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-mappingpackage="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-mappingpackage="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.xmlsessionFactory = new Configuration().configure().buildSessionFactory();} catch (Throwable ex) {// Make sure you log the exception, as it might be swallowedSystem.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);}}