Advanced Inheritance Mapping in Hibernate
Introduction to Inheritance Mapping
Inheritance mapping in Hibernate is a way to implement the Object-Oriented Programming principle of inheritance in a relational database. It allows you to define a base class and its subclasses, mapping them to a single table or multiple tables in the database.
In this tutorial, we will explore advanced techniques for inheritance mapping, focusing on various strategies such as Single Table, Joined Table, and Table Per Class.
Single Table Inheritance
Single Table Inheritance is a strategy where all classes in the inheritance hierarchy are mapped to a single table. A discriminator column is used to differentiate the types of entities.
Example
Consider an example of a Vehicle hierarchy:
@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "vehicle_type")
public abstract class Vehicle {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String make;
private String model;
}
@Entity
@DiscriminatorValue("Car")
public class Car extends Vehicle {
private int numberOfDoors;
}
@Entity
@DiscriminatorValue("Truck")
public class Truck extends Vehicle {
private double payloadCapacity;
}
This example shows a single table for Vehicle, Car, and Truck with a discriminator column called vehicle_type.
Joined Table Inheritance
Joined Table Inheritance creates a separate table for each class in the hierarchy. The base class table contains the common attributes, while the subclass tables contain specific attributes.
Example
Using the same Vehicle hierarchy, the mapping would look like this:
@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Vehicle {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String make;
private String model;
}
@Entity
public class Car extends Vehicle {
private int numberOfDoors;
}
@Entity
public class Truck extends Vehicle {
private double payloadCapacity;
}
In this case, the Vehicle table will contain common attributes, and the Car and Truck tables will contain their specific attributes.
Table Per Class Inheritance
Table Per Class Inheritance creates a separate table for each class in the hierarchy, including the base class. This means that each table contains all of its own attributes, including those inherited from the parent class.
Example
The Vehicle hierarchy using Table Per Class would look like this:
@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Vehicle {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String make;
private String model;
}
@Entity
public class Car extends Vehicle {
private int numberOfDoors;
}
@Entity
public class Truck extends Vehicle {
private double payloadCapacity;
}
Each class has its own table with all attributes, which can lead to data redundancy but offers better performance for certain queries.
Conclusion
In this tutorial, we explored advanced inheritance mapping techniques in Hibernate. Each strategy has its own advantages and trade-offs, and the choice of strategy depends on the specific requirements of your application.
Understanding these strategies allows you to design your database schema more effectively and leverage the power of Hibernate for managing inheritance hierarchies.