one-to-one双向关联之加载
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。