How to Avoid Hibernate from Creating Objects from an Abstract Class with the findAll() Method

Learn how to resolve Hibernate errors related to fetching abstract classes by leveraging the power of `-DiscriminatorColumn` and `-DiscriminatorValue`.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to avoid hibernate from creating object of abstract class when using findAll() method?

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Avoid Hibernate from Creating Objects from an Abstract Class with the findAll() Method

When working with Hibernate in Java, one common issue developers face is trying to fetch data from an abstract class, which can lead to errors. If you have an abstract class defined—like Employee, which is extended by concrete classes—and you attempt to use findAll() to retrieve a list of all employees, you'll likely run into problems. The error arises because Hibernate tries to instantiate an object from the abstract class, which it cannot do. Let’s dive into the solution for this issue and clarify how you can structure your Hibernate entities effectively.

Understanding the Problem

The Abstract Class Issue

In object-oriented programming, an abstract class is designed to serve as a base class for other classes. It cannot be instantiated directly, which means you can't create objects from it. In your case of the Employee class, you have three concrete classes like OfficeEmployee that extend Employee. Your application might look something like this:

[[See Video to Reveal this Text or Code Snippet]]

Why the Error Occurs

When you call the findAll() method to retrieve all employee records, Hibernate tries to create instances of the Employee class, but since it is abstract, it throws an error. To fetch all records successfully, you need to find a way for Hibernate to recognize which concrete class to instantiate.

Solution: Using Discriminator Columns

The simplest and most effective way to circumvent this issue is by employing the -DiscriminatorColumn annotation in your abstract class and -DiscriminatorValue in your concrete subclasses.

Step-by-Step Solution

1. Modify the Abstract Class

You need to add a -DiscriminatorColumn annotation within your Employee class. Here's how you can adjust your existing class:

[[See Video to Reveal this Text or Code Snippet]]

2. Adjust the Concrete Class

Next, add the -DiscriminatorValue annotation to your concrete class, like OfficeEmployee. This will help identify which type of employee is being instantiated.

[[See Video to Reveal this Text or Code Snippet]]

Effect of These Changes

By implementing the -DiscriminatorColumn, a new column named employee_type is added in the database which will store the type of each employee. This allows Hibernate to correctly instantiate objects based on the type when fetching data.

Final Notes

With these modifications, you should no longer encounter errors when fetching all employees. Hibernate will internally manage which concrete class to instantiate based on the value stored in the employee_type column. This approach enriches your database structure and ensures that your application can handle polymorphism effectively.

Using Hibernate effectively requires an understanding of how to structure your classes, particularly when dealing with abstract classes. Implementing discriminator columns is a powerful method to handle such challenges. Happy coding!