初学Mybatis之多对一查询 association 和一对多查询 collection

XML 映射器

多对一：关联（association）

一对多：集合（collection）

mysql 创建教师、学生表，插入数据

create table `teacher`(id int(10) primary key,`name` varchar(30) default null
) engine=InnoDB default charset=utf8;insert into teacher (id,`name`) values (1,'教师');create table `student`(id int(10) not null,`name` varchar(30) default null,tid int(10) default null,primary key (id),key fktid (tid),constraint fktid foreign key (tid) references `teacher` (id)
) engine=InnoDB default charset=utf8;insert into student (id,`name`,tid) values (1,'学生1',1);
insert into student (id,`name`,tid) values (2,'学生2',1);
insert into student (id,`name`,tid) values (3,'学生3',1);

pom.xml 导入相应 jar 包

  <dependencies><!-- mysql驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.33</version></dependency><!-- mybatis驱动 --><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId><version>3.5.16</version></dependency><!-- junit驱动 --><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.13.2</version></dependency><!-- lombok驱动 --><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.34</version></dependency></dependencies><!-- 在build中配置resources，防止资源导出失败 --><build><resources><resource><directory>src/main/java</directory><includes><include>**/*.xml</include><include>**/*.properties</include></includes></resource><resource><directory>src/main/resources</directory><includes><include>**/*.xml</include><include>**/*.properties</include></includes></resource></resources></build>

resources 目录下创建 db.properties

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=false&useUnicode=true&characterEncoding=UTF-8
username=root
password=root

resources 目录下创建 mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd"><!-- configuration核心配置文件 -->
<configuration><!-- 引入外部配置文件 --><properties resource="db.properties"/><settings><!-- 日志工厂实现 --><setting name="logImpl" value="STDOUT_LOGGING"/></settings><!-- 扫描实体类的包 --><typeAliases><package name="com.demo.pojo"/></typeAliases><environments default="development"><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="${driver}"/><property name="url" value="${url}"/><property name="username" value="${username}"/><property name="password" value="${password}"/></dataSource></environment></environments><mappers><mapper class="com.demo.dao.TeacherMapper"/><mapper class="com.demo.dao.StudentMapper"/></mappers></configuration>

MybatisUtils 工具类：

package com.demo.utils;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 java.io.IOException;
import java.io.InputStream;public class MybatisUtils {private static SqlSessionFactory sqlSessionFactory;static {try {//获取sqlSessionFactory对象String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();}}/*有了SqlSessionFactory，就可以获得SqlSession的实例SqlSession提供了在数据库执行SQL命令所需的所有方法可以通过SqlSession实例来执行已映射的SQL语句*/public static SqlSession getSqlSession(){return sqlSessionFactory.openSession(true);}
}

实体类与表字段一致

Teacher 类：

package com.demo.pojo;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {private int id;private String name;
}

Student 类：

（因为 tid 设置了外键，直接写 int tid 则无法关联，所以通过组合的方式关联进来）

package com.demo.pojo;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {private int id;private String name;//多个学生关联一个老师private Teacher teacher;
}

TeacherMapper 接口：

import com.demo.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;public interface TeacherMapper {@Select("select * from teacher where id = #{tid}")Teacher getTeacher(@Param("tid") int id);
}

同理，写个 StudentMapper 接口

resources 目录创建同包名

TeacherMapper.xml：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.demo.dao.TeacherMapper"></mapper>

同理，创建 StudentMapper.xml

在核心配置文件 mybatis-config.xml 绑定注册：

    <mappers><mapper class="com.demo.dao.TeacherMapper"/><mapper class="com.demo.dao.StudentMapper"/></mappers>

测试类：

package com.demo.dao;import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;public class MyTest {@Testpublic void test(){SqlSession sqlSession = MybatisUtils.getSqlSession();TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);System.out.println(mapper.getTeacher(1));sqlSession.close();}
}

输出结果：

测试环境搭建步骤：

1.导入 Lombok 依赖

2.新建实体类 Teacher、Student

3.建立 Mapper 接口

4.建立 Mapper.xml 文件

5.在核心配置文件中绑定注册 Mapper 接口或者文件

6.测试查询是否成功

多对一处理：

StudentMapper 实体类：

package com.demo.dao;import com.demo.pojo.Student;import java.util.List;public interface StudentMapper {//查询所有学生信息，以及对应教师信息List<Student> getStudent();
}

方法一：查找嵌套，根据查询出来的学生 tid，寻找对应的教师

StudentMapper.xml：

关联：association

集合：collection

javaType：对象类型

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.demo.dao.StudentMapper"><select id="getStudent" resultMap="tid">select * from student</select><resultMap id="tid" type="student"><result column="id" property="id"/><result column="name" property="name"/><!-- 关联：association --><!-- 集合：collection --><!-- javaType：对象类型 --><association property="teacher" column="tid" javaType="teacher" select="getTeacher"/></resultMap><select id="getTeacher" resultType="teacher">select * from teacher where id = #{tid}</select>
</mapper>

association 关联的 property 是 Student 类的组合

测试类：遍历数组

package com.demo.dao;import com.demo.pojo.Student;
import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;import java.util.List;public class MyTest {@Testpublic void getStudent(){SqlSession sqlSession = MybatisUtils.getSqlSession();StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List<Student> studentList = mapper.getStudent();for(Student student : studentList){System.out.println(student);}sqlSession.close();}
}

运行结果如下：

方法二：结果嵌套

StudentMapper.xml：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.demo.dao.StudentMapper"><select id="getStudent" resultMap="tid">select s.id sid,s.name sname,s.tid stid,t.name tname from student s,teacher t where s.tid = t.id;</select><resultMap id="tid" type="Student"><result property="id" column="sid"/><result property="name" column="sname"/><association property="teacher" javaType="Teacher"><result property="name" column="tname"/><result property="id" column="stid"/></association></resultMap>
</mapper>

结果如下：

Mybatis 多对一查询方式：

1.子查询

2.联表查询

一对多处理：

Student 类：

package com.demo.pojo;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {private int id;private String name;private int tid;
}

Teacher 类：

package com.demo.pojo;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;import java.util.List;@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {private int id;private String name;//一名教师教导多名学生private List<Student> students;
}

TeacherMapper 接口：

package com.demo.dao;import com.demo.pojo.Teacher;
import org.apache.ibatis.annotations.Param;import java.util.List;public interface TeacherMapper {List<Teacher> getTeacher();//获取指定教师下所有学生及教师的信息Teacher getTeacherById(@Param("tid") int id);
}

TeacherMapper.xml：

方法一：结果嵌套查询

javaType 指定属性的类型，ofType 应用于集合中的泛型信息

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.demo.dao.TeacherMapper"><select id="getTeacher" resultType="teacher">select * from teacher</select><!-- 结果嵌套查询 --><!-- resultMap 结果集映射 --><select id="getTeacherById" resultMap="TeacherStudent">select s.id sid,s.name sname,t.name tname,t.id tidfrom student s,teacher twhere s.tid = t.id and t.id = #{tid};</select><resultMap id="TeacherStudent" type="teacher"><result property="id" column="tid"/><result property="name" column="tname"/><!-- 集合：collection --><!-- javaType指定属性的类型，ofType应用于集合中的泛型信息 --><collection property="students" ofType="student"><result property="id" column="sid"/><result property="name" column="sname"/><result property="tid" column="tid"/></collection></resultMap>
</mapper>

测试类：

package com.demo.dao;import com.demo.pojo.Teacher;
import com.demo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;public class MyTest {@Testpublic void getTeacher(){SqlSession sqlSession = MybatisUtils.getSqlSession();TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);for (Teacher teacher : mapper.getTeacher()) {System.out.println(teacher);}sqlSession.close();}@Testpublic void getTeacherById(){SqlSession sqlSession = MybatisUtils.getSqlSession();TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);System.out.println(mapper.getTeacherById(1));sqlSession.close();}
}

运行结果：

方法二：查询嵌套

    <select id="getTeacherById" resultMap="TeacherStudent">select * from teacher where id = #{tid}</select><resultMap id="TeacherStudent" type="teacher"><collection property="students" javaType="ArrayList" ofType="student" select="tid" column="id"/></resultMap><select id="tid" resultMap="student">select * from student where tid = #{tid}</select>

总结：

1.关联：association（多对一）

2.集合：collection（一对多）

3. javaType 用来指定实体类中属性的类型

4. ofType 用来指定映射到 List 或集合中的 pojo 类型，泛型中的约束类型

注意点：

1.保证 SQL 可读性

2.注意一对多或多对一属性名和字段

3.使用 Log4j 日志排查错误

综上，还需了解 Mysql 引擎、InnoDB 底层原理、索引、索引优化等