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vo 使用和懒加载demo

分类: 文章 • 2025-05-06 10:37:28

懒加载，顾名思义就是懒得加载的意思吧，我是这么理解的。

当我们有两个表需要关联查询的时候，Author（用户表），和blog(博客表）。首先实体类之间的关系肯定是一对一。

然后我们现在有一个需求：查询出博客title是XXX的用户信息。

可以很清楚的知道将会怎么查询：select * from blog,Author where blog.author_id=author.id and blog.title="XXX";

使用懒加载其实又是一次查询操作：

1-->select * from blog where title="XXX"

2-->select * from author where id=author_id;

懒加载查询需要传递一些参数，还有一些其他条件,当执行第一个sql 的时候，mybatis会为author生成动态代理对象，将所有需要的信息封装到内部。

下面是相关一些代码及配置：

vo 使用和懒加载demo


创建表：

CREATE TABLE author
(
id INT NOT NULL,
username VARCHAR(10) NOT NULL,
PASSWORD VARCHAR(30) NOT NULL,
email VARCHAR(30),
bio VARCHAR(30)
);
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(1,'user1','user1','[email protected]','guy');
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(2,'user2','user2','[email protected]','guy');
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(3,'user3','user3','[email protected]','guy');
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(4,'user4','user4','[email protected]','guy');
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(5,'user5','user5','[email protected]','guy');
INSERT INTO author(id,username,PASSWORD,email,bio)
VALUES(6,'user6','user6','[email protected]','guy');




CREATE TABLE blog
(
id INT NOT NULL PRIMARY KEY,
title VARCHAR(30) DEFAULT 'My Blog',
author_id INT NOT NULL
);
INSERT INTO blog(id,title,author_id) VALUES(1,'just fun',1);
INSERT INTO blog(id,title,author_id) VALUES(2,'just funny',2);
INSERT INTO blog(id,author_id) VALUES(3,3);
INSERT INTO blog(id,author_id) VALUES(4,4);
INSERT INTO blog(id,title,author_id) VALUES(5,'hello one',5);
INSERT INTO blog(id,title,author_id) VALUES(6,'hello two',6);


实体类 Author.java和 VoAuthor.java 编写

package com.task.entity;

 

publicclass Author {

    privateintid;

    private String username;

    private String password;

    private String email;

    private String bio;

    publicint getId() {

       returnid;

    }

    publicvoid setId(int id) {

       this.id = id;

    }

    public String getUsername() {

       returnusername;

    }

    publicvoid setUsername(String username) {

       this.username = username;

    }

    public String getPassword() {

       returnpassword;

    }

    publicvoid setPassword(String password) {

       this.password = password;

    }

    public String getEmail() {

       returnemail;

    }

    publicvoid setEmail(String email) {

       this.email = email;

    }

    public String getBio() {

       returnbio;

    }

    publicvoid setBio(String bio) {

       this.bio = bio;

    }  

}

 

package com.task.entity;

 

publicclass VoAuthor{

private Author author;

private String title; //表blog 中的title 

 

public String getTitle() {

    returntitle;

}

 

publicvoid setTitle(String title) {

    this.title = title;

}

 

public Author getAuthor() {

    returnauthor;

}

 

publicvoid setAuthor(Author author) {

    this.author = author;

}

 

@Override

public String toString() {

    return"VoAuthor [author=" + author.getEmail() + ", title=" + title + "]";

}

}

 

 

 

 

 

编写 dao 层接口

package com.task.dao;

 

 

import com.task.entity.VoAuthor;

 

publicinterface AuthorDao {

 

    public VoAuthor getVoAuthor(VoAuthor author);

}

 

 

 

 

 

Mapper.xml 映射文件

<?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.task.dao.AuthorDao">

   

    <select id="getVoAuthor" parameterType="VoAuthor" resultMap="resultMap">

       SELECT * FROM blog where title=#{title}

    </select>

  

    <!--

        association:处理一对一关系

            property：和实体类属性名对应

            javaType：指定类型

            column：指定查询条件

            selectid：指定要执行的SQL

            fetchType：懒加载


       -->

    <resultMap type="com.task.entity.VoAuthor" id="resultMap">

        <association property="author" javaType="com.task.entity.Author" column="id" select="selectAuthor" fetchType="lazy"/>

    </resultMap>

    <select id="selectAuthor" parameterType="int" resultType="com.task.entity.Author">

        select * from author where id=#{id}

       

    </select>

</mapper>

 

 

 

 

 

Mapper配置文件

<?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"></properties>

    

    <settings>

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

        将积极加载改为消极加载，即按需加载 

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

    </settings>

    <typeAliases>

        <typeAlias type="com.task.entity.VoAuthor" alias="VoAuthor"/>

       

    </typeAliases>

 

   

    <environments default="development">

        <environment id="development">

            <transactionManager type="JDBC"></transactionManager>

            <dataSource type="POOLED">

                <property name="driver" value="${mysql.driver}"/>

                <property name="url" value="${mysql.url}"/>

                <property name="username" value="${mysql.username}"/>

                <property name="password" value="${mysql.password}"/>

            </dataSource>

        </environment>

       

    </environments>

    <mappers>

        <mapper resource="com/task/mapper/Authormapper.xml"/>

       

    </mappers>

</configuration>

 

 

 

 

Jdbc.properties 属性文件

mysql.driver=com.mysql.jdbc.Driver

mysql.url=jdbc:mysql://127.0.0.1:3306/test

mysql.username=root

mysql.password=123456

 

 

 

 

 

 

Log4j.properties属性文件

log4j.rootLogger=debug,c1

 

log4j.appender.c1=org.apache.log4j.ConsoleAppender

log4j.appender.c1.layout=org.apache.log4j.PatternLayout

 

 

 

 

 

测试类编写

package com.task.tests;

 

import java.io.IOException;

import java.io.Reader;

 

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 com.task.dao.AuthorDao;

import com.task.entity.VoAuthor;

 

publicclass Tests {

 

    publicstaticvoid main(String[] args) {

      

       try {

           Reader context=Resources.getResourceAsReader("config.xml");

           SqlSessionFactory factory=new SqlSessionFactoryBuilder().build(context);

           SqlSession session=factory.openSession();

           AuthorDao authorDao=session.getMapper(AuthorDao.class);

           VoAuthor author=new VoAuthor();

           author.setTitle("just fun");

            //只执行第一句SQL

           VoAuthor voAuthor=authorDao.getVoAuthor(author);

            //调用voAuthor类中toString时，会调用author.getEmail() ;这个时候执行第二句SQL

           System.out.println(voAuthor.toString());

       } catch (IOException e) {

           // TODO Auto-generated catch block

           e.printStackTrace();

       }

      

    }

}

 

 

 

 

结果：

Setting autocommit to false on JDBC Connection [[email protected]]

==>  Preparing: SELECT * FROM blog where title=?

==> Parameters: just fun(String)

<==      Total: 1

==>  Preparing: select * from author where id=?

==> Parameters: 1(Integer)

<==      Total: 1

VoAuthor [[email protected], title=just fun]



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