SSH学习四 Hibernate多对一的关系 + 增删改查

多对一关系实现起来不难，只需要在“多”的一方声明外键，声明外键在类的映射文件实现。

下面以老师学生为例，老师是1，学生是多

有一个有趣的地方，在Student类里面，没有声明teacherid这个属性，但是只要在配置文件声明了外键，那么就算类里面没有外键，配置文件的property也没有teacherid

下面是几个关键文件：

1、Student类（包含Student的增删改查）

package com.learn.po;

import org.hibernate.Session;
import org.hibernate.Transaction;

import com.ehr.dao.HibernateUtil;

public class Student {
private int sid;
public int getSid() {
return sid;
}
public void setSid(int sid) {
this.sid = sid;
}
private String nm;
private Teacher teacher;
public String getNm() {
return nm;
}
public void setNm(String nm) {
this.nm = nm;
}

public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public static void addStudent(){
//这里的逻辑是，Teacher类的对象是Student的成员函数，向Student表插入数据的时候，
//初始化Student，需要用Teacher对象，比如加小沈阳需要他的老师赵本上，所以先查找到赵本山，然后才能插入小沈阳，
//至于外键，是Hibernate自动维护的
Session session = HibernateUtil.getSessionFactory().openSession();
Teacher t = (Teacher)session.get(Teacher.class, 2);
Student s = new Student();
s.setNm("fanwei");
s.setTeacher(t);
HibernateUtil.addObject(s);
session.close();
}

public static void getQuery(int id){
Session session = HibernateUtil.getSessionFactory().openSession();
Student s = (Student)session.get(Student.class, 1);
/*找teacherid根本不进行查询，因为是外键，还在缓存存着*/
System.out.println(s.getTeacher().getTeacherid());
/*这里会在自动生成的Hibernate的sql语句会做内连接查询找到name*/
System.out.println(s.getTeacher().getNm());
}

public static void changeTeacher(){
Session session = HibernateUtil.getSessionFactory().openSession();
Teacher t = (Teacher)session.get(Teacher.class, 3);
Student s = (Student)session.get(Student.class, 1);
s.setTeacher(t);
session.beginTransaction().commit();
}

public static void deleteStu(){
Session session = HibernateUtil.getSessionFactory().openSession();

Student s = new Student();
s.setSid(2);
session.delete(s);
session.beginTransaction().commit();
}

public static void main(String[] args) {
//addStudent();
//getQuery(1);
//changeTeacher();
//deleteStu();
}
}


2、Student的映射文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping
package="com.learn.po">
<class name="Student" table="student" lazy="true">
<id name="sid" >
<generator class="identity"></generator>
</id>
<property name="nm"></property>

<many-to-one fetch="join" name="teacher" class="Teacher" column="teacherid"></many-to-one>
</class>
</hibernate-mapping>

3、Teacher的类
package com.learn.po;

import org.hibernate.Session;

import com.ehr.dao.HibernateUtil;

public class Teacher {
private int teacherid;
private String nm;
public String getNm() {
return nm;
}
public void setNm(String nm) {
this.nm = nm;
}
public int getTeacherid() {
return teacherid;
}
public void setTeacherid(int teacherid) {
this.teacherid = teacherid;
}

public static void addTeacher(){
Teacher t = new Teacher();
t.setNm("te1");
//这里很神奇，是优Hibernate识别出来是什么类，然后插入相应的表
HibernateUtil.addObject(t);
t.setNm("te2");
HibernateUtil.addObject(t);
}

public static void main(String args[]){

<pre name="code" class="java"><span style="white-space:pre"> </span>addTeacher();}}

4、Teacher的映射文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping
package="com.learn.po">

<class name="Teacher" table="teacher" lazy="true">
<id name="teacherid">
<generator class="identity"></generator>
</id>
<property name="nm"></property>
</class>
</hibernate-mapping>


5、Hibernate配置文件
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory name="foo"> <!-- 这个name没啥用 -->
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<!-- 另外解释 -->
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql://127.0.0.1/testdb</property>
<!-- url格式：mysql url+ 数据库名字-->
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">123</property>
<!-- 用户名及密码 -->
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<!-- 下面指的是相应类配置文件，需要把。改为/ -->
<mapping resource="com/learn/po/Teacher.hbm.xml"/>
<mapping resource="com/learn/po/Student.hbm.xml"/>
</session-factory>
</hibernate-configuration>

6\HibernateUtil.java
package com.ehr.dao;

import java.io.Serializable;

import org.hibernate.*;
import org.hibernate.cfg.*;
import org.hibernate.tool.hbm2ddl.SchemaExport;

public class HibernateUtil {

private static final SessionFactory sessionFactory;

static {
try {
// Create the SessionFactory from hibernate.cfg.xml
sessionFactory = new Configuration().configure()
.buildSessionFactory();
} catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
System.err.println("Initial SessionFactory creation failed." + ex);
throw new ExceptionInInitializerError(ex);
}
}

public static SessionFactory getSessionFactory() {
return sessionFactory;
}

public static Serializable addObject(Object o) {
// 获得session对象
Serializable s = null;
Session session = HibernateUtil.getSessionFactory().openSession();
// 开启事务
Transaction tx = session.beginTransaction();
try {
s = session.save(o);// 把对象保存到db，并且返回新建列的主键值
// 提交事务
tx.commit();
} catch (Exception e) {
System.out.println(e.getMessage());
tx.rollback();
} finally {
session.close();
}
return s;
}

public static void main(String[] args) {
Configuration cfg =new Configuration().configure();
SchemaExport e = new SchemaExport(cfg);
e.create(true, true);
}

}