Hibernate三种容器(List,Set,Map)映射技术之Set映射 和Hibernat的延迟

 环境: eclipse+myeclipse+mysql.

(1) 建表脚本

--删除表
drop table if exists person ;
drop table if exists email ;

--新建表
create table person
(
 id int AUTO_INCREMENT not null primary key ,
 name varchar(20)
) ;
create table email
(
 id int not null ,
 address varchar(50) not null ,
 foreign key (id) references person(id) on delete cascade
);

--提交
commit ;

(2) Person.java的编写.

package org.hibernate.listmapping.vo;

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

/**
 * Person generated by MyEclipse Persistence Tools
 */

public class Person implements java.io.Serializable {

 private int id ;
 private String name ;
 private Set email = new HashSet() ;
 public int getId() {
  return id;
 }
 public void setId(int id) {
  this.id = id;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public Set getEmail() {
  return email;
 }
 public void setEmail(Set email) {
  this.email = email;
 }
 

}

(3) Person.hbm.xml的编写.

<?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">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
 
 <class name="org.hibernate.listmapping.vo.Person" table="person">
  <id name="id" type="java.lang.Integer">
   <column name="id">
   </column>
   <generator class="native">
   </generator>
  </id>
  <property name="name" type="java.lang.String">
   <column name="name"></column>
  </property>
 
  <set name="email" table="email" lazy="false">
   <key column="id"></key>
   <element type="java.lang.String" >
    <column name="address"></column>
   </element>
  </set>
 
 </class>

</hibernate-mapping>

(4) hibernate.cfg.xml配置文件

<?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">

<!-- Generated by MyEclipse Hibernate Tools.                   -->
<hibernate-configuration>

<session-factory>
 <property name="connection.username">root</property>
 <property name="connection.url">
  jdbc:mysql://localhost:3306/hibernatetable
 </property>
 <property name="dialect">
  org.hibernate.dialect.MySQLDialect
 </property>
 <property name="myeclipse.connection.profile">mysql</property>
 <property name="connection.password">mysqladmin</property>
 <property name="connection.driver_class">
  com.mysql.jdbc.Driver
 </property>
 <property name="show_sql">true</property>
 <mapping resource="org/hibernate/listmapping/vo/Person.hbm.xml" />

</session-factory>

</hibernate-configuration>

(5)Insert测试类

package org.hibernate.listmapping.dao;

import org.hibernate.HibernateSessionFactory;
import org.hibernate.Session;
import org.hibernate.listmapping.vo.Person;

public class PersonDao {
 public void insert(Person p)
 {
  Session session = HibernateSessionFactory.getSession() ;
  session.save(p) ;
  session.beginTransaction().commit() ;
  session.close() ;
 }//end method insert
 
 public static void main(String[] args)
 {
  PersonDao pd = new PersonDao() ;
  Person p = new Person() ;
 
  //插入两个相同的email,在实际上数据库中只有一个,
  // set的特性.
  p.getEmail().add("[e-mail]") ;
  p.getEmail().add("[e-mail]") ;
 
  p.getEmail().add("[e-mail]") ;
  p.getEmail().add("[e-mail]") ;
 
  p.setName("zhangsan") ;
 
  pd.insert(p) ;
 }
}

(6)lazy  延迟加载.
 将此处的<set name="email" table="email" lazy="false">改成
  <set name="email" table="email" lazy="true">,就是Hibernate的延迟加载(懒加载机制了.

延迟加载测试类.

package org.hibernate.setDemo.dao;

import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.hibernate.Criteria;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;
import org.hibernate.criterion.Expression;
import org.hibernate.criterion.SimpleExpression;
import org.hibernate.setDemo.vo.Person;

public class PersonDao {
 
 private Session session  ;
 public PersonDao()
 {
  session = new Configuration().configure().buildSessionFactory()
    .openSession() ;
 }
 
 public void insert(Person p)
 {
  session.save(p) ;
  session.beginTransaction().commit() ;
  session.close() ;
 }
 
 public void update(Person p)
 {
  session.update(p) ;
  session.beginTransaction().commit() ;
  session.close() ;
 }
 
 public Person findById(int id)
 {
  Person p ;
  Criteria c = session.createCriteria(Person.class) ;
  SimpleExpression se = Expression.eq("id", id) ;
  c.add(se) ;
  List l =  c.list() ;
  if(l.size()>0)
  {
   p = (Person) l.get(0) ;
   return p ;
  }
  return null ;
 }
 
 public static void main(String []args)
 {
  PersonDao pd = new PersonDao() ;
  
  Person p = pd.findById(1) ;
  System.out.println(p.getName()) ;
  Set s = p.getEmail() ;
  Iterator i = s.iterator() ;
  while(i.hasNext())
  {
   String address = (String)i.next() ;
   System.out.println(address) ;
  }
  
 }
}

当我们的 Lazy = false时,运行上面的测试类控制台输出如下:
Hibernate: select this_.id as id0_0_, this_.name as name0_0_ from person this_ where this_.id=?
Hibernate: select email0_.id as id0_, email0_.address as address0_ from email email0_ where email0_.id=?
zhangsan
[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]
分析:我们可以看到,在查这个人的时候,附带的把这个人的地址都查出来放入内存中了.

当我们的 Lazy = true 时,运行上面的测试类控制台输出如下:
Hibernate: select this_.id as id0_0_, this_.name as name0_0_ from person this_ where this_.id=?
zhangsan
Hibernate: select email0_.id as id0_, email0_.address as address0_ from email email0_ where email0_.id=?
[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]
分析:我们可以很清楚的看到,是在我们要这个人的email地址的时候,才到数据库中去查,而不是事先去查.