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Speed Up Your Hibernate Applications with Second-Level Caching : Page 2

Newer Hibernate developers sometimes don't understand Hibernate caching and use it poorly as a result. However, when used correctly, it can be one of the most powerful ways to accelerate Hibernate applications.


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Cache Configuration

To activate second-level caching, you need to define the hibernate.cache.provider_class property in the hibernate.cfg.xml file as follows:

<hibernate-configuration> <session-factory> ... <property name="hibernate.cache.provider_class"> org.hibernate.cache.EHCacheProvider </property> ... </session-factory> </hibernate-configuration>

For testing purposes in Hibernate 3, you may also want to use the hibernate.cache.use_second_level_cache property, which allows you to activate (and deactivate) the second-level cache. By default, the second-level cache is activated and uses the EHCache provider.



A Practical Application

 
Figure 1. The Employee UML Class Diagram

The sample demo application for this article contains four simple tables: a list of countries, a list of airports, a list of employees, and a list of spoken languages. Each employee is assigned a country, and can speak many languages. Each country can have any number of airports. Figure 1 shows the UML class diagram for the application, and Figure 2 shows the database schema. The sample application source code contains the following SQL scripts, which you need in order to create and instantiate the corresponding database:

  • src/sql/create.sql: The SQL script used to create the database.
  • src/sql/init.sql: Test data

Note on Installing Maven 2
At the time of writing, the Maven 2 repository seemed to be missing some jars. To get around this problem, find the missing jars in the root directory of the application source code. To install them in the Maven 2 repository, go to the app directory and execute the following instructions:

$ mvn install:install-file -DgroupId=javax.security -DartifactId=jacc -Dversion=1.0
-Dpackaging=jar -Dfile=jacc-1.0.jar $ mvn install:install-file -DgroupId=javax.transaction -DartifactId=jta -Dversion=1.0.1B
-Dpackaging=jar -Dfile=jta-1.0.1B.jar

 
Figure 2. The Database Schema

Setting Up a Read-Only Cache
To begin with something simple, here's the Hibernate mapping for the Country class:

<hibernate-mapping package="com.wakaleo.articles.caching.businessobjects"> <class name="Country" table="COUNTRY" dynamic-update="true"> <meta attribute="implement-equals">true</meta> <cache usage="read-only"/> <id name="id" type="long" unsaved-value="null" > <column name="cn_id" not-null="true"/> <generator class="increment"/> </id> <property column="cn_code" name="code" type="string"/> <property column="cn_name" name="name" type="string"/> <set name="airports"> <key column="cn_id"/> <one-to-many class="Airport"/> </set> </class> </hibernate-mapping>

Suppose you need to display a list of all countries. You could implement this with a simple method in the CountryDAO class as follows:

public class CountryDAO { ... public List getCountries() { return SessionManager.currentSession() .createQuery( "from Country as c order by c.name") .list(); } }

Because this method is likely to be called often, you need to see how it behaves under pressure. So write a simple unit test that simulates five successive calls:

public void testGetCountries() { CountryDAO dao = new CountryDAO(); for(int i = 1; i <= 5; i++) { Transaction tx = SessionManager.getSession().beginTransaction(); TestTimer timer = new TestTimer("testGetCountries"); List countries = dao.getCountries(); tx.commit(); SessionManager.closeSession(); timer.done(); assertNotNull(countries); assertEquals(countries.size(),229); } }

You can run this test from either your preferred IDE or the command line using Maven 2 (the demo application provides the Maven 2 project files). The demo application was tested using a local MySQL database. When you run this test, you should get something like the following:

$mvn test -Dtest=CountryDAOTest ... testGetCountries: 521 ms. testGetCountries: 357 ms. testGetCountries: 249 ms. testGetCountries: 257 ms. testGetCountries: 355 ms. [surefire] Running com.wakaleo.articles.caching.dao.CountryDAOTest [surefire] Tests run: 1, Failures: 0, Errors: 0, Time elapsed: 3,504 sec

So each call takes roughly a quarter of a second, which is a bit sluggish by most standards. The list of countries probably doesn't change very often, so this class would be a good candidate for a read-only cache. So add one.

You can activate second-level caching classes in one of the two following ways:

  1. You activate it on a class-by-class basis in the *.hbm.xml file, using the cache attribute:

    <hibernate-mapping package="com.wakaleo.articles.caching.businessobjects"> <class name="Country" table="COUNTRY" dynamic-update="true"> <meta attribute="implement-equals">true</meta> <cache usage="read-only"/> ... </class> </hibernate-mapping>

  2. You can store all cache information in the hibernate.cfg.xml file, using the class-cache attribute:

    <hibernate-configuration> <session-factory> ... <property name="hibernate.cache.provider_class"> org.hibernate.cache.EHCacheProvider </property> ... <class-cache class="com.wakaleo.articles.caching.businessobjects.Country" usage="read-only" /> </session-factory> </hibernate-configuration>

Next, you need to configure the cache rules for this class. These rules determine the nitty-gritty details of how the cache will behave. The examples in this demo use EHCache, but remember that each cache implementation is different.

EHCache needs a configuration file (generally called ehcache.xml) at the classpath root. The EHCache configuration file is well documented on the project Web site. Basically, you define rules for each class you want to store, as well as a defaultCache entry for use when you don't explicitly give any rules for a class.

For the first example, you can use the following simple EHCache configuration file:

<ehcache> <diskStore path="java.io.tmpdir"/> <defaultCache maxElementsInMemory="10000" eternal="false" timeToIdleSeconds="120" timeToLiveSeconds="120" overflowToDisk="true" diskPersistent="false" diskExpiryThreadIntervalSeconds="120" memoryStoreEvictionPolicy="LRU" /> <cache name="com.wakaleo.articles.caching.businessobjects.Country" maxElementsInMemory="300" eternal="true" overflowToDisk="false" /> </ehcache>

This file basically sets up a memory-based cache for Countries with at most 300 elements (the country list contains 229 countries). Note that the cache never expires (the 'eternal=true' property).

Now, rerun the tests to see how the cache performs:

$mvn test -Dtest=CompanyDAOTest ... testGetCountries: 412 ms. testGetCountries: 98 ms. testGetCountries: 92 ms. testGetCountries: 82 ms. testGetCountries: 93 ms. [surefire] Running com.wakaleo.articles.caching.dao.CountryDAOTest [surefire] Tests run: 1, Failures: 0, Errors: 0, Time elapsed: 2,823 sec

As you would expect, the first query is unchanged since the first time around you have to actually load the data. However, all subsequent queries are several times faster.

Behind the Scenes
Before moving on, it is useful to look at what's going on behind the scenes. One thing you should know is that the Hibernate cache does not store object instances. Instead, it stores objects in their "dehydrated" form (to use Hibernate terminology), that is, as a set of property values. The following is a sample of the contents of the Country cache:

{ 30 => [bw,Botswana,30], 214 => [uy,Uruguay,214], 158 => [pa,Panama,158], 31 => [by,Belarus,31] 95 => [in,India,95] ... }

Notice how each ID is mapped to an array of property values. You may also have noticed that only the primitive properties are stored; there is no sign of the airports property. This is because the airports property is actually an association: a set of references to other persistent objects.

By default, Hibernate does not cache associations. It's up to you to decide which associations should be cached, and which associations should be reloaded whenever the cached object is retrieved from the second-level cache.

Association caching is a very powerful functionality. The next section takes a more detailed look at it.



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