十八 一对一对象的懒加载

一对一对象的懒加载，对于主的一方，lazy是无效的，它在访问数据的时候，还是会利用左连接，把主从表的数据
全部查询一遍 但是对于从对象，默认的是懒加载 lazy=true 如果你需要从对象的数据，如果你通过子实体和id直接去查询
从实体 get()方法 它只查询从对象的数据,并不会去查询主对象的数据


一对多 懒加载 重点
缺省的是懒加载的 比说说部门与员工 ，一个部门可能有几千员工 

比说我我们增加一个部门 同时增加几名员工  我们查询部门的信息 
package vo.util.test;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;

import org.hibernate.Session;
import org.hibernate.Transaction;

import vo.util.HibernateUtil;
import vo.util.bean.Department;
import vo.util.bean.Employee;

public class OneToMany {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
	
         add();
        query(1);
	}
    static Department add(){
    	Session session = null;
    	Transaction tx = null;
    	try{
    		session =HibernateUtil.getSession();
    		tx = session.beginTransaction();
    		//…你的代码save,delete,update,get…
    		Department dpart=new Department();
    		dpart.setName("政企事业部");
    		
    		Employee em1=new Employee();
    		em1.setName("员工李小仙");
    		em1.setDerpartment(dpart);
    		
    		Employee em2=new Employee();
    		em2.setName("员工张小三");
    		em2.setDerpartment(dpart);
    		
    		Map<String,Employee> ems=new HashMap<String, Employee>();
    	    ems.put(em1.getName(), em1);
    	    ems.put(em2.getName(), em2);
    	    
    	    dpart.setEms(ems);    
    		session.save(dpart);
    		session.save(em1);
    		session.save(em2);
    		
    		
    		tx.commit();
    		return dpart;
    	}finally{
    		if(session != null)session.close();
    	} 

    }
    
    static Department query(int id){
    	Session session=null;
    	
    	try{
    	   session=HibernateUtil.getSession();
	   //这里只是调用部门的实体类进行查询 
    	   Department d=(Department)session.get(Department.class, id);
    	   
    	   //System.out.println(d.getEms());
    	  
    		return d;
    	}finally{
    		if(session != null)session.close();
    	} 

    }
}


控制台输出 
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
打开连接成功！Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depaer_id) values (?, ?)
Hibernate: insert into Employee (name, depaer_id) values (?, ?)
Hibernate: update Employee set depaer_id=?, name=? where id=?
Hibernate: update Employee set depaer_id=?, name=? where id=?
Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?


发现 只是查询了部门的信息 并没有去查询员工的信息 说明默认的情况下 一对多 一的一方是懒加载的

然后我们看部门的映射文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping 
	package="vo.util.bean">

	<class name="Department">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<!-- 配置一对多的映射关系  -->
		<!-- 
		<set name="ems">
		 -->
		<!-- 关联关系的外键  -->
		<!--  
		<key column="depaer_id"/>
		-->
		   <!-- 关联关系的实体类配置 -->
		   <!-- 
		  <one-to-many  class="Employee" />
		</set>
		
		 -->
        <map name="ems">
        <key column="depaer_id"/>
		 <map-key type="string" column="name"/>
		 <one-to-many class="Employee"/>
		 </map>
	</class>
	
</hibernate-mapping>

部门这边的set节点我们没有去配置



当我们去通过部门去访问员工信息的时候 
才会去select  
给查询的方法加上   System.out.println(d.getEms());

就会去查询员工信息 了
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
打开连接成功！Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depaer_id) values (?, ?)
Hibernate: insert into Employee (name, depaer_id) values (?, ?)
Hibernate: update Employee set depaer_id=?, name=? where id=?
Hibernate: update Employee set depaer_id=?, name=? where id=?
Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
Hibernate: select ems0_.depaer_id as depaer3_1_, ems0_.id as id1_, ems0_.name as name1_, ems0_.id as id1_0_, ems0_.name as name1_0_, ems0_.depaer_id as depaer3_1_0_ from Employee ems0_ where ems0_.depaer_id=?
{}



总结 通过asm和cglib二个包实现；Domain是非final的。
1.session.load懒加载。
2.one-to-one(元素)懒加载：
	必需同时满足下面三个条件时才能实现懒加载
	(主表不能有constrained=true,所以主表没有懒加载)
	lazy!=false 2)constrained=true 3)fetch=select
3.one-to-many (元素)懒加载：1)lazy!=false 2)fetch=select
4.many-to-one (元素) ：1)lazy!=false 2)fetch=select
5.many-to-many (元素) ：1)lazy!=false 2)fetch=select
6.能够懒加载的对象都是被改写过的代理对象，当相关联的session没有关闭时，
访问这些懒加载对象（代理对象）的属性（getId和getClass除外）hibernate会初始化这些代理，
或用Hibernate.initialize(proxy)来初始化代理对象；当相关联的session关闭后，再访问懒加载的对象将出现异常。