Mybatis中多个对象包含同一个对象的处理
多个对象对应一个对象时,应该如何进行查询?
例如
文章目录
-
-
-
- 1.模拟数据库思想:连表查询
- 2.模拟面向对象的思想
-
-
关键字: association : 联系 ，关联 多个人可以关联一个人。
首先做一些准备,如:实体类,工具类和Mybatis核心文件
实体类:
//老师实体类
package com.MLXH.pojo;
public class Teacher {
private int id;
private String name;
public Teacher() {
}
public Teacher(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
//学生实体类
package com.MLXH.pojo;
public class Student {
private int id;
private String name;
private Teacher teacher;
public Student() {
}
public Student(int id, String name, Teacher teacher) {
this.id = id;
this.name = name;
this.teacher = teacher;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", teacher=" + teacher +
'}';
}
}
database.properties配置文件数据库需要的数据
driver = com.mysql.jdbc.Driver
url = jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=utf-8
username = root
password = 123456
Mybatis工具类:mybatis-config.xml
<configuration>
<properties resource="database.properties"/>
<settings>
<setting name="logImpl" value="LOG4J"/>
settings>
<typeAliases>
<package name="com.MLXH.pojo"/>
typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
dataSource>
environment>
environments>
<mappers>
<mapper resource="com/MLXH/dao/StudentMapper.xml"/>
mappers>
configuration>
工具类:主要是为了使获得sqlsession更为简单方便
package com.MLXH.utils;
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 java.io.IOException;
import java.io.InputStream;
//mybatis的工具类，重复的代码的提纯
public class MyBatisUtils {
//类变量不需要设置默认值；
private static SqlSessionFactory sqlSessionFactory;
static {
//在maven中，所有的资源文件一般都放在resources目录下，我们可以直接拿到。
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
//设置SqlSessionFactory公共的方法
public static SqlSessionFactory getSqlSessionFactory(){
return sqlSessionFactory;
}
//获得一个带事务自动提交功能的SqlSession公共的方法
public static SqlSession getSqlSession(){
//自动提交事务
return sqlSessionFactory.openSession(true);
}
}
StudentDao接口
package com.MLXH.dao;
import com.MLXH.pojo.Student;
import java.util.List;
public interface StudentDao {
//获得全部学生的信息以及对应的老师
List<Student> getStudents();
//获得全部学生的信息以及对应的老师
List<Student> getStudentsTwo();
}
针对于StudentDao接口的实现mapper文件:我们使用如下的方式来进行查询
1.模拟数据库思想:连表查询
StudentMapper.xml
<mapper namespace="com.MLXH.dao.StudentDao">
<select id="getStudents" resultMap="StudentTeacher">
select * from mybatis.student
select>
<resultMap id="StudentTeacher" type="Student">
<id column="id" property="id"/>
<result column="name" property="name"/>
<association column="tid" property="teacher" javaType="Teacher" select="getTeacher"/>
resultMap>
<select id="getTeacher" resultType="Teacher">
select * from mybatis.teacher where id = #{id}
select>
mapper>
测试类
@Test
public void getStudents(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
StudentDao mapper = sqlSession.getMapper(StudentDao.class);
List<Student> students = mapper.getStudents();
for (Student student : students) {
System.out.println("学生姓名："+student.getName()+"\t老师姓名："+student.getTeacher().getName());
}
}
2.模拟面向对象的思想
<mapper namespace="com.MLXH.dao.StudentDao">
<select id="getStudentsTwo" resultMap="StudentTeacher2">
select s.id,s.name,t.id as tid,t.name as tname
from mybatis.student as s, mybatis.teacher as t
where s.tid = t.id
select>
<resultMap id="StudentTeacher2" type="Student">
<id property="id" column="id"/>
<result property="name" column="name"/>
<association property="teacher" javaType="Teacher">
<id property="id" column="tid"/>
<result property="name" column="tname"/>
association>
resultMap>
mapper>
测试
@Test
public void getStudentsTwo(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
StudentDao mapper = sqlSession.getMapper(StudentDao.class);
List<Student> students = mapper.getStudentsTwo();
for (Student student : students) {
System.out.println("学生姓名："+student.getName()+"\t老师姓名："+student.getTeacher().getName());
}
}
总结:
- mybatis中遇到多对一的情况，要使用关联映射处理：使用association
- 两种处理思路：
- 数据库思想 ： 联表查询
- OOP思想 ：关联对象