What is Hibernate and how it works – a minimal introduction
During my post graduations we had to make a project based on the requirement, at that time I heard about a word hibernate. But at that time I was not so much curious about it. I only know that it is a kind of framework. My friends was used it in their project also but I was not aware about it so I use my own Java coding for all stuff.
But when I joined an organization I came to know that here also we are using hibernate but now the situation was very difficult because I did not know A, B, C… of it. But my senior level was very cooperative and they help me a lot to overcome from the situation. So now I want to share my knowledge with you people so that in future you will not face this problem. So let’s start with the basic question what, when, why and how?
What is Hibernate?
Hibernate is a free, open source Java package that makes it easy to work with relational databases. It provides a query service for Java. Hibernate lets you develop independent classes following common Java idiom – including association, inheritance, polymorphism, composition and the Java collections framework. Hibernate is basically used with J2EE (Java web application).
It is a very popular open source technology which fits well both with Java and .NET technologies. The Hibernate 3.0 core is 68,549 lines of Java code. Hibernate maps the Java classes to the database tables. It also provides the data query and retrieval facilities that significantly reduce the development time. Hibernate can be used in Java Swing applications, Java Servlet-based applications, or J2EE applications using EJB session beans. Hibernate is free software that is distributed under the GNU Lesser General Public License.
Source: wikipedia
When Hibernate?
Hibernate was started in 2001 by Gavin King as an alternative to using EJB2-style entity beans. Its mission was to simply offer better capabilities than EJB2(Enterprise Java beans) by simplifying the complexities and allowing for missing features. Early in 2003, the Hibernate development team began Hibernate2 releases which offered many significant improvements over the first release.
JBoss, Inc. (now part of Red Hat) later hired the lead Hibernate developers and worked with them in supporting Hibernate. On 9th Feb 2012 they have released a stable version of hibernate as Hibernate 4.1 final. It is developed in Java language so it uses JVM as a platform for the execution.
Why Hibernate?
It is the 
most important question that why we need hibernate into our application? So let’s discuss it in very simpler way. We know that when we develop any application, at that time for storing our data we required a database.
For example we are making a web base application for the Post-Office and here we have to keep track of all the information related to post, money order, courier etc. For the information storage point of view we use a MySQL as a backend database and we have code all things into a Java class like how to add data into database or how to perform operation on that data base. Now when we are going to deliver the product (i.e. our application) suddenly client requirement got changed and now they want ms access as their database because it is freely available and it can be easy to understand.
Can you imagine the situation now? You have a limited amount of time and you have to change your each and every class which is dealing with the database and the worst thing is that now you have to change all 50 files because it is a live application (this number can be 5000 also depend upon the scenario) . To handle this kind of situation hibernate came into picture. Hibernate provide an API (Application programming Interface) with all build in functions like insert (), delete (), update () etc. It also provides persistence architecture for a code (each class is independent). Here we follow a standard hibernate architecture in which we use xml for all the configuration settings.
How It Works?
Hibernate does not get in your way nor does it force you to change the way your objects behave. They do not need to implement any interfaces in order to become persist. All you need to do is create an XML "mapping document" telling Hibernate the classes you want to be able to store in a database, and how they relate to the tables and columns in that database, and then you can ask it to fetch data as objects, or store objects as data for you. At runtime, Hibernate reads the mapping document and dynamically builds Java classes to manage the translation between the database and Java program.
Now there is one question related to hibernate which was asked to me during my campus interview but at that time I was not able to answer it. Let’s find the answer
What is dirty checking?
Hibernate automatically detects object state changes in order to synchronize the updated state with the database, this is called dirty checking. An important note here is, Hibernate will compare objects by value, except for Collections, which are compared by identity. For this reason you should return exactly the same collection instance as Hibernate passed to the setter method to prevent unnecessary database updates.
Now I will discuss an example for a simple Java application, if you have basic knowledge in Java and JDBC (Java Database Connectivity), then you will understand it comfortably.
Suppose you have to make an application which will maintain records of the entire employee present in your office. So we will do it in the following way
Simple Design pattern to follow
Detailed step
Java Program will call the EmployeeDAO to perform any operation
Employee DAO will call HibernateUtil to get Hibernate Session.
Once Employee DAO is having valid Session it can call any Named Query based on the operation performed.
Description of each Class
EMPLOYEE DAO : is the class that will perform any operation on Table EMPLOYEE
HibernateUtil : This will provide the Hibernate Session to any DAO.
Hibernate.cfg.xml : File where we decide all the DB and Mapping file detail.
Employee.hbm.xml : File where we will declare all the named query specific to operation.
I hope the above example will help you to understand the basic flow of the project if you include hibernate into it. Hibernate is having a large set of methods and interface with it but here we have only discussed the basic need and flow of any hibernate application and still there are so many things which are untouched here.
