19.2. The Second Level Cache

A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a cluster or JVM-level (SessionFactory-level) cache on a class-by-class and collection-by-collection basis. You may even plug in a clustered cache. Be careful. Caches are never aware of changes made to the persistent store by another application (though they may be configured to regularly expire cached data).
You have the option to tell Hibernate which caching implementation to use by specifying the name of a class that implements org.hibernate.cache.CacheProvider using the property hibernate.cache.provider_class. Hibernate comes bundled with a number of built-in integrations with open-source cache providers (listed below); additionally, you could implement your own and plug it in as outlined above. Note that versions prior to 3.2 defaulted to use EhCache as the default cache provider; that is no longer the case as of 3.2.

Table 19.1. Cache Providers

Cache Provider class Type Cluster Safe Query Cache Supported
Hashtable (not intended for production use) org.hibernate.cache.HashtableCacheProvider memory yes
EHCache org.hibernate.cache.EhCacheProvider memory, disk yes
OSCache org.hibernate.cache.OSCacheProvider memory, disk yes  
SwarmCache org.hibernate.cache.SwarmCacheProvider clustered (ip multicast) yes (clustered invalidation)  
JBoss TreeCache org.hibernate.cache.TreeCacheProvider clustered (ip multicast), transactional yes (replication) yes (clock sync req.)

19.2.1. Cache mappings

The <cache> element of a class or collection mapping has the following form:
<cache 
    usage="transactional|read-write|nonstrict-read-write|read-only"
    region="RegionName"
    include="all|non-lazy"
/>
usage (required) specifies the caching strategy: transactional, read-write, nonstrict-read-write or read-only
region (optional, defaults to the class or collection role name) specifies the name of the second level cache region
include (optional, defaults to all) non-lazy specifies that properties of the entity mapped with lazy="true" may not be cached when attribute-level lazy fetching is enabled
Alternatively (preferrably?), you may specify <class-cache> and <collection-cache> elements in hibernate.cfg.xml.
The usage attribute specifies a cache concurrency strategy.