MyBatis之resultMap的作用和xml配置折腾

小鱼儿 2022-07-16 13:23 141阅读 0赞

# 简单的搭建。 #

[MyBatis实战——前身iBatis、基础环境搭建和如何"钓鱼"][MyBatis_iBatis] 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} 
      </select>  
    </mapper>

mybatis-config.xml的mappers映射改成：

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" />	
    		<result property="name" column="name" />  
    		<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" />	
    		<result property="name" column="name" />  
    		<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。

[MyBatis_iBatis]: http://blog.csdn.net/iaiti/article/details/47857703