one-to-one双向关联之加载

2020-11-09 来源：筏尚旅游网

关于hibernate的加载方式有两种，一种是get加载，一种是load加载，load属于延迟加载，使用了动态代理。这不是我们所关心的，我们看一看一对一双向关联在加载时有什么特点，想要观察特点，不得不看hibernate为我们生成的sql语句。 husband类与上一篇没有变化

关于hibernate的加载方式有两种，一种是get加载，一种是load加载，load属于延迟加载，使用了动态代理。这不是我们所关心的，我们看一看一对一双向关联在加载时有什么特点，想要观察特点，不得不看hibernate为我们生成的sql语句。

husband类与上一篇没有变化如下：

/**
 * 
 */
package com.maybe.test_1;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;

/**
 * @author MayBe
 *
 * function:
 */
@Entity
@Table(name="t_husband")
public class Husband {
	
	private Integer id;
	private String name;
	private Wife wife;
	@Id
	@GeneratedValue
	public Integer getId() {
	return id;
	}
	
	public String getName() {
	return name;
	}
	public void setId(Integer id) {
	this.id = id;
	}
	public void setName(String name) {
	this.name = name;
	}

	@OneToOne(mappedBy="husband")
	public Wife getWife() {
	return wife;
	}

	public void setWife(Wife wife) {
	this.wife = wife;
	}
	

}

wife类也没有变化：

/**
 * 
 */
package com.maybe.test_1;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

/**
 * @author MayBe
 *
 * function:
 */
@Entity
@Table(name="t_wife")
public class Wife {
	
	private Integer id;
	private String name;
	private Husband husband;
	@Id
	@GeneratedValue
	public Integer getId() {
	return id;
	}
	public String getName() {
	return name;
	}
	public void setId(Integer id) {
	this.id = id;
	}
	public void setName(String name) {
	this.name = name;
	}
	@OneToOne
	public Husband getHusband() {
	return husband;
	}
	public void setHusband(Husband husband) {
	this.husband = husband;
	}
	

}

junit测试方法如下：

@Test
	public void One_to_oneLoadTest1(){
	//一对一双向关联

	Session s = sessionFacotry.getCurrentSession();
	
	s.beginTransaction();
	
	Husband hus = (Husband)s.get(Husband.class, 2);	

 System.out.println("*****************");
	
	Wife wif = (Wife)s.get(Wife.class, 3);


	s.getTransaction().commit();
	
	}

假设我们在数据中存在一个id为2的husband，他与id为2的wife是关联的。在数据库有一个id为3的wife，她与id为3的husband是关联的，我们运行一下这个测试，输出sql语句如下：

2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
 2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata
 2014-02-04 16:52:20 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]
 2014-02-04 16:52:21 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete
 Hibernate: 
 select
 husband0_.id as id1_0_0_,
 husband0_.name as name2_0_0_,
 wife1_.id as id1_1_1_,
 wife1_.husband_id as husband_3_1_1_,
 wife1_.name as name2_1_1_ 
 from
 t_husband husband0_ 
 left outer join
 t_wife wife1_ 
 on husband0_.id=wife1_.husband_id 
 where
 husband0_.id=?
*****************
Hibernate: 
 select
 wife0_.id as id1_1_0_,
 wife0_.husband_id as husband_3_1_0_,
 wife0_.name as name2_1_0_,
 husband1_.id as id1_0_1_,
 husband1_.name as name2_0_1_ 
 from
 t_wife wife0_ 
 left outer join
 t_husband husband1_ 
 on wife0_.husband_id=husband1_.id 
 where
 wife0_.id=?
Hibernate: 
 select
 wife0_.id as id1_1_1_,
 wife0_.husband_id as husband_3_1_1_,
 wife0_.name as name2_1_1_,
 husband1_.id as id1_0_0_,
 husband1_.name as name2_0_0_ 
 from
 t_wife wife0_ 
 left outer join
 t_husband husband1_ 
 on wife0_.husband_id=husband1_.id 
 where
 wife0_.husband_id=?

首先加载husband对象时会把相关联的字段全部加载出来。

但是可以看出来加载wife表的时候会执行两次查询，虽然这两次查询的内容是一样的，但是第一次是查询出的是主表的信息，第二次是为了查出从表信息，这是hibernate的默认加载策略，因为t_wife表有外键，如果加载的表有从表，他会默认加载同时夹在两个表的信息。是否加载从表信息是由fetch进行设置的。

我们把wife类one-to-one新添加一个属性

/**
 * 
 */
package com.maybe.test_1;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

/**
 * @author MayBe
 *
 * function:
 */
@Entity
@Table(name="t_wife")
public class Wife {
	
	private Integer id;
	private String name;
	private Husband husband;
	@Id
	@GeneratedValue
	public Integer getId() {
	return id;
	}
	public String getName() {
	return name;
	}
	public void setId(Integer id) {
	this.id = id;
	}
	public void setName(String name) {
	this.name = name;
	}
	@OneToOne(fetch=FetchType.LAZY)
	public Husband getHusband() {
	return husband;
	}
	public void setHusband(Husband husband) {
	this.husband = husband;
	}
	
}

我们把抓取方式改成懒抓取，加载输出如下所示：

2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata
2014-02-04 17:07:38 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema
2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband
2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]
2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []
2014-02-04 17:07:39 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]
2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife
2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]
2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]
2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]
2014-02-04 17:07:40 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate:
select
husband0_.id as id1_0_0_,
husband0_.name as name2_0_0_,
wife1_.id as id1_1_1_,
wife1_.husband_id as husband_3_1_1_,
wife1_.name as name2_1_1_
from
t_husband husband0_
left outer join
t_wife wife1_
on husband0_.id=wife1_.husband_id
where
husband0_.id=?
*****************
Hibernate:
select
wife0_.id as id1_1_0_,
wife0_.husband_id as husband_3_1_0_,
wife0_.name as name2_1_0_
from
t_wife wife0_
where
wife0_.id=?

这样wife类只加载自己主表的信息，不考虑从表的信息了( 当然把husband也是如此，可以自己去试试)。

hibernate默认的加载策略为eager加载，也就是急抓取，会把所关联的信息一次性全部得到，而lazy则都是在你用到从表才会去加载。

我们改变一下测试类内容：

	public void One_to_oneLoadTest1() {
	// 一对一双向关联

	Session s = sessionFacotry.getCurrentSession();

	s.beginTransaction();

	Husband hus = (Husband) s.get(Husband.class, 2);

 	

	System.out.println("*****************");
	Wife wif = (Wife) s.get(Wife.class, 3);
	

	System.out.println(wif.getHusband().getName());
	s.getTransaction().commit();

	}

输出如下：

2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:193 - HHH000102: Fetching database metadata
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:205 - HHH000396: Updating schema
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_husband
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, name]
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: []
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_husban__3213e83f0f624af8]
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:66 - HHH000261: Table found: hibernate.dbo.t_wife
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:67 - HHH000037: Columns: [id, husband_id, name]
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:69 - HHH000108: Foreign keys: [fk_fi3kodkmubgryyblf4935y4dk]
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.TableMetadata:70 - HHH000126: Indexes: [pk__t_wife__3213e83f1332dbdc]
2014-02-04 17:10:51 INFO org.hibernate.tool.hbm2ddl.SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate:
select
husband0_.id as id1_0_0_,
husband0_.name as name2_0_0_,
wife1_.id as id1_1_1_,
wife1_.husband_id as husband_3_1_1_,
wife1_.name as name2_1_1_
from
t_husband husband0_
left outer join
t_wife wife1_
on husband0_.id=wife1_.husband_id
where
husband0_.id=?
*****************
Hibernate:
select
wife0_.id as id1_1_0_,
wife0_.husband_id as husband_3_1_0_,
wife0_.name as name2_1_0_
from
t_wife wife0_
where
wife0_.id=?
Hibernate:
select
husband0_.id as id1_0_0_,
husband0_.name as name2_0_0_,
wife1_.id as id1_1_1_,
wife1_.husband_id as husband_3_1_1_,
wife1_.name as name2_1_1_
from
t_husband husband0_
left outer join
t_wife wife1_
on husband0_.id=wife1_.husband_id
where
husband0_.id=?
Hibernate:
select
wife0_.id as id1_1_0_,
wife0_.husband_id as husband_3_1_0_,
wife0_.name as name2_1_0_
from
t_wife wife0_
where
wife0_.husband_id=?
GossipMan

这样才会加载从表的信息，对于一些其他的语句，可能是hibernate自动生成的其他语句，但只要理解其中的思想就可以了，不要在意这些细节。

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one-to-one双向关联之加载