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If you have queries that run over and over, with the same parameters, query caching provides performance gains.
Caching introduces overhead in the area of transactional processing. For example, if you cache results of a query against an object, Hibernate needs to keep track of whether any changes have been committed against the object, and invalidate the cache accordingly. In addition, the benefit from caching query results is limited, and highly dependent on the usage patterns of your application. For these reasons, Hibernate disables the query cache by default.
Procedure 6.1. Enabling the query cache
Set the hibernate.cache.use_query_cache property to true.
This setting creates two new cache regions:
org.hibernate.cache.internal.StandardQueryCache holds the cached query results.
org.hibernate.cache.spi.UpdateTimestampsCache holds timestamps of the most recent updates to
queryable tables. These timestamps validate results served from the query cache.
Adjust the cache timeout of the underlying cache region
If you configure your underlying cache implementation to use expiry or timeouts, set the cache timeout of the
underlying cache region for the UpdateTimestampsCache to a higher value than the timeouts of any
of the query caches. It is possible, and recommended, to set the UpdateTimestampsCache region never to
expire. To be specific, a LRU (Least Recently Used) cache expiry policy is never appropriate.
Enable results caching for specific queries
Since most queries do not benefit from caching of their results, you need to enable caching for individual
queries, e ven after enabling query caching overall. To enable results caching for a particular query, call
org.hibernate.Query.setCacheable(true). This call allows the query to look for
existing cache results or add its results to the cache when it is executed.
The query cache does not cache the state of the actual entities in the cache. It caches identifier values and results of value type. Therefore, always use the query cache in conjunction with the second-level cache for those entities which should be cached as part of a query result cache.
For fine-grained control over query cache expiration policies, specify a named cache region for a particular
query by calling Query.setCacheRegion().
Example 6.1. Method setCacheRegion
List blogs = sess.createQuery("from Blog blog where blog.blogger = :blogger")
.setEntity("blogger", blogger)
.setMaxResults(15)
.setCacheable(true)
.setCacheRegion("frontpages")
.list();
To force the query cache to refresh one of its regions and disregard any cached results in the region, call
org.hibernate.Query.setCacheMode(CacheMode.REFRESH). In conjunction with the region defined for the
given query, Hibernate selectively refreshes the results cached in that particular region. This is much more
efficient than bulk eviction of the region via org.hibernate.SessionFactory.evictQueries().
Hibernate is compatible with several second-level cache providers. None of the providers support all of Hibernate's possible caching strategies. Section 6.2.3, “Second-level cache providers for Hibernate” lists the providers, along with their interfaces and supported caching strategies. For definitions of caching strategies, see Section 6.2.2, “Caching strategies”.
You can configure your cache providers using either annotations or mapping files.
Entities.
By default, entities are not part of the second-level cache, and their use is not recommended. If you
absolutely must use entities, set the shared-cache-mode element in
persistence.xml, or use property javax.persistence.sharedCache.mode
in your configuration. Use one of the values in Table 6.1, “Possible values for Shared Cache Mode”.
Table 6.1. Possible values for Shared Cache Mode
| Value | Description |
|---|---|
| ENABLE_SELECTIVE |
Entities are not cached unless you explicitly mark them as cachable. This is the default and recommended value. |
| DISABLE_SELECTIVE |
Entities are cached unless you explicitly mark them as not cacheable. |
| ALL |
All entities are always cached even if you mark them as not cacheable. |
| NONE |
No entities are cached even if you mark them as cacheable. This option basically disables second-level caching. |
Set the global default cache concurrency strategy The cache concurrency strategy with the hibernate.cache.default_cache_concurrency_strategy configuration property. See Section 6.2.2, “Caching strategies” for possible values.
When possible, define the cache concurrency strategy per entity rather than globally. Use the
@org.hibernate.annotations.Cache annotation.
Example 6.2. Configuring cache providers using annotations
@Entity
@Cacheable
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class Forest { ... }
You can cache the content of a collection or the identifiers, if the collection contains other entities. Use
the @Cache annotation on the Collection property.
@Cache can take several attributes.
Attributes of @Cache annotation
The given cache concurrency strategy, which may be:
NONE
READ_ONLY
NONSTRICT_READ_WRITE
READ_WRITE
TRANSACTIONAL
The cache region. This attribute is optional, and defaults to the fully-qualified class name of the class, or the qually-qualified role name of the collection.
Whether or not to include all properties.. Optional, and can take one of two possible values.
A value of all includes all properties. This is the default.
A value of non-lazy only includes non-lazy properties.
Example 6.3. Configuring cache providers using mapping files
<cache
usage="transactional"
region="RegionName"
include="all"
/>
Just as in the Example 6.2, “Configuring cache providers using annotations”, you can provide attributes in the mapping file. There are some specific differences in the syntax for the attributes in a mapping file.
The caching strategy. This attribute is required, and can be any of the following values.
transactional
read-write
nonstrict-read-write
read-only
The name of the second-level cache region. This optional attribute defaults to the class or collection role name.
Whether properties of the entity mapped with lazy=true can be cached when
attribute-level lazy fetching is enabled. Defaults to all and can also be
non-lazy.
Instead of <cache>, you can use <class-cache> and
<collection-cache> elements in hibernate.cfg.xml.
A read-only cache is good for data that needs to be read often but not modified. It is simple, performs well, and is safe to use in a clustered environment.
Some applications only rarely need to modify data. This is the case if two transactions are unlikely to
try to update the same item simultaneously. In this case, you do not need strict transaction isolation,
and a nonstrict-read-write cache might be appropriate. If the cache is used in a JTA environment, you must
specify hibernate.transaction.manager_lookup_class. In other environments, ensore
that the transaction is complete before you call Session.close() or
Session.disconnect().
A read-write cache is appropriate for an application which needs to update data regularly. Do not use a
read-write strategy if you need serializable transaction isolation. In a JTA environment, specify a
strategy for obtaining the JTA TransactionManager by setting the property
hibernate.transaction.manager_lookup_class. In non-JTA environments, be sure the
transaction is complete before you call Session.close() or
Session.disconnect().
To use the read-write strategy in a clustered environment, the underlying cache implementation must support locking. The build-in cache providers do not support locking.
The transactional cache strategy provides support for transactional cache providers such as JBoss
TreeCache. You can only use such a cache in a JTA environment, and you must first specify
hibernate.transaction.manager_lookup_class.
Actions that add an item to internal cache of the Session
save()
update()
saveOrUpdate()
load()
get()
list()
iterate()
scroll()
Syncing or removing a cached item.
The state of an object is synchronized with the database when you call method
flush(). To avoid this synchronization, you can remove the object and all collections
from the first-level cache with the evict() method. To remove all items from the
Session cache, use method Session.clear().
Example 6.4. Evicting an item from the first-level cache
ScrollableResult cats = sess.createQuery("from Cat as cat").scroll(); //a huge result set
while ( cats.next() ) {
Cat cat = (Cat) cats.get(0);
doSomethingWithACat(cat);
sess.evict(cat);
}
Determining whether an item belongs to the Session cache.
The Session provides a contains() method to determine if an instance belongs to the
session cache.
Example 6.5. Second-level cache eviction
You can evict the cached state of an instance, entire class, collection instance or entire collection role,
using methods of SessionFactory.
sessionFactory.getCache().containsEntity(Cat.class, catId); // is this particular Cat currently in the cache
sessionFactory.getCache().evictEntity(Cat.class, catId); // evict a particular Cat
sessionFactory.getCache().evictEntityRegion(Cat.class); // evict all Cats
sessionFactory.getCache().evictEntityRegions(); // evict all entity data
sessionFactory.getCache().containsCollection("Cat.kittens", catId); // is this particular collection currently in the cache
sessionFactory.getCache().evictCollection("Cat.kittens", catId); // evict a particular collection of kittens
sessionFactory.getCache().evictCollectionRegion("Cat.kittens"); // evict all kitten collections
sessionFactory.getCache().evictCollectionRegions(); // evict all collection data
The CacheMode controls how a particular session interacts with the second-level cache.
| CacheMode.NORMAL | reads items from and writes them to the second-level cache. |
| CacheMode.GET | reads items from the second-level cache, but does not write to the second-level cache except to update data. |
| CacheMode.PUT | writes items to the second-level cache. It does not read from the second-level cache. It bypasses the effect of hibernate.cache.use_minimal_puts and forces a refresh of the second-level cache for all items read from the database. |
After enabling statistics, you can browse the contents of a second-level cache or query cache region.
Procedure 6.2. Enabling Statistics
Set hibernate.generate_statistics to true.
Optionally, set hibernate.cache.use_structured_entries to true, to cause
Hibernate to store the cache entries in a human-readable format.
Example 6.6. Browsing the second-level cache entries via the Statistics API
Map cacheEntries = sessionFactory.getStatistics()
.getSecondLevelCacheStatistics(regionName)
.getEntries();