MyBatis之resultMap的作用和xml配置折腾

简单的搭建。

MyBatis实战——前身iBatis、基础环境搭建和如何"钓鱼" 这篇写了基础的环境搭建和代码操作。

我这里是使用了xml加interface结合的方法进行操作。现有Employee类。如果是简单的select，可以看看下面简单的例子。

Employee.Java

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public class Employee {//省略get set 方法

private int id;

private String first_name;

private String last_name;

private int salary;

}

EmployeeMapper.java

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package com.mybatis3.mappers;

import model.Employee;

public interface EmployeeMapper

{

Employee findEmployeeById(Integer a);

}

EmployeeMapper.xml(和EmployeeMapper.java位于同个package中

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<?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.mybatis3.mappers.EmployeeMapper">

<select id="findEmployeeById" resultType="model.Employee">

select * from Employee where id = #{abc} <!-- 后面发现这个参数名这里填什么都无所谓，能映射到 -->

</select>

</mapper>

mybatis-config.xml的mappers映射改成：

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<mapper class="com.mybatis3.mappers.EmployeeMapper" />

进行查询操作。

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public class Factory {

public static void main(String[] args) {

String configxml = "mybatis-config.xml";

InputStream is = null;

try {

is = Resources.getResourceAsStream(configxml);

} catch (IOException e) {

e.printStackTrace();

}

SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);

SqlSession s = sf.openSession();

EmployeeMapper employeeMapper = s.getMapper(EmployeeMapper.class);

Employee e = employeeMapper.findEmployeeById(47);

System.out.println(e.getFirst_name());

System.out.println(e.getLast_name());

System.out.println(e.getSalary());

s.close();

}

}

这是单表查询的时候，那么问题就来了。如果是多表复杂的查询。现在有了一张新表Employer，假如你是框架的设计者，会让Employee的字段全部丢到Employer中吗，显然不合适。

这是MyBatis提供了resultmap的方法。先看看单独一个类也可以使用resultmap进行映射。

Employer类

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public class Employer {

private int id;//省略set get 方法

private int employee_id;

private String name;

private Employee em;

}

EmployerMapper.xml

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<?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.mybatis3.mappers.EmployerMapper">

<resultMap id="Employer" type="model.Employer">

<id property="id" column="id" /> <!-- id相当于resultmap的引用的唯一标识-->

<result property="name" column="name" /> <!-- property对应javabean的属性名，column对应数据库字段名 -->

<result property="employee_id" column="employee_id" />

</resultMap>

<select id="findEmployeeById" resultMap="Employer">

select * from Employer where id = #{abc}

</select>

</mapper>

测试：

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public class Factory {

public static void main(String[] args) {

String configxml = "mybatis-config.xml";

InputStream is = null;

try {

is = Resources.getResourceAsStream(configxml);

} catch (IOException e) {

e.printStackTrace();

}

SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);

SqlSession s = sf.openSession();

EmployerMapper employerMapper = s.getMapper(EmployerMapper.class);

Employer e2 = employerMapper.findEmployeeById(1);

System.out.println(e2.getEmployee_id());

System.out.println(e2.getName());

s.close();

}

}

折腾resultMap的id,result属性：

接下来开始折腾了。将EmployerMapper.xml里面改成只剩：

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<resultMap id="Employer" type="model.Employer">

</resultMap>

<select id="findEmployeeById" resultMap="Employer">

select * from Employer where id = #{abc}

</select>

发现还是能正常映射。但是如果改javabean的一个属性:

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private int employeeid;

public int getEmployeeid() {

return employeeid;

}

public void setEmployeeid(int employee_id) {

this.employeeid = employee_id;

}

这样最后取值是取不到的，因为映射不到。那究竟是属性名还是通过getset方法的名字映射呢。再折腾：

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private int employeeid;

public int getEmployee_id() {

return employeeid;

}

public void setEmployee_id(int employee_id) {

this.employeeid = employee_id;

}

employeeid属性名与数据库字段名不对应，但是get，set的方法名是和数据库字段名对应的。运行正常。

如果不想在配置这里瞎折腾，JavaBean字段名保持和数据库字段名是最好的选择。

折腾resultMap extends属性，表关联查询。

Employer.xml

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<?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.mybatis3.mappers.EmployerMapper">

<resultMap id="Employer" type="model.Employer">

</resultMap>

<resultMap type="model.Employer" id="EmployerWithEmployee"

extends="Employer">

<result property="em.id" column="id" />

<result property="em.first_name" column="first_name" />

<result property="em.last_name" column="last_name" />

<result property="em.salary" column="salary" />

</resultMap>

<select id="findEmployeeById2" resultMap="EmployerWithEmployee">

select employee_id,name,b.first_name,b.salary,b.last_name from Employer a

left join Employee b on a.employee_id=b.id where a.id = #{abc}

</select>

</mapper>

测试：

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SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);

SqlSession s = sf.openSession();

EmployerMapper employerMapper = s.getMapper(EmployerMapper.class);

Employer e2 = employerMapper.findEmployeeById2(1);

System.out.println(e2.getEmployee_id());

System.out.println(e2.getName());

System.out.println(e2.getEm().getFirst_name());

System.out.println(e2.getEm().getLast_name());

注意这次的em不再是和上面第一个折腾的一样，是和方法名一致的。这次是和属性名一致。不然会报错。

而且MyBatis的关联查询就这么简单，还是一样的关联语句，只不过加了一些映射配置。

折腾association

extends拓展看起来挺好，但是讲到解耦的话，你extends出来是个不伦不类的东西。本身我就有Employee的存在。

所以，这时association出现了。

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<mapper namespace="com.mybatis3.mappers.EmployerMapper">

<resultMap id="Employee" type="model.Employee">

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

<result property="first_name" column="first_name" />

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

<result property="salary" column="salary" />

</resultMap>

<resultMap id="Employer" type="model.Employer">

<id property="id" column="id" /> <!-- id相当于resultmap的引用的唯一标识-->

<result property="name" column="name" /> <!-- property对应javabean的属性名，column对应数据库字段名 -->

<result property="employee_id" column="employee_id" />

<association property="em" resultMap="Employee" />

</resultMap>

<select id="findEmployeeById2" resultMap="Employer">

select employee_id,name,b.first_name,b.salary,b.last_name from Employer a

left join Employee b on a.employee_id=b.id where a.id = #{abc}

</select>

</mapper>

测试代码不变：

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SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);

SqlSession s = sf.openSession();

EmployerMapper employerMapper = s.getMapper(EmployerMapper.class);

Employer e2 = employerMapper.findEmployeeById2(1);

System.out.println(e2.getEmployee_id());

System.out.println(e2.getName());

System.out.println(e2.getEme().getFirst_name());

System.out.println(e2.getEme().getLast_name());

s.close();

但是不同于resultmap使用extends，result标签是不可少的，一旦少了，将获取不到其属性值。

经过上面种种折腾之后终于知道作者讲的作用是什么：最后就明白了书上所讲的ResultMap的作用，映射SQL选择声明的结果到JavaBean属性中。通过一对一和一对多associations可以将简单选择声明映射复杂的声明。