一对一也就是 A 表的一条记录对应 B 表的一条记录，下面的测试数据中，从employee 表来看，一个员工对应一个部门，是一对一关系，如果从部门角度来看，则是一对多的关系，一个部门对应多个员工，本节主要研究一对一的关系。

数据表建立

新建数据表department，有两个字段，插入两条数据如下：

id dept_name
1 CIA
2 FSB

新建数据表employee，有三个字段，其中dept_id是外键，关联department表的主键id。插入数据如下：

id last_name dept_id
1 Tom 1
2 Jerry 2
3 Neo 1
4 Cypher 2

新建maven工程，添加依赖，主要是mybatis和mysql

<dependencies>

    <dependency>

        <groupId>org.mybatis</groupId>

        <artifactId>mybatis</artifactId>

        <version>3.4.6</version>

    </dependency>

    <dependency>

        <groupId>mysql</groupId>

        <artifactId>mysql-connector-java</artifactId>

        <version>5.1.47</version>

    </dependency>

</dependencies>

3、编写数据库表对应的实体。

对于department表，对应实体如下：

package com.yefengyu.mybatis.entity;

public class Department

{

    private Integer id;

    private String deptName;

    public Integer getId()

    {

        return id;

    }

    public void setId(Integer id)

    {

        this.id = id;

    }

    public String getDeptName()

    {

        return deptName;

    }

    public void setDeptName(String deptName)

    {

        this.deptName = deptName;

    }

    @Override

    public String toString()

    {

        return "Department{" +

               "id=" + id +

               ", deptName='" + deptName + '\'' +

               '}';

    }

}

对于employee表，实体如下，注意在Employee实体中把外键直接变成对于Department对象的引用。

package com.yefengyu.mybatis.entity;

public class Employee

{

    private Integer id;

    private String lastName;

    private Department department;

    public Integer getId()

    {

        return id;

    }

    public void setId(Integer id)

    {

        this.id = id;

    }

    public String getLastName()

    {

        return lastName;

    }

    public void setLastName(String lastName)

    {

        this.lastName = lastName;

    }

    public Department getDepartment()

    {

        return department;

    }

    public void setDepartment(Department department)

    {

        this.department = department;

    }

    @Override

    public String toString()

    {

        return "Employee{" +

               "id=" + id +

               ", lastName='" + lastName + '\'' +

               ", department=" + department +

               '}';

    }

}

编写mapper接口

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Employee;

public interface EmployeeMapper

{

    Employee getEmployee(Integer id);

}

根据员工ID查询员工信息和部门信息

编写mapper映射文件（本节重点）

一对一查询mapper文件有三种编写方式，分别是：

1、级联属性法

<?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="com.yefengyu.mybatis.mapper.EmployeeMapper">

    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">

        <id column="id" property="id"/>

        <result column="last_name" property="lastName"/>

        <result column="d_id" property="department.id"/>

        <result column="dept_name" property="department.deptName"/>

    </resultMap>

    <select id="getEmployee" resultMap="employee">

        select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name

        from employee e, department d

        where e.id = #{id} and e.dept_id = d.id

    </select>

</mapper>

2、association嵌套结果集

<?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="com.yefengyu.mybatis.mapper.EmployeeMapper">

    <!--

         使用association定义关联的单个对象的封装规则；

    -->

    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">

        <id column="id" property="id"/>

        <result column="last_name" property="lastName"/>

        <!--  association可以指定联合的javaBean对象

            property="department"：指定哪个属性是联合的对象

            javaType:指定这个属性对象的类型[不能省略]

        -->

        <association property="department" javaType="com.yefengyu.mybatis.entity.Department">

            <id column="d_id" property="id"/>

            <result column="dept_name" property="deptName"/>

        </association>

    </resultMap>

    <select id="getEmployee" resultMap="employee">

        select e.id id, e.last_name last_name, e.dept_id dept_id, d.id d_id,d.dept_name dept_name

        from employee e, department d

        where e.id = #{id} and e.dept_id = d.id

    </select>

</mapper>

3、association分步查询法

1）首先需要创建一个关于department的接口

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Department;

public interface DepartmentMapper

{

    public Department getDeptById(Integer id);

}

2）其次编写该接口的映射文件

<?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="com.yefengyu.mybatis.mapper.DepartmentMapper">

    <select id="

getDeptById

" resultType="com.yefengyu.mybatis.entity.Department">

        select id,dept_name from department where id = #{id}

    </select>

</mapper>

3）现在定义一个查询employee的接口（级联属性法和association嵌套结果集测试时已经建立该接口）

package com.yefengyu.mybatis.mapper;

import com.yefengyu.mybatis.entity.Employee;

public interface EmployeeMapper

{

    Employee getEmployee(Integer id);

}

4）编写该接口对应的映射文件

<?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="com.yefengyu.mybatis.mapper.EmployeeMapper">

    <resultMap id="employee" type="com.yefengyu.mybatis.entity.Employee">

        <id column="id" property="id"/>

        <result column="last_name" property="lastName"/>

        <!-- association定义关联对象的封装规则

          select:表明当前属性是调用select指定的方法查出的结果

          column:指定将哪一列的值传给这个方法

          流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性

       -->

        <association property="department"

                     select="com.yefengyu.mybatis.mapper.DepartmentMapper.getDeptById"

                     column="dept_id">

        </association>

    </resultMap>

    <select id="getEmployee" resultMap="employee">

        select * from employee where id = #{id}

    </select>

</mapper>

延迟加载：

每次查询Employee对象的时候，都将部门信息一起查询出来。但是想要部门信息在我们使用的时候再去查询，如果不需要就不查询，怎么实现？分步查询的基础上加两个配置：

<!--显示的指定每个我们需要更改的配置的值，即使他是默认的。防止版本更新带来的问题  -->

<setting name="lazyLoadingEnabled" value="true"/>

<setting name="aggressiveLazyLoading" value="false"/>

Mybatis仅支持association关联对象和collection关联集合对象的延迟加载，association指的就是一对一，collection指的就是一对多查询。

新建一个mybatis全局配置文件，详细信息见官网

<?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>

    <settings>

        <setting name="mapUnderscoreToCamelCase" value="true"/>

        <setting name="logImpl" value="STDOUT_LOGGING" />

    </settings>

    <environments default="development">

        <environment id="development">

            <transactionManager type="JDBC"/>

            <dataSource type="POOLED">

                <property name="driver" value="com.mysql.jdbc.Driver"/>

                <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis?characterEncoding=utf8&amp;allowMultiQueries=true"/>

                <property name="username" value="root"/>

                <property name="password" value="123456"/>

            </dataSource>

        </environment>

    </environments>

    <mappers>

        <mapper resource="mapper/EmployeeMapper.xml"/>

        <mapper resource="mapper/DepartmentMapper.xml"/>

    </mappers>

</configuration>

测试

通用测试代码：

public static void main(String[] args)

    throws IOException

{

    InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml");

    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);

    SqlSession sqlSession = sqlSessionFactory.openSession();

    try

    {

        EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

        Employee employee = mapper.getEmployee(1);

        System.out.println(employee);

    }

    finally

    {

        sqlSession.close();

    }

}

准对于延迟加载的特殊测试，对于上面测试代码，如果只打印员工名

System.out.println(employee.getLastName());

那么不会查询部门表

Created connection 1776957250.

Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

==>  Preparing: select * from employee where id = ?

==> Parameters: 1(Integer)

<==    Columns: id, last_name, dept_id

<==        Row: 1, tom, 1

<==      Total: 1

tom

Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

Returned connection 1776957250 to pool.

如果打印部门相关信息，例如通用测试的代码（由于实体都重写了toString方法，因此打印employee的时候也打印了department，所以也是使用了部门信息），此时的日志为：

Created connection 1776957250.

Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

==>  Preparing: select * from employee where id = ?

==> Parameters: 1(Integer)

<==    Columns: id, last_name, dept_id

<==        Row: 1, tom, 1

<==      Total: 1

==>  Preparing: select id,dept_name from department where id = ?

==> Parameters: 1(Integer)

<==    Columns: id, dept_name

<==        Row: 1, A

<==      Total: 1

Employee{id=1, lastName='tom', department=Department{id=1, deptName='A'}}

Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@69ea3742]

Returned connection 1776957250 to pool.

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