鉴别器：mybatis可以使用discriminator判断某列的值，然后根据某列的值改变封装行为。比如封装Employee：根据gender值
1、如果查出的是女生：就把部门信息查询出来，否则不查询；
2、如果是男生，把last_name这一列的值赋值给email;
1、搭建数据库
tbl_employee表【d_id 是外键 对应 tbl_dept表】
id last_name gender email d_id 1 mike 0 [email protected] 1 2 book 0 [email protected] 2 3 tom 1 [email protected] 2 4 jerry 1 [email protected] 1
tbl_dept表：
id dept_name 1 开发部 2 测试部
2、编辑 javabean
Employee.java
package com.mybatis.bean;
public class Employee {
private int id;
private String lastName;
private String email;
private String gender;
private Department department;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
@Override
public String toString() {
return "Employee [id=" + id + ", lastName=" + lastName + ", email=" + email + ", gender=" + gender + "]";
}
}
Department.java
package com.mybatis.bean;
import java.util.List;
public class Department {
private Integer id;
private String departmentName;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getDepartmentName() {
return departmentName;
}
public void setDepartmentName(String departmentName) {
this.departmentName = departmentName;
}
@Override
public String toString() {
return "Department [id=" + id + ", departmentName=" + departmentName
+ "]";
}
}
3、编辑接口文件
EmployeeMapper.java
package com.mybatis.mapper;
import com.mybatis.bean.Employee;
public interface EmployeeMapper {
public Employee getEmpByIdStep(Integer id);
}
DepartmentMapper.java
package com.mybatis.mapper;
import com.mybatis.bean.Department;
public interface DepartmentMapper {
public Department getDeptById(Integer id);
}
4、编辑sql映射文件
EmployeeMapper.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.mybatis.mapper.EmployeeMapperPlus">
<!--自定义某个javaBean的封装规则
type：自定义规则的Java类型
id:唯一id方便引用
-->
<!-- public Employee getEmpById(Integer id); -->
<select id="getEmpByIdStep" resultMap="MyEmpDis">
select * from tbl_employee where id=#{id}
</select>
<!-- =======================鉴别器============================ -->
<!-- <discriminator javaType=""></discriminator>
鉴别器：mybatis可以使用discriminator判断某列的值，然后根据某列的值改变封装行为
封装Employee：
如果查出的是女生：就把部门信息查询出来，否则不查询；
如果是男生，把last_name这一列的值赋值给email;
-->
<resultMap type="com.mybatis.bean.Employee" id="MyEmpDis">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<!--
column：指定判定的列名
javaType：列值对应的java类型 ，string 是 mybatis 对String类型 直接默认设置 的别名 -->
<discriminator javaType="string" column="gender">
<!--女生 resultType:指定封装的结果类型；不能缺少。/resultMap-->
<case value="0" resultType="com.mybatis.bean.Employee">
<association property="department"
select="com.mybatis.mapper.DepartmentMapper.getDeptById"
column="d_id">
</association>
</case>
<!--男生 ;如果是男生，把last_name这一列的值赋值给email; -->
<case value="1" resultType="com.mybatis.bean.Employee">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="last_name" property="email"/>
<result column="gender" property="gender"/>
</case>
</discriminator>
</resultMap>
</mapper>
DepartmentMapper.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.mybatis.mapper.DepartmentMapper">
<!--public Department getDeptById(Integer id); -->
<select id="getDeptById" resultType="com.mybatis.bean.Department">
select id,dept_name departmentName from tbl_dept where id=#{id}
</select>
</mapper>
5、编辑全局配置文件：
mybatis-config.xml
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/mybatis" /> <property name="username" value="root" /> <property name="password" value="Kitty521!" /> </dataSource> </environment> </environments> <!-- 将我们写好的sql映射文件（EmployeeMapper.xml）一定要注册到全局配置文件（mybatis-config.xml）中 如果 数据库全局文件 和 子配置文件 不在同一个目录 ，就需要 /目录/目录/.../EmployeeMapper_old.xml --> <mappers> <!-- 新方法操作mybatis 需要 的配置文件 --> <mapper resource="EmployeeMapperPlus.xml" /> <mapper resource="DepartmentMapper.xml" /> </mappers> </configuration>
特别注意：分步查询需要 调用两句sql语句。因为这两句sql语句分布在两个不同的sql映射文件，所以两个sql映射文件都需要注册到 全局配置文件中。