[原]Hibernate一对多(单向)

[标题]:Hibernate一对多(单向)
[时间]:2009-6-12
[摘要]:单向一对多关联只需要在"一方"进行配置即可，"多方"无需额外配置。
[关键字]:外键，inverse，Hibernate，Set，一对多，单向，ORM，mapping，关系数据库，映射
[环境]:MyEclipse7 ， JDK6，Hibernate3.2，Tomcat6，MySQL 5.1.34-community
[作者]:Winty (wintys@gmail.com) http://www.blogjava.net/wintys

[正文]:
 Hibernate一对多关联，例如一个用户有多张银行卡(只考虑用户到银行卡的单向一对多关联)。由于是学习Hibernate原理，并没有使用工具自动生成代码等。
 单向一对多关联只需要在"一方"进行配置即可，"多方"无需额外配置。

a.Java程序中所要做的一对多:
public class User{
 ...
 private Set<Card> cards;
 ...
}

public class Card{
 ...
}

b.Hibernate中所要做的一对多:
User.hbm.xml:
...
<set name="cards" inverse="false" cascade="all">
<key column="userId" />
<one-to-many class="wintys.hibernate.onetomany.Card" />
</set>
...

c.数据库中的一对多:
而对应的数据库中，只要相应在Card对应的物理表中添加外键userId(不要设为NOT NULL)即可。

详细的MyEclipse WebProject如下:
1、实体类:
用户类User.java:

package wintys.hibernate.onetomany;

import java.util.Set;

public class User {
 private String id;
 private String name;
 private Set<Card> cards;

 public String getId() {
 return id;
 }
 public void setId(String id) {
 this.id = id;
 }
 public String getName() {
 return name;
 }
 public void setName(String name) {
 this.name = name;
 }
 public void setCards(Set<Card> cards) {
 this.cards = cards;
 }
 public Set<Card> getCards() {
 return cards;
 }
}

银行卡类Card.java:

package wintys.hibernate.onetomany;

public class Card {
    private String id;
    private float balance;

    public Card(){
    }
    public Card(float balance){
        this.balance = balance;
    }
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public float getBalance() {
        return balance;
    }
    public void setBalance(float balance) {
        this.balance = balance;
    }
}

2、数据库表:
数据库是MySQL 5.1.34-community。

用户表:

CREATE TABLE myuser(
    id               VARCHAR(50) NOT NULL,
    name         VARCHAR(100),
    PRIMARY KEY(id)
);

银行卡表:

CREATE TABLE mycard(
    id               VARCHAR(50) NOT NULL,
    balance      FLOAT(7,2),
    userId       VARCHAR(50),
    PRIMARY KEY(id)
);

3、映射文件:
用户类映射文件/src/wintys/hibernate/onetomany/User.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">


<hibernate-mapping>
 <class name="wintys.hibernate.onetomany.User" table="myuser" catalog="db">
 <id name="id" type="string">
 <column name="id" not-null="true"/>
 <generator class="uuid.hex" />
 </id>
 <property name="name" type="java.lang.String" column="name" />

 <set name="cards" inverse="false" cascade="all">
 <key column="userId" />
 <one-to-many class="wintys.hibernate.onetomany.Card" />
 </set>
 </class>
</hibernate-mapping>

银行卡映射文件/src/wintys/hibernate/onetomany/User.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">


<hibernate-mapping>
 <class name="wintys.hibernate.onetomany.Card" table="mycard" catalog="db">
 <id name="id" type="string">
 <column name="id" not-null="true"/>
 <generator class="uuid.hex" />
 </id>
 <property name="balance" />
 </class>
</hibernate-mapping>

Hibernate配置文件:/src/hibernate.cfg.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
 "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">


<hibernate-configuration>

<session-factory>
 <property name="connection.username">root</property>
 <property name="connection.url">
 jdbc:mysql://localhost:3306/db?useUnicode=true&characterEncoding=utf-8
 </property>
 <property name="dialect">
 org.hibernate.dialect.MySQLDialect
 </property>
 <property name="myeclipse.connection.profile">MySQLDriver</property>
 <property name="connection.password">root</property>
 <property name="connection.driver_class">
 com.mysql.jdbc.Driver
 </property>
 <property name="show_sql">true</property>
 <mapping resource="wintys/hibernate/onetomany/User.hbm.xml" />
 <mapping resource="wintys/hibernate/onetomany/Card.hbm.xml" />

</session-factory>

</hibernate-configuration>

4、使用测试:
/src/wintys/hibernate/onetomany/HibernateDAO.java:

package wintys.hibernate.onetomany;
import java.util.List;

public interface HibernateDAO {
public void insert();
public List<User> selectAll();
}

/src/wintys/hibernate/onetomany/HibernateDAOBean.java:

package wintys.hibernate.onetomany;

import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.Transaction;

public class HibernateDAOBean implements HibernateDAO {

 public void insert() throws HibernateException {
 Transaction tc = null;
 try{
 Set<Card> cards = new HashSet<Card>();
 Card c1,c2,c3;
 c1 = new Card(7641.96f);
 c2 = new Card(654.8f);
 c3 = new Card(3650f);

 cards.add(c1);
 cards.add(c2);
 cards.add(c3);

 User user = new User();
 user.setName("Tom");
 user.setCards(cards);

 Session session = HibernateUtil.getSession();
 tc = session.beginTransaction();

 /*
 配置文件中的cascade="true"时，所以无需手动保存c1,c2,c3
 session.save(c1);
 session.save(c2);
 session.save(c3);
 */
 session.save(user);

 tc.commit();
 }catch(HibernateException e){
 try{
 if(tc != null)
 tc.rollback();
 }catch(Exception ex){
 System.err.println(ex.getMessage());
 }
 System.err.println(e.getMessage());
 }finally{
 HibernateUtil.closeSession();
 }
 }

 @SuppressWarnings("unchecked")
 public List<User> selectAll() throws HibernateException {
 List<User> users = null;
 Transaction tc = null;
 try{
 Session session = HibernateUtil.getSession();
 tc = session.beginTransaction();

 Query query = session.createQuery("from User");
 users = query.list();

 tc.commit();
 }catch(HibernateException e){
 try{
 if(tc != null){
 tc.rollback();
 users = null;
 }
 }catch(Exception ex){
 System.err.println(ex.getMessage());
 }
 System.err.println(e.getMessage());
 }finally{
 //HibernateUtil.closeSession();
 }

 return users;
 }
}

/src/wintys/hibernate/onetomany/HibernateUtil.java:

package wintys.hibernate.onetomany;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/**
* Hibernate Session管理
* @author Winty
*/
public class HibernateUtil {
 private static SessionFactory factory = null;
 private static ThreadLocal<Session> threadLocal;

 static {
 try{
 factory = new Configuration()
 .configure()
 .buildSessionFactory();
 }catch(HibernateException e){
 System.err.println(e.getMessage());
 }

 threadLocal = new ThreadLocal<Session>();
 }

 private HibernateUtil(){
 }

 public static Session getSession()throws HibernateException{
 Session session = threadLocal.get();
 if(session == null){
 session = factory.openSession();
 threadLocal.set(session);
 }

 return session;
 }

 public static void closeSession()throws HibernateException{
 Session session = threadLocal.get();
 if(session != null){
 session.close();
 }
 threadLocal.set(null);
 }
}

/index.jsp:

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%>
<%@ page import="wintys.hibernate.onetomany.*"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
 <title>My JSP 'index.jsp' starting page</title>
</head>

<body>
 <%
 List<User> users = null;
 HibernateDAO dao = new HibernateDAOBean();
 dao.insert();
 users = dao.selectAll();

 Iterator<User> it = users.iterator();
 while(it.hasNext()){
 User user = it.next();
 String id = user.getId();
 String name = user.getName();
 out.println("id:" + id + " ");
 out.println("name:" + name + " ");
 out.println("cards: ");

 Set<Card> cards = user.getCards();
 Iterator<Card> itc = cards.iterator();
 while(itc.hasNext()){
 Card card = itc.next();
 String cardId = card.getId();
 float balance = card.getBalance();
 out.println("    cardId:" + cardId + " ");
 out.println("    balance:" + balance + " ");
 }
 out.println("<hr/>");
 }

 %>

</body>
</html>

5、运行结果:
控制台显示:

......
Hibernate: insert into db.myuser (name, id) values (?, ?)
Hibernate: insert into db.mycard (balance, id) values (?, ?)
Hibernate: insert into db.mycard (balance, id) values (?, ?)
Hibernate: insert into db.mycard (balance, id) values (?, ?)
Hibernate: update db.mycard set userId=? where id=?
Hibernate: update db.mycard set userId=? where id=?
Hibernate: update db.mycard set userId=? where id=?
......

index.jsp页面显示:
id:402881e421d4d0be0121d4d20e140005
name:Tom
cards:
    cardId:402881e421d4d0be0121d4d20e230008
    balance:654.8
    cardId:402881e421d4d0be0121d4d20e230006
    balance:7641.96
    cardId:402881e421d4d0be0121d4d20e230007
    balance:3650.0

6、注意的问题:
a、错误提示:Field 'userId' doesn't have a default value。
    一开始把"userId"设成NOT NULL，但是Hibernate先执行的是:
"insert into db.mycard (balance, id) values (?, ?)"
然后才执行"update db.mycard set userId=? where id=?"，
而userId在insert时是没有写入值的，所以就会报错。把userId的NOT NULL去掉即可。
b、User.hbm.xml中要设置cascade="all"，或其它有效值，不然，在保存User对象时，相关的Card对象不会被保存。
c、User.hbm.xml中set标签的inverse属性不能设置为"true"，inverse的默认值是"false"，所以不加inverse也可以。看书上说:在一对多的关联关系实现中，最好设置inverse="true"，将有助于性能的改善。所以一开始就用了inverse="true"，User和Card对象都分别正确写入数据库了，但是就是userId字段没有被自动写入。
myuser表:
+--------------------------------------------+------+
| id                                 | name |
+--------------------------------------------+------+
| 402881e421d4d0be0121d4d20e140005 | Tom |
+--------------------------------------------+------+
mycard表:
+--------------------------------------------+---------+---------
| id                                | balance | userId
+--------------------------------------------+---------+---------
| 402881e421d4d0be0121d4d20e230006 | 7641.96 | NULL
| 402881e421d4d0be0121d4d20e230007 | 3650.00 | NULL
|
| 402881e421d4d0be0121d4d20e230008 | 654.80 | NULL
+--------------------------------------------+---------+---------
搞了半天，原来在本例应该把inverse设为false。inverse还是很有用的，只是用错了地方。

[参考资料]:
[1] Hibernate 一对多外键单向关联--熔岩 : http://lavasoft.blog.51cto.com/62575/39317
[2] Hibernate应用（二）单向一对多及双向一对多的配置: http://suhaoyuan.spaces.live.com/Blog/cns!2659D3AC8253C554!217.entry
[3] Hibernate单向一对多应注意的问题 : http://blog.chinaunix.net/u2/88320/showart_1716296.html
[4] hibernate一对多例子-已更新(二) : http://www.blogjava.net/wujun/archive/2008/08/04/39700.html

原创作品，转载请注明出处。
作者:Winty (wintys@gmail.com)
博客:http://www.blogjava.net/wintys

posted on 2009-06-13 00:12 天堂露珠阅读(12829) 评论(4) 编辑收藏所属分类: Hibernate

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