Composite Primary key

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.

Ex. In amzon.com, has to display set of mobile from DB. for every request, if i am going to db, and fetch the record,then it create multiple hits and increase the data traffic.It will impact the application performance.Instead of this, we can store the request in Cache at initial hit and from there we can seve th user request.

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

First-level cache:

The first-level cache is the Session cache and is a mandatory cache through which all requests must pass. The Session object keeps an object under its own power before committing it to the database.For how many sessions we are creating, it is maintain a session area in memory called session cache.

Insert operation: when you are inserting the data, and updating the same data in DB. then, it will not directly insert the data in the DB,What it will do. Hibernate intelligently identifies that, same data is updating. ,and last update will be triggered to the Database.

Limitation of First level Cache:

Once you close the session, the cache will get destroyed.

For ex: when we use select * from customer, and close the session, then it will again hit the DB.So to overcome this problem, we use Second Level Cache.

By default first level is enabled, in our code, we as a java developer don't enable/disable it forcefully in our code.

Second Level Cache:

First level cache will be enabled by default, but for enable second level cache we need to follow some settings, let us see few points regarding this..

Second level cache was introduced in hibernate 3.0
When ever we are loading any object from the database, then hibernate verify whether that object is available in the local cache memory of that particular session [ means first level cache ], if not available then hibernate verify whether the object is available in global cache or factory cache [ second level cache ], if not available then hibernate will hit the database and loads the object from there, and then first stores in the local cache of the session [ first level ] then in the global cache [ second level cache ]
When another session need to load the same object from the database, then hibernate copies that object from global cache [ second level cache ] into the local cache of this new session

Second level cache in the hibernate is of from 4 vendors…

Easy Hibernate [EHCache] Cache from hibernate framework
Open Symphony [OS] cache from Open Symphony
Swarm Cache
Tree Cache from JBoss

We need one provider class, here we are going to see hibernate provider class that is EHCache

How to enable second level cache in hibernate

To enable second level cache in the hibernate, then the following 3 changes are required

1. Add provider class in hibernate configuration file (hibernate.cfg.xml )

<property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>

2. Configure cache element for a class in hibernate mapping file (hbm file )

3. Create ehcache.xml file

<?xml version="1.0"?>

<cache name="testCache1"

maxElementsInMemory="10000"

eternal="false"

timeToIdleSeconds="3600"

timeToLiveSeconds="10"

overflowToDisk="true"

diskPersistent="true"

diskExpiryThreadIntervalSeconds="120"

maxElementsOnDisk="10000"

</ehcache>

- Note: this must write soon after <class>

Important points on this second level cache

create xml file called ehcache.xml and store in at class path location [ no confusions, i mean in the place where you have mapping and configuration XML’s ] in web application.

Lets take an example, we have 2 pojo classes in our application like Student, Employee.

If we load student object from the database, then as its the first time hibernate will hits the database and fetch this student object data and stores in the session1 cache memory [ First level cache ], then in the global cache [ second level cache ] provided if we write <cache usage=”read-only” /> in the student mapping file
I mean hibernate will stores in the local session memory by default, but it only stores in the global cache [ second level cache ] only if we write <cache usage=”read-only” /> in the student mapping file, if not so hibernate wont stores in the global cache
Now take another session like session 2 for example, if session 2 also load the student object then hibernate will loads from the global cache [ second level cache ] as student object is available at global [Actually when ever we want to load any object hibernate first will checks at local, then global then database right hope you remembered this ], now if session 3 modify that student object then hibernate will thorows an error because we have written <cache usage=”read-only” /> in student mapping file
We can avoid this by writing <cache usage=”read-write” />
so remember <cache /> element has that much importance

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