list与iterate的不同之处:
还是用上一次的例子,话题topic和版块category
Topic.java:
Category.java:
添加数据(每一个板块ci下有一个话题ti)
下面进行list与iterate的不同测试
1.list取所有
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
2.iterate先取ID,等用到的时候在根据ID来取对象。
Hibernate:
select
topic0_.id as col_0_0_
from
Topic topic0_
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
1-t0
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
2-t1
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
3-t2
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
4-t3
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
5-t4
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
6-t5
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
7-t6
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
8-t7
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
9-t8
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
10-t9
所以我们可以看出,先取ID,等用到的时候在根据ID来取对象。
3.session中list第二次发出,仍会到数据库查询。
测试代码:
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
4.iterate第二次,首先找session级缓存
测试代码:
Hibernate:
select
category0_.id as col_0_0_
from
Category category0_
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c0
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c1
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c2
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c3
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c4
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c5
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c6
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c7
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c8
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c9
Hibernate:
select
category0_.id as col_0_0_
from
Category category0_
c0
c1
c2
c3
c4
c5
c6
c7
c8
c9
看出了第二次使用的session缓存里面的数据,而没有去数据库再查。
总结:
原因是:先看list,list取完10个数据以后session中有了10个数据的缓存,第二次list再去取的时候,它仍然要再去数据库里加载一次,然后再刷新缓存,再把原来对象重新刷一遍。也就是list不会去读session的缓存,原因是作为一个查询来说,查询条件很难认定,不是list不想利用,而是很难利用。
iterate就不一样了,首先在第一遍的时候,他首先把id取出来,由于后面我们做了遍历,所以他把id对应的对象全都放在了缓存中,第二次再做遍历的时候,这个时候只要去缓存中查就行了。
什么时候用list?什么时候用iterate?工作的时候用list就行了。
小技巧:第一次用list,第二次再去访问的时候就可以用iterate了。因为iterate会首先检查缓存,缓存中有了,就不会再去数据库里去查了。
还是用上一次的例子,话题topic和版块category
Topic.java:
package com.bjsxt.hibernate; import java.util.ArrayList; import java.util.Date; import java.util.List; import javax.persistence.Entity; import javax.persistence.FetchType; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.ManyToOne; import javax.persistence.NamedQueries; import javax.persistence.NamedQuery; import javax.persistence.OneToMany; import javax.persistence.Temporal; import javax.persistence.TemporalType; import org.hibernate.annotations.BatchSize; @Entity public class Topic { private int id; private String title; private Category category; private Date createDate; private List<Msg> msgs = new ArrayList<Msg>(); @OneToMany(mappedBy="topic") public List<Msg> getMsgs() { return msgs; } public void setMsgs(List<Msg> msgs) { this.msgs = msgs; } @Temporal(TemporalType.TIME) public Date getCreateDate() { return createDate; } public void setCreateDate(Date createDate) { this.createDate = createDate; } @ManyToOne(fetch=FetchType.LAZY) public Category getCategory() { return category; } public void setCategory(Category category) { this.category = category; } @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } }
Category.java:
package com.bjsxt.hibernate; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import org.hibernate.annotations.BatchSize; @Entity public class Category { private int id; private String name; @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
添加数据(每一个板块ci下有一个话题ti)
@Test public void testSave() { Session session = sf.openSession(); session.beginTransaction(); //每一个版块ci下面有一个话题ti for(int i=0; i<10; i++) { Category c = new Category(); c.setName("c" + i); Topic t=new Topic(); t.setCategory(c); t.setTitle("t"+i); t.setCreateDate(new Date()); session.save(c); session.save(t); } session.getTransaction().commit(); session.close(); }
下面进行list与iterate的不同测试
1.list取所有
@Test public void testOneAddNProblem1(){ Session session = sf.openSession(); session.beginTransaction(); List<Topic> topics=(List<Topic>)session.createQuery("from Topic").list(); for(Topic t:topics){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); }结果:
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
2.iterate先取ID,等用到的时候在根据ID来取对象。
@Test public void testListAndIterate(){ Session session = sf.openSession(); session.beginTransaction(); Iterator<Topic> topics=(Iterator<Topic>)session.createQuery("from Topic").iterate(); for(Topic t=topics.next();t!=null;t=topics.next()){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); }测试结果:
Hibernate:
select
topic0_.id as col_0_0_
from
Topic topic0_
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
1-t0
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
2-t1
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
3-t2
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
4-t3
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
5-t4
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
6-t5
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
7-t6
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
8-t7
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
9-t8
Hibernate:
select
topic0_.id as id2_0_,
topic0_.category_id as category4_2_0_,
topic0_.createDate as createDate2_0_,
topic0_.title as title2_0_
from
Topic topic0_
where
topic0_.id=?
10-t9
所以我们可以看出,先取ID,等用到的时候在根据ID来取对象。
3.session中list第二次发出,仍会到数据库查询。
测试代码:
@Test public void testListAndIterate2(){ Session session = sf.openSession(); session.beginTransaction(); //Iterator<Topic> topics=(Iterator<Topic>)session.createQuery("from Topic").iterate(); Query q=session.createQuery("from Topic"); List<Topic> topics=q.list(); for(Topic t:topics){ System.out.println(t.getId()+"-"+t.getTitle()); } List<Topic> topics2=q.list(); for(Topic t:topics2){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); }测试结果:
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
Hibernate:
select
topic0_.id as id2_,
topic0_.category_id as category4_2_,
topic0_.createDate as createDate2_,
topic0_.title as title2_
from
Topic topic0_
1-t0
2-t1
3-t2
4-t3
5-t4
6-t5
7-t6
8-t7
9-t8
10-t9
4.iterate第二次,首先找session级缓存
测试代码:
@Test public void testListAndIterate3(){ Session session = sf.openSession(); session.beginTransaction(); Iterator<Category> categories = (Iterator<Category>)session.createQuery("from Category").iterate(); while(categories.hasNext()) { Category c = categories.next(); System.out.println(c.getName()); } Iterator<Category> categories2 = (Iterator<Category>)session.createQuery("from Category").iterate(); while(categories2.hasNext()) { Category c = categories2.next(); System.out.println(c.getName()); } session.getTransaction().commit(); session.close(); }测试结果:
Hibernate:
select
category0_.id as col_0_0_
from
Category category0_
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c0
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c1
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c2
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c3
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c4
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c5
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c6
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c7
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c8
Hibernate:
select
category0_.id as id0_0_,
category0_.name as name0_0_
from
Category category0_
where
category0_.id=?
c9
Hibernate:
select
category0_.id as col_0_0_
from
Category category0_
c0
c1
c2
c3
c4
c5
c6
c7
c8
c9
看出了第二次使用的session缓存里面的数据,而没有去数据库再查。
总结:
原因是:先看list,list取完10个数据以后session中有了10个数据的缓存,第二次list再去取的时候,它仍然要再去数据库里加载一次,然后再刷新缓存,再把原来对象重新刷一遍。也就是list不会去读session的缓存,原因是作为一个查询来说,查询条件很难认定,不是list不想利用,而是很难利用。
iterate就不一样了,首先在第一遍的时候,他首先把id取出来,由于后面我们做了遍历,所以他把id对应的对象全都放在了缓存中,第二次再做遍历的时候,这个时候只要去缓存中查就行了。
什么时候用list?什么时候用iterate?工作的时候用list就行了。
小技巧:第一次用list,第二次再去访问的时候就可以用iterate了。因为iterate会首先检查缓存,缓存中有了,就不会再去数据库里去查了。
主要应付面试
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