There is a trick that pays off when there are ResultSets or entities in a web application that are not updated so often: second-level caching. Getting second-level cache to work means that entities or
ResultSets can be stored in the cache and not retrieved from the database in future calls.
There are many cache providers, among them is EhCache which we will be using in this example. In order to get second-level cache to work, first we have to tell hibernate about it. Add this in the hibernate configuration file (hibernate.cfg.xml):
<property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>
<property name="cache.use_query_cache">true</property>
<property name="cache.use_second_level_cache">true</property>
These properties tell hibernate that we will be using EhCache for entity and query caching. Then, create a file named ehcache.xml (the default name of the EhCache configuration file) and put it in the same location with hibernate.cfg.xml, typically in the classpath root. A sample configuration for ehcache.xml is the following:
<ehcache>
<diskStore path="java.io.tmpdir"/>
<defaultCache
maxElementsInMemory="20000"
eternal="false"
timeToIdleSeconds="300"
timeToLiveSeconds="300"
overflowToDisk="true"
diskPersistent="false"
diskExpiryThreadIntervalSeconds="300"
memoryStoreEvictionPolicy="LRU" />
<cache name="com.app.entities.City"
maxElementsInMemory="15000"
eternal="true"
overflowToDisk="false" />
...
</ehcache>
In short, this tells EhCache to use the java temp folder and to store a maximum of 20000 entities in the cache (cities for instance). The configuration for each entity overrides the global configuration in the ehcache.xml file. In this example, the global setting defines a TimeToLiveSeconds value which is overriden for the City entities, causing them to be stored eternally in the cache. The same holds for the cache location: global configuration allows the usage of the hard disk but for the entities cached, only the memory will be used (overflowToDisk=false).
But, what happens when an entity changes? There is no problem as long as the entity is configured through hibernate:
@Entity
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE)
public class City {
private Long id;
private String name;
....
}
The @Cache annotation in this example uses a READ_WRITE strategy, which tells hibernate to update the cache when the entity is updated. If the entity is not so probable to be changed, a read-only strategy will suffice (CacheConcurrencyStrategy.READ_ONLY).
The good part is that ResultSets can be stored as well. Suppose you have a query that returns the most popular cities by country. Define it as cacheable and it will be evaluated against the database only once:
public List<City> findPopularCitiesByCountryId(Long countryId) {
List<City> cities = session.createQuery("FROM City AS c WHERE
c.country.id=? ORDER BY c.popularity DESC")
.setCacheable(true)
.setLong(0, countryId)
.setMaxResults(20)
.list();
return cities;
}
The trick is the setCacheable property of the query. Running the application in debug mode, you will see in the output for the first time
[net.sf.ehcache.Cache]-[DEBUG] org.hibernate.cache.StandardQueryCache cache - Miss
[org.hibernate.cache.StandardQueryCache]-[DEBUG] query results were not found in cache
[org.hibernate.cache.ReadWriteCache]-[DEBUG] Cache miss: com.app.entities.City#1412
[org.hibernate.event.def.DefaultLoadEventListener]-[DEBUG] object not resolved in any cache
…but in subsequent calls you will be happy to see, that even if the query cache fails:
[net.sf.ehcache.Cache]-[DEBUG] org.hibernate.cache.UpdateTimestampsCache cache - Miss
[org.hibernate.cache.EhCache]-[DEBUG] Element for cities is null
the entities will be retrieved from the cache:
[org.hibernate.cache.ReadWriteCache]-[DEBUG] Cache lookup: com.app.entities.City#1412
[org.hibernate.cache.ReadWriteCache]-[DEBUG] Cache hit: com.app.entities.City#1412
There you go, you just boosted your application’s performance. Take care though, common sense orders that you should only cache ResultSets or entities that are frequently accessed and rarely modified. You can find more info at the official hibernate manual:
http://www.hibernate.org/hib_docs/v3/reference/en/html/performance.html