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);
	}

}

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