Introduction to Inheritance Mapping
What is Inheritance Mapping?
Inheritance mapping is a fundamental concept in object-relational mapping (ORM) frameworks like Hibernate. It allows developers to model the inheritance relationships between classes in a database. Inheritance is a core principle of object-oriented programming, enabling a new class to inherit properties and methods from an existing class.
In Hibernate, there are several strategies to represent inheritance in the database, including:
- Single Table Inheritance
- Joined Table Inheritance
- Table Per Class Inheritance
Types of Inheritance Mapping Strategies
1. Single Table Inheritance
In this strategy, all classes in the inheritance hierarchy are mapped to a single database table. A discriminator column is used to determine which class a particular row belongs to.
Example:
Consider a base class Animal and derived classes Dog and Cat. The single table might look like:
| ID | TYPE | NAME |
+----+-----------+--------+
| 1 | Dog | Buddy |
| 2 | Cat | Whiskers |
+----+-----------+--------+
2. Joined Table Inheritance
This strategy involves creating separate tables for each class in the hierarchy, but each derived class table will have a foreign key reference to the base class table.
Example:
Using the same classes Animal, Dog, and Cat, the database schema might look like:
+----+--------+
| ID | NAME |
+----+--------+
| 1 | Buddy |
| 2 | Whiskers |
+----+--------+
Dog Table:
+----+-----------+
| ID | BARK_SOUND|
+----+-----------+
| 1 | Woof |
+----+-----------+
Cat Table:
+----+-----------+
| ID | MEOW_SOUND|
+----+-----------+
| 2 | Meow |
+----+-----------+
3. Table Per Class Inheritance
In this approach, each class has its own table, and there is no shared table for the base class. This leads to data duplication but provides a clean separation of concerns.
Example:
For the Animal, Dog, and Cat classes, the tables might look like:
+----+--------+
| ID | NAME |
+----+--------+
| 1 | Buddy |
+----+--------+
Dog Table:
+----+-----------+--------+
| ID | NAME | BARK_SOUND|
+----+-----------+--------+
| 1 | Buddy | Woof |
+----+-----------+--------+
Cat Table:
+----+-----------+-----------+
| ID | NAME | MEOW_SOUND|
+----+-----------+-----------+
| 2 | Whiskers | Meow |
+----+-----------+-----------+
Conclusion
Understanding inheritance mapping is crucial for effectively using Hibernate in your applications. Each strategy has its own advantages and disadvantages depending on the use case and requirements. Single Table Inheritance is efficient in terms of performance but can lead to a sparse table. Joined Table Inheritance provides better normalization but may require more joins in queries. Table Per Class gives clean separation but can lead to data duplication.
Choosing the right inheritance mapping strategy will depend on the specific needs of your application and the complexity of the class hierarchies you are dealing with.