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[原]Hibernate一對多(單向)

[標(biāo)題]:Hibernate一對多(單向)
[時間]:2009-6-12
[摘要]:單向一對多關(guān)聯(lián)只需要在"一方"進(jìn)行配置即可，"多方"無需額外配置。
[關(guān)鍵字]:外鍵，inverse，Hibernate，Set，一對多，單向，ORM，mapping，關(guān)系數(shù)據(jù)庫，映射
[環(huán)境]:MyEclipse7 ， JDK6，Hibernate3.2，Tomcat6，MySQL 5.1.34-community
[作者]:Winty (wintys@gmail.com) http://www.aygfsteel.com/wintys

[正文]:
 Hibernate一對多關(guān)聯(lián)，例如一個用戶有多張銀行卡(只考慮用戶到銀行卡的單向一對多關(guān)聯(lián))。由于是學(xué)習(xí)Hibernate原理，并沒有使用工具自動生成代碼等。
 單向一對多關(guān)聯(lián)只需要在"一方"進(jìn)行配置即可，"多方"無需額外配置。

a.Java程序中所要做的一對多:
public class User{
 ...
 private Set<Card> cards;
 ...
}

public class Card{
 ...
}

b.Hibernate中所要做的一對多:
User.hbm.xml:
...
<set name="cards" inverse="false" cascade="all">
<key column="userId" />
<one-to-many class="wintys.hibernate.onetomany.Card" />
</set>
...

c.數(shù)據(jù)庫中的一對多:
而對應(yīng)的數(shù)據(jù)庫中，只要相應(yīng)在Card對應(yīng)的物理表中添加外鍵userId(不要設(shè)為NOT NULL)即可。

詳細(xì)的MyEclipse WebProject如下:
1、實體類:
用戶類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;
 }
}

銀行卡類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、數(shù)據(jù)庫表:
數(shù)據(jù)庫是MySQL 5.1.34-community。

用戶表:

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

銀行卡表:

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

3、映射文件:
用戶類映射文件/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>

銀行卡映射文件/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、使用測試:
/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"時，所以無需手動保存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、運(yùn)行結(jié)果:
控制臺顯示:

......
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頁面顯示:
id:402881e421d4d0be0121d4d20e140005
name:Tom
cards:
    cardId:402881e421d4d0be0121d4d20e230008
    balance:654.8
    cardId:402881e421d4d0be0121d4d20e230006
    balance:7641.96
    cardId:402881e421d4d0be0121d4d20e230007
    balance:3650.0

6、注意的問題:
a、錯誤提示:Field 'userId' doesn't have a default value。
    一開始把"userId"設(shè)成NOT NULL，但是Hibernate先執(zhí)行的是:
"insert into db.mycard (balance, id) values (?, ?)"
然后才執(zhí)行"update db.mycard set userId=? where id=?"，
而userId在insert時是沒有寫入值的，所以就會報錯。把userId的NOT NULL去掉即可。
b、User.hbm.xml中要設(shè)置cascade="all"，或其它有效值，不然，在保存User對象時，相關(guān)的Card對象不會被保存。
c、User.hbm.xml中set標(biāo)簽的inverse屬性不能設(shè)置為"true"，inverse的默認(rèn)值是"false"，所以不加inverse也可以。看書上說:在一對多的關(guān)聯(lián)關(guān)系實現(xiàn)中，最好設(shè)置inverse="true"，將有助于性能的改善。所以一開始就用了inverse="true"，User和Card對象都分別正確寫入數(shù)據(jù)庫了，但是就是userId字段沒有被自動寫入。
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，原來在本例應(yīng)該把inverse設(shè)為false。inverse還是很有用的，只是用錯了地方。

[參考資料]:
[1] Hibernate 一對多外鍵單向關(guān)聯(lián)--熔巖 : http://lavasoft.blog.51cto.com/62575/39317
[2] Hibernate應(yīng)用（二）單向一對多及雙向一對多的配置: http://suhaoyuan.spaces.live.com/Blog/cns!2659D3AC8253C554!217.entry
[3] Hibernate單向一對多應(yīng)注意的問題 : http://blog.chinaunix.net/u2/88320/showart_1716296.html
[4] hibernate一對多例子-已更新(二) : http://www.aygfsteel.com/wujun/archive/2008/08/04/39700.html

原創(chuàng)作品，轉(zhuǎn)載請注明出處。
作者:Winty (wintys@gmail.com)
博客:http://www.aygfsteel.com/wintys

posted on 2009-06-13 00:12 天堂露珠閱讀(12840) 評論(4) 編輯收藏所屬分類: Hibernate

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