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【hibernate框架】性能优化之1+N问题

2015-02-25 15:12 232 查看

1+N问题/典型的面试题

a)Lazy

b)BatchSize

c)join fetch

什么是1+N:如果我在一个对象里面关联另外一个对象，同时fetch=FetchType.EAGER,

最典型的是@ManyToOne。本来我用一条Sql语句就可以解决的，结果发了1条外加N条sql语句。

1+N问题实例剖析

Category.java:

package com.bjsxt.hibernate;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@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;
}
}

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;

@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
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;
}

}

建表之后先填入记录：

@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();
}

//测试1加N问题

@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();

}

先取出Topic的所有信息，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_

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id=?

1-t0

2-t1

3-t2

4-t3

5-t4

6-t5

7-t6

8-t7

9-t8

10-t9

我只是想取出topic而已，为什么多了那么多category？因为在实体类上，topic类的category属性上面的@ManyToOne的fetch默认是EAGER，所以取出topic的时候，会顺带着把topic关联的category出取出来。取category的过程就是，每取一个topic的时候，根据topic中的category_id取category表中按照id号取出相应的category对象。

这就是1+N问题，就是本来你该发一条sql语句，结果发了N条sql语句。

解决方案1：

如果上面的@MangToOne改为@ManyToOne(fetch=FetchType.LAZY)

结果就变成了：

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

只有你取category的时候他才会发出sql语句(按需而发)

解决方案2：

使用QBC面向对象的查询语句：

List<Topic> topics=(List<Topic>)session.createCriteria(Topic.class).list();

这是因为Criteria默认的是用了"表连接"的方式来取数据，两张表做连接来取，而不是另外单独发sql语句来取。

测试结果:

Hibernate:

select

this_.id as id2_0_,

this_.category_id as category4_2_0_,

this_.createDate as createDate2_0_,

this_.title as title2_0_

from

Topic this_

1-t0

2-t1

3-t2

4-t3

5-t4

6-t5

7-t6

8-t7

9-t8

10-t9

解决方案3:(这里@ManyToOne的fetch默认仍是EAGER)

@BatchSize(size=5)

首先在Category的类上面加@BatchSize(size=5)：

package com.bjsxt.hibernate;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

import org.hibernate.annotations.BatchSize;

@Entity
@BatchSize(size=5)//当你去加载Category的时候一次性加载5条
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;
}
}

测试：

@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();

}

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_

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id in (

?, ?, ?, ?, ?

)

Hibernate:

select

category0_.id as id0_0_,

category0_.name as name0_0_

from

Category category0_

where

category0_.id in (

?, ?, ?, ?, ?

)

1-t0

2-t1

3-t2

4-t3

5-t4

6-t5

7-t6

8-t7

9-t8

10-t9

发现关于Category的查询语句变成了两条，每条查询五个记录。(取出了10条)

这样就没有必要一条一条的去取了。要设成10只要发一个就可以了。

BatchSize只是提高了效率，少发多条sql语句，其实并没有完全解决1+N问题。

解决方案4：join fetch

@Test
public void testOneAddNProblem2(){
Session session = sf.openSession();
session.beginTransaction();
List<Topic> topics=(List<Topic>)session.createQuery("from Topic t left join fetch t.category c").list();
for(Topic t:topics){
System.out.println(t.getId()+"-"+t.getTitle());
}
session.getTransaction().commit();
session.close();

}

测试结果：

Hibernate:

select

topic0_.id as id2_0_,

category1_.id as id0_1_,

topic0_.category_id as category4_2_0_,

topic0_.createDate as createDate2_0_,

topic0_.title as title2_0_,

category1_.name as name0_1_

from

Topic topic0_

left outer join

Category category1_

on topic0_.category_id=category1_.id

1-t0

2-t1

3-t2

4-t3

5-t4

6-t5

7-t6

8-t7

9-t8

10-t9
只发出了一条sql语句，原因是用了"Topic t left join fetch"，做了一个外连接，做了一个外连接的话就没有必要每拿出一条来单独再去取了。

转载请注明出处:http://blog.csdn.net/acmman/article/details/43937551

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