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 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("123@163.com") ;
? p.getEmail().add("123@163.com") ;
?
? p.getEmail().add("124@163.com") ;
? p.getEmail().add("125@163.com") ;
?
? 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
126@163.com
125@163.com
123@163.com
124@163.com
127@163.com
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=?
126@163.com
125@163.com
123@163.com
124@163.com
127@163.com
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.
?
