Working with Cached Associations

Suppose you need to display the list of employees (with employee names, languages spoken, etc.) for a given country. The following is the Hibernate mapping of the Employee class:

<hibernate-mapping package="com.wakaleo.articles.caching.businessobjects">
<class name="Employee" table="EMPLOYEE" dynamic-update="true">
<meta attribute="implement-equals">true</meta>
<id name="id" type="long" unsaved-value="null" >
<column name="emp_id" not-null="true"/>
<generator class="increment"/>
</id>
<property column="emp_surname" name="surname" type="string"/>
<property column="emp_firstname" name="firstname" type="string"/>
<many-to-one name="country"
column="cn_id"
class="com.wakaleo.articles.caching.businessobjects.Country"
not-null="true" />
<!-- Lazy-loading is deactivated to demonstrate caching behavior -->
<set name="languages" table="EMPLOYEE_SPEAKS_LANGUAGE" lazy="false">
<key column="emp_id"/>
<many-to-many column="lan_id" class="Language"/>
</set>
</class>
</hibernate-mapping>

Suppose you really need to load the languages spoken by an employee every time you use the Employee object. To force Hibernate to automatically load the languages set, you set the lazy attribute to false. (This is just for the sake of this example. In general, deactivating lazy loading is not a good idea. Do it only when absolutely necessary.) You will also need a DAO class to fetch the employees. The following one would do the trick:

public class EmployeeDAO {
public List getEmployeesByCountry(Country country) {
return SessionManager.currentSession()
.createQuery(
"from Employee as e where e.country = :country "
+ " order by e.surname, e.firstname")
.setParameter("country",country)
.list();
}
}

Next, write some simple unit tests to see how it performs. As in the previous example, you should see how it performs when called repeatedly:

public class EmployeeDAOTest extends TestCase {
CountryDAO countryDao = new CountryDAO();
EmployeeDAO employeeDao = new EmployeeDAO();
/**
* Ensure that the Hibernate session is available
* to avoid the Hibernate initialisation interfering with
* the benchmarks
*/
protected void setUp() throws Exception {
super.setUp();
SessionManager.getSession();
}
public void testGetNZEmployees() {
TestTimer timer = new TestTimer("testGetNZEmployees");
Transaction tx = SessionManager.getSession().beginTransaction();
Country nz = countryDao.findCountryByCode("nz");
List kiwis = employeeDao.getEmployeesByCountry(nz);
tx.commit();
SessionManager.closeSession();
timer.done();
}
public void testGetAUEmployees() {
TestTimer timer = new TestTimer("testGetAUEmployees");
Transaction tx = SessionManager.getSession().beginTransaction();
Country au = countryDao.findCountryByCode("au");
List aussis = employeeDao.getEmployeesByCountry(au);
tx.commit();
SessionManager.closeSession();
timer.done();
}
public void testRepeatedGetEmployees() {
testGetNZEmployees();
testGetAUEmployees();
testGetNZEmployees();
testGetAUEmployees();
}
}

If you run a test using the above configuration, you should get something like the following:

$mvn test -Dtest=EmployeeDAOTest
...
testGetNZEmployees: 1227 ms.
testGetAUEmployees: 883 ms.
testGetNZEmployees: 907 ms.
testGetAUEmployees: 873 ms.
testGetNZEmployees: 987 ms.
testGetAUEmployees: 916 ms.
[surefire] Running com.wakaleo.articles.caching.dao.EmployeeDAOTest
[surefire] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 3,684 sec

So loading the 50 or so employees assigned to each country takes about a second each time. That's way too slow. This is typical of the N+1 query problem. If you activate the SQL logs, you will see one query on the EMPLOYEE table, followed by literally hundreds of queries on the LANGUAGE table: whenever Hibernate retrieves an employee object from the cache, it reloads all the associated languages. So how can you improve on this? The first thing to do is activate read/write caching on the Employee class as follows:

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

You should also activate caching on the Language class. Read-only caching should do here:

<class name="Language" table="SPOKEN_LANGUAGE" dynamic-update="true">
<meta attribute="implement-equals">true</meta>
<cache usage="read-only"/>
...
</class>
</hibernate-mapping>

Then, you will need to configure the cache rules by adding the following entries to the ehcache.xml file:

<cache name="com.wakaleo.articles.caching.businessobjects.Employee"
maxElementsInMemory="5000"
eternal="false"
overflowToDisk="false"
timeToIdleSeconds="300"
timeToLiveSeconds="600"
/>
<cache name="com.wakaleo.articles.caching.businessobjects.Language"
maxElementsInMemory="100"
eternal="true"
overflowToDisk="false"
/>

This is fine, but it doesn't solve the N+1 query problem: 50 or so extra queries will still be executed whenever you load an Employee. This is a case where you need to activate caching on the language association in the Employee.hbm.xml mapping file, as follows:

<hibernate-mapping package="com.wakaleo.articles.caching.businessobjects">
<class name="Employee" table="EMPLOYEE" dynamic-update="true">
<meta attribute="implement-equals">true</meta>
<id name="id" type="long" unsaved-value="null" >
<column name="emp_id" not-null="true"/>
<generator class="increment"/>
</id>
<property column="emp_surname" name="surname" type="string"/>
<property column="emp_firstname" name="firstname" type="string"/>
<many-to-one name="country"
column="cn_id"
class="com.wakaleo.articles.caching.businessobjects.Country"
not-null="true" />
<!-- Lazy-loading is deactivated to demonstrate caching behavior -->
<set name="languages" table="EMPLOYEE_SPEAKS_LANGUAGE" lazy="false">
<cache usage="read-write"/>
<key column="emp_id"/>
<many-to-many column="lan_id" class="Language"/>
</set>
</class>
</hibernate-mapping>

In this configuration, you should get near-optimal performance:

$mvn test -Dtest=EmployeeDAOTest
...
testGetNZEmployees: 1477 ms.
testGetAUEmployees: 940 ms.
testGetNZEmployees: 65 ms.
testGetAUEmployees: 65 ms.
testGetNZEmployees: 76 ms.
testGetAUEmployees: 52 ms.
[surefire] Running com.wakaleo.articles.caching.dao.EmployeeDAOTest
[surefire] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 0,228 sec

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.

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.

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.

Using Query Caches

In certain cases, it is useful to cache the exact results of a query, not just certain objects. For example, the getCountries() method probably should return exactly the same country list each time it is called. So, in addition to caching the Country class, you could also cache the query results themselves.

To do this, you need to set the hibernate.cache.use_query_cache property in the hibernate.cfg.xml file to true, as follows:

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

Then, you use the setCacheable() method as follows on any query you wish to cache:

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

To guarantee the non-staleness of cache results, Hibernate expires the query cache results whenever cached data is modified in the application. However, it cannot anticipate any changes made by other applications directly in the database. So you should not use any second-level caching (or configure a short expiration timeout for class- and collection-cache regions) if your data has to be up-to-date all the time.