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:1521: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">1</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