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。
