Hibernate之lazy懒加载

Hibernate Lazy采用CGLIB，可以对类进行代理，用在：

one-to-one，many-to-one: 取false,proxy/noproxy

Hibernate lazy概念：只有真正使用对象时，才会创建，对于Hibernate来说，只有真正使用时才会发出sql.

测试：one-to-one

Card.java

package com.po;public class Card {private int id;private String cardNo;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getCardNo() {return cardNo;}public void setCardNo(String cardNo) {this.cardNo = cardNo;}}

Person.java

package com.po;public class Person {private int id;private String name;private Card card;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 Card getCard() {return card;}public void setCard(Card card) {this.card = card;}}

Card.hbm.xml

<hibernate-mapping>    <class name="com.po.Card" table="card">        <id name="id" type="integer">            <generator class="native" />        </id>        <property name="cardNo" type="string">        </property>         </class></hibernate-mapping>

Person.hbm.xml

<hibernate-mapping>    <class name="com.po.Person" table="person">        <id name="id" type="integer">            <!-- foreign外键关联  -->            <generator class="foreign">               <param name="property">card</param>            </generator>        </id>        <property name="name" type="string">        </property>         <one-to-one name="card" constrained="true" lazy="false" fetch="join" />    </class></hibernate-mapping>

先导出对应生成的数据表

@Testpublic void testCreateDB(){Configuration config = new Configuration().configure();SchemaExport export = new SchemaExport(config);export.create(true, true);}

保存测试数据

@Testpublic void testSave(){Configuration config = new Configuration().configure();SessionFactory factory = config.buildSessionFactory();Session session = factory.openSession();session.beginTransaction();    Card card = new Card();    card.setCardNo("411257");        Person person = new Person();    person.setName("zhujingjing");    person.setCard(card);        session.save(person);session.getTransaction().commit();} 

当Person.hbm.xml配置为

 <one-to-one name="card" constrained="true" />

 

@Testpublic void testGetPerson(){Configuration config = new Configuration().configure();SessionFactory factory = config.buildSessionFactory();Session session = factory.openSession();session.beginTransaction();Person person = (Person)session.get(Person.class, 1);System.out.println(person.getName());    session.getTransaction().commit();}

此时发出的sql语句为

Hibernate: select person0_.id as id0_0_, person0_.name as name0_0_ from person person0_ where person0_.id=?zhujingjing

可见只发出了一条sql语句，查询Person的SQL

 

而如果配置文件加上lazy="false"

<one-to-one name="card" constrained="true" lazy="false" />

发出的sql语句为：

Hibernate: select person0_.id as id0_0_, person0_.name as name0_0_ from person person0_ where person0_.id=?Hibernate: select card0_.id as id1_0_, card0_.cardNo as cardNo1_0_ from card card0_ where card0_.id=?zhujingjing

两条sql语句，而我们没有进行查询Card啊，为什么会查询呢，因为我们设置了lazy懒加载为false,会把相关的都查询出来。

通过设置fetch="join"可避免延迟加载

<one-to-one name="card" constrained="true" lazy="false" fetch="join" />

 

Hibernate: select person0_.id as id0_1_, person0_.name as name0_1_, card1_.id as id1_0_, card1_.cardNo as cardNo1_0_ from person person0_ inner join card card1_ on person0_.id=card1_.id where person0_.id=?zhujingjing

此时发出一条sql 利用inner join进行查询

fetch 取值 join;select 默认为select