【hibernate框架】性能优化之1+N问题

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。

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