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 ="last_name"="lastName"="gender"="gender"="email"="email"discriminator javaType="string" column>
            case value="0" resultType="com.gong.mybatis.bean.Employee">
                association property="dept" select="com.gong.mybatis.dao.DepartmentMapper.getDeptById"
                column="d_id"</associationcase="1"/>
                />
            discriminator>
    resultMapselect id="getEmpByIdStep" resultMap>
        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

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

进行测试：

package com.gong.mybatis.test;

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

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

 com.gong.mybatis.bean.Department;
 com.gong.mybatis.bean.Employee;
 com.gong.mybatis.dao.DepartmentMapper;
 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
    void test()  IOException {
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        try {
            EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.);
            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=[e-mail],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。