Hibernate Cache

Cache In Hibernate
HIBERNATE中的CACHE有两级. 一级是在Session范围内的CACHE. 即每个Session有自己的一个CACHE, 当前操作的对象都会被保留在CACHE中. 但是Session关闭后这个CACHE也就没有. 可见这级CACHE的生命期是很短的. 另一级CACHE是在SessionFactory范围的, 可以被来自同一个SessionFactory的Session共享. 在HIBERNATE的文档中称其为SECOND LEVEL CACHE. 显然后者的优势较明显, 也比较复合当前的使用环境.

还有一个类型的CACHE就是QueryCache. 它的作用就是缓存一个Query以及Query返回对象的Identifier以及对象的类型. 有了QueryCache后就可以高效的使用SECOND LEVEL CACHE.

Second Level Cache
这里使用ehcache来作为HIBERNATE的SECOND LEVEL CACHE. 由于这是HIBERNATE默认的CACHE提供者, 所以无须做什么设置. 两个很简单的POJO, Person & Address, 一对多的关系. HBM如下:
[code:1]
<hibernate-mapping
package="goncha.hb.bean">
<class name="Person" table="hb.person">
<!-- enable second level cache -->
<cache usage="read-write"/>

<id name="id">
<generator class="native"/>
</id>

<property name="name" not-null="true" unique="true"/>
<property name="age" type="java.lang.Integer" not-null="true"/>

<!-- relationship -->
<set name="addresses" lazy="true" inverse="true" cascade="save-update">
<key column="owner_id"/>
<one-to-many class="Address"/>
</set>
</class>
</hibernate-mapping>

<hibernate-mapping
package="goncha.hb.bean">
<class name="Address" table="hb.address">
<!-- enable second level cache -->
<cache usage="read-write"/>

<id name="id">
<generator class="native"/>
</id>

<property name="location" not-null="true" unique="true"/>
<property name="phone" not-null="true" unique="true"/>

<many-to-one name="owner" column="owner_id" not-null="true"/>
</class>
</hibernate-mapping>
[/code:1]
这两个类都使用了SECOND LEVEL CACHE, 并且两者之间存在关系映射.

ehcache的配置文件ehcache.xml:
[code:1]
<ehcache>

<diskStore path="java.io.tmpdir"/>

 

<defaultCache

maxElementsInMemory="10000"

eternal="false"

timeToIdleSeconds="120"

timeToLiveSeconds="120"

overflowToDisk="true"

/>

 

<cache name="goncha.hb.bean.Person"

maxElementsInMemory="10"

eternal="false"

timeToIdleSeconds="100"

timeToLiveSeconds="100"

overflowToDisk="false"

/>

 

<cache name="goncha.hb.bean.Address"

maxElementsInMemory="10"

eternal="false"

timeToIdleSeconds="100"

timeToLiveSeconds="100"

overflowToDisk="false"

/>

 

</ehcache>
[/code:1]

一段简单的测试代码, 可以清楚的看到HIBERNATE如何来使用SECOND LEVEL CACHE. 在运行这段代码前需要作一些工作: 在HIBERNATE的配置文件中允许输出生成的SQL语句; 在LOG4J的配置文件中, 处了net.sf.hibernate.cache在DEBUG级别, 其余都在WARN级别, 为了没有干扰 :)
[code:1]
# log4j.properties
log4j.logger.net.sf.hibernate=warn
log4j.logger.net.sf.hibernate.cache=debug

# hibernate.properties
hibernate.show_sql true
[/code:1]

至此就可以运行以下代码
[code:1]
//: Main.java

package goncha.hb;

import java.util.Iterator;
import java.util.List;
import java.util.Set;

import net.sf.hibernate.Query;
import net.sf.hibernate.Session;
import net.sf.hibernate.SessionFactory;
import net.sf.hibernate.Transaction;
import net.sf.hibernate.cfg.Configuration;

import goncha.hb.bean.*;

public class Main {

public static void main(String[] args) throws Exception {
SessionFactory sessions = buildSessionFactory();

pushDataIntoCache(sessions);
popDataFromCache(sessions);
}

static SessionFactory buildSessionFactory() throws Exception {
Configuration config = new Configuration();
config.addClass(Person.class).addClass(Address.class);
return config.buildSessionFactory();
}

static void pushDataIntoCache(SessionFactory sessions) throws Exception {
System.out.println("======= Push some data into cache =======");

Session sess = sessions.openSession();
Transaction tx = sess.beginTransaction();

Person goncha = (Person)sess.load(Person.class, new Integer(1));
System.out.println(goncha);

Person chengang = (Person)sess.load(Person.class, new Integer(2));
System.out.println(chengang);

goncha.getAddresses().size();
tx.commit();
sess.close();
}

static void popDataFromCache(SessionFactory sessions) throws Exception {
System.out.println("======= Pop some data into cache =======");

Session sess = sessions.openSession();
Transaction tx = sess.beginTransaction();

Person goncha = (Person)sess.load(Person.class, new Integer(1));
System.out.println(goncha);

Person chengang = (Person)sess.load(Person.class, new Integer(2));
System.out.println(chengang);

Person tommy = (Person)sess.load(Person.class, new Integer(3));
System.out.println(tommy);

Person mary = (Person)sess.load(Person.class, new Integer(4));
System.out.println(mary);

List persons = sess.find("from Person as person");

Set addresses = goncha.getAddresses();
Iterator it = addresses.iterator();
while(it.hasNext()) {
System.out.println((Address)it.next());
}

tx.commit();
sess.close();
}
}
[/code:1]

以下是程序的输出
[code:1]
Buildfile: build.xml

init:

build:

run:
[java] 23:14:04,878 DEBUG CacheFactory:32 - cache for: goncha.hb.bean.Person usage strategy: read-write
[java] 23:14:05,156 DEBUG CacheFactory:32 - cache for: goncha.hb.bean.Address usage strategy: read-write
[java] Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@17bd6a1 [ connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@8b819f [ acquireIncrement -> 2, autoCommitOnClose -> false, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, idleConnectionTestPeriod -> 3000, initialPoolSize -> 2, maxIdleTime -> 5000, maxPoolSize -> 2, maxStatements -> 100, minPoolSize -> 2, nestedDataSource -> com.mchange.v2.c3p0.DriverManagerDataSource@147ee05 [ description -> null, driverClass -> null, factoryClassLocation -> null, jdbcUrl -> jdbc:postgresql:test, properties -> {user=pgsql, password=pgsql} ] , propertyCycle -> 300, testConnectionOnCheckout -> false ] , factoryClassLocation -> null, numHelperThreads -> 3 ]
[java] 23:14:07,390 INFO UpdateTimestampsCache:35 - starting update timestamps cache at region: net.sf.hibernate.cache.UpdateTimestampsCache
[java] 23:14:07,396 INFO QueryCache:39 - starting query cache at region: net.sf.hibernate.cache.QueryCache
[java] __======= Push some data into cache =======__
[java] 23:14:07,492 DEBUG ReadWriteCache:68 - Cache lookup: 1
[java] 23:14:07,494 DEBUG ReadWriteCache:84 - Cache miss: 1
[java] Hibernate: select person0_.id as id0_, person0_.name as name0_, person0_.age as age0_ from hb.person person0_ where person0_.id=?
[java] 23:14:07,584 DEBUG ReadWriteCache:132 - Caching: 1
[java] 23:14:07,594 DEBUG ReadWriteCache:143 - Cached: 1
[java] Person ['goncha'; 23]
[java] 23:14:07,599 DEBUG ReadWriteCache:68 - Cache lookup: 2
[java] 23:14:07,601 DEBUG ReadWriteCache:84 - Cache miss: 2
[java] Hibernate: select person0_.id as id0_, person0_.name as name0_, person0_.age as age0_ from hb.person person0_ where person0_.id=?
[java] 23:14:07,609 DEBUG ReadWriteCache:132 - Caching: 2
[java] 23:14:07,611 DEBUG ReadWriteCache:143 - Cached: 2
[java] Person ['chengang'; 23]
[java] Hibernate: select addresses0_.id as id__, addresses0_.owner_id as owner_id__, addresses0_.id as id0_, addresses0_.location as location0_, addresses0_.phone as phone0_, addresses0_.owner_id as owner_id0_ from hb.address addresses0_ where addresses0_.owner_id=?
[java] 23:14:07,624 DEBUG ReadWriteCache:132 - Caching: 1
[java] 23:14:07,630 DEBUG ReadWriteCache:143 - Cached: 1
[java] 23:14:07,632 DEBUG ReadWriteCache:132 - Caching: 2
[java] 23:14:07,633 DEBUG ReadWriteCache:143 - Cached: 2
[java] 23:14:07,635 DEBUG ReadWriteCache:132 - Caching: 6
[java] 23:14:07,637 DEBUG ReadWriteCache:143 - Cached: 6
[java] __======= Pop some data into cache =======__
[java] 23:14:07,669 DEBUG ReadWriteCache:68 - Cache lookup: 1
[java] 23:14:07,671 DEBUG ReadWriteCache:78 - Cache hit: 1
[java] Person ['goncha'; 23]
[java] 23:14:07,675 DEBUG ReadWriteCache:68 - Cache lookup: 2
[java] 23:14:07,678 DEBUG ReadWriteCache:78 - Cache hit: 2
[java] Person ['chengang'; 23]
[java] 23:14:07,680 DEBUG ReadWriteCache:68 - Cache lookup: 3
[java] 23:14:07,682 DEBUG ReadWriteCache:84 - Cache miss: 3
[java] Hibernate: select person0_.id as id0_, person0_.name as name0_, person0_.age as age0_ from hb.person person0_ where person0_.id=?
[java] 23:14:07,691 DEBUG ReadWriteCache:132 - Caching: 3
[java] 23:14:07,693 DEBUG ReadWriteCache:143 - Cached: 3
[java] Person ['tommy'; 21]
[java] 23:14:07,698 DEBUG ReadWriteCache:68 - Cache lookup: 4
[java] 23:14:07,699 DEBUG ReadWriteCache:84 - Cache miss: 4
[java] Hibernate: select person0_.id as id0_, person0_.name as name0_, person0_.age as age0_ from hb.person person0_ where person0_.id=?
[java] 23:14:07,704 DEBUG ReadWriteCache:132 - Caching: 4
[java] 23:14:07,706 DEBUG ReadWriteCache:143 - Cached: 4
[java] Person ['mary'; 18]
[java] Hibernate: select person0_.id as id, person0_.name as name, person0_.age as age from hb.person person0_
[java] 23:14:07,768 DEBUG ReadWriteCache:132 - Caching: 5
[java] 23:14:07,770 DEBUG ReadWriteCache:143 - Cached: 5
[java] 23:14:07,772 DEBUG ReadWriteCache:132 - Caching: 6
[java] 23:14:07,774 DEBUG ReadWriteCache:143 - Cached: 6
[java] Hibernate: select addresses0_.id as id__, addresses0_.owner_id as owner_id__, addresses0_.id as id0_, addresses0_.location as location0_, addresses0_.phone as phone0_, addresses0_.owner_id as owner_id0_ from hb.address addresses0_ where addresses0_.owner_id=?
[java] 23:14:07,809 DEBUG ReadWriteCache:132 - Caching: 1
[java] 23:14:07,811 DEBUG ReadWriteCache:152 - Item was already cached: 1
[java] 23:14:07,816 DEBUG ReadWriteCache:132 - Caching: 2
[java] 23:14:07,817 DEBUG ReadWriteCache:152 - Item was already cached: 2
[java] 23:14:07,819 DEBUG ReadWriteCache:132 - Caching: 6
[java] 23:14:07,820 DEBUG ReadWriteCache:152 - Item was already cached: 6
[java] Address ['goncha'; 'newyork']
[java] Address ['goncha'; 'shanghai']
[java] Address ['goncha'; 'guangzhou']

 

BUILD SUCCESSFUL
Total time: 7 seconds
[/code:1]

这个结果很出乎意外, HERBNATE只能在Session.load()方法中使用CACHE. pushDataIntoCache()方法成功地给CACHE注入两个Person对象("goncha", "chengang")以及三个Address对象(与"goncha"关联). 再看看popDataFromCache()方法. 使用Session.load()时, "goncha"与"chengang"对应的Person对象都是从CACHE中获得, 其余两个是CACHE MISS的, 正常举动. 而Session.find()的数据获取与CACHE没有关系, 完全由JDBC来操办. 最后只是把从JDBC获得的对象注入CACHE. 通过关系查找对象的过程和Session.find()没有异样, 甚至CACHE还报出对象已存在的消息.

不知这样的测试是否会偏面. 但是在具体的使用环境中就是Session.find()和Relationship Collection占了多数. 像上面的例子那样, CACHE不仅没有用到还白白占了内存. 所以在HIBERNATE中使用CACHE还是需要根据具体情况来定制.

Query Cache
老实说, 要做到在JDBC查询之前决定哪些数据需要从JDBC来还是CACHE来不是件容易事. 但是HIBERNATE还是很好地完成了这个任务. 前面说过QueryCache用来缓存查询语句, 及查询结果集中对象的Identifier与Type. 当再次使用已缓存的Query时, 就可以通过对象的Identifier与Type在SECOND LEVEL CACHE中查找实际的对象.

使用QueryCache时需要在hibernate配置文件中设置如下属性:
[code:1]
hibernate.cache.use_query_cache true
[/code:1]

在程序中需要为Query对象设置Cachable属性:
[code:1]
Query query = sess.createQuery("from AccountingPeriod as period "
+ "where period.id.dealerId = ? order by period.id.orderType asc, "
+ "period.id.accountingPeriod asc");
query.setCacheable(true);
query.setInteger(0, dealerId.intValue());
List rs = query.list();
[/code:1]