Mybatis 实战： 一对一关系
文章目录
一 背景
某大型企业，集团公司旗下有若干子公司，每个公司一名 CEO。同时，集团文件要求各子公司的 CEO 不得兼任。此时，CEO 与子公司的关系就是一对一的，分别对 CEO 和公司进行建模。要求输入公司 ID 即可查询到该公司的信息，包括 CEO 详细信息。
二 操作步骤
2.1 新建项目
mvn archetype:generate -DgroupId=com.ivandu.mybatis -D artifactId=mybatis -DarchetypeArtifactId=maven-archetype-quickstart2.2 关联远程已有仓库
git init
git add .
git commit -m "一对一"
git remote add origin https://github.com/cxwn/mybatis.git
git push -u -f origin master2.3 创建数据库、表
根据背景描述，我们可以创建如下两张表：
create table ceo
(
id_ceo int primary key auto_increment, -- ID
name_ceo varchar(20), -- 姓名
age_ceo int -- 年龄
);
insert into ceo(name_ceo, age_ceo)
values ('John', 39);
insert into ceo(name_ceo, age_ceo)
values ('Bill', 45);
insert into ceo(name_ceo, age_ceo)
values ('Musk', 40);
create table company
(
id_com int primary key auto_increment, -- 公司ID
name_com varchar(20), -- 公司名称
id_ceo_com int unique,
foreign key (id_ceo_com) references ceo (id_ceo)
);
insert into company(name_com, id_ceo_com)
values ('ChinaACompany', 1);
insert into company(name_com, id_ceo_com)
values ('ChinaCCompany', 3);
insert into company(name_com, id_ceo_com)
values ('ChinaBCompany', 2);2.4 项目配置
2.4.1 pom.xml
<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.ivandu.mybatis</groupId>
<artifactId>mybatis</artifactId>
<packaging>jar</packaging>
<version>1.0.0</version>
<name>mybatis</name>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>
<maven.compiler.compilerVersion>11</maven.compiler.compilerVersion>
</properties>
<dependencies>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.7</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.26</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.17</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.13.2</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
</plugin>
</plugins>
<resources>
<resource>
<directory>src/main/java</directory>
<includes>
<include>**/*.xml</include>
</includes>
</resource>
<resource>
<directory>src/main/resources</directory>
<includes>
<include>**/*.*</include>
</includes>
</resource>
</resources>
</build>
</project>2.4.2 src/resources/mybatis.cfg.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<typeAliases>
<package name="com.ivandu.mybatis.model"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://10.1.1.88:3306/mybatis?useSSL=false"/>
<property name="username" value="mybatis"/>
<property name="password" value="drh123"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/ivandu/mybatis/mapper/CompanyMapper.xml"/>
</mappers>
</configuration>2.4.3 src/resources/log4j.properties
# Global logging configuration
log4j.rootLogger=INFO, stdout
# 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%n2.5 创建实例类 Model
2.5.1 CEO 类
package com.ivandu.mybatis.model;
public class CEO {
private Integer id;
private String name;
private Integer age;
public CEO(Integer id, String name, Integer age) {
this.id = id;
this.name = name;
this.age = age;
}
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;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
}2.5.2 Company 类
package com.ivandu.mybatis.model;
public class Company {
private Integer id;
private String name;
private CEO ceo;
public Company(){}
public Company(Integer id, String name, CEO ceo) {
this.id = id;
this.name = name;
this.ceo = ceo;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getId() {
return this.id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public CEO getCeo() {
return ceo;
}
public void setCeo(CEO ceo) {
this.ceo = ceo;
}
}2.6 创建 Mapper
2.6.1 CompanyMapper.java
package com.ivandu.mybatis.mapper;
import com.ivandu.mybatis.model.Company;
public interface CompanyMapper {
Company selectCompanyById(Integer id) throws Exception;
}2.6.2 CompanyMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.ivandu.mybatis.mapper.CompanyMapper">
<select id="selectCompanyById" parameterType="int" resultMap="companymap">
select *
from company com,
ceo
where com.id_ceo_com = ceo.id_ceo
and com.id_com = #{id}
</select>
<!-- resultMap: 映射实体类和字段之间的一一对应的关系 -->
<resultMap id="companymap" type="Company">
<id property="id" column="id_com"/>
<result property="name" column="name_com"/>
<!-- 一对一关联映射：association -->
<association property="ceo" javaType="CEO">
<id property="id" column="id_ceo" javaType="Integer"/>
<result property="name" column="name_ceo" javaType="String"/>
<result property="age" column="age_ceo" javaType="Integer"/>
</association>
</resultMap>
</mapper>2.7 测试运行
package com.ivandu.mybatis;
import com.ivandu.mybatis.mapper.CompanyMapper;
import com.ivandu.mybatis.model.Company;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.BeforeClass;
import org.junit.Test;
import java.io.InputStream;
public class OneToOneTest {
public static SqlSessionFactory sqlSessionFactory;
@BeforeClass
public static void init() throws Exception{
String resource = "mybatis.cfg.xml";
InputStream reader = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
reader.close();
}
@Test
public void selectCompanyById(){
Company company;
SqlSession sqlSession = sqlSessionFactory.openSession();
try (sqlSession) {
CompanyMapper companyMapper = sqlSession.getMapper(CompanyMapper.class);
company = companyMapper.selectCompanyById(2);
System.out.println(company.getId() + "\t" +
company.getName() + "\t" +
company.getCeo().getAge() + "\t" +
company.getCeo().getId() + "\t" +
company.getCeo().getName());
} catch (Exception e) {
e.printStackTrace();
}
}
}三 总结
3.1 错误及故障排除
Error querying database. Cause: org.apache.ibatis.reflection.ReflectionException: Error instantiating class com.ivandu.mybatis.model.Company with invalid types (Integer,String,CEO) or values (1,ChinaBigCompany,1). Cause: java.lang.IllegalArgumentException: argument type mismatch.
解决：实体类中加入无参构造函数。3.2 参考资料
3.3 源码
GitHub：https://github.com/cxwn/mybatis.git tag:2.0.0
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原始发表：2021-09-29 ，如有侵权请联系 cloudcommunity@tencent.com 删除
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