Cache definition-

Cache is a temporary data which is used to speed up the performance.

Types of caching in hibernate-

1-Session Cache/First level cache in hibernate-

• First level cache is enabled by default.
• First level cache can’t be disabled.
• Objects are retrieved from current session.
  Example-
  i.e. for session object (s1) and Employee class.
  Session s1=sessionfactory.openSession();
  
  Employee emp1=(Employee)s1.load(Employee.class,1);  // query will execute and  data will be saved into first level cache
  
  Employee emp2=(Employee)session1.load(Employee.class,1); // query will not execute, and data will be fetched from first level cache

2- SessionFactory Cache/Second level cache in hibernate-

• Second level cache is disabled by default.
• Second level cache is configured to use.
• Objects can be retrieved from various sessions.
  Example-
  Session s1=sessionfactory.openSession();
  Employee emp1=(Employee)s1.load(Employee.class,1); //query will execute and data will be saved into first level(by default) and into Second level cache as well(only if configured)
  
  Session s2=sessionfactory.openSession();
  Employee emp2=(Employee)s2.load(Employee.class,1); //query will not execute and data will be fetched from second level cache.

  Note- Data can’t be fetched from first level cache here because different session has been created, and first level cache only works with same session.

• Steps to enable Second level cache-(we are choosing “ehCache” vendor, because it is widely used)
  
  Step1- Add cache usage in hbm file, for which you want to use second level cache
  i.e.<hibernate-mapping>
  <class name=”ClassName” table=”tableName”>
  <cache usage=”read-only” />
  
  i.e.- write-only, read-write only, restricted read-write only, read-only is widely used and famous.
  Step2- Add these 3 lines in cfg file,
  i.e.
  <property name=”cache.provider_class”> org.hibernate.cache.EhCacheProvider
  </property>

  <property name=”hibernate.cache.use_second_level_cache”>true</property>

  <property name=”hibernate.cache.region.factory_class”> org.hibernate.cache.ehcache.EhCacheRegionFactory
  </property>

  step3- Add 2 jars for hibernate second level cache. 
  hibernate-ehCache-x.x.x.jar, similar to hibernate-core version.
  ehCache-x.x.jar, latest version.

Extra-Query Level cache in hibernate-

• Query Cache is used to cache the results of a query.
  i.e.
  Session session3=factory.openSession();
  Query query=session3.createQuery(“from Employee e where e.id=1”); //query will execute
  query.setCacheable(true);
  Employee em=(Employee)query.getSingleResult();
  
  Query query1=session3.createQuery(“from Employee e where e.id=1”); //will come from query cache, query will not run
  query1.setCacheable(true);
  Employee em1=(Employee)query1.getSingleResult();

• Stepes to enable query level cache-
  step 1- Only add this line to cfg file.
  i.e.  <property name=”hibernate.cache.use_query_cache”>true</property>
  Done!!