Mybatis——鉴别器映射

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鉴别器映射

有时一个单独的数据库查询会返回很多种不同数据类型的结果集。

discriminator鉴别器元素可以用来处理这种情况

数据库表结构 

 

创建实体类、映射接口和映射文件 

实体类employee

package org.example.demo2;import lombok.Data;@Data
public class Employee {private Integer employeeId;private String employeeName;
}

 实体类HourlyEmployee

package org.example.demo2;import lombok.Data;@Data
public class HourlyEmployee extends Employee{private Double rate;
}

 实体类SalaryEmployee

package org.example.demo2;import lombok.Data;@Data
public class SalaryEmployee extends Employee{private Double salary;
}

 EmployeeMapper

package org.example.mapper;import org.example.demo2.Employee;public interface EmployeeMapper {Employee findEmployeeById(Integer employeeId);
}

EmployeeMapper.xml 

使用resultMap的extends属性来映射两个子类的属性

使用discriminator元素映射employee_type字段

discriminator元素常用的两个属性如下：

column：该属性用于设置需要进行鉴别比较值的列

javaType：该属性用于指定列的类型

discriminator元素可以有1个或者多个case子元素，case元素包含三个属性

value：该值用来匹配column指定字段的值

resultMap：当value值和column的值匹配时的结果映射，优先级高于resultType

resultType：当value值和column的值匹配时的结果类型

<?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="org.example.mapper.EmployeeMapper"><resultMap id="employeeMap" type="org.example.demo2.Employee"><id property="employeeId" column="employee_id"/><result property="employeeName" column="employee_name"/></resultMap><resultMap id="hourlyMap" type="org.example.demo2.HourlyEmployee" extends="employeeMap"><result property="rate" column="rate"/></resultMap><resultMap id="salaryMap" type="org.example.demo2.SalaryEmployee" extends="employeeMap"><result property="salary" column="salary"/></resultMap><resultMap id="selectMap" type="org.example.demo2.Employee"><discriminator javaType="String" column="employee_type"><case value="HE" resultMap="hourlyMap"></case><case value="SE" resultMap="salaryMap"></case></discriminator></resultMap><select id="findEmployeeById" resultMap="selectMap">select * from t_employee where employee_id = #{employeeId};</select>
</mapper>

 测试类

 

package org.example;import org.apache.ibatis.session.SqlSession;
import org.example.demo.Order;
import org.example.demo.User;
import org.example.demo2.Employee;
import org.example.demo2.SalaryEmployee;
import org.example.mapper.EmployeeMapper;
import org.example.mapper.UserMapper;
import org.example.utils.MybatisUtil;import java.util.List;public class test {public static void main(String[] args) {test test1 = new test();test1.selectAllEmployee();}public void selectAllEmployee() {SqlSession sqlSession = MybatisUtil.getSqlSession();EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);SalaryEmployee salary = (SalaryEmployee) mapper.findEmployeeById(2);System.out.println(salary.getSalary());sqlSession.close();}
}

 

 

 

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