Hibernate - ORM Overview

JDBC stands for Java Database Connectivity. It provides a set of Java API for accessing the relational databases from Java program. These Java APIs enables Java programs to execute SQL statements and interact with any SQL compliant database.
JDBC provides a flexible architecture to write a database independent application that can run on different platforms and interact with different DBMS without any modification.
Clean and simple SQL processing
Good performance with large data
Very good for small applications
Simple syntax so easy to learn
Complex if it is used in large projects
Large programming overhead
No encapsulation
Hard to implement MVC concept
Query is DBMS specific
When we work with an object-oriented system, there is a mismatch between the object model and the relational database. RDBMSs represent data in a tabular format whereas object-oriented languages, such as Java or C# represent it as an interconnected graph of objects.
Consider the following Java Class with proper constructors and associated public function −
public class Employee {
   private int id;
   private String first_name; 
   private String last_name;   
   private int salary;  

   public Employee() {}
   public Employee(String fname, String lname, int salary) {
      this.first_name = fname;
      this.last_name = lname;
      this.salary = salary;
   }
   
   public int getId() {
      return id;
   }
   
   public String getFirstName() {
      return first_name;
   }
   
   public String getLastName() {
      return last_name;
   }
   
   public int getSalary() {
      return salary;
   }
}
Consider the above objects are to be stored and retrieved into the following RDBMS table −
create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);
First problem, what if we need to modify the design of our database after having developed a few pages or our application? Second, loading and storing objects in a relational database exposes us to the following five mismatch problems −
Granularity
Sometimes you will have an object model, which has more classes than the number of corresponding tables in the database.
Inheritance
RDBMSs do not define anything similar to Inheritance, which is a natural paradigm in object-oriented programming languages.
Identity
An RDBMS defines exactly one notion of 'sameness': the primary key. Java, however, defines both object identity (a==b) and object equality (a.equals(b)).
Associations
Object-oriented languages represent associations using object references whereas an RDBMS represents an association as a foreign key column.
Navigation
The ways you access objects in Java and in RDBMS are fundamentally different.
The Object-Relational Mapping (ORM) is the solution to handle all the above impedance mismatches.
ORM stands for Object-Relational Mapping (ORM) is a programming technique for converting data between relational databases and object oriented programming languages such as Java, C#, etc.
An ORM system has the following advantages over plain JDBC −
An ORM solution consists of the following four entities −
There are several persistent frameworks and ORM options in Java. A persistent framework is an ORM service that stores and retrieves objects into a relational database.
Enterprise JavaBeans Entity Beans
Java Data Objects
Castor
TopLink
Spring DAO
Hibernate
And many more

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