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。