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
代码语言：javascript复制 <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
代码语言：javascript复制 <select id="getDeptById" resultType="com.gong.mybatis.bean.Department">
select * from tbl_department where id=#{id}
</select>进行测试：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制 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：
代码语言：javascript复制 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。
