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：

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;

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">

<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;

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