Hibernate之lazy懒加载详解编程语言

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

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

保存测试数据

@Test 
	public 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" />

@Test 
	public 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