MyBatis 延遲加載（懶加載）—— association 延遲加載、collection 延遲加載

一、概述

1、延遲加載：

• 就是在需要用到數據時才進行加載，不需要用到數據時就不加載數據。延遲加載也稱懶加載

2、好處：

• 先從單表查詢，需要時再從關聯表去關聯查詢，大大提高數據庫性能，因爲查詢單表要比關聯查詢多張錶速度要快。

3、壞處：

• 因爲只有當需要用到數據時，纔會進行數據庫查詢，這樣在大批量數據查詢時，因爲查詢工作也要消耗時間，所以可能造成用戶等待時間變長，造成用戶體驗下降。

4、MyBatis 多表查詢
具體查詢步驟請 點擊
在多表查詢時，使用了resultMap來實現一對一，一對多，多對多關係的操作。主要是通過 association、collection 實現一對一及一對多映射。association、collection 具備延遲加載功能。

二、association 延遲加載、collection 延遲加載

association : 用於一對多（一個用戶對應多個賬號：User ——> Account）

collection ：用於多對一

第一步：封裝對數據庫映射的實體類 User.java、Account.java

package cn.lemon.domain;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class User implements Serializable {
    private Integer id;
    private Date birthday;
    private String username;
    private String sex;
    private String address;

    private List<Account> accountList = new ArrayList<>();

    public Integer getId() {
        return id;
    }

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

    public Date getBirthday() {
        return birthday;
    }

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

    public String getUsername() {
        return username;
    }

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

    public String getSex() {
        return sex;
    }

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

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public List<Account> getAccountList() {
        return accountList;
    }

    public void setAccountList(List<Account> accountList) {
        this.accountList = accountList;
    }

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

package cn.lemon.domain;

import java.io.Serializable;

public class Account implements Serializable {
    private Integer id;
    private Integer uid;
    private Double money;

    private User user;//關聯屬性，記錄關聯關係

    public Integer getId() {
        return id;
    }

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

    public Integer getUid() {
        return uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    public Double getMoney() {
        return money;
    }

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

    public User getUser() {
        return user;
    }

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

    @Override
    public String toString() {
        return "Account{" +
                "id=" + id +
                ", uid=" + uid +
                ", money=" + money +
                '}';
    }
}

第二步：新建持久層（dao）接口 IUserDao.java 、IAccountDao.java，操作數據庫

package cn.lemon.dao;

import cn.lemon.domain.User;

import java.util.List;

public interface IUserDao {
    public User findById(Integer userId);

    public List<User> findAll();
}

package cn.lemon.dao;

import cn.lemon.domain.Account;

import java.util.List;

public interface IAccountDao {
    public List<Account> findAll();
}

第三步：新建配置文件 SqlMapConfig.xml （配置懶加載）和 jdbcConfig.properties

<?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">
<!--mybatis配置文件-->
<configuration>
    <!--配置屬性或使用屬性文件-->
    <properties resource="jdbc.properties"/>

    <!--全局屬性配置-->
    <settings>
        <!--開啓Mybatis支持延遲加載-->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!--關閉積極延遲加載：任何屬性都會引起延遲加載-->
        <setting name="aggressiveLazyLoading" value="false"></setting>
    </settings>

    <!--給包定義別名-->
    <typeAliases>
        <!--定義單個別名-->
        <!--<typeAlias type="com.lxs.domain.User" alias="user"></typeAlias>-->
        <!--配置包定義別名，別名=包中的類的名字（首字母大寫小寫都可以)-->
        <package name="cn.lemon.domain"/>
    </typeAliases>
    <!--配置環境-->
    <environments default="mysql">
        <environment id="mysql">
            <!--事務類型-->
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <!--配置連接池(數據源)-->
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <!--配置映射文件-->
    <mappers>
        <!--配置單個映射文件-->
        <!--<mapper resource="com/lxs/dao/IUserDao.xml"></mapper>-->
        <package name="cn.lemon.dao"/>
    </mappers>
</configuration>

driver=com.mysql.jdbc.Driver
url=jdbc:mysql:///db_mybatis
username=root
password=lemon

第四步：編寫持久層接口的映射文件 IUserDao.xml、IAccountDao.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="cn.lemon.dao.IUserDao">
    <select id="findById" resultType="user" parameterType="int">
      SELECT * FROM user WHERE id = #{uid}
    </select>

    <select id="findAll" resultMap="userMap">
        SELECT * FROM user
    </select>

    <resultMap id="userMap" type="user">
        <id column="id" property="id"></id>
        <!--collection：映射一對多的一的一方(主表方)的集合屬性
                ofType：集合的元素類型，可以寫別名
                select: 查詢關聯集合的select語句
                column：查詢關聯集合的語句的參數
        -->
        <collection property="accountList" ofType="Account" select="cn.lemon.dao.IAccountDao.findByUid" column="id">
            <id column="id" property="id"/>
        </collection>
    </resultMap>
</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="cn.lemon.dao.IAccountDao">
    <select id="findAll" resultMap="accountMap">
      select * from account
    </select>

    <resultMap id="accountMap" type="account">
        <id column="id" property="id"/><!--字段名等於屬性名可以省略-->
        <result column="uid" property="uid"/>
        <!--<result property="money" column="money"></result>-->

        <!--association映射多對一，多的一方的實體屬性
                * select： 調用select映射的語句查詢關聯的user數據
                * column： 調用select映射的語句的輸入參數
        -->
        <association property="user" javaType="user" select="cn.lemon.dao.IUserDao.findById" column="uid">
            <!--主鍵不能省略-->
            <id column="id" property="id"/>
            <!-- 同名屬性可以省略 -->
            <result property="username" column="username"/>
            <!--<result property="birthday" column="birthday"></result>-->
            <!--<result property="sex" column="sex"></result>-->
            <!--<result property="address" column="address"></result>-->
        </association>
    </resultMap>

    <select id="findByUid" resultType="Account" parameterType="int">
        SELECT * FROM account WHERE uid= #{aid}
    </select>
</mapper>

第五步：測試類

package cn.lemon.dao;

import cn.lemon.domain.User;
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.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class IUserDaoTest {
    private InputStream inputStream;
    private SqlSessionFactory sqlSessionFactory;
    private SqlSession sqlSession;
    private IUserDao iUserDao;

    @Before
    public void init() throws Exception {
        inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        sqlSession = sqlSessionFactory.openSession(true);
        iUserDao = sqlSession.getMapper(IUserDao.class);
    }

    @Test
    public void findById() {
    }

    @Test
    public void findAll() {
        List<User> userList = iUserDao.findAll();
        for (User user : userList) {
            System.out.println(user);
            System.out.println("====================================================================");
        }
        System.out.println("-------------------------------------------------------------------------");
        System.out.println("第一個用戶的賬戶" + userList.get(0).getAccountList());
    }

    @After
    public void destory() throws Exception {
        sqlSession.close();
        inputStream.close();
    }
}

package cn.lemon.dao;

import cn.lemon.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.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

import static org.junit.Assert.*;

public class IAccountDaoTest {
    private InputStream inputStream;
    private SqlSessionFactory sqlSessionFactory;
    private SqlSession sqlSession;
    private IAccountDao iAccountDao;

    @Before
    public void init() throws Exception {
        inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        sqlSession = sqlSessionFactory.openSession(true);
        iAccountDao = sqlSession.getMapper(IAccountDao.class);
    }

    @Test
    public void findAll() {
        List<Account> accountList = iAccountDao.findAll();
        for (Account account : accountList) {
            System.out.println(account);
            System.out.println("--------------------------------------------------------------------------------");
        }
        System.out.println("***************************************************************************");
        System.out.println("第一個賬戶的用戶爲：" + accountList.get(0).getUser());//當使用用戶時才查詢用戶，稱爲懶加載
    }

    @After
    public void destory() throws Exception {
        sqlSession.close();
        inputStream.close();
    }
}