Hibernate Inheritance Table per Class Hierarchy

This tutorial explains Hibernate Inheritance Table per Class Hierarchy and an implementation of the same.

1. Introduction

Java is an Object Oriented Language and supports Inheritance which is one of the most visible paradigms of Object-Relational mismatch. Object Oriented Systems can model both “is a”and “has a” relationship whereas Relational Model supports only “has a” relationship between two entities. Hibernate can help us map such Objects with relational tables.

Basically there are three ways in which we can implement Inheritance in Hibernate. We shall use one of the ways here Table per Class Hierarchy.

2. Example

Consider the below Object Model where in we have Base Class Payment and its subclasses OnlineTransfer, ChequePayment, CashPayment which are basically specific forms of Payments.

Class Diagram

Class Diagram

3. Database Table Design

In Table per Class Hierarchy method, we save the entire Class hierarchy in a single Database table. A discriminator is a key to uniquely identify the Class type of the Class hierarchy. The Table Design would look like this for the above Class Diagram.

Table Design

Table Design

As we mentioned earlier only one Table i.e. PAYMENT is created having all the attributes of subclasses also.

4. Project Setup

Lets create a simple Maven project with the below pom.xml

In the above pom.xml we added dependencies for Hibernate 3.6.10, MySQL Connector 5.1.25,  Javassist and Sl4J.

5. Database Table Creation

6. Hibernate Entity Class & Hibernate Mapping File

Lets us create a Hibernate Entity  Class for Payment.java, CashPayment.java, ChequePayment.java, OnlineTransfer.java

and the Hibernate Mapping File Payment.hbm.xml as

Note that we have defined only one hibernate mapping (hbm) file Payment.hbm.xml. All the entities CashPayment,ChequePayment and Employee Entity classes are defined within the same mapping file.

<discriminator> tag is used to define the Discriminator column.

<subclass> tag is used to map the subclass of Payment. Note that we have not used the usual <class>tag to map the subclasses as it is in the hierarchy tree.

The discriminator-value for ChequePayment is defined as “CHEQUE” , CashPayment is defined “CASH” and for OnlineTransfer is “ONLINE”  and the default value is defined as “OTHERS“, when Hibernate persists the data it will accordingly populate this value based on the Sub Class Type.

7. Hibernate Configuration

Below is the Hibernate Configuration File hibernate.cfg.xml

7. Main Program

8. Output

Output

Output

From the above Console Output we could see that irrespective of whatever class we persist all the data is inserted into PAYMENT Table.

Conclusion

Conclusion

9. Project Structure

Project Structure

Project Structure

Download Hibernate Inheritance Table per Class Hierarchy Example

I hope this has been useful for you and I’d like to thank you for reading. If you like this article, please leave a helpful comment and share it with your friends.

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