hibernate---性能优化, 1+N问题

session级缓存 保存一个hashmap, 读出来的对象放在里面, 如果读出来50条放50条, 如果另起session原来的50条就被清除.可以手动session.clear清除.

如果同一个session里一直读, 内存里越来越多. 所以要用session.clear()处理.

java有内存泄露吗? 语法上来说没有, 但是写程序的时候会有, 用了资源要回收, 比如打开连接池.

1+N问题: 如果一个对象里关联另外一个对象, 同时fetchtype为eager, 比如many to one, 要取many里面对象的时候, 被关联对象也会被取出来, 就会取出来1+N条.

获取Topic里面的内容的时候, 被关联对象category也被取出:

category.java:

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)
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 org.hibernate.annotations.BatchSize;

@Entity
@NamedQueries(
		{
			@NamedQuery(name="topic.selectCertainTopic", query="from Topic t where t.id = :id")
		}
		)
		/*
@NamedNativeQueries(
		{
			@NamedNativeQuery(name="topic.select2_5Topic", query="select * from topic limit 2, 5")
		}
		)
		*/

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;
	}
	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;
	}

}

msg.java:

package com.bjsxt.hibernate;

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

@Entity
public class Msg {
	private int id;
	private String cont;
	private Topic topic;
	@ManyToOne
	public Topic getTopic() {
		return topic;
	}
	public void setTopic(Topic topic) {
		this.topic = topic;
	}
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}

	public String getCont() {
		return cont;
	}
	public void setCont(String cont) {
		this.cont = cont;
	}

}

hibernate.cfg.xml:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost/hibernate</property>
        <property name="connection.username">root</property>
        <property name="connection.password">linda0213</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
       <!--
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost::SXT</property>
        <property name="connection.username">scott</property>
        <property name="connection.password">tiger</property>
          <property name="dialect">org.hibernate.dialect.OracleDialect</property>
       -->

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size"></property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        <property name="format_sql">true</property>

        <!-- Drop and re-create the database schema on startup
        <property name="hbm2ddl.auto">update</property>
         -->
        <!--  

        <mapping resource="com/bjsxt/hibernate/Group.hbm.xml"/>
         <mapping resource="com/bjsxt/hibernate/User.hbm.xml"/>
         -->
         <mapping class="com.bjsxt.hibernate.Category"/>
          <mapping class="com.bjsxt.hibernate.Msg"/>
          <mapping class="com.bjsxt.hibernate.Topic"/>

    </session-factory>

</hibernate-configuration>

test文件:

package com.bjsxt.hibernate;

import java.util.Date;
import java.util.List;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateQLTest {
	private static SessionFactory sf;

	@BeforeClass
	public static void beforeClass() {
		sf = new AnnotationConfiguration().configure().buildSessionFactory();
	}
	@AfterClass
	public static void afterClass() {
		sf.close();
	}

	@Test
	public void testSchemaExport() {
		new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
	}

	@Test
	public void testSave() {
		Session session = sf.openSession();
		session.beginTransaction();

		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();
	}

	//N+1
	@Test
	public void testQuery1() {
		Session session = sf.openSession();
		session.beginTransaction();
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		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();

	}

	@Test
	public void testQuery2() {
		Session session = sf.openSession();
		session.beginTransaction();
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		List<Topic> topics = (List<Topic>)session.createQuery("from Topic").list();

		for(Topic t : topics) {
			System.out.println(t.getId() + "-" + t.getTitle());
			System.out.println(t.getCategory().getName());
		}
		session.getTransaction().commit();
		session.close();

	}

	//@BatchSize
	@Test
	public void testQuery3() {
		Session session = sf.openSession();
		session.beginTransaction();
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		List<Topic> topics = (List<Topic>)session.createQuery("from Topic").list();

		for(Topic t : topics) {
			System.out.println(t.getId() + "-" + t.getTitle());
			System.out.println(t.getCategory().getName());
		}
		session.getTransaction().commit();
		session.close();

	}
	//join fetch
	@Test
	public void testQuery4() {
		Session session = sf.openSession();
		session.beginTransaction();
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		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());
			System.out.println(t.getCategory().getName());
		}
		session.getTransaction().commit();
		session.close();

	}
	public static void main(String[] args) {
		beforeClass();
	}
}

此时输出结果会顺带打印出 category的查询信息,

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

解决方案

1. 在topic.java 里many to one下写入fetch=FetchType.LAZY

2. 在test文件里用Creteria替换Query.因为Creteria是表连接的方式, 和4一样.

3. 在category里@Entity下面加上 @BatchSize(size=5), 这样10条记录的话, 发出2条sql就可以了.

4, 使用join fetch:

//join fetch
	@Test
	public void testQuery4() {
		Session session = sf.openSession();
		session.beginTransaction();
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		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());
			System.out.println(t.getCategory().getName());
		}
		session.getTransaction().commit();
		session.close();

	}

如何选择使用哪个?

lazy:就取对象属性, 不取关联对象的属性

join fetch:马上就用关联对象的属性