Hibernate Caching

        Hibernate has two kinds of cache, which are the first-level cache and the second-level cache. We also call the first-leve cache session cache, and the second-level cache process-level cache. The first-level cache is mandatory and can’t be turned off; However, the second-level cache in Hibernate is optional. Because the process-level cache caches objects across sessions and has process or cluster scope, in some situation, turning on the process-level cache can improve the performance of the application. In fact, before accessing the database to load an object, Hibernate will first look in the Session cache and then in the process-level cache.

Query cache

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.

Enabling the query cache

1.      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.

2.     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. 3. 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.

Query cache regions

For fine-grained control over query cache expiration policies, specify a named cache region for a particular query by calling Query.setCacheRegion().

Session session = SessionFactory.openSession();

Query query = session.createQuery("FROM EMPLOYEE");

query.setCacheable(true);

query.setCacheRegion("employee");

List users = query.list();

SessionFactory.closeSession();

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().

Second-level cache(process scoped cache)

A hibernate SessionFactory aims to create sessions, initializes JDBC connections and pool them. The second level cache lives in the SessionFactory level so all session can share it. This is why it’s called a process scoped cache. The second level cache is not enabled by default. One has to configure the cache strategy for hibernate entities and the cache provider.

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.

 Possible values for Shared Cache Mode

ENABLE_SELECTIVE: Entities are not cached unless you explicitly mark them as cacheable. 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.

 Caching strategies

read-only : 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.

nonstrict read-write :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().

read-write :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.

transactional :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.

Second-level cache providers for Hibernate Cache Interface Supported strategies

Strategy/Provider	Read-only	Nonstrictread-write	Read-write	Transactional
EHCache	X	X	X
OSCache	X	X	X
SwarmCache	X	X
JBoss Cache	X			X
HashTable (testing only)	X	X	X

Configuring cache providers

You will specify a cache provider in hibernate.cfg.xml configuration file. We choose EHCache as our second-level cache provider:

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE hibernate-configuration SYSTEM

"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

   <session-factory>

   <property name="hibernate.dialect">

      org.hibernate.dialect.MySQLDialect

   </property>

   <property name="hibernate.connection.driver_class">

      com.mysql.jdbc.Driver

   </property>

   <!-- Assume students is the database name -->

   <property name="hibernate.connection.url">

      jdbc:mysql://localhost/test

   </property>

   <property name="hibernate.connection.username">

      root

   </property>

   <property name="hibernate.connection.password">

      root123

   </property>

   <property name="hibernate.cache.provider_class">

      org.hibernate.cache.EhCacheProvider

   </property>

   <!-- List of XML mapping files -->

   <mapping resource="Employee.hbm.xml"/>

</session-factory>

</hibernate-configuration>

Now, you need to specify the properties of the cache regions. EHCache has its own configuration file,ehcache.xml, which should be in the CLASSPATH of the application. A cache configuration in ehcache.xml for the Employee class may look like this:

<diskStore path="java.io.tmpdir"/>

<defaultCache

maxElementsInMemory="1000"

eternal="false"

timeToIdleSeconds="120"

timeToLiveSeconds="120"

overflowToDisk="true"

/>

<cache name="Employee"

maxElementsInMemory="500"

eternal="true"

timeToIdleSeconds="0"

timeToLiveSeconds="0"

overflowToDisk="false"

/>

Using annotations

@Entity 

@Cacheable

@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)

public class Forest { ... }

Using <cache> element in hbm.xml file

<cache

    usage="transactional"

    region="RegionName"

    include="all"

/>

Usage:The caching strategy. This attribute is required, and can be any of the following values.

o    transactional

o    read-write

o    nonstrict-read-write

o    read-only

Region: The name of the second-level cache region. This optional attribute defaults to the class or collection role name.

Include: 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 inhibernate.cfg.xml.

 

Managing the caches

Whenever you pass an object to save(), update() or saveOrUpdate(), and whenever you retrieve an object using load(), get(), list(), iterate() or scroll(), that object is added to the internal cache of the Session.

When flush() is subsequently called, the state of that object will be synchronized with the database. If you do not want this synchronization to occur, or if you are processing a huge number of objects and need to manage memory efficiently, the evict() method can be used to remove the object and its collections 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);

}

The Session also provides a contains() method to determine if an instance belongs to the session cache.

To evict all objects from the session cache, call Session.clear()

For the second-level cache, there are methods defined on SessionFactory for evicting the cached state of an instance, entire class, collection instance or entire collection role.

sessionFactory.evict(Cat.class, catId); //evict a particular Cat

sessionFactory.evict(Cat.class);  //evict all Cats

sessionFactory.evictCollection("Cat.kittens", catId); //evict a particular collection of kittens

sessionFactory.evictCollection("Cat.kittens"); //evict all kitten collections

The CacheMode controls how a particular session interacts with the second-level cache:

o    CacheMode.NORMAL: will read items from and write items to the second-level cache

o    CacheMode.GET: will read items from the second-level cache. Do not write to the second-level cache except when updating data

o    CacheMode.PUT: will write items to the second-level cache. Do not read from the second-level cache

o    CacheMode.REFRESH: will write items to the second-level cache. Do not read from the second-level cache. Bypass the effect of hibernate.cache.use_minimal_puts forcing a refresh of the second-level cache for all items read from the database

To browse the contents of a second-level or query cache region, use the Statistics API:

Map cacheEntries = sessionFactory.getStatistics()

        .getSecondLevelCacheStatistics(regionName)

        .getEntries();

You will need to enable statistics and, optionally, force Hibernate to keep the cache entries in a more readable format:

hibernate.generate_statistics true

hibernate.cache.use_structured_entries true

Session Cache

Fist level cache in hibernate is enabled by default and you do not need to do anything to get this functionality working. In fact, you can not disable it even forcefully.  it is associated with Session object. As we know session object is created on demand from session factory and it is lost, once the session is closed. Similarly, first level cache associated with session object is available only till session object is live. It is available to session object only and is not accessible to any other session object in any other part of application.

Important facts

1.       First level cache is associated with “session” object and other session objects in application can not see it.

2.       The scope of cache objects is of session. Once session is closed, cached objects are gone forever.

3.       First level cache is enabled by default and you can not disable it.

4.       When we query an entity first time, it is retrieved from database and stored in first level cache associated with hibernate session.

5.       If we query same object again with same session object, it will be loaded from cache and no sql query will be executed.

6.       The loaded entity can be removed from session using evict() method. The next loading of this entity will again make a database call if it has been removed using evict() method.

7.       The whole session cache can be removed using clear() method. It will remove all the entities stored in cache. evict() is used to remove a particular object from cache associated with session, and clear() method is used to remove all cached objects associated with session.