【hibernate框架】性能优化之list_iterate的不同之处-阿里云开发者社区

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:

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

转载请注明出处:http://blog.csdn.net/acmman/article/details/43937591

【hibernate框架】性能优化之list_iterate的不同之处

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