Mybatis之延迟加载机制

1.  延迟加载的含义： 用到的时候才会去进行相关操作

2.  延迟加载的例子：

2.1 spring的BeanFactory，在getBean（）的时候才创建Bean

2.2 物理分页查询，只有点击某一页时采取数据库查询该页的数据

3. Mybatis启用延迟加载机制

<setting name="lazyLoadingEnabled" value="true"/>

4. Mybatis延迟加载的策略：

　　侵入式延迟加载机制【如果只查询主表数据而不进行使用，级联表的数据不会被查询；如果使用了主表数据，即使级联表的数据没有使用，也会查询】

4.1 实例一：只查询主表数据而不进行使用

package com.blueStarWei.test;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import com.blueStarWei.entity.TPersonInfo;
import com.blueStarWei.mappers.PersonAddressMapper;
import com.blueStarWei.utils.SqlSessionFactoryUtil;

public class TestPersonAddress {

    public static void main(String[] args) {
        SqlSession session = SqlSessionFactoryUtil.openSession();
        PersonAddressMapper mapper = session.getMapper(PersonAddressMapper.class);

        List<TPersonInfo> info = mapper.findAllWithAddress();

    }
}

4.1.1 日志

[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Opening JDBC Connection
[org.apache.ibatis.datasource.pooled.PooledDataSource]-Created connection 836514715.
[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@31dc339b]
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==>  Preparing: select * from t_person_info
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==> Parameters:
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-<==      Total: 1

4.2 实例二：查询并使用主表数据

package com.blueStarWei.test;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import com.blueStarWei.entity.TPersonInfo;
import com.blueStarWei.mappers.PersonAddressMapper;
import com.blueStarWei.utils.SqlSessionFactoryUtil;

public class TestPersonAddress {

    public static void main(String[] args) {
        SqlSession session = SqlSessionFactoryUtil.openSession();
        PersonAddressMapper mapper = session.getMapper(PersonAddressMapper.class);

        List<TPersonInfo> info = mapper.findAllWithAddress();

        for (TPersonInfo tPersonInfo : info) {
            System.out.println(tPersonInfo.getName());
        }
    }
}

4.2.2 日志

[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Opening JDBC Connection
[org.apache.ibatis.datasource.pooled.PooledDataSource]-Created connection 836514715.
[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@31dc339b]
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==>  Preparing: select * from t_person_info
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==> Parameters:
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-<==      Total: 1
[com.blueStarWei.mappers.AddressMapper.findById]-==>  Preparing: SELECT * FROM t_address t where t.id = ?
[com.blueStarWei.mappers.AddressMapper.findById]-==> Parameters: 1(Integer)
[com.blueStarWei.mappers.AddressMapper.findById]-<==      Total: 1

5. Mybatis禁用侵入式延迟加载【使用到数据才会去查找相关表】

<setting name="aggressiveLazyLoading" value="false"/>

5.1 实例一：只使用主表数据

package com.blueStarWei.test;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import com.blueStarWei.entity.TPersonInfo;
import com.blueStarWei.mappers.PersonAddressMapper;
import com.blueStarWei.utils.SqlSessionFactoryUtil;

public class TestPersonAddress {

    public static void main(String[] args) {
        SqlSession session = SqlSessionFactoryUtil.openSession();
        PersonAddressMapper mapper = session.getMapper(PersonAddressMapper.class);

        List<TPersonInfo> info = mapper.findAllWithAddress();

        for (TPersonInfo tPersonInfo : info) {
            System.out.println(tPersonInfo.getName());
        }
    }
}

5.1.1 日志

[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Opening JDBC Connection
[org.apache.ibatis.datasource.pooled.PooledDataSource]-Created connection 836514715.
[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@31dc339b]
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==>  Preparing: select * from t_person_info
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==> Parameters:
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-<==      Total: 1

5.2 实例二：使用级联表数据

package com.blueStarWei.test;

import java.util.List;

import org.apache.ibatis.session.SqlSession;

import com.blueStarWei.entity.TPersonInfo;
import com.blueStarWei.mappers.PersonAddressMapper;
import com.blueStarWei.utils.SqlSessionFactoryUtil;

public class TestPersonAddress {

    public static void main(String[] args) {
        SqlSession session = SqlSessionFactoryUtil.openSession();
        PersonAddressMapper mapper = session.getMapper(PersonAddressMapper.class);

        List<TPersonInfo> info = mapper.findAllWithAddress();

        for (TPersonInfo tPersonInfo : info) {
            System.out.println(tPersonInfo.getAddress());
        }
    }
}

5.2.1 日志

[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Opening JDBC Connection
[org.apache.ibatis.datasource.pooled.PooledDataSource]-Created connection 836514715.
[org.apache.ibatis.transaction.jdbc.JdbcTransaction]-Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@31dc339b]
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==>  Preparing: select * from t_person_info
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-==> Parameters:
[com.blueStarWei.mappers.PersonAddressMapper.findAllWithAddress]-<==      Total: 1
[com.blueStarWei.mappers.AddressMapper.findById]-==>  Preparing: SELECT * FROM t_address t where t.id = ?
[com.blueStarWei.mappers.AddressMapper.findById]-==> Parameters: 1(Integer)
[com.blueStarWei.mappers.AddressMapper.findById]-<==      Total: 1

6. 补充

6.1 启用log4j打印日志

导入log4j.jar， 将log4j.properties文件与ybatis-config.xml放到一起【会自动加载，无需配置】

7.附录：

7.1 建表语句【Mysql】

CREATE TABLE `t_person_info` (
   `id` int(2) NOT NULL AUTO_INCREMENT,
   `name` varchar(10) DEFAULT NULL,
   `age` int(3) DEFAULT NULL,
   `addressId` int(3) DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) 

CREATE TABLE `t_address` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `country` varchar(10) DEFAULT NULL,
   `city` varchar(10) DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) 

7.2 mybatis-config.xml

<?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>
        <!-- 延迟加载的全局开关。当开启时，所有的关联对象会延迟加载 。默认false-->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 禁用侵入式延迟加载 -->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

    <!-- 类型别名：给Java类起一个简单的名字 -->
    <typeAliases>
        <package name="com.blueStarWei.entity"/>
    </typeAliases>

    <!-- 环境信息 -->
    <environments default="development">
        <environment id="development">
            <!-- myBatis支持两种事务管理：JDBC和MANAGED(托管)
                 MANAGED一般商业服务器才有此功能，例如JBOSS-->
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driverClassName}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <package name="com.blueStarWei.mappers"/>
    </mappers>
</configuration>

7.3 TpersonInfo.java

package com.blueStarWei.entity;

public class TPersonInfo {

    private Integer id;
    private String name;
    private Integer age;
    private Address address;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }

    public Address getAddress() {
        return address;
    }
    public void setAddress(Address address) {
        this.address = address;
    }

}

7.4 Addreess.java

package com.blueStarWei.entity;

public class Address {

    private int id;
    private String country;
    private String city;

    public String getCountry() {
        return country;
    }
    public void setCountry(String country) {
        this.country = country;
    }
    public String getCity() {
        return city;
    }
    public void setCity(String city) {
        this.city = city;
    }

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    @Override
    public String toString() {
        return "Address [country=" + country + ", city=" + city + "]";
    }

}

7.5 log4j.properties

log4j.rootLogger=debug,console

log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n

7.6 mybatis相关架包：

https://pan.baidu.com/s/1248sNRHReiK8S3pENKfmlA  密码：xpe7