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MyBatis系列：（7）一对多映射
0、准备SQL语句（mysql）
CREATE TABLE Cars( cid INT(5) PRIMARY KEY, cname VARCHAR(10) ); CREATE TABLE Wheels( wid INT(5) PRIMARY KEY, wname VARCHAR(10), wcar_id INT(5), CONSTRAINT wheels_fk FOREIGN KEY(wcar_id) REFERENCES Cars(cid) ); INSERT INTO Cars(cid,cname) VALUES(1,"BMW"); INSERT INTO Wheels(wid,wname,wcar_id) VALUES(1,‘第1个轮子‘,1); INSERT INTO Wheels(wid,wname,wcar_id) VALUES(2,‘第2个轮子‘,1); INSERT INTO Wheels(wid,wname,wcar_id) VALUES(3,‘第3个轮子‘,1); INSERT INTO Wheels(wid,wname,wcar_id) VALUES(4,‘第4个轮子‘,1); SELECT * FROM Cars; SELECT * FROM Wheels; SELECT c.cid,c.cname,w.wid,w.wname FROM cars c INNER JOIN wheels w ON c.cid=w.wcar_id
1、entity类
Car.java
package com.rk.entity;
import java.util.ArrayList;
import java.util.List;
public class Car {
private Integer id;
private String name;
private List<Wheel> wheels = new ArrayList<Wheel>();
public Car(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Wheel> getWheels() {
return wheels;
}
public void setWheels(List<Wheel> wheels) {
this.wheels = wheels;
}
@Override
public String toString() {
return "Car [id=" + id + ", name=" + name + ", wheels=" + wheels + "]";
}
}Wheel.java
package com.rk.entity;
public class Wheel {
private Integer id;
private String name;
private Car car;
public Wheel(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Car getCar() {
return car;
}
public void setCar(Car car) {
this.car = car;
}
@Override
public String toString() {
return "Wheel [id=" + id + ", name=" + name + "]";
}
}2、mybatis的映射文件
CarMapper.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="carNamespace">
<resultMap type="com.rk.entity.Car" id="carMap">
<id property="id" column="cid"/>
<result property="name" column="cname"/>
<collection property="wheels" resultMap="wheelNamespace.wheelMap"></collection>
</resultMap>
<select id="findCarByWheelId" parameterType="int" resultMap="carMap">
select c.cid,c.cname
from cars c inner join wheels w on c.cid=w.wcar_id
where w.wid=#{wheelId}
</select>
</mapper>WheelMapper.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="wheelNamespace">
<resultMap type="com.rk.entity.Wheel" id="wheelMap">
<id property="id" column="wid"/>
<result property="name" column="wname"/>
</resultMap>
<select id="findWheelsByCarName" parameterType="string" resultMap="wheelMap">
select w.wid,w.wname
from cars c inner join wheels w on c.cid=w.wcar_id
and c.cname=#{carName}
</select>
</mapper>3、mybatis的主配置文件（引入映射文件）
mybatis.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>
<properties resource="db.properties"></properties>
<typeAliases>
<typeAlias type="com.rk.entity.Emp" alias="emp"/>
</typeAliases>
<environments default="mysql_developement">
<environment id="mysql_developement">
<transactionManager type="JDBC"></transactionManager>
<dataSource type="POOLED">
<property name="driver" value="http://www.mamicode.com/${mysql.driver}"/>
<property name="url" value="http://www.mamicode.com/${mysql.url}"/>
<property name="username" value="http://www.mamicode.com/${mysql.username}"/>
<property name="password" value="http://www.mamicode.com/${mysql.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/rk/entity/CarMapper.xml"/>
<mapper resource="com/rk/entity/WheelMapper.xml"/>
</mappers>
</configuration>db.properties
mysql.driver=com.mysql.jdbc.Driver mysql.url=jdbc:mysql://127.0.0.1:3306/testdb mysql.username=root mysql.password=root
4、MyBatisUtils.java
package com.rk.utils;
import java.io.IOException;
import java.io.Reader;
import java.sql.Connection;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
public class MyBatisUtils {
private static ThreadLocal<SqlSession> threadLocal = new ThreadLocal<SqlSession>();
private static SqlSessionFactory sqlSessionFactory;
static{
try {
Reader reader = Resources.getResourceAsReader("mybatis.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
} catch (IOException e) {
e.printStackTrace();
throw new RuntimeException(e);
}
}
private MyBatisUtils(){}
public static SqlSession getSqlSession(){
SqlSession sqlSession = threadLocal.get();
if(sqlSession == null){
sqlSession = sqlSessionFactory.openSession();
threadLocal.set(sqlSession);
}
return sqlSession;
}
public static void closeSqlSession(){
SqlSession sqlSession = threadLocal.get();
if(sqlSession != null){
sqlSession.close();
threadLocal.remove();
}
}
public static void main(String[] args) {
SqlSession sqlSession = MyBatisUtils.getSqlSession();
Connection conn = sqlSession.getConnection();
System.out.println(conn);
}
}5、CarWheelDao.java
package com.rk.dao;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import com.rk.entity.Car;
import com.rk.entity.Wheel;
import com.rk.utils.MyBatisUtils;
public class CarWheelDao {
public Car findCarByWheelId(int wheelId){
SqlSession sqlSession = null;
try{
sqlSession = MyBatisUtils.getSqlSession();
Car car = sqlSession.selectOne("carNamespace.findCarByWheelId", wheelId);
return car;
}
catch(Exception e){
e.printStackTrace();
sqlSession.rollback();
throw new RuntimeException(e);
}
finally{
MyBatisUtils.closeSqlSession();
}
}
public List<Wheel> findWheelsByCarName(String carName){
SqlSession sqlSession = null;
try{
sqlSession = MyBatisUtils.getSqlSession();
List<Wheel> list = sqlSession.selectList("wheelNamespace.findWheelsByCarName", carName);
return list;
}
catch(Exception e){
e.printStackTrace();
sqlSession.rollback();
throw new RuntimeException(e);
}
finally{
MyBatisUtils.closeSqlSession();
}
}
public static void main(String[] args) {
CarWheelDao dao = new CarWheelDao();
Car car = dao.findCarByWheelId(2);
System.out.println(car);
List<Wheel> wheels = dao.findWheelsByCarName("BMW");
if(wheels != null && wheels.size()>0){
for(Wheel wheel : wheels){
System.out.println(wheel);
}
}
}
}MyBatis系列：（7）一对多映射
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