Hibernate – Table Per Concrete Class Using Annotation

Hibernate is a framework that provides some abstraction layer, meaning that the programmer does not have to worry about the implementations, it does the implementations for you internally like writing queries to perform CRUD operations, establishing a connection with the database, etc. It is an open-source, non-invasive, lightweight java ORM(Object-relational mapping) framework that is used to develop persistence logic that is independent of Database software.

Hibernate is capable of storing the inherited properties of an object along with its new properties in its database when an object is saved in the database. In Hibernate, inheritance between POJO classes is applied when multiple POJO classes of a module contain some common properties.

Hibernate Inheritance Mapping

Object-oriented can model both “is a” and “has a” relationship. The relational model supports only the “has a” relationship between two entities. Hibernate helps in mapping such Objects with relational tables. There are three inheritance mapping strategies defined in the Hibernate.

• Table Per Hierarchy 
• Table Per Concrete class
• Table Per Subclass

Table per Concrete class (using Annotations)

Table per Concrete Class is one of the inheritance strategies in hibernate. If we want to keep each concrete class object of inheritance in separate tables of the database then we can proceed with the table per concrete class strategy.

In a Table per Concrete Class strategy:

• Hibernate stores each derived class object of hierarchy in a separate table of the database.
• Data that belongs to a parent class is scattered across a number of subclass tables, which represent concrete classes.
• The discriminator is not required, so we can avoid discriminator-related annotations.

In this strategy, each subclass table will have the subclass-specific attributes and the attributes inherited from the parent class.

Important Annotations

Annotation 1: @Inheritance 

This annotation defines the inheritance strategy to be used for an entity class hierarchy. It is specified on the entity class that is the root of the entity class hierarchy. If the @Inheritance annotation is not specified or if no inheritance is specified for an entity class hierarchy, the SINGLE_TABLE mapping strategy is used.

Syntax: @Inheritance

@Entity
@Table("name = students")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
// Class 
public class Student {

    // Insert code here
}

Annotation 2: @AttributeOverride

The @AttributeOverride annotation is used to override the mapping of a Basic (whether explicit or default) property or field or Id property or field. This annotation may be applied to an entity that extends a mapped superclass or to an embedded field or property to override a basic mapping defined by the mapped superclass or embeddable class. If the annotation is not specified, the column is mapped the same as in the original mapping.

Syntax: @AttributeOverride

@Entity
@Table(name="students")
@AttributeOverrides({

    @AttributeOverride(name="name", column=@Column(name="NAME")),

    @AttributeOverride(name="age", column=@Column(name="AGE"))

})
// Class 
public class Student {

    // Insert code here
}

Implementation of Table Per Concrete class (Using Annotations)

In this example, we will be creating three persistence classes with Employee as the parent class and P_Employee and C_Employee as the two subclasses.

Hierarchy of Classes

We have 3 tables Employee, P_Employee, and C_Employee. The mapping of the subclass repeats the properties of the parent class.

Creating  Database Table to persist Concrete classes:

CREATE TABLE `Employee` (
        `Id` BIGINT(20) NOT NULL AUTO_INCREMENT,
       `name` VARCHAR(50) NOT NULL DEFAULT '0',
       `age` BIGINT(3) NOT NULL DEFAULT '0',
        PRIMARY KEY (`id`)
)

CREATE TABLE `P_Employee` (
       `Id` BIGINT(20) NOT NULL AUTO_INCREMENT,
      `name` VARCHAR(50) NOT NULL DEFAULT '0',
      `age` BIGINT(3) NOT NULL DEFAULT '0',
     `salary` BIGINT(11) NULL DEFAULT NULL,
     PRIMARY KEY (`id`)
)

CREATE TABLE `C_Employee` (
      `Id` BIGINT(20) NOT NULL AUTO_INCREMENT,
      `name` VARCHAR(50) NOT NULL DEFAULT '0',
      `age` BIGINT(3) NOT NULL DEFAULT '0',
      `hourlyrate` BIGINT(11) NULL DEFAULT NULL,
     `duration` BIGINT(11) NULL DEFAULT NULL,
     PRIMARY KEY (`id`)
)

The project structure (IntelliJ IDEA)  is as follows: 

Project Structure

Creating the Employee, P_Employee, and C_Employee classes for the above hierarchy:

File: Employee.java

File: P_Employee.java

File: C_Employee.java

Creating a hibernate.cfg.xml configuration file and adding the entries of mapping resources:

Following are the dependencies used in the pom.xml file:

Creating the class that stores the persistent object

File: Main.java

Output:

Employee records successfully persisted.

The Main class is used to persist Employee, P_Employee, and C_Employee object instances. This is the usual way of mapping Table Per Concrete Class using Annotations.

kaalel

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