前面提到的关联查询中，bean里面的 某个属性就是一个对象，属于 一对一映射：可以采用一步查询，级联赋值 或者 采用分步查询 ，单独赋值。现在如果 bean的某个属性是 一个集合，属于一对多映射，那么 查询结果 就不能用级联赋值了。级联赋值 是 一对一 关系映射中的 一种方法。
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
数据库测试：
select d.id did, d.dept_name dept_name, e.id eid, e.last_name last_name, e.email email, e.gender gender from tbl_dept d left join tbl_employee e on d.id = e.d_id where d.id =1
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;
private List<Employee> employees;
public List<Employee> getEmployees() {
return employees;
}
public void setEmployees(List<Employee> employees) {
this.employees = employees;
}
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
DepartmentMapper.java
package com.mybatis.mapper;
import com.mybatis.bean.Department;
public interface DepartmentMapper {
//增强版 还能查出员工
public Department getDeptByIdPlus(Integer id);
}
4、编辑sql映射文件
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 class Department {
private Integer id;
private String departmentName;
private List<Employee> emps;
did dept_name || eid last_name email gender
-->
<!--嵌套结果集的方式，使用collection标签定义关联的集合类型的属性封装规则 -->
<resultMap type="com.mybatis.bean.Department" id="MyDept">
<id column="did" property="id"/>
<result column="dept_name" property="departmentName"/>
<!--
collection定义关联集合类型的属性的封装规则
ofType:指定集合里面元素的类型
-->
<collection property="employees" ofType="com.mybatis.bean.Employee">
<!-- 定义这个集合中元素的封装规则 -->
<id column="eid" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
</collection>
</resultMap>
<!-- public Department getDeptByIdPlus(Integer id); -->
<select id="getDeptByIdPlus" resultMap="MyDept">
SELECT d.id did,d.dept_name dept_name,
e.id eid,e.last_name last_name,e.email email,e.gender gender
FROM tbl_dept d
LEFT JOIN tbl_employee e
ON d.id=e.d_id
WHERE d.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> <mapper resource="DepartmentMapper.xml" /> </mappers> </configuration>
6、单元测试：
MybatisTest.java
package com.mybatis.test;
import java.io.IOException;
import java.io.InputStream;
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.mybatis.bean.Department;
import com.mybatis.bean.Employee;
import com.mybatis.mapper.DepartmentMapper;
import com.mybatis.mapper.EmployeeMapperPlus;
/**
* 1、接口式编程
* 原生： Dao ====> DaoImpl
* mybatis： Mapper ====> xxMapper.xml
*
* 2、SqlSession代表和数据库的一次会话；用完必须关闭；
* 3、SqlSession和connection一样她都是非线程安全。每次使用都应该去获取新的对象。(就是不要放在共享成员变量里面，A线程用完释放，B线程再用就为空了)
* 4、mapper接口没有实现类，但是mybatis会为这个接口生成一个代理对象。
* （将接口和xml进行绑定）
* EmployeeMapper empMapper = sqlSession.getMapper(EmployeeMapper.class);
* 5、两个重要的配置文件：
* mybatis的全局配置文件：包含数据库连接池信息，事务管理器信息等...系统运行环境信息
* sql映射文件：保存了每一个sql语句的映射信息：
* 将sql抽取出来。
*
*/
public class MybatisTest {
/********************************************************************/
public SqlSessionFactory getSqlSessionFactory() throws IOException {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
return new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public void test_new() throws IOException {
// 1、获取sqlSessionFactory对象
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
// 2、获取sqlSession对象
SqlSession openSession = sqlSessionFactory.openSession();
try {
// 3、获取接口的实现类对象
//会为接口自动的创建一个代理对象，代理对象去执行增删改查方法
// EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.class);
// Employee employee = mapper.getEmpByIdStep(1);
// System.out.println(mapper.getClass());
// System.out.println(employee);
// System.out.println(employee.getDepartment());
DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
Department department = mapper.getDeptByIdPlus(1);
System.out.println(mapper.getClass());
System.out.println(department);
System.out.println(department.getEmployees());
} finally {
openSession.close();
}
}
}