MyBatis系列之关联查询(一对一)

我们以一个例子demo的形式，说明Mybatis的一对一关联查询

设计teachers表

classes表设置外键

classes表

classes表是课程表，teachers表是教师信息表

这里是一对一的关联关系，当然实际中一般不是这样的，一般可以是多对多，不过这里只是举例子

项目的结构，如图：

Classes.java:

      package com.mybatis.entity;
      import java.util.List;
      public class Classes {
     	private int id;
     	private String name;
     	private Teacher teacher;
     	private List<Student> students;
     	public List<Student> getStudents() {
     		return students;
      	}
     	public void setStudents(List<Student> students) {
     		this.students = students;
      	}
     	public int getId() {
     		return id;
      	}
     	public void setId(int id) {
     		this.id = id;
      	}
     	public String getName() {
     		return name;
      	}
     	public void setName(String name) {
     		this.name = name;
      	}
     	public Teacher getTeacher() {
     		return teacher;
      	}
     	public void setTeacher(Teacher teacher) {
     		this.teacher = teacher;
      	}
     	@Override
     	public String toString() {
     		return "Classes [id=" + id + ", name=" + name + "]";
      	}
      }
  
 

Teacher.java:

      package com.mybatis.entity;
      public class Teacher {
     	private int id;
     	private String name;
     	public int getId() {
     		return id;
      	}
     	public void setId(int id) {
     		this.id = id;
      	}
     	public String getName() {
     		return name;
      	}
     	public void setName(String name) {
     		this.name = name;
      	}
      }
  
 

这里以两种方式进行使用

classesMapper.xml:

      <?xml version="1.0" encoding="UTF-8" ?>
      <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
      "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
      <!--
       为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
      -->
      <mapper namespace="com.mybatis.mapper.classesMapper">
      <!--
       在select标签中编写查询的SQL语句， 设置select标签的id属性为findById，id属性值必须是唯一的，不能够重复
       parameterType可以在MyBatis的配置文件里设置，也可以直接用包名+类名
       -->
     	<select id="findById" parameterType="int" resultMap="ClassesResultMap">
      	select * from classes c,teachers t where c.teacher_id=t.t_id and c.c_id=#{id}
     	</select>
     	<resultMap type="Classes" id="ClassesResultMap">
     		<id property="id" column="c_id" />
     		<result property="name" column="c_name" />
     		<association property="teacher" column="teacher_id" javaType="Teacher">
     			<id property="id" column="t_id" />
     			<result property="name" column="t_name" />
     		</association>
     	</resultMap>
     	<select id="findById2" parameterType="int" resultMap="ClassesResultMap2">
      	select * from classes where c_id=#{id}
     	</select>
     	<resultMap type="Classes" id="ClassesResultMap2">
     		<id property="id" column="c_id" />
     		<result property="name" column="c_name" />
     		<association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacherInfo">
     		</association>
     	</resultMap>
     	<select id="getTeacherInfo" parameterType="int" resultType="Teacher">
      	select t_id,t_name from teachers where t_id=#{id}
     	</select>
      </mapper>
  
 

然后对mybatis进行配置：

      <?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>
     	<!-- 数据库配置文件 -->
      <properties resource="db.properties" />
        <!-- 实体类,简称 -设置别名 -->
     	<typeAliases>
     		<typeAlias alias="User" type="com.mybatis.entity.User" />
     		<typeAlias alias="Classes" type="com.mybatis.entity.Classes" />
     		<typeAlias alias="Student" type="com.mybatis.entity.Student" />
     		<typeAlias alias="Teacher" type="com.mybatis.entity.Teacher" />
     	</typeAliases>
     	<!-- 读取db.properties里的数据，进行数据库配置 -->
     	<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>
     	<!--
       注册userMapper.xml文件
       <mappers>
       <mapper resource="com/mybatis/mapper/userMapper.xml"/>
       </mappers>
       -->
     	<mappers>
     		<mapper class="com.mybatis.mapper.UserMapper"/>
     		<mapper resource="com/mybatis/mapper/classesMapper.xml"/>
     	</mappers>
      </configuration>
  
 

db,properties:

      driver=com.mysql.jdbc.Driver
      url=jdbc\:mysql\://localhost\:3306/db_mysql?useUnicode\=true&characterEncoding\=utf-8
      username=root
      password=111
  
 

编写测试类：

      String resource = "mybatis-config.xml";
     		try{
     			//加载Mybatis的配置文件
      			Reader reader = Resources.getResourceAsReader(resource);
     			//构建sqlSession的工厂
       SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
     			//创建可以执行sql的SqlSession
       SqlSession session = sessionFactory.openSession();
      			UserMapper mapper = session.getMapper(UserMapper.class);
      			User user = mapper.findById(1);
       System.out.println(user);
       Classes c = session.selectOne("com.mybatis.mapper.classesMapper.findById1",1);
       System.out.println(c);
      		}catch (Exception e) {
      			e.printStackTrace();
      		}
  
 

文章来源: smilenicky.blog.csdn.net，作者：smileNicky，版权归原作者所有，如需转载，请联系作者。

原文链接：smilenicky.blog.csdn.net/article/details/51292271