Hibernate延迟加载 lazy loading

延迟加载在Hibernate中是默认延迟加载；

测试代码一：

HibernateTest.java

代码：

/**  *  */ package com.b510.examples;

import java.util.Set;

import org.hibernate.Session;

/**  *  * @author XHW  *  * @date 2011-7-18  *  */ public class HibernateTest {  public static void main(String[] args) {   new HibernateTest().update();  }  public void update(){   Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();   session.beginTransaction();  Category category=(Category)session.get(Category.class, 1);   System.out.println("id:"+category.getId()+"  ,name:"+category.getName()+", description:"+category.getDescription());   Set<Product> products=category.getProducts();   session.getTransaction().commit();    } }

运行结果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         category0_.id as id1_0_,         category0_.name as name1_0_,         category0_.description as descript3_1_0_     from         users.category category0_     where         category0_.id=? id:1  ,name:java, description:java好啊

这里我们看到我们关心的是id，name和description属性，

虽然有：  Set<Product> products=category.getProducts(); 代码，即：不处理集合对象。但是我们只要的是：

  System.out.println("id:"+category.getId()+"  ,name:"+category.getName()+", description:"+category.getDescription()); 输出的是id，name和description属性值，其他的我们不管，所以Hibernate用了lazy loading(延迟加载)，带来的好处就是我们不关心的

数据，不用现在加载，当我们要用的时候，才去加载

测试代码二：

HibernateTest.java

代码：

/**  *  */ package com.b510.examples;

import java.util.Set;

import org.hibernate.Session;

/**  *  * @author XHW  *  * @date 2011-7-18  *  */ public class HibernateTest {  public static void main(String[] args) {   new HibernateTest().update();  }  public void update(){   Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();   session.beginTransaction();   Category category=(Category)session.get(Category.class, 1);   System.out.println("id:"+category.getId()+"  ,name:"+category.getName()+", description:"+category.getDescription());   Set<Product> products=category.getProducts();   for(Product product:products){    System.out.println("ID:  "+product.getId()+"  name:"+product.getName()+" price: "+product.getPrice());   }     session.getTransaction().commit();    } }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         category0_.id as id1_0_,         category0_.name as name1_0_,         category0_.description as descript3_1_0_     from         users.category category0_     where         category0_.id=? id:1  ,name:java, description:java好啊 Hibernate:  select         products0_.category_id as category2_1_,         products0_.id as id1_,         products0_.id as id0_0_,         products0_.category_id as category2_0_0_,         products0_.name as name0_0_,         products0_.price as price0_0_,         products0_.descripton as descripton0_0_     from         users.product products0_     where         products0_.category_id=? ID:  1  name:java SE应用程序设计 price: 78.00 这里可以明确的告诉我们，当我们要加载Set集合的时候，这时候才去加载，而上面的例子，说明的是我们不加载的时候

Hibernate就延迟加载

取消延迟加载：

Category.hbm.xml

代码：

<?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="com.b510.examples.Category" table="category" catalog="users">         <id name="id" type="java.lang.Integer">             <column name="id" />             <generator class="increment" />         </id>         <property name="name" type="java.lang.String">             <column name="name" length="500" />         </property>         <property name="description" type="java.lang.String">             <column name="description" length="500" />         </property>         <set name="products" inverse="true" lazy="false">             <key>                 <column name="category_id" />             </key>             <one-to-many class="com.b510.examples.Product" />         </set>     </class> </hibernate-mapping>

测试代码：

HIbernateTest.java

代码：

/**  *  */ package com.b510.examples;

import java.util.Set;

import org.hibernate.Session;

/**  *  * @author XHW  *  * @date 2011-7-18  *  */ public class HibernateTest {  public static void main(String[] args) {   new HibernateTest().update();  }  public void update(){   Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();   session.beginTransaction();   Category category=(Category)session.get(Category.class, 1);   System.out.println("id:"+category.getId()+"  ,name:"+category.getName()+", description:"+category.getDescription());   Set<Product> products=category.getProducts();   session.getTransaction().commit();    } }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         category0_.id as id1_0_,         category0_.name as name1_0_,         category0_.description as descript3_1_0_     from         users.category category0_     where         category0_.id=? Hibernate:     select         products0_.category_id as category2_1_,         products0_.id as id1_,         products0_.id as id0_0_,         products0_.category_id as category2_0_0_,         products0_.name as name0_0_,         products0_.price as price0_0_,         products0_.descripton as descripton0_0_     from         users.product products0_     where         products0_.category_id=? id:1  ,name:java, description:java好啊 和测试代码一的运行结果相互比较，我们会发现，这次运行结果用了两条select语句。但是我们会发现

第二条select语句，对于我们的需求是没有必要的，他只有一个用处就是占用我们的程序执行时间。当然，

这是我们不希望看到的结果。

一般情况下，Hibernate会默认给我们设置延迟加载。lazy="true" ,这样会提升我们的系统性能，所以一般情况下，我们不会去

设置lazy="false",当然在特殊的情况下，我们必须要取消延迟加载的时候，我们就把lazy="false"，就可以了