In earlier we said that hibernate much more than an ORM tool i.e. Hibernate provide the lots of other features. In which Cache is very important feature one of them. Hibernate is actually a very powerful, consistent, and reliable database mapping tool. Mapping between objects in Java to relational databases has many facets that you must be aware of. Hibernate does a particularly good job of making the process simple to start, and providing the facilities to allow it to scale well and meet exceedingly complex mapping demands.

Caching in Hibernate 

Caching is all about application performance optimization and it sits between your application and the database to avoid the number of database hits as many as possible to give a better performance for performance critical applications.

Caching is important to Hibernate as well which utilizes a multilevel caching schemes as explained below:

Caching in Hibernate-First Level and Second Level Cache in Hibernate
One of the primary concerns of mappings between a database and our Java application is performance.  This is the common concern of the all guys who working with hibernate and spent the more time in ORM tools for performance-enhancing changes to particular queries and retrievals. Today I want to discuss about some facets of the Hibernate infrastructure that are implemented to handle certain performance concerns –
  1. The first-level cache – Session (Earlier hibernate already provide this level of cache)
  2. The second-level cache –Session-factory-level cache
  3. and the query cache.

The first-level cache:

The first level cache type is the session cache. The session cache caches object within the current session but this is not enough for long level i.e. session factory scope.

The second-level cache:

The second-level cache is called ‘second-level’ because there is already a cache operating for you in Hibernate for the duration you have a session open. A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a SessionFactory-level cache on a class-by-class and collection-by-collection basis.
                 second-level cache
— Across sessions in an Application
— Across applications (different applications on same servers with same database)
— Across clusters (different applications on different servers with same database)
NOTE: Be careful Caches are never aware of changes made to the persistent store by another application i.e. suppose one application deploy one server with using hibernate and get the data from database and put to the cache for further using purpose but another application deployed another server which does not using any ORM tool so it does mot know about Cache and direct interacting with database and may be update data of database. Now data in Cache is invalid.

 

Hibernate uses first-level cache by default and you have nothing to do to use first-level cache. Let’s go straight to the optional second-level cache. Not all classes benefit from caching, so it’s important to be able to disable the second-level cache.

The ‘second-level’ cache exists as long as the session factory is alive. The second-level cache holds on to the ‘data’ for all properties and associations (and collections if requested) for individual entities that are marked to be cached.

In hibernate configuration file (hibernate.cfg.xml)  we wrote the following line.
For Disabling the second level of cache we have to made following change to hibernate configuration file.
<!-- Disable the second-level cache -->  
<property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

 

For Enabling the second level of cache we have to made following change to hibernate configuration file.
<!-- Enable the second-level cache -->  
<property name="cache.use_second_level_cache">true</property>
   <property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>
1. Cache Strategy using with Annotation as some changes made to the Model class also.
@Cacheable
@Cache(usage = CacheConcurrencyStrategy.READ_ONLY)
@Entity
@Table(name="STUDENT")
public class Student implements Serializable 
{
  ....
2. Cache Strategy using with Mapping file(.hbm.xml) .
<hibernate-mapping>
   <class name="Student" table="STUDENT">
      
         This class contains the student detail. 
      
      <cache usage="read-only">
      <id column="ID" name="id" type="java.lang.Long">
         <generator class="native">
      </generator></id>
      <property column="STUDENT_NAME" name="studentName" type="java.lang.String">
      <property column="lCOURSE" name="course" type="java.lang.String">
      <property column="ROLL_NUMBER" name="rollNumber" type="java.lang.Long">
   </property></property></property></cache></class>
</hibernate-mapping>

Concurrency strategies:

A concurrency strategy is a mediator which responsible for storing items of data in the cache and retrieving them from the cache. If you are going to enable a second-level cache, you will have to decide, for each persistent class and collection, which cache concurrency strategy to use.
  1. Transactional: Use this strategy for read-mostly data where it is critical to prevent stale data in concurrent transactions,in the rare case of an update.
  2. Read-write: Again use this strategy for read-mostly data where it is critical to prevent stale data in concurrent transactions,in the rare case of an update.
  3. Nonstrict-read-write: This strategy makes no guarantee of consistency between the cache and the database. Use this strategy if data hardly ever changes and a small likelihood of stale data is not of critical concern.
  4. Read-only: A concurrency strategy suitable for data which never changes. Use it for reference data only.
You have to download one more jar (ehcache-core-2.5.2) and configure with your application as given in below..
First Level and Second Level Cache in Hibernate
Now look the following example which describe the Second Level Of Cache i.e. Across the sessions.

HibernateTestDemo.java

package com.sdnext.hibernate.tutorial;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;

import com.sdnext.hibernate.tutorial.dto.Student;


public class HibernateTestDemo {

 /**
  * @param args
  */
 public static void main(String[] args) 
 {
  SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
  Session session = sessionFactory.openSession();
  session.beginTransaction();
  
  Student student = (Student) session.get(Student.class, 1);
  
  session.getTransaction().commit();
  session.close();
  
  Session session2 = sessionFactory.openSession();
  session2.beginTransaction();

  Student student2 = (Student) session2.get(Student.class, 1);

  session2.getTransaction().commit();
  session2.close();
 }
}
Output:

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).

log4j:WARN Please initialize the log4j system properly.

Hibernate: select student0_.ID as ID0_0_, student0_.COURSE as COURSE0_0_, student0_.ROLL_NUMBER as ROLL3_0_0_, student0_.STUDENT_NAME as STUDENT4_0_0_ from STUDENT student0_ where student0_.ID=?

Second Level Cache in Hibernate

 

As we seen the output there is only one query execute but we called get method for same model object two times i.e. when retrieve the model object first time it cache to the second level cache when we call second get of second session object for the same model object it does not execute select query again because hibernate return this model object from the session.

In the Next Chapter we will discuss more about the Query Caching in Hibernate.

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