jpa抓取策略详解fetch(lazy ,eager)

在jpa中jpa默认的加载方式是lazy方式也就是在实际使用到数据的时候才加载相关数据，使用lazy时可以不用显示注明fetch=FetchType.LAZY

实体bean：carage

package com.hibernate.jpa.bean1;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
@Entity
public class Garage {

	/**
	 * many to one 多对一
	 */
	private Integer gid;
	private String garagenum;
	private Set<Auto> autos = new HashSet<Auto>();
	
	@Id @GeneratedValue
	public Integer getGid() {
		return gid;
	}
	public void setGid(Integer gid) {
		this.gid = gid;
	}
	@Column(length=20)
	public String getGaragenum() {
		return garagenum;
	}
	public void setGaragenum(String garagenum) {
		this.garagenum = garagenum;
	}
	@OneToMany(cascade={CascadeType.PERSIST},mappedBy="garage")
	public Set<Auto> getAutos() {
		return autos;
	}
	public void setAutos(Set<Auto> autos) {
		this.autos = autos;
	}
	public void addGarageAuto(Auto auto) {
		auto.setGarage(this);
		this.autos.add(auto);
	}

}

实体bean：auto

package com.hibernate.jpa.bean1;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
@Entity
public class Auto {

	/**
	 * one to many 一对多关联
	 */
	private Integer autoId;
	private String autotype;
	private String autonum;
	private Garage garage;

	@Id @GeneratedValue
	public Integer getAutoId() {
		return autoId;
	}
	public void setAutoId(Integer autoId) {
		this.autoId = autoId;
	}
	public String getAutotype() {
		return autotype;
	}
	public void setAutotype(String autotype) {
		this.autotype = autotype;
	}
	public String getAutonum() {
		return autonum;
	}
	public void setAutonum(String autonum) {
		this.autonum = autonum;
	}
	@ManyToOne()
	@JoinColumn(name="garageid")
	public Garage getGarage() {
		return garage;
	}
	public void setGarage(Garage garage) {
		this.garage = garage;
	}

}

junit的测试方法

	@Test public void query() {
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");
		EntityManager em = factory.createEntityManager();
		
		Garage garage = em.find(Garage.class, 1);

		
		em.close();
		factory.close();
	}

调用query方法的时候发出的sql语句是：

Hibernate: select garage0_.gid as gid1_0_, garage0_.garagenum as garagenum1_0_ from Garage garage0_ where garage0_.gid=?

也就是仅仅获取了garage对象，而没有获取与garage关联的auto对象

-----------------

（二）在Garage.java中添加fetch=FetchType.EAGER字段

@OneToMany(cascade={CascadeType.PERSIST},fetch=FetchType.EAGER,mappedBy="garage")
public Set<Auto> getAutos() {
return autos;
}

再次运行query方法，这一次发出的sql语句是：

Hibernate: select garage0_.gid as gid1_1_, garage0_.garagenum as garagenum1_1_, autos1_.garageid as garageid3_, autos1_.autoId as autoId3_, autos1_.autoId as autoId0_0_, autos1_.autonum as autonum0_0_, autos1_.autotype as autotype0_0_, autos1_.garageid as garageid0_0_ from Garage garage0_ left outer join Auto autos1_ on garage0_.gid=autos1_.garageid where garage0_.gid=?

这一次由于将jpa默认的抓取策略改为fetch=FetchType.EAGER

所以jpa在加载数据的时候一次性的加载了和garage相关联的数据

说明：由于fetch=FetchType.EAGER加载数据的时候是一次性加载可能会造成不必要的性能浪费，使用是应该慎重考虑

评论

6 楼 木南飘香 2011-02-24  

楼主很强悍！谢谢了

5 楼 renci 2010-06-05  

楼主我搞错了..sorry

4 楼 renci 2010-06-05  

楼主,我配置的多对多,在配置为lazy时用get怎么取不出来数据

调试的时候用eclipse察看出了这个错
com.sun.jdi.InvocationException occurred invoking method.

但是日志里没哟任何错误,请问这是怎么回事啊?

3 楼 fejay 2009-11-04  

thanks。搞明白了

2 楼 sefcertyu 2009-09-27  

在jpa中jpa默认的加载方式是lazy方式也就是在实际使用到数据的时候才加载相关数据，使用lazy时可以不用显示注明fetch=FetchType.LAZY

releasa 写道

不错，好，牛，好。。呵呵

呵呵，还望多提意见

1 楼 releasa 2009-09-27  

不错，好，牛，好。。呵呵