本篇博客是之前博客hibernate关联对象的增删改查------查 的后继,本篇代码的设定都在前文已经写好,因此读这篇之前,请先移步上一篇博客
//代码片5
SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
Session session = sessionFactory.getCurrentSession();
session.beginTransaction();
Dream d=new Dream();
d.setDescription("marry glt");
Person p=new Person();
p.setName("dlf");
d.setPerson(p);
session.save(d);
session.save(p);
session.getTransaction().commit();
session = sessionFactory.getCurrentSession();
session.beginTransaction();
Dream dream=(Dream) session.get(Dream.class, 1);
System.out.println(dream.getPerson().getName()+" ddddd");
session.getTransaction().commit(); select
dream0_.id as id0_1_,
dream0_.description as descript2_0_1_,
dream0_.personId as personId0_1_,
person1_.id as id1_0_,
person1_.myname as myname1_0_
from
Dream dream0_
left outer join
Person person1_
on dream0_.personId=person1_.id
where
dream0_.id=? //代码6
public void testGetPerson() {
Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
Person p = (Person)s.get(Person.class, 2);
s.getTransaction().commit();
}这个时候它发的sql语句是: select
person0_.id as id1_0_,
person0_.myname as myname1_0_
from
Person person0_
where
person0_.id=?并没有去主动获得person对应的dream。 @OneToMany(mappedBy="person",fetch=FetchType.EAGER)
public Set<Dream> getDreams() {
return dreams;
}此时再运行代码6,发的sql语句就是: select
person0_.id as id1_1_,
person0_.myname as myname1_1_,
dreams1_.personId as personId3_,
dreams1_.id as id3_,
dreams1_.id as id0_0_,
dreams1_.description as descript2_0_0_,
dreams1_.personId as personId0_0_
from
Person person0_
left outer join
Dream dreams1_
on person0_.id=dreams1_.personId
where
person0_.id=?我们就能知道,如果想在取一的时候同时取多的一方,就在一的一方上加上fetch=feachType.eager。 //代码7
public void testGetPerson() {
Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
Person p = (Person)s.get(Person.class, 2);
System.out.println(p.getDreams().size());
s.getTransaction().commit();
}此时不设置person的fetch值。(保持默认的lazy)Hibernate:
select
person0_.id as id1_0_,
person0_.myname as myname1_0_
from
Person person0_
where
person0_.id=?
Hibernate:
select
dreams0_.personId as personId1_,
dreams0_.id as id1_,
dreams0_.id as id0_0_,
dreams0_.description as descript2_0_0_,
dreams0_.personId as personId0_0_
from
Dream dreams0_
where
dreams0_.personId=?此时我们可以得到一个结论,如果在session里,我们只是获得"一"的那一方,hibernate默认不会去取多的那一方;但是如果在session里,访问了获得的"一"里面"多"的那一方数据(就是访问了person里面的dream)。就会发关联sql。 //代码8
public void testGetPerson() {
testSavePerson();
Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
Person p = (Person)s.get(Person.class, 2);
s.getTransaction().commit();
System.out.println(p.getDreams().size());
}就是把获得多的那一方数据的代码放到了session的外面。Hibernate:
select
person0_.id as id1_1_,
person0_.myname as myname1_1_,
dreams1_.personId as personId3_,
dreams1_.id as id3_,
dreams1_.id as id0_0_,
dreams1_.description as descript2_0_0_,
dreams1_.personId as personId0_0_
from
Person person0_
left outer join
Dream dreams1_
on person0_.id=dreams1_.personId
where
person0_.id=?
2 //dream的size是2可是如果我把fetch=FetchType.eager去掉,在运行代码8,就会报错:<span style="color:#FF0000;">failed to lazily initialize a collection of role: com.bjsxt.hibernate.Person.dreams, no session or session was closed</span>说的很清楚,session已经关闭了。
//代码9
public void testGetPerson() {
testSavePerson();
Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
Person p = (Person)s.get(Person.class, 2);
s.getTransaction().commit();
System.out.println(p.getDreams().size());
s.getTransaction().commit();
}此时的person的fetch还是默认的lazy。
关于取对象这部分,hibernate还是比较麻烦的,我认为最好的方法就是保持默认的情况,一旦有了问题再查api文档,或其他的资料。
那既然这样,我为什么还要写这篇博客呢?反正最好的方法就是保持默认情况嘛。
因为:
世之奇伟、瑰怪、非常之观，常在于险远，而人之所罕至焉，故非有志者不能至也。
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原文：http://blog.csdn.net/dlf123321/article/details/46845725