MyBatis之resultMap的作用和xml配置折腾

原文博客：http://blog.csdn.net/iaiti/article/details/53227880
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

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

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

<?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：

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

进行查询操作。

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

public class Employer {      private int id;//省略set get 方法      private int employee_id;      private String name;      private Employee em;  }  

EmployerMapper.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">  <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：

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：

<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:

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：

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

<?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：

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。

<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：

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。