mybatis文件映射之鉴别器discriminator标签

mybatis可以使用鉴别器判断某列的值，然后根据某列的值改变封装行为。

比如说：

如果是女生（gender=0）我们将部门信息提取出来，否则不提取；

如果是男生（gender=1），把last_name的值赋值给email；

EmployeeMapper.xml

    <resultMap type="com.gong.mybatis.bean.Employee" id="MyEmpDis">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="gender" property="gender"/>
        <result column="email" property="email"/>
        <discriminator javaType="string" column="gender">
            <case value="0" resultType="com.gong.mybatis.bean.Employee">
                <association property="dept" select="com.gong.mybatis.dao.DepartmentMapper.getDeptById"
                column="d_id">
                </association>
            </case>
            <case value="1" resultType="com.gong.mybatis.bean.Employee">
                <id column="id" property="id"/>
                <result column="last_name" property="lastName"/>
                <result column="gender" property="gender"/>
                <result column="last_name" property="email"/>
            </case>
        </discriminator>
    </resultMap>
    <select id="getEmpByIdStep" resultMap="MyEmpDis">
        select * from tbl_employee where id=#{id}
    </select>

说明：distriminator中的javaType指定数据库中字段对应着java中的属性的类型，column属性为列名称。case标签中value属性标识数据库中列的值，resultType指定封装的结果类型，不可少。

DepartmentMapper.xml

    <select id="getDeptById" resultType="com.gong.mybatis.bean.Department">
        select * from tbl_department where id=#{id}
    </select>

进行测试：

package com.gong.mybatis.test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.gong.mybatis.bean.Department;
import com.gong.mybatis.bean.Employee;
import com.gong.mybatis.dao.DepartmentMapper;
import com.gong.mybatis.dao.EmployeeMapperPlus;

public class TestMybatis2 {
    
    public SqlSessionFactory getSqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(resource);
        return new SqlSessionFactoryBuilder().build(is);
    }

    @Test
    public void test() throws IOException {
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        try {
            EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.class);
            Employee employee = mapper.getEmpByIdStep(3);
            System.out.println(employee.getLastName());
            System.out.println(employee.getDept());
            openSession.commit();
        } finally {
            openSession.close();
        }
        
    }

}

首先测试的是gender=0的情况：

DEBUG 01-20 20:47:37,503 ==> Preparing: select * from tbl_employee where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:47:37,590 ==> Parameters: 3(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:47:37,757 <== Total: 1 (BaseJdbcLogger.java:145) 小红 DEBUG 01-20 20:47:37,759 ==> Preparing: select * from tbl_department where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:47:37,760 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:47:37,763 <== Total: 1 (BaseJdbcLogger.java:145) Department [id=1, deptName=开发部]

可以查到部门的信息。

接下来是gender=1的情况：

Employee employee = mapper.getEmpByIdStep(1);

DEBUG 01-20 20:53:32,581 ==> Preparing: select * from tbl_employee where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:53:32,647 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:53:32,708 <== Total: 1 (BaseJdbcLogger.java:145) xiximayou null

发现部门信息是查不到的。

再进行gender=1时的情况：

            Employee employee = mapper.getEmpByIdStep(1);
            System.out.println(employee);
            System.out.println(employee.getDept());

DEBUG 01-20 20:54:39,440 ==> Preparing: select * from tbl_employee where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:54:39,480 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:54:39,555 <== Total: 1 (BaseJdbcLogger.java:145) Employee [id=1, lastName=xiximayou, gender=1, email=xiximayou, dept=null] null

email被修改，且部门信息查不到。

gender=0的情况：

        Employee employee = mapper.getEmpByIdStep(3);
        System.out.println(employee);
        System.out.println(employee.getDept());

DEBUG 01-20 20:56:19,177 ==> Preparing: select * from tbl_employee where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:56:19,221 ==> Parameters: 3(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:56:19,384 <== Total: 1 (BaseJdbcLogger.java:145) DEBUG 01-20 20:56:19,385 ==> Preparing: select * from tbl_department where id=? (BaseJdbcLogger.java:145) DEBUG 01-20 20:56:19,386 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 01-20 20:56:19,389 <== Total: 1 (BaseJdbcLogger.java:145) Employee [id=3, lastName=小红, gender=0, email=xiaohong@qq.com, dept=Department [id=1, deptName=开发部]] Department [id=1, deptName=开发部]

email未被修改，且可以查到部门的信息。