【Hibernate】解析hibernate中的缓存

Hibernate中的缓存一共有三种，一级缓存、二级缓存、查询缓存。缓存除了使用Hibernate自带的缓存，还可以使用redis进行缓存，或是MongoDB进行缓存。

所使用的Demo：

User.java文件

package cn.test.bean;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="user")//表示对应的表名
public class User {

    @Id
    @Column(name="uid")
    private int id;
    @Column(name="uname")
    private String name;
    @Column(name="upass")
    private String password;
    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;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }

}

User.java

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">

<!-- Generated by MyEclipse Hibernate Tools.                   -->
<hibernate-configuration>
    <session-factory>
        <property name="dialect">
            org.hibernate.dialect.MySQLDialect
        </property>
        <property name="connection.url">
            jdbc:mysql://localhost:3306/test?useUnicode=true&amp;characterEncoding=utf8
        </property>
        <property name="connection.username">root</property>
        <property name="connection.password">517839</property>
        <property name="connection.driver_class">
            com.mysql.jdbc.Driver
        </property>
        <property name="show_sql">true</property>
        <property name="format_sql">true</property>

        <!-- 加载映射描述信息 -->
        <mapping class="cn.test.bean.User" />

    </session-factory>
</hibernate-configuration>

hibernate.cfg.xml

其中：

        <property name="show_sql">true</property>
        <property name="format_sql">true</property>

表示开启打印底层执行的SQL日志。

下面这是图片反应了hibernate缓存的大致流程：

1,一级缓存

每个 Session 对象创建出来，就会分配一块缓存空间，可以存储 session 对象访问的对象信息。 session 关闭后会自动清除缓存，手动清除可以用session.clear() ， session.evict(obj) 。 Session 一级缓存是独享。
load/get/save/update/saveorupdate 方法处理的对象都会放入缓存中

Configuration conf = new Configuration();
conf.configure("hibernate.cfg.xml");//读取连接参数和映射描述信息
SessionFactory factory = conf.buildSessionFactory();
Session session = factory.openSession();

User user1 = (User)session.load(User.class,1);
System.out.println(user1.getName());//honny，如果不调用用getName()方法，那么数据不会显示，因为load()默认使用的是一种延迟加载的机制，只有使用到数据的时候才会到数据库中查询

//先从session缓存中查找，如果没找到再去数据库获取
User user2 = (User)session.load(User.class,1);
System.out.println(user2.getName());//honny

System.out.println(user1==user2);//true,因为user1和user2使用的是同一个session

然后再来看一看控制台：

从控制台中，我们也可以看出上只执行了一次SQL查询。

一级查询的优缺点：

优点：可以减少查询数据库的次数，加快查询速度。
缺点：在批量操作中容易导致内存溢出问题。

2,二级缓存

二级缓存是SessionFactory 对象缓存，可以被创建出的多个 Session 对象共享。

下面是一张图片体现一级缓存和二级缓存的关系：

从这个我们就看出了二级缓存包含了一级缓存。

二级缓存默认是关闭的，如果要使用需要手动开启，下面是开启过程：

1.导入ehcache 工具包和 ehcache.xml 配置文件(配置文件放到src路径下)

echache工具包包括：ehcache-core-2.4.3.jar，hibernate-ehcache-4.2.21.Final.jar，slf4j-api-1.6.1.jar

ehcache.xml 文件

<!--
  ~ Hibernate, Relational Persistence for Idiomatic Java
  ~
  ~ Copyright (c) 2007, Red Hat Middleware LLC or third-party contributors as
  ~ indicated by the @author tags or express copyright attribution
  ~ statements applied by the authors. All third-party contributions are
  ~ distributed under license by Red Hat Middleware LLC.
  ~
  ~ This copyrighted material is made available to anyone wishing to use, modify,
  ~ copy, or redistribute it subject to the terms and conditions of the GNU
  ~ Lesser General Public License, as published by the Free Software Foundation.
  ~
  ~ This program is distributed in the hope that it will be useful,
  ~ but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  ~ or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
  ~ for more details.
  ~
  ~ You should have received a copy of the GNU Lesser General Public License
  ~ along with this distribution; if not, write to:
  ~ Free Software Foundation, Inc.
  ~ 51 Franklin Street, Fifth Floor
  ~ Boston, MA  02110-1301  USA
  -->
<ehcache>

    <!-- Sets the path to the directory where cache .data files are created.

         If the path is a Java System Property it is replaced by
         its value in the running VM.

         The following properties are translated:
         user.home - User's home directory
         user.dir - User's current working directory
         java.io.tmpdir - Default temp file path -->
    <diskStore path="./target/tmp"/>

    <!--Default Cache configuration. These will applied to caches programmatically created through
        the CacheManager.

        The following attributes are required for defaultCache:

        maxInMemory       - Sets the maximum number of objects that will be created in memory
        eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element
                            is never expired.
        timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used
                            if the element is not eternal. Idle time is now - last accessed time
        timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used
                            if the element is not eternal. TTL is now - creation time
        overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache
                            has reached the maxInMemory limit.

        -->
    <defaultCache
        maxElementsInMemory="10000"
        eternal="false"
        timeToIdleSeconds="120"
        timeToLiveSeconds="120"
        overflowToDisk="true"
        />

    <!--Predefined caches.  Add your cache configuration settings here.
        If you do not have a configuration for your cache a WARNING will be issued when the
        CacheManager starts

        The following attributes are required for defaultCache:

        name              - Sets the name of the cache. This is used to identify the cache. It must be unique.
        maxInMemory       - Sets the maximum number of objects that will be created in memory
        eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element
                            is never expired.
        timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used
                            if the element is not eternal. Idle time is now - last accessed time
        timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used
                            if the element is not eternal. TTL is now - creation time
        overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache
                            has reached the maxInMemory limit.

        -->

    <!-- Sample cache named sampleCache1
        This cache contains a maximum in memory of 10000 elements, and will expire
        an element if it is idle for more than 5 minutes and lives for more than
        10 minutes.

        If there are more than 10000 elements it will overflow to the
        disk cache, which in this configuration will go to wherever java.io.tmp is
        defined on your system. On a standard Linux system this will be /tmp"
        -->
    <cache name="sampleCache1"
        maxElementsInMemory="10000"
        eternal="false"
        timeToIdleSeconds="300"
        timeToLiveSeconds="600"
        overflowToDisk="true"
        />

    <!-- Sample cache named sampleCache2
        This cache contains 1000 elements. Elements will always be held in memory.
        They are not expired. -->
    <cache name="sampleCache2"
        maxElementsInMemory="1000"
        eternal="true"
        timeToIdleSeconds="0"
        timeToLiveSeconds="0"
        overflowToDisk="false"
        /> -->

    <!-- Place configuration for your caches following -->

</ehcache>

ehcache.xml

2.在 hibernate.cfg.xml 中配置参数开启二级缓存，启用 ehcache

<property name="hibernate.cache.use_sencond_level_cache">true</property>
<property name="hibernate.cache.region.factory_class">
org.hibernate.cache.ehcache.EhCacheRegionFactory
</property>

3.在要缓存的对象类型中，指定 @Cache 注解标记

@Entity
@Table(name="user")//表示对应的表名
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE)
public class User {
    //........
}

到这里hibernate的二级缓存配置就配好了，下面来测试一下：

Configuration conf = new Configuration();
conf.configure("hibernate.cfg.xml");//读取连接参数和映射描述信息
SessionFactory factory = conf.buildSessionFactory();
Session session1 = factory.openSession();
User user1=(User)session1.load(User.class, 1);
System.out.println(user1.getName());//honny

Session session2 = factory.openSession();
//先从缓存中查找，如果没有查到再去数据库中取
User user2=(User)session2.load(User.class, 1);
System.out.println(user2.getName());//honny

下面是控制台打印的打印：

我们可以看出，用同一个SessionFactory的两个不同session对象查询相同的数据，只从数据库中取了一次。

3,查询缓存

一级和二级缓存，只能缓存单个对象，如果需要缓存一个结果集，必须使用查询缓存。

查询缓存默认也是关闭的，如需使用需要手动开启，下面是开启过程：

1.针对的对象必需已经开启了二级缓存

2.在 hibernate.cfg.xml 中添加开启查询缓存的配置

<property name="hibernate.cache.use_query_cache">true</property>

3.在查询执行前，调用 query.setCacheable(true);

下面看一看测试：

        String hql="from User";
        Configuration conf=new Configuration();
        conf.configure("hibernate.cfg.xml");
        SessionFactory factory=conf.buildSessionFactory();
        Session session1 = factory.openSession();
        Query query1 = session1.createQuery(hql);
        query1.setCacheable(true);//设置开启缓存
        List list1 = query1.list();
        for(Object user:list1){
            System.out.println(((User)user).getName());
        }
        System.out.println("------------------------");
        Session session2 = factory.openSession();
        Query query2 = session2.createQuery(hql);
        query2.setCacheable(true);
        List list2 =query2.list();
        for(Object user:list2){
            System.out.println(((User)user).getName());
        }

然后来看一看控制台：

从控制台中，我们可以看出，底层查询的数据库的过程也只执行了一次。

上面就是hibernate的三种缓存。最后总结一下，并不是所有的方法都会产生缓存效果，只有“load/get/save/update/saveorupdate”才会产生缓存效果。三种缓存中一级缓存是默认开启的，二级缓存和三级缓存默认是关闭的。