MyBatis 学习（五）之高级映射

1 association 和 collection 介绍

2 案例分析

3 一对一关联和一对多关联

4 参考文档

1 association 和 collection 介绍

在之前的 SQL 映射文件中提及了 resultMap 元素的 association 和 collection 标签，这两个标签是用来关联查询的，它们的属性几乎一致，以下是对它们属性的描述，红色标注的是常用属性。

属性	描述
property	实体类属性名
column	数据库字段名或者其别名
javaType	实体类属性的 Java 类型
jdbcType	数据库列的数据类型
ofType	指定关联对象的类型。它通常用于泛型类型的情况，以确保正确的类型转换
select	指定一个子查询，用于加载关联的对象
fetchType	用于控制加载策略，比如立即加载还是延迟加载。常见的取值有 lazy（延迟加载）和 eager（立即加载）
resultMap	引用预定义的结果映射，以便更灵活地配置关联对象的映射规则
resultSet	指定结果集的名称，以便在多结果集的情况下进行区分
autoMapping	是否自动映射所有列到目标对象的属性上。默认为 true，表示自动映射；设置为 false 则表示不自动映射
columnPrefix	当使用内连接查询时，这个属性可以用来指定前缀，以区分关联表的列名
foreignColumn	指定外键对应的列名
notNullColumn	指定一个列名，只有当该列的值不为 null 时，才会执行关联查询
typeHandler	自定义的类型处理器

2 案例分析

以典型的员工 (Employee) 和部门 (Department) 为例

一个员工只能在一个部门：Employee -> Department（一对一）
一个部门可以包含多个员工：Department -> Employee（一对多）

以用户 (User) 和角色 (Role) 为例，两者成了一个双向的一对多，从而变成了多对多，不做介绍

一个用户可以拥有多个角色：User -> Role（一对多）
一个角色可以赋予多个用户：Role -> User（一对多）

表 department 和表 employee

# 创建 department 表
drop table if exists department;
create table department(
department_id int(11) primary key auto_increment, #主键，自增
department_name varchar(255)
)charset=utf8;# 插入数据
insert into department(department_name) values
('开发部'),
('人力资源部'),
('市场营销部'),
('财务部'),
('行政部'),
('监察部'),
('客服服务部');# 创建 employee 表
drop table if exists employee;
create table employee(
employee_id int(11) primary key auto_increment, #主键，自增
employee_name varchar(255),
employee_age int(11),
employee_sex char(32),
employee_email varchar(255),
employee_address varchar(255),
department_id int(11)
)charset=utf8;# 插入数据
insert into employee values
(1, '唐浩荣', 23, 1, '15477259875@163.com', '中国上海浦东区', 1),
(2, '黄飞鸿', 32, 1, '86547547@qq.com', '大清广东', 2),
(3, '十三姨', 18, 0, '520520520@gmail.com', '大清广东', 3),
(4, '纳兰元述', 28, 1, '545627858@qq.com', '大清京师', 5),
(5, '梁宽', 31, 1, '8795124578@qq.com', '大清广东', 7),
(6, '蔡徐坤', 20, 0, '4257895124@gmail.com', '四川成都', 4),
(7, '杨超越', 21, 0, '8746821252@qq.com', '中国北京', 7),
(8, '马保国', 66, 1, '6666666666@qq.com', '广东深圳', 6),
(9, '马牛逼', 45, 1, 'asdfg45678@163.com', '湖北武汉', 3);

3 一对一关联和一对多关联

工程目录

Department 类

public class Department {//部门idprivate Integer deptId;//部门名称private String deptName;//部门有哪些员工private List<Employee> employees;// Getter、Setter、toString 方法省略
}

Employee 类

public class Employee {//员工idprivate Integer empId;//员工名称private String empName;//员工年龄private Integer empAge;//员工性别private Integer empSex;//员工邮箱private String empEmail;//员工地址private String empAddress;//员工所属部门，和部门表构成一对一的关系，一个员工只能在一个部门private Department department;// Getter、Setter、toString 方法省略
}

创建 DepartmentMapper

public interface DepartmentMapper {//查询所有数据@Select("select * from department")@Results(id = "deptMap1", value = {@Result(property = "deptId", column = "department_id"),@Result(property = "deptName", column = "department_name"),// 一对多关联对象// 根据 department_id 来比较@Result(property = "employees", column = "department_id",many = @Many(select = "com.mapper.EmployeeMapper.selectEmpByDeptId"))})List<Department> selectAll();// 根据 id 查找部门@Select("select * from department where department_id = #{id}")@Results(id = "deptMap2", value = {@Result(property = "deptId", column = "department_id"),@Result(property = "deptName", column = "department_name")})Department findDepartmentById(int id);
}

创建 EmployeeMapper

public interface EmployeeMapper {//查询所有数据@Select("select * from employee")@Results(id = "empMap1", value = {@Result(property = "empId", column = "employee_id", id = true),@Result(property = "empName", column = "employee_name"),@Result(property = "empAge", column = "employee_age"),@Result(property = "empSex", column = "employee_sex"),@Result(property = "empEmail", column = "employee_email"),@Result(property = "empAddress", column = "employee_address"),// 一对一关联对象// 根据 department_id 来比较@Result(property = "department", column = "department_id",one = @One(select = "com.mapper.DepartmentMapper.findDepartmentById"))})List<Employee> selectAll();//根据员工id查询数据@Select("select * from employee where employee_id = #{id}")@ResultMap("empMap1")Employee selectEmpByEmpId(@Param("id") int empId);// 根据 department_id 查询数据@Select("select * from employee where department_id = #{id}")@ResultMap("empMap1")Employee selectEmpByDeptId(@Param("id") int deptId);
}

log4j.properties

log4j.rootLogger=DEBUG, Console
#Console
log4j.appender.Console=org.apache.log4j.ConsoleAppender
log4j.appender.Console.layout=org.apache.log4j.PatternLayout
log4j.appender.Console.layout.ConversionPattern=%d [%t] %-5p [%c] - %m%n
log4j.logger.java.sql.ResultSet=INFO
log4j.logger.org.apache=INFO
log4j.logger.java.sql.Connection=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG

mysql.properties

url=jdbc:mysql://localhost:3306/study?useSSL=false&useUnicode=true&characterEncoding=UTF-8&serverTimezone=UTC
name=root
password=123456
driver=com.mysql.cj.jdbc.Driver

MyBatis 配置文件 mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><!-- 通过properties标签，读取java配置文件的内容 --><properties resource="mysql.properties" /><!-- 配置环境.--><environments default="development"><environment id="development"><!--配置事务的类型--><transactionManager type="JDBC"></transactionManager><!--dataSource 元素使用标准的 JDBC 数据源接口来配置 JDBC 连接对象源 --><dataSource type="POOLED"><!--配置连接数据库的4个基本信息--><property name="url" value="${url}" /><property name="username" value="${name}" /><property name="password" value="${password}" /><property name="driver" value="${driver}" /></dataSource></environment></environments><!--通过包 package 引入 SQL 映射文件--><mappers><package name="com.mapper"/></mappers>
</configuration>

创建 EmployeeTest 测试类

public class EmployeeTest {//定义 SqlSessionSqlSession sqlSession = null;//定义 EmployeeMapper 对象private EmployeeMapper mapper = null;@Beforepublic void getSqlSession() throws IOException {//加载 mybatis 全局配置文件 Resources// 原 InputStream is = MybatisTest.class.getClassLoader().getResourceAsStream("mybatis-config.xml");InputStream is = Resources.getResourceAsStream("mybatis-config.xml");//创建 SqlSessionFactory 对象SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);//根据 sqlSessionFactory 产生 sessionsqlSession = sqlSessionFactory.openSession();// 创建Mapper接口的的代理对象，getMapper方法底层会通过动态代理生成 EmployeeMapper 的代理实现类mapper = sqlSession.getMapper(EmployeeMapper.class);}//查询所有员工数据@Testpublic void testSelectAll() {List<Employee> listEmployee = mapper.selectAll();for (Employee employee : listEmployee) {System.out.println(employee);}sqlSession.close();}//根据员工 id 查询数据@Testpublic void testSelectById() {Employee employee = mapper.selectEmpByEmpId(1);System.out.println(employee);sqlSession.close();}
}

测试结果

查询所有员工数据，包括员工所在部门（一个员工属于一个部门）

创建 DepartmentTest 测试类

public class DepartmentTest {//定义 SqlSessionSqlSession sqlSession = null;//定义 DepartmentMapper 对象private DepartmentMapper mapper = null;@Beforepublic void getSqlSession() throws IOException {//加载 mybatis 全局配置文件 Resources// 原 InputStream is = MybatisTest.class.getClassLoader().getResourceAsStream("mybatis-config.xml");InputStream is = Resources.getResourceAsStream("mybatis-config.xml");//创建 SqlSessionFactory 对象SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);//根据 sqlSessionFactory 产生 sessionsqlSession = sqlSessionFactory.openSession();// 创建Mapper接口的的代理对象，getMapper方法底层会通过动态代理生成 DepartmentMapper 的代理实现类mapper = sqlSession.getMapper(DepartmentMapper.class);}//查询所有部门数据@Testpublic void testSelectAll() {List<Department> listDepartment = mapper.selectAll();for (Department department : listDepartment) {System.out.println(department);}sqlSession.close();}//根据部门 id 查询 数据@Testpublic void testSelectById() {Department department = mapper.findDepartmentById(1);System.out.println(department);sqlSession.close();}
}

测试结果

查询所有部门信息，包括该部门有哪些员工（一个部门有多个员工）