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。
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