Hibernate一对多单向配置详细说明实例教程，index.jsp页面显示:
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:
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public class User{ ... private Set<Card> cards; ...}public class Card{ ...} |
b.Hibernate中所要做的一对多:
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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(不为设为NOT NULL)即可。
详细的MyEclipse WebProject如下:
1、实体类:
用户类User.java:
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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:
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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:
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CREATE TABLE myuser( id VARCHAR(50) NOT NULL, name VARCHAR(100), PRIMARY KEY(id)); |
银行卡表:
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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:
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<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"<!-- Mapping file autogenerated by MyEclipse Persistence Tools--><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:
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<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"<!-- Mapping file autogenerated by MyEclipse Persistence Tools--><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:
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<?xml version='1.0' encoding='UTF-8'?><!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN"<!-- Generated by MyEclipse Hibernate Tools. --><hibernate-configuration><session-factory> <property name="connection.username">root</property> <property name="connection.url"> </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:
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/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:
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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:
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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:
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<%@ 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 + "<br />"); out.println("name:" + name + "<br />"); out.println("cards:<br />"); 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 + "<br />"); out.println(" balance:" + balance + "<br />"); } out.println("<hr/>"); } %> </body></html> |
5、运行结果:
控制台显示:
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......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=?...... |
id:402881e421d4d0be0121d4d20e140005
name:Tom
cards:
cardId:402881e421d4d0be0121d4d20e230008
balance:654.8
cardId:402881e421d4d0be0121d4d20e230006
balance:7641.96
cardId:402881e421d4d0be0121d4d20e230007
balance:3650.0
6、Hibernate一对多单向配置注意的问题:
a、hibernate错误提示:Field ‘userId’ doesn’t have a default value解决方法。
刚开始把”userId”设成NOT NULL，但是Hibernate先执行的是:
“insert into db.mycard (balance, id) values (?, ?)”
然后再执行”update db.mycard set userId=? where id=?”，
但userId在insert时没有写入值的，因此就会报错。这时将userId的NOT NULL去掉就可以了。
b、User.hbm.xml要设置cascade=”all”，或者是其它有效值，要不，保存User对象的时候，有关的Card对象不会被保存。
c、User.hbm.xml中set标签的inverse属性不能设为”true”，inverse的默认为”false”，因此不加inverse也行的。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
+——————————————–+———+———
问题的原因原来在本例应该把inverse设置为false。inverse是很有用的，不过用错了地方。 本文链接地址: Hibernate一对多单向配置详细说明实例教程inverse属性用法