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

@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