAO;
import com.southwind.entity.Student;
public class Test {
public static void main(String[] args) {
InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml');
SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
SqlSession sqlSession = sqlSessionFactory.openSession();
StudentDAO studentDAO = sqlSession.getMapper(StudentDAO.class);
Student student = studentDAO.getById(1);
System.out.println(student.getName());
}
}
可以看到，执行了两条SQL，分别查询出Student对象和级联的Classes对象，但是此时我们只需要输出Student的name，没有必要去查询Classes对象，开启延迟加载，即可解决这个问题。
9.config.xml中开启延迟加载
configuration>
settings>
setting name='logImpl' value='STDOUT_LOGGING'/>
setting name='lazyLoadingEnabled' value='true' />
setting name='aggressiveLazyLoading' value='false'/>
settings>
typeAliases>
typeAlias type='com.southwind.entity.Student' alias='student'/>
typeAlias type='com.southwind.entity.Classes' alias='classes'/>
typeAliases>
environments default='development'>
environment id='development'>
transactionManager type='JDBC' />
dataSource type='POOLED'>
property name='driver' value='com.mysql.jdbc.Driver' />
property name='url' value='jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=UTF-8' />
property name='username' value='root' />
property name='password' value='root' />
dataSource>
environment>
environments>
mappers>
mapper resource='com/southwind/dao/StudentDAO.xml'/>
mappers>
configuration>
10.再次查询Student，输出name，可以看到只打印了一条SQL语句，查询Student对象。
11.查询Student，输出级联Classes对象的name。
package com.southwind.test;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.southwind.dao.StudentDAO;
import com.southwind.entity.Student;
public class Test {
public static void main(String[] args) {
InputStream is = Test.class.getClassLoader().getResourceAsStream('config.xml');
SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
SqlSession sqlSession = sqlSessionFactory.openSession();
StudentDAO studentDAO = sqlSession.getMapper(StudentDAO.class);
Student student = studentDAO.getById(1);
System.out.println(student.getClasses().getName());
}
}
可以看到执行了两条SQL，因为此时需要用到Student对象所级联的Classes对象，按需加载，所以在执行查询Student的SQL的同时，也需要执行查询Classes的SQL。
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