Mybatis延迟加载

延迟加载测试观察延迟加载的方式

1.延迟加载

输出语句为：
System.out.println(account.getAid());
测试结果

可以看出，只需要获取id，所以就不用再获取account的user属性
通过日志可以看出只会有一个查询语句

select * from account

2.非延迟加载（积极加载）

会有两条查询语句

Mybatis的延迟加载，两种配置方式：

注意：
如果配置了全局为延迟加载，在映射文件中配置了积极加载，那么改映射文件配置的那个方法，将会积极加载。

1.在核心配置文件中配置（全局配置）

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

2.在映射文件中配置（局部配置）

  <association fetchType="lazy" property="user" javaType="user"
                     column="userId" select="com.william.dao.UserDao.findId"></association>

一、目录结构

二、maven

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.william</groupId>
    <artifactId>Mybatis_Day04_OTO_lazy</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
    </dependencies>

</project>

三、domain实体类

1、 Account

package com.william.domain;

/**
 * @author ：lijunxuan
 * @date ：Created in 2020/2/2  20:35
 * @description ：
 * @version: 1.0
 */
public class Account {
    private Integer aid;
    private String accountname;
    private Float money;
    private Integer userId;
    private User user;

    @Override
    public String toString() {
        return "Account{" +
                "aid=" + aid +
                ", accountname='" + accountname + '\'' +
                ", money=" + money +
                ", userId=" + userId +
                ", user=" + user +
                '}';
    }

    public Integer getAid() {
        return aid;
    }

    public void setAid(Integer aid) {
        this.aid = aid;
    }

    public String getAccountname() {
        return accountname;
    }

    public void setAccountname(String accountname) {
        this.accountname = accountname;
    }

    public Float getMoney() {
        return money;
    }

    public void setMoney(Float money) {
        this.money = money;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }
}

2、User

package com.william.domain;

/**
 * @author ：lijunxuan
 * @date ：Created in 2020/2/2  20:37
 * @description ：
 * @version: 1.0
 */
public class User {
   private Integer id;
   private String username;
   private String PASSWORD;
   private String sex;
   private String birthday;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", PASSWORD='" + PASSWORD + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday='" + birthday + '\'' +
                '}';
    }

    public Integer getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPASSWORD() {
        return PASSWORD;
    }

    public void setPASSWORD(String PASSWORD) {
        this.PASSWORD = PASSWORD;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getBirthday() {
        return birthday;
    }

    public void setBirthday(String birthday) {
        this.birthday = birthday;
    }
}

四、Dao 接口

1、AccountDao

package com.william.dao;

import com.william.domain.Account;

import java.util.List;

/**
 * @author ：lijunxuan
 * @date ：Created in 2020/2/3  11:58
 * @description ：
 * @version: 1.0
 */
public interface AccountDao {
    List<Account> findAll();
}

2、UserDao

package com.william.dao;

public interface UserDao {
   void findId(Integer id);
}

五、映射文件

延迟加载

1、AccountDao.xml

<?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.william.dao.AccountDao">
    <resultMap id="resultMap" type="account">
        <id column="aid" property="aid"></id>
        <result column="accountname" property="accountname"></result>
        <result column="money" property="money"></result>
        <result column="userId" property="userId"></result>
        <!--association ; 关联 联想 配置一对一映射-->
        <!--
        property ：属性名
        javaType ： 属性的类型
        -->
        <association fetchType="lazy" property="user" javaType="user" column="userId" select="com.william.dao.UserDao.findId"></association>
    </resultMap>
    <select id="findAll" resultMap="resultMap">
        select * from account
    </select>
</mapper>

2、UserDao.xml

<?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.william.dao.UserDao">
    <select id="findId" parameterType="int" resultType="user">
        select * from user where id=#{id}
    </select>
</mapper>

六、核心配置文件

1、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="db.properties"></properties>
    <typeAliases>
        <package name="com.william.domain"></package>
    </typeAliases>
    <environments default="develoment">
        <environment id="develoment">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"></property>
                <property name="url" value="${jdbc.url}"></property>
                <property name="username" value="${jdbc.user}"></property>
                <property name="password" value="${jdbc.password}"></property>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--<dao resource="com/william/Mapper.xml"></dao>-->
        <package name="com.william.dao"></package>
    </mappers>
</configuration>

也可以进行全局配置延迟加载

<?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="db.properties"></properties>
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>
    <typeAliases>
        <package name="com.william.domain"></package>
    </typeAliases>
    <environments default="develoment">
        <environment id="develoment">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"></property>
                <property name="url" value="${jdbc.url}"></property>
                <property name="username" value="${jdbc.user}"></property>
                <property name="password" value="${jdbc.password}"></property>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--<dao resource="com/william/Mapper.xml"></dao>-->
        <package name="com.william.dao"></package>
    </mappers>
</configuration>

七、数库配置文件

1、db.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/web02
jdbc.user=root
jdbc.password=root

2、log4j.properties

# Set root category priority to INFO and its only appender to CONSOLE.
#log4j.rootCategory=INFO, CONSOLE            debug   info   warn error fatal
log4j.rootCategory=debug, CONSOLE, LOGFILE, info

# Set the enterprise logger category to FATAL and its only appender to CONSOLE.
log4j.logger.org.apache.axis.enterprise=FATAL, CONSOLE

# CONSOLE is set to be a ConsoleAppender using a PatternLayout.
log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender
log4j.appender.CONSOLE.layout=org.apache.log4j.PatternLayout
log4j.appender.CONSOLE.layout.ConversionPattern=%d{ISO8601} %-6r [%15.15t] %-5p %30.30c %x - %m\n

# LOGFILE is set to be a File appender using a PatternLayout.
log4j.appender.LOGFILE=org.apache.log4j.FileAppender
log4j.appender.LOGFILE.File=d:\axis.log
log4j.appender.LOGFILE.Append=true
log4j.appender.LOGFILE.layout=org.apache.log4j.PatternLayout
log4j.appender.LOGFILE.layout.ConversionPattern=%d{ISO8601} %-6r [%15.15t] %-5p %30.30c %x - %m\n

八、测试类

1、MybatisTest.java

package com.william;

import com.william.dao.AccountDao;
import com.william.domain.Account;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.util.List;

/**
 * @author ：lijunxuan
 * @date ：Created in 2020/2/3  12:18
 * @description ：
 * @version: 1.0
 */
public class MybatisTest {
    /**
     * 一对一映射
     */
    @Test
    public void testOTO(){
        try {
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("Mybatis-config.xml"));
            SqlSession sqlSession = sqlSessionFactory.openSession(true);
            AccountDao mapper = sqlSession.getMapper(AccountDao.class);
            List<Account> list = mapper.findAll();
            for (Account account : list) {
                System.out.println(account.getAid());
            }
            sqlSession.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

九、懒加载测试结果

来源：CSDN

作者：L-李俊漩

链接：https://blog.csdn.net/sinat_39179993/article/details/104158768