What is ORM?

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 −

Advantages

• Let’s business code access objects rather than DB tables.
• Hides details of SQL queries from OO logic.
• Based on JDBC ‘under the hood.’
• No need to deal with the database implementation.
• Entities based on business concepts rather than database structure.
• Transaction management and automatic key generation.
• Fast development of application.

Solutions

• An API to perform basic CRUD operations on objects of persistent classes.
• A language or API to specify queries that refer to classes and properties of classes.
• A configurable facility for specifying mapping metadata.
• A technique to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions.

Java ORM Frameworks

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.

Why Object Relational Mapping (ORM)?

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.

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