MyBatis入门——关联映射

博主： CairBin
发布时间：2024 年 04 月 01 日
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分类： Java

MyBatis关联映射

为什么要关联映射

实际开发中，对数据库操作常常会涉及多张表，所以在OOP中就涉及对象与对象的关联关系。针对多表操作，MyBatis提供关联映射。

关联关系概述

一对一：A类中定义B类的属性b，B类中定义A类属性a。
一对多：一个A类类型对应多个B类。
多对多：在A类中定义B的集合，B中定义A的集合。

嵌套查询与嵌套结果

嵌套查询是通过执行另外一条SQL映射语句来返回预期复杂类型：

嵌套查询是在查询SQL嵌套一个子查询
嵌套查询会执行多条SQL语句
嵌套查询SQL语句编写相对简单

嵌套结果是使用嵌套结果映射来处理的联合结果子集：

嵌套结果是一个嵌套的多表查询SQL
嵌套结果只会执行一条复杂的SQL语句
嵌套结果SQL语句写起来相对麻烦

对于嵌套查询有一个问题，那就是执行多条SQL语句导致性能开销很大。于是就有了MyBatis的延迟加载——fetchType。

实践

创建表

我们还是用之前的账户连接数据库，终端命令可以如下

mysql -u db_mybatis -p

输入密码敲击回车，然后切换数据库到db_mybatis

Windows用户可以用终端或者sqlyog执行下面语句

如果你没有这个数据库请回到前面的章节创建它。

USE db_mybatis;

我们要使用一个新的customer表，所以把之前项目留下的删除掉

DROP TABLE IF EXISTS customer;

重新创建它

CREATE TABLE customer(
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) DEFAULT NULL,
  `jobs` varchar(50) DEFAULT NULL,
  `phone` varchar(16) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

给这张表插入两条数据方便我们接下来写代码用

INSERT  INTO customer(`id`,`username`,`jobs`,`phone`) VALUES (1,'zhangsan','teacher','11111111111'),(2,'lisi','student','11111111112');

创建一张idcard表，并设置主键（主码）为id

如果之前存在请删除

DROP TABLE IF EXISTS `idcard`;

CREATE TABLE idcard(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `code` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

同样插入两条数据（身份证号是我瞎编的，为了防止巧合不一定符合真实格式，但长度与现实一样）

INSERT  INTO idcard(`id`,`code`) VALUES(1,'37010219800X051118'),(2,'370203199X09092222');

创建persion表，并插入数据

DROP TABLE IF EXISTS person;

CREATE TABLE person(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `card_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_person` (`card_id`),
  CONSTRAINT `FK_person` FOREIGN KEY (`card_id`) REFERENCES `idcard` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO person(`id`,`name`,`age`,`card_id`) values (1,'张三',18,1),(2,'李四',18,2);

创建product表并插入数据

DROP TABLE IF EXISTS product;

CREATE TABLE product(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(32) DEFAULT NULL,
  `price` double DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO product(`id`,`name`,`price`) values (1,'笔记本电脑',8888.8),(2,'华为手机',6666.6);

创建user表并插入数据

DROP TABLE IF EXISTS user;

CREATE TABLE user(
  `id` int(11) NOT NULL,
  `name` varchar(20) DEFAULT NULL,
  `password` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO user(`id`,`name`,`password`) values (1,'zhangsan','654321'),(2,'lisi','123456');

创建orders表

DROP TABLE IF EXISTS orders;
CREATE TABLE orders(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `number` varchar(32) DEFAULT NULL,
  `user_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_orders` (`user_id`),
  CONSTRAINT `FK_orders` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

INSERT INTO `orders`(`id`,`number`,`user_id`) VALUES (1,'201901',1);

创建orderitem表，还是一样，如果之前存在先删除。这个表一定放后面，否则创建外键会报错。

DROP TABLE IF EXISTS orderitem;

CREATE TABLE orderitem(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `orders_id` int(11) DEFAULT NULL,
  `product_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_orderitem` (`orders_id`),
  KEY `FK_orderitem_product` (`product_id`),
  CONSTRAINT `FK_orderitem_product` FOREIGN KEY (`product_id`) REFERENCES `product` (`id`),
  CONSTRAINT `FK_orderitem` FOREIGN KEY (`orders_id`) REFERENCES `orders` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO orderitem(`id`,`orders_id`,`product_id`) VALUES (1,1,1),(2,1,2);

构建项目

我们新创建一个Maven项目top.cairbin.test5

pom.xml添加依赖包

<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>top.cairbin</groupId>
  <artifactId>test5</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>test5</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    
    <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
    <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.4</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.33</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.mybatis.generator/mybatis-generator-core -->
        <dependency>
            <groupId>org.mybatis.generator</groupId>
            <artifactId>mybatis-generator-core</artifactId>
            <version>1.4.0</version>
        </dependency>
    
        <dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>
    
  </dependencies>
</project>

创建与src平级的resources目录，并右键->Build Path->Use as Source Folder

在里面添加一个数据库配置文件db.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/db_mybatis
jdbc.username=db_mybatis
jdbc.password=db_mybatis

配置MyBatis，在同目录下创建mybatis-config.xml，这个文件会使用db.properties里面的配置，以${...}的方式调用，在<properties resource="db.properties" />里声明位置。

<?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" />
    <settings>
        <!-- 延迟加载的开关 -->
        <setting name="lazyLoadingEnabled" value="false" />
        <!-- 设置积极加载（true）或按需加载（false） -->
        <setting name="aggressiveLazyLoading" value="true" />
        <setting name="mapUnderscoreToCamelCase" value="true" />
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
    <typeAliases>
        <package name="top.cairbin.test5" />
    </typeAliases>
    <!--1.配置环境 ，默认的环境id为mysql -->
    <environments default="mysql">
        <!--1.2.配置id为mysql的数据库环境 -->
        <environment id="mysql">
            <!-- 使用JDBC的事务管理 -->
            <transactionManager type="JDBC" />
            <!--数据库连接池 -->
            <dataSource type="POOLED">
                <!-- 数据库驱动 -->
                <property name="driver" value="${jdbc.driver}" />
                <!-- 连接数据库的url -->
                <property name="url" value="${jdbc.url}" />
                <!-- 连接数据库的用户名 -->
                <property name="username" value="${jdbc.username}" />
                <!-- 连接数据库的密码 -->
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
    </environments>
    <!--2.配置Mapper的位置 -->
    <mappers>
        <mapper resource="top/cairbin/test5/mapper/CustomerMapper.xml" />
        <mapper resource="top/cairbin/test5/mapper/UserMapper.xml" />
        <mapper resource="top/cairbin/test5/mapper/IdCardMapper.xml" />
        <mapper resource="top/cairbin/test5/mapper/OrdersMapper.xml" />
        <mapper resource="top/cairbin/test5/mapper/PersonMapper.xml" />
        <mapper resource="top/cairbin/test5/mapper/ProductMapper.xml" />
    </mappers>
</configuration>

注意上方的<mapper>的目录，我们一会介绍。

然后添加log4j的配置文件log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.top.cairbin.test5=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

Dao层

我们先把对应的类创建了再说，到src/main/java下top.cairbin.test5创建一个子包，名称为top.cairbin.test5.dao

在我们新创建的包下面编写若干个类

首先是Person类，此时我们的IdCard还没创建，IDE提示错误很正常

package top.cairbin.test5.dao;

public class Person {
    private Integer id;
    private String name;
    private Integer age;
    private Integer cardId;
    private IdCard card;

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

    public void setCardId(Integer cardId) {
        this.cardId = cardId;
    }
    
    public void setCard(IdCard card) {
        this.card = card;
    }
    
    public IdCard getCard() {
        return this.card;
    }    
}

接下来创建IdCard

package top.cairbin.test5.dao;

public class IdCard {
    private Integer id;
    private String code;

    public Integer getId() {
        return id;
    }

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

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code == null ? null : code.trim();
    }
}

然后是Customer类

package top.cairbin.test5.dao;

public class Customer {
    private Integer id;
    private String username;
    private String jobs;
    private String phone;

    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 == null ? null : username.trim();
    }

    public String getJobs() {
        return jobs;
    }

    public void setJobs(String jobs) {
        this.jobs = jobs == null ? null : jobs.trim();
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone == null ? null : phone.trim();
    }
}

接着是Orders

package top.cairbin.test5.dao;

package top.cairbin.test5.dao;

import java.util.List;

public class Orders {
    private Integer id;
    private String number;
    private Integer userId;
    
    private List<Product> productList;

    public Integer getId() {
        return id;
    }

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

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number == null ? null : number.trim();
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }
    
    public void setProductList(List<Product> productList) {
        this.productList = productList;
    }

    public List<Product> getProductList(){
        return this.productList;
    }   
}

Product类

package top.cairbin.test5.dao;

public class Product {
    private Integer id;
    private String name;
    private Double price;

    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 == null ? null : name.trim();
    }

    public Double getPrice() {
        return price;
    }

    public void setPrice(Double price) {
        this.price = price;
    }
}

User类

package top.cairbin.test5.dao;

import java.util.List;

public class User {
    private Integer id;
    private String name;
    private String password;

    private List<Orders> ordersList;
    
    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 == null ? null : name.trim();
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }
    
    public void setOrdersList(List<Orders> ordersList) {
        this.ordersList = ordersList;
    }
    
    public List<Orders> getOrdersList(){
        return this.ordersList;
    }   
}

Util层

这一层我们来放置一些工具类。创建包top.cairbin.test5.util

然后在下面创建MyBatisHelper类

package top.cairbin.test5.util;

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;
public class MyBatisHelper {
    private static SqlSessionFactory sqlSessionFactory = null;
    // 初始化SqlSessionFactory对象
    static {
        try {
            // 使用MyBatis提供的Resources类加载MyBatis的配置文件
            Reader reader = 
                    Resources.getResourceAsReader("mybatis-config.xml");
            // 构建SqlSessionFactory工厂
            sqlSessionFactory = 
                    new SqlSessionFactoryBuilder().build(reader);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    // 获取SqlSession对象的静态方法
    public static SqlSession getSession() {
        return sqlSessionFactory.openSession();
    }
}

Mapper层

接下来我们再为top.cairbin.test5创建一个子包top.cairbin.test5.mapper，该层主要实现数据持久化。

与上面不一样的是，我们这里定义的是接口

首先是CustomerMapper接口

package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.Customer;

public interface CustomerMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(Customer record);

    int insertSelective(Customer record);

    Customer selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(Customer record);

    int updateByPrimaryKey(Customer record);
}

到这里你可能会问，谁去实现它们呢，还记得mybatis-config.xml里那些<mapper>吗，它们每一个都指定resource到Mapper层，也就是说我们要在这里写XML文件让MyBatis实现这些方法。

第一个是CustomerMapper.xml，注意此处的namespace与<resultMap>的id，下文同理。

<?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="top.cairbin.test5.mapper.CustomerMapper">
  <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.Customer">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="username" jdbcType="VARCHAR" property="username" />
    <result column="jobs" jdbcType="VARCHAR" property="jobs" />
    <result column="phone" jdbcType="VARCHAR" property="phone" />
  </resultMap>
  <sql id="Base_Column_List">
    id, username, jobs, phone
  </sql>
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from customer
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from customer
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.Customer">
    insert into customer (id, username, jobs, 
      phone)
    values (#{id,jdbcType=INTEGER}, #{username,jdbcType=VARCHAR}, #{jobs,jdbcType=VARCHAR}, 
      #{phone,jdbcType=VARCHAR})
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.Customer">
    insert into customer
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="username != null">
        username,
      </if>
      <if test="jobs != null">
        jobs,
      </if>
      <if test="phone != null">
        phone,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="username != null">
        #{username,jdbcType=VARCHAR},
      </if>
      <if test="jobs != null">
        #{jobs,jdbcType=VARCHAR},
      </if>
      <if test="phone != null">
        #{phone,jdbcType=VARCHAR},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.Customer">
    update customer
    <set>
      <if test="username != null">
        username = #{username,jdbcType=VARCHAR},
      </if>
      <if test="jobs != null">
        jobs = #{jobs,jdbcType=VARCHAR},
      </if>
      <if test="phone != null">
        phone = #{phone,jdbcType=VARCHAR},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.Customer">
    update customer
    set username = #{username,jdbcType=VARCHAR},
      jobs = #{jobs,jdbcType=VARCHAR},
      phone = #{phone,jdbcType=VARCHAR}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

对了，请检查你XML中的方法的parameterType属性以及resultType对应类型的包是否正确！！！

然后是IdCardMapper接口

package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.IdCard;

public interface IdCardMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(IdCard record);

    int insertSelective(IdCard record);

    IdCard selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(IdCard record);

    int updateByPrimaryKey(IdCard record);
}

同样在IdCardMapper.xml里实现这些方法

<?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="top.cairbin.test5.mapper.IdCardMapper">
  <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.IdCard">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="code" jdbcType="VARCHAR" property="code" />
  </resultMap>
  <sql id="Base_Column_List">
    id, code
  </sql>
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from idcard
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from idcard
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.IdCard">
    insert into idcard (id, code)
    values (#{id,jdbcType=INTEGER}, #{code,jdbcType=VARCHAR})
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.IdCard">
    insert into idcard
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="code != null">
        code,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="code != null">
        #{code,jdbcType=VARCHAR},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.IdCard">
    update idcard
    <set>
      <if test="code != null">
        code = #{code,jdbcType=VARCHAR},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.IdCard">
    update idcard
    set code = #{code,jdbcType=VARCHAR}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

然后是ProductMapper接口以及ProductMapper.xml

package top.cairbin.test5.mapper;
import top.cairbin.test5.dao.Product;

public interface ProductMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(Product record);

    int insertSelective(Product record);

    Product selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(Product record);

    int updateByPrimaryKey(Product record);
}

<?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="top.cairbin.test5.mapper.ProductMapper">
  <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.Product">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="price" jdbcType="DOUBLE" property="price" />
  </resultMap>
  <sql id="Base_Column_List">
    id, name, price
  </sql>
  
  <select id="findProductByOrderId" parameterType="Integer"   resultType="Product">
        SELECT * from product where id IN(
           SELECT product_id FROM orderitem  WHERE orders_id = #{id}
        )
    </select>

  
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from product
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from product
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.Product">
    insert into product (id, name, price
      )
    values (#{id,jdbcType=INTEGER}, #{name,jdbcType=VARCHAR}, #{price,jdbcType=DOUBLE}
      )
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.Product">
    insert into product
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="name != null">
        name,
      </if>
      <if test="price != null">
        price,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="name != null">
        #{name,jdbcType=VARCHAR},
      </if>
      <if test="price != null">
        #{price,jdbcType=DOUBLE},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.Product">
    update product
    <set>
      <if test="name != null">
        name = #{name,jdbcType=VARCHAR},
      </if>
      <if test="price != null">
        price = #{price,jdbcType=DOUBLE},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.Product">
    update product
    set name = #{name,jdbcType=VARCHAR},
      price = #{price,jdbcType=DOUBLE}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

对于剩下来的几个Dao层类对应的Mapper需要特别注意下，因为它们的类里都包含复合类型，这也是我们这一章节真正要学习的东西——关联映射。

PersonMapper接口

package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.Person;

public interface PersonMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(Person record);

    int insertSelective(Person record);

    Person selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(Person record);

    int updateByPrimaryKey(Person record);
}

PersonMapper.xml文件，请重点关注findPersonById和findPersonById2这两个地方。

<?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="top.cairbin.test5.mapper.PersonMapper">

    <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.Person">
        <id column="id" jdbcType="INTEGER" property="id" />
        <result column="name" jdbcType="VARCHAR" property="name" />
        <result column="age" jdbcType="INTEGER" property="age" />
        <result column="card_id" jdbcType="INTEGER" property="cardId" />
  </resultMap>
  <sql id="Base_Column_List">
    id, name, age, card_id
  </sql>
  
  <!-- 嵌套查询：通过执行另外一条SQL映射语句来返回预期的特殊类型 -->
    <select id="findPersonById" parameterType="Integer" 
    resultMap="IdCardWithPersonResult">
        SELECT * from person where id=#{id}
    </select>
    <resultMap type="Person" id="IdCardWithPersonResult">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <!-- 一对一：association使用select属性引入另外一条SQL语句 -->
        <association property="card" column="card_id" javaType="top.cairbin.test5.dao.IdCard"
            select="top.cairbin.test5.mapper.IdCardMapper.selectByPrimaryKey" />
    </resultMap>

  
      <!-- 嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 -->
      <select id="findPersonById2" parameterType="Integer" 
                                       resultMap="IdCardWithPersonResult2">
        SELECT p.*,idcard.code
        from person p,idcard idcard
        where p.card_id=idcard.id 
        and p.id= #{id}
    </select>
    <resultMap type="Person" id="IdCardWithPersonResult2">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <association property="card" javaType="IdCard">
            <id property="id" column="id" />
            <result property="code" column="code" />
        </association>
    </resultMap>

  
  
  
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from person
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from person
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.Person">
    insert into person (id, name, age, 
      card_id)
    values (#{id,jdbcType=INTEGER}, #{name,jdbcType=VARCHAR}, #{age,jdbcType=INTEGER}, 
      #{cardId,jdbcType=INTEGER})
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.Person">
    insert into person
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="name != null">
        name,
      </if>
      <if test="age != null">
        age,
      </if>
      <if test="cardId != null">
        card_id,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="name != null">
        #{name,jdbcType=VARCHAR},
      </if>
      <if test="age != null">
        #{age,jdbcType=INTEGER},
      </if>
      <if test="cardId != null">
        #{cardId,jdbcType=INTEGER},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.Person">
    update person
    <set>
      <if test="name != null">
        name = #{name,jdbcType=VARCHAR},
      </if>
      <if test="age != null">
        age = #{age,jdbcType=INTEGER},
      </if>
      <if test="cardId != null">
        card_id = #{cardId,jdbcType=INTEGER},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.Person">
    update person
    set name = #{name,jdbcType=VARCHAR},
      age = #{age,jdbcType=INTEGER},
      card_id = #{cardId,jdbcType=INTEGER}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

对于上面这么长的文件，最重要的地方是这里

<!-- 一对一：association使用select属性引入另外一条SQL语句 -->
<association property="card" column="card_id"   javaType="top.cairbin.test5.dao.IdCard"
select="top.cairbin.test5.mapper.IdCardMapper.selectByPrimaryKey" />

为了看到效果，我们在mybatis-config.xml里将延迟加载关掉。

<settings>
    <!-- 延迟加载的开关 -->
    <!-- 为了让大家看到关联关系的效果，我们在这里关闭了延迟加载 -->  
    <setting name="lazyLoadingEnabled" value="false" />  
    <setting name="aggressiveLazyLoading" value="true"/>  
</settings>

创建UserMapper接口和UserMapper.xml

package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.User;

public interface UserMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(User record);

    int insertSelective(User record);

    User selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(User record);

    int updateByPrimaryKey(User record);
}

<?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="top.cairbin.test5.mapper.UserMapper">
  <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.User">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="password" jdbcType="VARCHAR" property="password" />
    
    <collection property="ordersList" column="id" ofType="Orders" 
             select="top.cairbin.test5.mapper.OrdersMapper.findOrdersWithUser">
    </collection>

    
  </resultMap>
  <sql id="Base_Column_List">
    id, name, password
  </sql>
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from user
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from user
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.User">
    insert into user (id, name, password
      )
    values (#{id,jdbcType=INTEGER}, #{name,jdbcType=VARCHAR}, #{password,jdbcType=VARCHAR}
      )
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.User">
    insert into user
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="name != null">
        name,
      </if>
      <if test="password != null">
        password,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="name != null">
        #{name,jdbcType=VARCHAR},
      </if>
      <if test="password != null">
        #{password,jdbcType=VARCHAR},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.User">
    update user
    <set>
      <if test="name != null">
        name = #{name,jdbcType=VARCHAR},
      </if>
      <if test="password != null">
        password = #{password,jdbcType=VARCHAR},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.User">
    update user
    set name = #{name,jdbcType=VARCHAR},
      password = #{password,jdbcType=VARCHAR}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

最后OrdersMapper和OrdersMapper.xml

package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.Orders;

public interface OrdersMapper {
    int deleteByPrimaryKey(Integer id);

    int insert(Orders record);

    int insertSelective(Orders record);

    Orders selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(Orders record);

    int updateByPrimaryKey(Orders record);
}

<?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="top.cairbin.test5.mapper.OrdersMapper">
  <resultMap id="BaseResultMap" type="top.cairbin.test5.dao.Orders">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="number" jdbcType="VARCHAR" property="number" />
    <result column="user_id" jdbcType="INTEGER" property="userId" />
  </resultMap>
  <sql id="Base_Column_List">
    id, number, user_id
  </sql>
  
  <select id="findOrdersWithUser" parameterType="Integer" 
              resultMap="BaseResultMap">
        select * from orders WHERE user_id=#{id}    
    </select>

  
  
  <select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from orders
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
    delete from orders
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <insert id="insert" parameterType="top.cairbin.test5.dao.Orders">
    insert into orders (id, number, user_id
      )
    values (#{id,jdbcType=INTEGER}, #{number,jdbcType=VARCHAR}, #{userId,jdbcType=INTEGER}
      )
  </insert>
  <insert id="insertSelective" parameterType="top.cairbin.test5.dao.Orders">
    insert into orders
    <trim prefix="(" suffix=")" suffixOverrides=",">
      <if test="id != null">
        id,
      </if>
      <if test="number != null">
        number,
      </if>
      <if test="userId != null">
        user_id,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
      <if test="id != null">
        #{id,jdbcType=INTEGER},
      </if>
      <if test="number != null">
        #{number,jdbcType=VARCHAR},
      </if>
      <if test="userId != null">
        #{userId,jdbcType=INTEGER},
      </if>
    </trim>
  </insert>
  <update id="updateByPrimaryKeySelective" parameterType="top.cairbin.test5.dao.Orders">
    update orders
    <set>
      <if test="number != null">
        number = #{number,jdbcType=VARCHAR},
      </if>
      <if test="userId != null">
        user_id = #{userId,jdbcType=INTEGER},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="top.cairbin.test5.dao.Orders">
    update orders
    set number = #{number,jdbcType=VARCHAR},
      user_id = #{userId,jdbcType=INTEGER}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

我们编写测试类，在src/test/java下的top.cairbin.test5里

PersonTest类来测试一对一的关系

package top.cairbin.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import top.cairbin.test5.dao.Person;
import top.cairbin.test5.util.*;

public class PersonTest {
    @Test
    public void findPersonByIdTest() {
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2.使用MyBatis嵌套查询的方式查询id为1的人的信息
        Person person = session.selectOne("top.cairbin.test5.mapper." 
                                   + "PersonMapper.selectByPrimaryKey", 1);
        // 3、输出查询结果信息
        System.out.println(person);
        // 4、关闭SqlSession
        session.close();
    }

    //测试一对一（嵌套结果）：
    @Test
    public void findPersonByIdTest2() {
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2.使用MyBatis嵌套查询的方式查询id为1的人的信息
        Person person = session.selectOne("top.cairbin.test5.mapper." 
                                   + "PersonMapper.findPersonById2", 1);
        // 3、输出查询结果信息
        System.out.println(person);
        // 4、关闭SqlSession
        session.close();
    }
}

点击运行，两个方法所用时间差距不小。

我们再编写测试类UserTest来测试一对多

package top.cairbin.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import top.cairbin.test5.dao.User;
import top.cairbin.test5.util.MyBatisHelper;

public class UserTest {
    @Test
    public void findUserTest() {
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2、查询id为1的用户信息，注意包名为mapper层的
        User user = session.selectOne("top.cairbin.test5.mapper."
                                + "UserMapper.selectByPrimaryKey", 1);
        // 3、输出查询结果信息
        System.out.println(user);
        // 4、关闭SqlSession
        session.close();
    }

}

最后编写OrdersTest来测试多对多

package top.cairbin.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import top.cairbin.test5.dao.Orders;
import top.cairbin.test5.util.MyBatisHelper;

public class OrdersTest {
    @Test
    public void findOrdersTest(){
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2、查询id为1的订单中的商品信息
        Orders orders = session.selectOne("top.cairbin.test5.mapper."
                               + "OrdersMapper.selectByPrimaryKey", 1);
        // 3、输出查询结果信息
        System.out.println(orders);
        // 4、关闭SqlSession
        session.close();
    }
}

总结

在本章节，我们学习了MyBatis的关联映射。此外，你会发现我们写代码不再像以前一样只放在一个包里，而是创建好几个子包分层放置不同的代码，这有利于代码逻辑清晰并且方便维护，这种分层设计已经有一个正式项目的雏形了，当你写完这些东西的时候项目目录应该与我差不多。

最后修改：2024 年 04 月 01 日

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anyaer
完了，我的主题没有comments.php文件
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这个设备的配置够玩openwrt不(๑•̀ㅁ•́ฅ)
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MyBatis入门——关联映射

CairBin • 2024 年 04 月 01 日

<h1>MyBatis关联映射</h1><h2>为什么要关联映射</h2>实际开发中，对数据库操作常常会涉及多张表，所以在OOP中就涉及对象与对象的关联关系。针对多表操作，MyBatis提供关联映射。<h2>关联关系概述</h2><ul><li>一对一：A类中定义B类的属性b，B类中定义A类属性a。</li><li>一对多：一个A类类型对应多个B类。</li><li>多对多：在A类中定义B的集合，B中定义A的集合。</li></ul><h2>嵌套查询与嵌套结果</h2>嵌套查询是通过执行另外一条SQL映射语句来返回预期复杂类型：<ul><li>嵌套查询是在查询SQL嵌套一个子查询</li><li>嵌套查询会执行多条SQL语句</li><li>嵌套查询SQL语句编写相对简单</li></ul>嵌套结果是使用嵌套结果映射来处理的联合结果子集：<ul><li>嵌套结果是一个嵌套的多表查询SQL</li><li>嵌套结果只会执行一条复杂的SQL语句</li><li>嵌套结果SQL语句写起来相对麻烦</li></ul>对于嵌套查询有一个问题，那就是执行多条SQL语句导致性能开销很大。于是就有了MyBatis的延迟加载——fetchType。<h2>实践</h2><h3>创建表</h3>我们还是用之前的账户连接数据库，终端命令可以如下<pre><code class="lang-sql">mysql -u db_mybatis -p</code></pre>输入密码敲击回车，然后切换数据库到<code>db_mybatis</code>Windows用户可以用终端或者<code>sqlyog</code>执行下面语句如果你没有这个数据库请回到前面的章节创建它。<pre><code class="lang-sql">USE db_mybatis;</code></pre>我们要使用一个新的<code>customer</code>表，所以把之前项目留下的删除掉<pre><code class="lang-sql">DROP TABLE IF EXISTS customer;</code></pre>重新创建它<pre><code class="lang-sql">CREATE TABLE customer(
 `id` int(32) NOT NULL AUTO_INCREMENT,
 `username` varchar(50) DEFAULT NULL,
 `jobs` varchar(50) DEFAULT NULL,
 `phone` varchar(16) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;</code></pre>给这张表插入两条数据方便我们接下来写代码用<pre><code class="lang-sql">INSERT INTO customer(`id`,`username`,`jobs`,`phone`) VALUES (1,'zhangsan','teacher','11111111111'),(2,'lisi','student','11111111112');</code></pre>创建一张<code>idcard</code>表，并设置主键（主码）为<code>id</code>如果之前存在请删除<pre><code class="lang-sql">DROP TABLE IF EXISTS `idcard`;</code></pre><pre><code class="lang-sql">CREATE TABLE idcard(
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `code` varchar(50) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;</code></pre>同样插入两条数据（身份证号是我瞎编的，为了防止巧合不一定符合真实格式，但长度与现实一样）<pre><code class="lang-sql">INSERT INTO idcard(`id`,`code`) VALUES(1,'37010219800X051118'),(2,'370203199X09092222');</code></pre>创建<code>persion</code>表，并插入数据<pre><code class="lang-sql">DROP TABLE IF EXISTS person;

CREATE TABLE person(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `card_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_person` (`card_id`),
  CONSTRAINT `FK_person` FOREIGN KEY (`card_id`) REFERENCES `idcard` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO person(`id`,`name`,`age`,`card_id`) values (1,'张三',18,1),(2,'李四',18,2);</code></pre>创建<code>product</code>表并插入数据<pre><code class="lang-sql">DROP TABLE IF EXISTS product;

CREATE TABLE product(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(32) DEFAULT NULL,
  `price` double DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO product(`id`,`name`,`price`) values (1,'笔记本电脑',8888.8),(2,'华为手机',6666.6);</code></pre>创建<code>user</code>表并插入数据<pre><code class="lang-sql">DROP TABLE IF EXISTS user;

CREATE TABLE user(
  `id` int(11) NOT NULL,
  `name` varchar(20) DEFAULT NULL,
  `password` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO user(`id`,`name`,`password`) values (1,'zhangsan','654321'),(2,'lisi','123456');</code></pre>创建<code>orders</code>表<pre><code class="lang-sql">DROP TABLE IF EXISTS orders;
CREATE TABLE orders(
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `number` varchar(32) DEFAULT NULL,
 `user_id` int(11) DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `FK_orders` (`user_id`),
 CONSTRAINT `FK_orders` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

INSERT INTO `orders`(`id`,`number`,`user_id`) VALUES (1,'201901',1);</code></pre>创建<code>orderitem</code>表，还是一样，如果之前存在先删除。这个表一定放后面，否则创建外键会报错。<pre><code class="lang-sql">DROP TABLE IF EXISTS orderitem;

CREATE TABLE orderitem(
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `orders_id` int(11) DEFAULT NULL,
  `product_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_orderitem` (`orders_id`),
  KEY `FK_orderitem_product` (`product_id`),
  CONSTRAINT `FK_orderitem_product` FOREIGN KEY (`product_id`) REFERENCES `product` (`id`),
  CONSTRAINT `FK_orderitem` FOREIGN KEY (`orders_id`) REFERENCES `orders` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO orderitem(`id`,`orders_id`,`product_id`) VALUES (1,1,1),(2,1,2);</code></pre><h3>构建项目</h3>我们新创建一个Maven项目<code>top.cairbin.test5</code><code>pom.xml</code>添加依赖包<pre><code class="lang-xml">&lt;project xmlns=&quot;http://maven.apache.org/POM/4.0.0&quot; xmlns:xsi=&quot;http://www.w3.org/2001/XMLSchema-instance&quot;
 xsi:schemaLocation=&quot;http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd&quot;&gt;
 &lt;modelVersion&gt;4.0.0&lt;/modelVersion&gt;

&lt;groupId&gt;top.cairbin&lt;/groupId&gt;
  &lt;artifactId&gt;test5&lt;/artifactId&gt;
  &lt;version&gt;0.0.1-SNAPSHOT&lt;/version&gt;
  &lt;packaging&gt;jar&lt;/packaging&gt;

&lt;name&gt;test5&lt;/name&gt;
  &lt;url&gt;http://maven.apache.org&lt;/url&gt;

&lt;properties&gt;
    &lt;project.build.sourceEncoding&gt;UTF-8&lt;/project.build.sourceEncoding&gt;
  &lt;/properties&gt;

&lt;dependencies&gt;
 
 &lt;dependency&gt;
 &lt;groupId&gt;junit&lt;/groupId&gt;
 &lt;artifactId&gt;junit&lt;/artifactId&gt;
 &lt;version&gt;4.12&lt;/version&gt;
 &lt;scope&gt;test&lt;/scope&gt;
 &lt;/dependency&gt;
 &lt;!-- https://mvnrepository.com/artifact/org.mybatis/mybatis --&gt;
 &lt;dependency&gt;
 &lt;groupId&gt;org.mybatis&lt;/groupId&gt;
 &lt;artifactId&gt;mybatis&lt;/artifactId&gt;
 &lt;version&gt;3.5.4&lt;/version&gt;
 &lt;/dependency&gt;
 &lt;!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java --&gt;
 &lt;dependency&gt;
 &lt;groupId&gt;mysql&lt;/groupId&gt;
 &lt;artifactId&gt;mysql-connector-java&lt;/artifactId&gt;
 &lt;version&gt;8.0.33&lt;/version&gt;
 &lt;/dependency&gt;
 &lt;!-- https://mvnrepository.com/artifact/org.mybatis.generator/mybatis-generator-core --&gt;
 &lt;dependency&gt;
 &lt;groupId&gt;org.mybatis.generator&lt;/groupId&gt;
 &lt;artifactId&gt;mybatis-generator-core&lt;/artifactId&gt;
 &lt;version&gt;1.4.0&lt;/version&gt;
 &lt;/dependency&gt;
 
 &lt;dependency&gt;
 &lt;groupId&gt;log4j&lt;/groupId&gt;
 &lt;artifactId&gt;log4j&lt;/artifactId&gt;
 &lt;version&gt;1.2.17&lt;/version&gt;
&lt;/dependency&gt;
 
 &lt;/dependencies&gt;
&lt;/project&gt;</code></pre>创建与<code>src</code>平级的<code>resources</code>目录，并右键-&gt;<code>Build Path</code>-&gt;<code>Use as Source Folder</code>在里面添加一个数据库配置文件<code>db.properties</code><pre><code class="lang-properties">jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/db_mybatis
jdbc.username=db_mybatis
jdbc.password=db_mybatis</code></pre>配置MyBatis，在同目录下创建<code>mybatis-config.xml</code>，这个文件会使用<code>db.properties</code>里面的配置，以<code>${...}</code>的方式调用，在<code>&lt;properties resource=&quot;db.properties&quot; /&gt;</code>里声明位置。<pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot; ?&gt;
&lt;!DOCTYPE configuration PUBLIC &quot;-//mybatis.org//DTD Config 3.0//EN&quot;
 &quot;http://mybatis.org/dtd/mybatis-3-config.dtd&quot;&gt;
&lt;configuration&gt;
 &lt;properties resource=&quot;db.properties&quot; /&gt;
 &lt;settings&gt;
 &lt;!-- 延迟加载的开关 --&gt;
 &lt;setting name=&quot;lazyLoadingEnabled&quot; value=&quot;false&quot; /&gt;
 &lt;!-- 设置积极加载（true）或按需加载（false） --&gt;
 &lt;setting name=&quot;aggressiveLazyLoading&quot; value=&quot;true&quot; /&gt;
 &lt;setting name=&quot;mapUnderscoreToCamelCase&quot; value=&quot;true&quot; /&gt;
 &lt;setting name=&quot;logImpl&quot; value=&quot;STDOUT_LOGGING&quot; /&gt;
 &lt;/settings&gt;
 &lt;typeAliases&gt;
 &lt;package name=&quot;top.cairbin.test5&quot; /&gt;
 &lt;/typeAliases&gt;
 &lt;!--1.配置环境 ，默认的环境id为mysql --&gt;
 &lt;environments default=&quot;mysql&quot;&gt;
 &lt;!--1.2.配置id为mysql的数据库环境 --&gt;
 &lt;environment id=&quot;mysql&quot;&gt;
 &lt;!-- 使用JDBC的事务管理 --&gt;
 &lt;transactionManager type=&quot;JDBC&quot; /&gt;
 &lt;!--数据库连接池 --&gt;
 &lt;dataSource type=&quot;POOLED&quot;&gt;
 &lt;!-- 数据库驱动 --&gt;
 &lt;property name=&quot;driver&quot; value=&quot;${jdbc.driver}&quot; /&gt;
 &lt;!-- 连接数据库的url --&gt;
 &lt;property name=&quot;url&quot; value=&quot;${jdbc.url}&quot; /&gt;
 &lt;!-- 连接数据库的用户名 --&gt;
 &lt;property name=&quot;username&quot; value=&quot;${jdbc.username}&quot; /&gt;
 &lt;!-- 连接数据库的密码 --&gt;
 &lt;property name=&quot;password&quot; value=&quot;${jdbc.password}&quot; /&gt;
 &lt;/dataSource&gt;
 &lt;/environment&gt;
 &lt;/environments&gt;
 &lt;!--2.配置Mapper的位置 --&gt;
 &lt;mappers&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/CustomerMapper.xml&quot; /&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/UserMapper.xml&quot; /&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/IdCardMapper.xml&quot; /&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/OrdersMapper.xml&quot; /&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/PersonMapper.xml&quot; /&gt;
 &lt;mapper resource=&quot;top/cairbin/test5/mapper/ProductMapper.xml&quot; /&gt;
 &lt;/mappers&gt;
&lt;/configuration&gt;</code></pre>注意上方的<code>&lt;mapper&gt;</code>的目录，我们一会介绍。然后添加log4j的配置文件<code>log4j.properties</code><pre><code class="lang-properties"># Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.top.cairbin.test5=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n</code></pre><h3>Dao层</h3>我们先把对应的类创建了再说，到<code>src/main/java</code>下<code>top.cairbin.test5</code>创建一个子包，名称为<code>top.cairbin.test5.dao</code><img src="http://img.cairbin.top/img/202403220056991.png" alt="" title=""style=""><img src="http://img.cairbin.top/img/202403220056749.png" alt="" title=""style="">在我们新创建的包下面编写若干个类首先是<code>Person</code>类，此时我们的<code>IdCard</code>还没创建，IDE提示错误很正常<pre><code class="lang-java">package top.cairbin.test5.dao;

public class Person {
    private Integer id;
    private String name;
    private Integer age;
    private Integer cardId;
    private IdCard card;

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

public void setCardId(Integer cardId) {
 this.cardId = cardId;
 }
 
 public void setCard(IdCard card) {
 this.card = card;
 }
 
 public IdCard getCard() {
 return this.card;
 } 
}</code></pre>接下来创建<code>IdCard</code><pre><code class="lang-java">package top.cairbin.test5.dao;

public class IdCard {
    private Integer id;
    private String code;

public Integer getId() {
        return id;
    }

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

public String getCode() {
        return code;
    }

public void setCode(String code) {
 this.code = code == null ? null : code.trim();
 }
}</code></pre>然后是<code>Customer</code>类<pre><code class="lang-java">package top.cairbin.test5.dao;

public class Customer {
    private Integer id;
    private String username;
    private String jobs;
    private String phone;

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 == null ? null : username.trim();
    }

public String getJobs() {
        return jobs;
    }

public void setJobs(String jobs) {
        this.jobs = jobs == null ? null : jobs.trim();
    }

public String getPhone() {
        return phone;
    }

public void setPhone(String phone) {
 this.phone = phone == null ? null : phone.trim();
 }
}</code></pre>接着是<code>Orders</code><pre><code class="lang-java">package top.cairbin.test5.dao;

package top.cairbin.test5.dao;

import java.util.List;

public class Orders {
    private Integer id;
    private String number;
    private Integer userId;
    
    private List&lt;Product&gt; productList;

public Integer getId() {
        return id;
    }

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

public String getNumber() {
        return number;
    }

public void setNumber(String number) {
        this.number = number == null ? null : number.trim();
    }

public Integer getUserId() {
        return userId;
    }

public void setUserId(Integer userId) {
        this.userId = userId;
    }
    
    public void setProductList(List&lt;Product&gt; productList) {
        this.productList = productList;
    }

public List&lt;Product&gt; getProductList(){
 return this.productList;
 } 
}</code></pre><code>Product</code>类<pre><code class="lang-java">package top.cairbin.test5.dao;

public class Product {
    private Integer id;
    private String name;
    private Double price;

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 == null ? null : name.trim();
    }

public Double getPrice() {
        return price;
    }

public void setPrice(Double price) {
 this.price = price;
 }
}</code></pre><code>User</code>类<pre><code class="lang-java">package top.cairbin.test5.dao;

import java.util.List;

public class User {
    private Integer id;
    private String name;
    private String password;

private List&lt;Orders&gt; ordersList;
    
    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 == null ? null : name.trim();
    }

public String getPassword() {
        return password;
    }

public void setPassword(String password) {
 this.password = password == null ? null : password.trim();
 }
 
 public void setOrdersList(List&lt;Orders&gt; ordersList) {
 this.ordersList = ordersList;
 }
 
 public List&lt;Orders&gt; getOrdersList(){
 return this.ordersList;
 } 
}</code></pre><h3>Util层</h3>这一层我们来放置一些工具类。创建包<code>top.cairbin.test5.util</code>然后在下面创建<code>MyBatisHelper</code>类<pre><code class="lang-java">package top.cairbin.test5.util;

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;
public class MyBatisHelper {
 private static SqlSessionFactory sqlSessionFactory = null;
 // 初始化SqlSessionFactory对象
 static {
 try {
 // 使用MyBatis提供的Resources类加载MyBatis的配置文件
 Reader reader = 
 Resources.getResourceAsReader(&quot;mybatis-config.xml&quot;);
 // 构建SqlSessionFactory工厂
 sqlSessionFactory = 
 new SqlSessionFactoryBuilder().build(reader);
 } catch (Exception e) {
 e.printStackTrace();
 }
 }
 // 获取SqlSession对象的静态方法
 public static SqlSession getSession() {
 return sqlSessionFactory.openSession();
 }
}</code></pre><h3>Mapper层</h3>接下来我们再为<code>top.cairbin.test5</code>创建一个子包<code>top.cairbin.test5.mapper</code>，该层主要实现数据持久化。与上面不一样的是，我们这里定义的是接口首先是<code>CustomerMapper</code>接口<pre><code class="lang-java">package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.Customer;

public interface CustomerMapper {
    int deleteByPrimaryKey(Integer id);

int insert(Customer record);

int insertSelective(Customer record);

Customer selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(Customer record);

int updateByPrimaryKey(Customer record);
}</code></pre>到这里你可能会问，谁去实现它们呢，还记得<code>mybatis-config.xml</code>里那些<code>&lt;mapper&gt;</code>吗，它们每一个都指定<code>resource</code>到Mapper层，也就是说我们要在这里写XML文件让MyBatis实现这些方法。第一个是<code>CustomerMapper.xml</code>，注意此处的<code>namespace</code>与<code>&lt;resultMap&gt;</code>的<code>id</code>，下文同理。<pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.CustomerMapper&quot;&gt;
 &lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.Customer&quot;&gt;
 &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
 &lt;result column=&quot;username&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;username&quot; /&gt;
 &lt;result column=&quot;jobs&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;jobs&quot; /&gt;
 &lt;result column=&quot;phone&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;phone&quot; /&gt;
 &lt;/resultMap&gt;
 &lt;sql id=&quot;Base_Column_List&quot;&gt;
 id, username, jobs, phone
 &lt;/sql&gt;
 &lt;select id=&quot;selectByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot; resultMap=&quot;BaseResultMap&quot;&gt;
 select 
 &lt;include refid=&quot;Base_Column_List&quot; /&gt;
 from customer
 where id = #{id,jdbcType=INTEGER}
 &lt;/select&gt;
 &lt;delete id=&quot;deleteByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot;&gt;
 delete from customer
 where id = #{id,jdbcType=INTEGER}
 &lt;/delete&gt;
 &lt;insert id=&quot;insert&quot; parameterType=&quot;top.cairbin.test5.dao.Customer&quot;&gt;
 insert into customer (id, username, jobs, 
 phone)
 values (#{id,jdbcType=INTEGER}, #{username,jdbcType=VARCHAR}, #{jobs,jdbcType=VARCHAR}, 
 #{phone,jdbcType=VARCHAR})
 &lt;/insert&gt;
 &lt;insert id=&quot;insertSelective&quot; parameterType=&quot;top.cairbin.test5.dao.Customer&quot;&gt;
 insert into customer
 &lt;trim prefix=&quot;(&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 id,
 &lt;/if&gt;
 &lt;if test=&quot;username != null&quot;&gt;
 username,
 &lt;/if&gt;
 &lt;if test=&quot;jobs != null&quot;&gt;
 jobs,
 &lt;/if&gt;
 &lt;if test=&quot;phone != null&quot;&gt;
 phone,
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;trim prefix=&quot;values (&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 #{id,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;username != null&quot;&gt;
 #{username,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;jobs != null&quot;&gt;
 #{jobs,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;phone != null&quot;&gt;
 #{phone,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;/insert&gt;
 &lt;update id=&quot;updateByPrimaryKeySelective&quot; parameterType=&quot;top.cairbin.test5.dao.Customer&quot;&gt;
 update customer
 &lt;set&gt;
 &lt;if test=&quot;username != null&quot;&gt;
 username = #{username,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;jobs != null&quot;&gt;
 jobs = #{jobs,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;phone != null&quot;&gt;
 phone = #{phone,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/set&gt;
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
 &lt;update id=&quot;updateByPrimaryKey&quot; parameterType=&quot;top.cairbin.test5.dao.Customer&quot;&gt;
 update customer
 set username = #{username,jdbcType=VARCHAR},
 jobs = #{jobs,jdbcType=VARCHAR},
 phone = #{phone,jdbcType=VARCHAR}
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
&lt;/mapper&gt;</code></pre>对了，请检查你XML中的方法的<code>parameterType</code>属性以及<code>resultType</code>对应类型的包是否正确！！！然后是<code>IdCardMapper</code>接口<pre><code class="lang-java">package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.IdCard;

public interface IdCardMapper {
    int deleteByPrimaryKey(Integer id);

int insert(IdCard record);

int insertSelective(IdCard record);

IdCard selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(IdCard record);

int updateByPrimaryKey(IdCard record);
}</code></pre>同样在<code>IdCardMapper.xml</code>里实现这些方法<pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.IdCardMapper&quot;&gt;
 &lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.IdCard&quot;&gt;
 &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
 &lt;result column=&quot;code&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;code&quot; /&gt;
 &lt;/resultMap&gt;
 &lt;sql id=&quot;Base_Column_List&quot;&gt;
 id, code
 &lt;/sql&gt;
 &lt;select id=&quot;selectByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot; resultMap=&quot;BaseResultMap&quot;&gt;
 select 
 &lt;include refid=&quot;Base_Column_List&quot; /&gt;
 from idcard
 where id = #{id,jdbcType=INTEGER}
 &lt;/select&gt;
 &lt;delete id=&quot;deleteByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot;&gt;
 delete from idcard
 where id = #{id,jdbcType=INTEGER}
 &lt;/delete&gt;
 &lt;insert id=&quot;insert&quot; parameterType=&quot;top.cairbin.test5.dao.IdCard&quot;&gt;
 insert into idcard (id, code)
 values (#{id,jdbcType=INTEGER}, #{code,jdbcType=VARCHAR})
 &lt;/insert&gt;
 &lt;insert id=&quot;insertSelective&quot; parameterType=&quot;top.cairbin.test5.dao.IdCard&quot;&gt;
 insert into idcard
 &lt;trim prefix=&quot;(&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 id,
 &lt;/if&gt;
 &lt;if test=&quot;code != null&quot;&gt;
 code,
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;trim prefix=&quot;values (&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 #{id,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;code != null&quot;&gt;
 #{code,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;/insert&gt;
 &lt;update id=&quot;updateByPrimaryKeySelective&quot; parameterType=&quot;top.cairbin.test5.dao.IdCard&quot;&gt;
 update idcard
 &lt;set&gt;
 &lt;if test=&quot;code != null&quot;&gt;
 code = #{code,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/set&gt;
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
 &lt;update id=&quot;updateByPrimaryKey&quot; parameterType=&quot;top.cairbin.test5.dao.IdCard&quot;&gt;
 update idcard
 set code = #{code,jdbcType=VARCHAR}
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
&lt;/mapper&gt;</code></pre>然后是<code>ProductMapper</code>接口以及<code>ProductMapper.xml</code><pre><code class="lang-java">package top.cairbin.test5.mapper;
import top.cairbin.test5.dao.Product;

public interface ProductMapper {
    int deleteByPrimaryKey(Integer id);

int insert(Product record);

int insertSelective(Product record);

Product selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(Product record);

int updateByPrimaryKey(Product record);
}</code></pre><pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.ProductMapper&quot;&gt;
 &lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.Product&quot;&gt;
 &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
 &lt;result column=&quot;name&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;name&quot; /&gt;
 &lt;result column=&quot;price&quot; jdbcType=&quot;DOUBLE&quot; property=&quot;price&quot; /&gt;
 &lt;/resultMap&gt;
 &lt;sql id=&quot;Base_Column_List&quot;&gt;
 id, name, price
 &lt;/sql&gt;
 
 &lt;select id=&quot;findProductByOrderId&quot; parameterType=&quot;Integer&quot; resultType=&quot;Product&quot;&gt;
 SELECT * from product where id IN(
 SELECT product_id FROM orderitem WHERE orders_id = #{id}
 )
 &lt;/select&gt;

import top.cairbin.test5.dao.Person;

public interface PersonMapper {
    int deleteByPrimaryKey(Integer id);

int insert(Person record);

int insertSelective(Person record);

Person selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(Person record);

int updateByPrimaryKey(Person record);
}</code></pre><code>PersonMapper.xml</code>文件，请重点关注<code>findPersonById</code>和<code>findPersonById2</code>这两个地方。<pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.PersonMapper&quot;&gt;

&lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.Person&quot;&gt;
        &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
        &lt;result column=&quot;name&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;name&quot; /&gt;
        &lt;result column=&quot;age&quot; jdbcType=&quot;INTEGER&quot; property=&quot;age&quot; /&gt;
        &lt;result column=&quot;card_id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;cardId&quot; /&gt;
  &lt;/resultMap&gt;
  &lt;sql id=&quot;Base_Column_List&quot;&gt;
    id, name, age, card_id
  &lt;/sql&gt;
  
  &lt;!-- 嵌套查询：通过执行另外一条SQL映射语句来返回预期的特殊类型 --&gt;
    &lt;select id=&quot;findPersonById&quot; parameterType=&quot;Integer&quot; 
    resultMap=&quot;IdCardWithPersonResult&quot;&gt;
        SELECT * from person where id=#{id}
    &lt;/select&gt;
    &lt;resultMap type=&quot;Person&quot; id=&quot;IdCardWithPersonResult&quot;&gt;
        &lt;id property=&quot;id&quot; column=&quot;id&quot; /&gt;
        &lt;result property=&quot;name&quot; column=&quot;name&quot; /&gt;
        &lt;result property=&quot;age&quot; column=&quot;age&quot; /&gt;
        &lt;!-- 一对一：association使用select属性引入另外一条SQL语句 --&gt;
        &lt;association property=&quot;card&quot; column=&quot;card_id&quot; javaType=&quot;top.cairbin.test5.dao.IdCard&quot;
            select=&quot;top.cairbin.test5.mapper.IdCardMapper.selectByPrimaryKey&quot; /&gt;
    &lt;/resultMap&gt;

&lt;!-- 嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 --&gt;
      &lt;select id=&quot;findPersonById2&quot; parameterType=&quot;Integer&quot; 
                                       resultMap=&quot;IdCardWithPersonResult2&quot;&gt;
        SELECT p.*,idcard.code
        from person p,idcard idcard
        where p.card_id=idcard.id 
        and p.id= #{id}
    &lt;/select&gt;
    &lt;resultMap type=&quot;Person&quot; id=&quot;IdCardWithPersonResult2&quot;&gt;
        &lt;id property=&quot;id&quot; column=&quot;id&quot; /&gt;
        &lt;result property=&quot;name&quot; column=&quot;name&quot; /&gt;
        &lt;result property=&quot;age&quot; column=&quot;age&quot; /&gt;
        &lt;association property=&quot;card&quot; javaType=&quot;IdCard&quot;&gt;
            &lt;id property=&quot;id&quot; column=&quot;id&quot; /&gt;
            &lt;result property=&quot;code&quot; column=&quot;code&quot; /&gt;
        &lt;/association&gt;
    &lt;/resultMap&gt;

&lt;select id=&quot;selectByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot; resultMap=&quot;BaseResultMap&quot;&gt;
 select 
 &lt;include refid=&quot;Base_Column_List&quot; /&gt;
 from person
 where id = #{id,jdbcType=INTEGER}
 &lt;/select&gt;
 &lt;delete id=&quot;deleteByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot;&gt;
 delete from person
 where id = #{id,jdbcType=INTEGER}
 &lt;/delete&gt;
 &lt;insert id=&quot;insert&quot; parameterType=&quot;top.cairbin.test5.dao.Person&quot;&gt;
 insert into person (id, name, age, 
 card_id)
 values (#{id,jdbcType=INTEGER}, #{name,jdbcType=VARCHAR}, #{age,jdbcType=INTEGER}, 
 #{cardId,jdbcType=INTEGER})
 &lt;/insert&gt;
 &lt;insert id=&quot;insertSelective&quot; parameterType=&quot;top.cairbin.test5.dao.Person&quot;&gt;
 insert into person
 &lt;trim prefix=&quot;(&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 id,
 &lt;/if&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 name,
 &lt;/if&gt;
 &lt;if test=&quot;age != null&quot;&gt;
 age,
 &lt;/if&gt;
 &lt;if test=&quot;cardId != null&quot;&gt;
 card_id,
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;trim prefix=&quot;values (&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 #{id,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 #{name,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;age != null&quot;&gt;
 #{age,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;cardId != null&quot;&gt;
 #{cardId,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;/insert&gt;
 &lt;update id=&quot;updateByPrimaryKeySelective&quot; parameterType=&quot;top.cairbin.test5.dao.Person&quot;&gt;
 update person
 &lt;set&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 name = #{name,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;age != null&quot;&gt;
 age = #{age,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;cardId != null&quot;&gt;
 card_id = #{cardId,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;/set&gt;
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
 &lt;update id=&quot;updateByPrimaryKey&quot; parameterType=&quot;top.cairbin.test5.dao.Person&quot;&gt;
 update person
 set name = #{name,jdbcType=VARCHAR},
 age = #{age,jdbcType=INTEGER},
 card_id = #{cardId,jdbcType=INTEGER}
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
&lt;/mapper&gt;</code></pre>对于上面这么长的文件，最重要的地方是这里<pre><code class="lang-xml">&lt;!-- 一对一：association使用select属性引入另外一条SQL语句 --&gt;
&lt;association property=&quot;card&quot; column=&quot;card_id&quot; javaType=&quot;top.cairbin.test5.dao.IdCard&quot;
select=&quot;top.cairbin.test5.mapper.IdCardMapper.selectByPrimaryKey&quot; /&gt;</code></pre>为了看到效果，我们在<code>mybatis-config.xml</code>里将延迟加载关掉。<pre><code class="lang-xml">&lt;settings&gt;
 &lt;!-- 延迟加载的开关 --&gt;
 &lt;!-- 为了让大家看到关联关系的效果，我们在这里关闭了延迟加载 --&gt; 
 &lt;setting name=&quot;lazyLoadingEnabled&quot; value=&quot;false&quot; /&gt; 
 &lt;setting name=&quot;aggressiveLazyLoading&quot; value=&quot;true&quot;/&gt; 
&lt;/settings&gt;</code></pre><img src="http://img.cairbin.top/img/202403220218139.png" alt="" title=""style="">创建<code>UserMapper</code>接口和<code>UserMapper.xml</code><pre><code class="lang-java">package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.User;

public interface UserMapper {
    int deleteByPrimaryKey(Integer id);

int insert(User record);

int insertSelective(User record);

User selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(User record);

int updateByPrimaryKey(User record);
}</code></pre><pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.UserMapper&quot;&gt;
 &lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.User&quot;&gt;
 &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
 &lt;result column=&quot;name&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;name&quot; /&gt;
 &lt;result column=&quot;password&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;password&quot; /&gt;
 
 &lt;collection property=&quot;ordersList&quot; column=&quot;id&quot; ofType=&quot;Orders&quot; 
 select=&quot;top.cairbin.test5.mapper.OrdersMapper.findOrdersWithUser&quot;&gt;
 &lt;/collection&gt;

&lt;/resultMap&gt;
 &lt;sql id=&quot;Base_Column_List&quot;&gt;
 id, name, password
 &lt;/sql&gt;
 &lt;select id=&quot;selectByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot; resultMap=&quot;BaseResultMap&quot;&gt;
 select 
 &lt;include refid=&quot;Base_Column_List&quot; /&gt;
 from user
 where id = #{id,jdbcType=INTEGER}
 &lt;/select&gt;
 &lt;delete id=&quot;deleteByPrimaryKey&quot; parameterType=&quot;java.lang.Integer&quot;&gt;
 delete from user
 where id = #{id,jdbcType=INTEGER}
 &lt;/delete&gt;
 &lt;insert id=&quot;insert&quot; parameterType=&quot;top.cairbin.test5.dao.User&quot;&gt;
 insert into user (id, name, password
 )
 values (#{id,jdbcType=INTEGER}, #{name,jdbcType=VARCHAR}, #{password,jdbcType=VARCHAR}
 )
 &lt;/insert&gt;
 &lt;insert id=&quot;insertSelective&quot; parameterType=&quot;top.cairbin.test5.dao.User&quot;&gt;
 insert into user
 &lt;trim prefix=&quot;(&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 id,
 &lt;/if&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 name,
 &lt;/if&gt;
 &lt;if test=&quot;password != null&quot;&gt;
 password,
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;trim prefix=&quot;values (&quot; suffix=&quot;)&quot; suffixOverrides=&quot;,&quot;&gt;
 &lt;if test=&quot;id != null&quot;&gt;
 #{id,jdbcType=INTEGER},
 &lt;/if&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 #{name,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;password != null&quot;&gt;
 #{password,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/trim&gt;
 &lt;/insert&gt;
 &lt;update id=&quot;updateByPrimaryKeySelective&quot; parameterType=&quot;top.cairbin.test5.dao.User&quot;&gt;
 update user
 &lt;set&gt;
 &lt;if test=&quot;name != null&quot;&gt;
 name = #{name,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;if test=&quot;password != null&quot;&gt;
 password = #{password,jdbcType=VARCHAR},
 &lt;/if&gt;
 &lt;/set&gt;
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
 &lt;update id=&quot;updateByPrimaryKey&quot; parameterType=&quot;top.cairbin.test5.dao.User&quot;&gt;
 update user
 set name = #{name,jdbcType=VARCHAR},
 password = #{password,jdbcType=VARCHAR}
 where id = #{id,jdbcType=INTEGER}
 &lt;/update&gt;
&lt;/mapper&gt;</code></pre>最后<code>OrdersMapper</code>和<code>OrdersMapper.xml</code><pre><code class="lang-java">package top.cairbin.test5.mapper;

import top.cairbin.test5.dao.Orders;

public interface OrdersMapper {
    int deleteByPrimaryKey(Integer id);

int insert(Orders record);

int insertSelective(Orders record);

Orders selectByPrimaryKey(Integer id);

int updateByPrimaryKeySelective(Orders record);

int updateByPrimaryKey(Orders record);
}</code></pre><pre><code class="lang-xml">&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE mapper PUBLIC &quot;-//mybatis.org//DTD Mapper 3.0//EN&quot; &quot;http://mybatis.org/dtd/mybatis-3-mapper.dtd&quot;&gt;
&lt;mapper namespace=&quot;top.cairbin.test5.mapper.OrdersMapper&quot;&gt;
 &lt;resultMap id=&quot;BaseResultMap&quot; type=&quot;top.cairbin.test5.dao.Orders&quot;&gt;
 &lt;id column=&quot;id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;id&quot; /&gt;
 &lt;result column=&quot;number&quot; jdbcType=&quot;VARCHAR&quot; property=&quot;number&quot; /&gt;
 &lt;result column=&quot;user_id&quot; jdbcType=&quot;INTEGER&quot; property=&quot;userId&quot; /&gt;
 &lt;/resultMap&gt;
 &lt;sql id=&quot;Base_Column_List&quot;&gt;
 id, number, user_id
 &lt;/sql&gt;
 
 &lt;select id=&quot;findOrdersWithUser&quot; parameterType=&quot;Integer&quot; 
 resultMap=&quot;BaseResultMap&quot;&gt;
 select * from orders WHERE user_id=#{id} 
 &lt;/select&gt;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import top.cairbin.test5.dao.Person;
import top.cairbin.test5.util.*;

public class PersonTest {
    @Test
    public void findPersonByIdTest() {
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2.使用MyBatis嵌套查询的方式查询id为1的人的信息
        Person person = session.selectOne(&quot;top.cairbin.test5.mapper.&quot; 
                                   + &quot;PersonMapper.selectByPrimaryKey&quot;, 1);
        // 3、输出查询结果信息
        System.out.println(person);
        // 4、关闭SqlSession
        session.close();
    }

//测试一对一（嵌套结果）：
 @Test
 public void findPersonByIdTest2() {
 // 1、通过工具类生成SqlSession对象
 SqlSession session = MyBatisHelper.getSession();
 // 2.使用MyBatis嵌套查询的方式查询id为1的人的信息
 Person person = session.selectOne(&quot;top.cairbin.test5.mapper.&quot; 
 + &quot;PersonMapper.findPersonById2&quot;, 1);
 // 3、输出查询结果信息
 System.out.println(person);
 // 4、关闭SqlSession
 session.close();
 }
}</code></pre>点击运行，两个方法所用时间差距不小。 <img src="http://img.cairbin.top/img/202403220338937.png" alt="" title=""style="">我们再编写测试类<code>UserTest</code>来测试一对多<pre><code class="lang-java">package top.cairbin.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import top.cairbin.test5.dao.User;
import top.cairbin.test5.util.MyBatisHelper;

public class UserTest {
    @Test
    public void findUserTest() {
        // 1、通过工具类生成SqlSession对象
        SqlSession session = MyBatisHelper.getSession();
        // 2、查询id为1的用户信息，注意包名为mapper层的
        User user = session.selectOne(&quot;top.cairbin.test5.mapper.&quot;
                                + &quot;UserMapper.selectByPrimaryKey&quot;, 1);
        // 3、输出查询结果信息
        System.out.println(user);
        // 4、关闭SqlSession
        session.close();
    }

}</code></pre><img src="http://img.cairbin.top/img/202403220339629.png" alt="" title=""style="">最后编写<code>OrdersTest</code>来测试多对多<pre><code class="lang-java">package top.cairbin.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import top.cairbin.test5.dao.Orders;
import top.cairbin.test5.util.MyBatisHelper;

public class OrdersTest {
 @Test
 public void findOrdersTest(){
 // 1、通过工具类生成SqlSession对象
 SqlSession session = MyBatisHelper.getSession();
 // 2、查询id为1的订单中的商品信息
 Orders orders = session.selectOne(&quot;top.cairbin.test5.mapper.&quot;
 + &quot;OrdersMapper.selectByPrimaryKey&quot;, 1);
 // 3、输出查询结果信息
 System.out.println(orders);
 // 4、关闭SqlSession
 session.close();
 }
}</code></pre><img src="http://img.cairbin.top/img/202403220342141.png" alt="" title=""style=""><h2>总结</h2>在本章节，我们学习了MyBatis的关联映射。此外，你会发现我们写代码不再像以前一样只放在一个包里，而是创建好几个子包分层放置不同的代码，这有利于代码逻辑清晰并且方便维护，这种分层设计已经有一个正式项目的雏形了，当你写完这些东西的时候项目目录应该与我差不多。<img src="http://img.cairbin.top/img/202403220342671.png" alt="" title=""style="">

MyBatis入门——关联映射

MyBatis关联映射

为什么要关联映射

关联关系概述

嵌套查询与嵌套结果

实践

创建表

构建项目

Dao层

Util层

Mapper层

总结

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MyBatis入门——关联映射

MyBatis关联映射

为什么要关联映射

关联关系概述

嵌套查询与嵌套结果

实践

创建表

构建项目

Dao层

Util层

Mapper层

总结

发表评论 取消回复

MyBatis入门——关联映射

发表评论取消回复