hibernate里两张表的多对多关系映射...

http://dev.csdn.net/article/61/61933.shtmA表和B表　这两张表有一个字段关联。那在pojo里，A对象是B对象的一个属性。。Hibernate关联对象的Lazy属性设置了true，当你利用Hibernate查询出一个对象A以后，如果A还有其他的Set对象，此时是没有初始化的。当在程序中调用A.get集合对象时Hibernate才会查询数据库，初始化其集合属性。而如果你查询A以后Session会关闭只知道这些

Hibernate中多对多关系映射2007-05-12 00:23

说明:本文在一个java项目中实现hibernate多对对映射
环境:eclipse3.1,oracle10g,hibernate2
1 新建三张表:categories,category_item,item;categories存储类别,item存储类别下的具体item，category_item存储categories和item两张表中的记录映射关系;sql语句如下:
categories表
create table WORK.CATEGORIES
(
    ID     NUMBER not null,
    NAME VARCHAR2(32)
)
tablespace USERS
    pctfree 10
    initrans 1
    maxtrans 255
    storage
    (
      initial 64K
      minextents 1
      maxextents unlimited
    );
alter table WORK.CATEGORIES
    add constraint CATEGORIES_ID primary key (ID)
    using index
    tablespace USERS
    pctfree 10
    initrans 2
    maxtrans 255
    storage
    (
      initial 64K
      minextents 1
      maxextents unlimited
    );
    category_item表
    create table WORK.CATEGORY_ITEM
(
    CATEGORY_ID NUMBER,
    ITEM_ID       NUMBER
)
tablespace USERS
    pctfree 10
    initrans 1
    maxtrans 255
    storage
    (
      initial 64K
      minextents 1
      maxextents unlimited
    );
alter table WORK.CATEGORY_ITEM
    add constraint CATEGORIES_IITEM_ID foreign key (CATEGORY_ID)
    references WORK.CATEGORIES (ID) on delete cascade;
alter table WORK.CATEGORY_ITEM
    add constraint ITEM_CATEGORIES_ID foreign key (ITEM_ID)
    references WORK.ITEM (ID) on delete cascade;
item表
create table WORK.ITEM
(
    ID     NUMBER not null,
    NAME VARCHAR2(32),
    BASE_PRICE LONG
)
tablespace USERS
    pctfree 10
    initrans 1
    maxtrans 255
    storage
    (
      initial 64K
      minextents 1
      maxextents unlimited
    );
alter table WORK.ITEM
    add constraint ITEM_ID primary key (ID)
    using index
    tablespace USERS
    pctfree 10
    initrans 2
    maxtrans 255
    storage
    (
      initial 64K
      minextents 1
      maxextents unlimited
    );
2 在eclipse中新建一个java项目，再新建一个源文件夹src,再在其下新建包:hibernate.manytomany，除hibernate.cfg.xml文件位于src下，其它文件均位于此包下
     持久类及其对应的映射文件:
Category.java
     package hibernate.manytomany;import java.util.HashSet;
import java.util.Set;public class Category {
Long id;
String name;
Set items = new HashSet();
public Category() {
}
public Long getId() {
    return id;
}
public void setId(Long id) {
    this.id = id;
}
public Set getItems() {
    return items;
}
public void setItems(Set items) {
    this.items = items;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
}
Category.hbm.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
      "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
      "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" ><hibernate-mapping>
<class name="hibernate.manytomany.Category" table="categories">
    <id name="id" type="long" column="ID">
     <generator class="increment" />
    </id>
    <property name="name" type="string" column="name" />
    <set name="items" table="category_item" lazy="true"    cascade="save-update">
     <key column="category_id" />
     <many-to-many class="hibernate.manytomany.Item" column="item_id" />
    </set>
</class>
</hibernate-mapping>Item.java
package hibernate.manytomany;import java.util.HashSet;
import java.util.Set;public class Item {
Long id;
String name;
double basePrice;
Set category = new HashSet();
public Item() {
}
public Item(String name, double basePrice) {
    this.name = name;
    this.basePrice = basePrice;
}
public double getBasePrice() {
    return basePrice;
}
public void setBasePrice(double basePrice) {
    this.basePrice = basePrice;
}
public Set getCategory() {
    return category;
}
public void setCategory(Set category) {
    this.category = category;
}
public Long getId() {
    return id;
}
public void setId(Long id) {
    this.id = id;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
}
Item.hbm.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
      "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
      "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" ><hibernate-mapping>
<class name="hibernate.manytomany.Item" table="item" >
    <id name="id" type="long" column="ID">
     <generator class="increment" />
    </id>
    <property name="name" type="string" column="name" />
    <property name="basePrice" type="double" column="BASE_PRICE" />
    <set name="category" table="category_item" lazy="true" inverse="true" cascade="save-update">
     <key column="ITEM_ID" />
     <many-to-many class="hibernate.manytomany.Category" column="CATEGORY_ID" />
    </set>
</class>
</hibernate-mapping>
3 session管理类:HibernateUtil.java
package hibernate.manytomany;import net.sf.hibernate.*;
import net.sf.hibernate.cfg.Configuration;
public class HibernateUtil {
private static SessionFactory sessionFactory;
public static final ThreadLocal session = new ThreadLocal();
static {
    try {
     // 实例化一个SessionFactory对象
     //System.out.println("通过静态模块创建一个SessionFactory");
     sessionFactory = new Configuration().configure()
       .buildSessionFactory();
    } catch (HibernateException ex) {
     throw new RuntimeException("Configuration problem:"
       + ex.getMessage(), ex);
    }
}
public static Session currentSession() throws HibernateException {
    Session s = (Session) session.get();
    // 当原session为空或已关闭时,打一个新的Session
    if (s == null || !s.isOpen()) {
     s = sessionFactory.openSession();
     session.set(s);
    }
    return s;
}
public static void closeSession() throws HibernateException {
    Session s = (Session) session.get();
    session.set(null);
    if (s != null) {
     s.close();
    }
}
}
4.hibernate.cfg.xml源文件如下:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN"
                                           "http://hibernate.sourceforge.net/hibernate-configuration-2.0.dtd">
<hibernate-configuration>
<session-factory>
    <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
    <property name="hibernate.connection.url">jdbc:oracle:thin:@localhost:1521:orcl</property>
    <property name="hibernate.connection.username">work</property>
    <property name="hibernate.connection.password">caecaodb</property>
    <property name="hibernate.show_sql">true</property>
    <property name="dialect">net.sf.hibernate.dialect.OracleDialect</property>
    <property name="hibernate.jdbc.batch_size">0</property>
    <property name="hibernate.connection.isloation">2</property>    <mapping resource="hibernate/manytomany/Category.hbm.xml"/>
    <mapping resource="hibernate/manytomany/Item.hbm.xml"/>
</session-factory>
</hibernate-configuration>5　测试类Test.java
package hibernate.manytomany;import net.sf.hibernate.HibernateException;
import net.sf.hibernate.Session;
import net.sf.hibernate.Transaction;
public class Test    {
public static void main(String[] args) throws HibernateException {
    // TODO 自动生成方法存根
    new Test().saveCategory();
}
public void saveCategory() throws HibernateException {
    Session session = HibernateUtil.currentSession();
    Transaction tx = session.beginTransaction();
    Item item1 = new Item("NEC500", 1000);
    Item item2 = new Item("BELL4560", 1800);    Category category1 = new Category();
    category1.setName("CellPhone");
    category1.getItems().add(item1);
    category1.getItems().add(item2);    Category category2 = new Category();
    category2.setName("NECSeries");
    category2.getItems().add(item1);
    session.save(category1);
    session.save(category2);
    tx.commit();
    session.close();
}
}
运行Test.java，观察数据库中三张表的记录，体现出了多对多关系
注意:
1.多对多关系要通过一个中间表来存储记录的多对多关系
2.要把其中一方的映射文件中的set标签中的inverse设为true

这方面的例子网上蛮多的就是通过<many-to-many>来操作，两表之间再加个关联表即可！

多对多共有四种形式：
角色表 js 用户表 yh 用户角色表 yhjs
第一种形式：三张表分别对应一个持久化类，自动生成一个复合主键类，js 和yh 各要配置一个set，yhjs中有一个复合主键类的对象,还有yh 和js的id，还有yh 和js的对象。复合主键类有个变量分别是js 和yh的id。
第二种：比较简单的一种，就是三个表对应两个持久化类，js 和yh 各要配置一个set，每个set都配置成<many-to-many>详细说太多了，我写不完了~~

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eg:学生和课程的关系多对多课程public class Cource {
private long id;
private String name;
private Set students;

public long getId() {
return id;
}
private void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}

}<?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="com.java.model.Cource" table="hiber_cource">
<id name="id">
<column name="cid"></column>
<generator class="increment"></generator>
</id>

<property name="name"></property>

<set name="students" cascade="all" table="hiber_stu_cour">
<key column="cid"></key>
<many-to-many class="com.java.model.Student" column="sid"></many-to-many>
</set>
</class>
</hibernate-mapping>学生package com.java.model;import java.io.Serializable;
import java.util.Set;public class Student implements Serializable{
private long id;
private String name;
private Teacher teacher;
private Set cources;

public long getId() {
return id;
}
private void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Set getCources() {
return cources;
}
public void setCources(Set cources) {
this.cources = cources;
}
}<?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="com.java.model.Student" table="hiber_student">
<id name="id">
<column name="sid"></column>
<generator class="increment"></generator>
</id>

<property name="name">
<column name="name" ></column>
</property>
<set name="cources" cascade="all" table="hiber_stu_cour">
<key column="sid"></key>
<many-to-many class="com.java.model.Cource" column="cid"></many-to-many>
</set>

</class>
</hibernate-mapping>

调试易

hibernate里两张表的多对多关系映射...

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