Understanding the Problem: Overriding EJB 3 Session Bean Methods with Generics

Java Enterprise Edition (Jakarta EE) provides a robust environment for building scalable applications. Within this framework, Enterprise Java Beans (EJB) are designed for building scalable and transactional enterprise applications. However, developers often encounter challenges when trying to incorporate generics with EJB 3 session beans, particularly when overriding methods.

In this blog post, we will address the question of how to override an EJB 3 session bean method with a generic argument. The issue arises when overridden methods don’t seem to execute as expected. By examining a practical example, we will clarify whether this problem stems from a limitation within the Java or EJB 3 framework, or if it’s a misunderstanding of generics and inheritance.

The Scenario: An Example with EJB 3

Let’s take a look at a coding example to illustrate the problem. Suppose we have the following EJB 3 interface and implementations:

public interface Repository<E> {
   public void delete(E entity);
}

public abstract class AbstractRepository<E> implements Repository<E> {
   public void delete(E entity){
      //...
   }
}

public interface FooRepository<Foo> {
   //other methods
}

@Local(FooRepository.class)
@Stateless
public class FooRepositoryImpl extends AbstractRepository<Foo> implements FooRepository {
   @Override
   public void delete(Foo entity){
      // do something before deleting the entity
      super.delete(entity);
   }
   //other methods
}

Accessing the Repository

We then have another bean that uses the FooRepository:

@EJB
private FooRepository fooRepository;

public void someMethod(Foo foo) {
    fooRepository.delete(foo);
}

The Problem

In this setup, you might expect the overridden delete method in FooRepositoryImpl to execute. However, only the implementation of delete defined in AbstractRepository executes. This raises the question—what went wrong?

The Solution: Ensuring Proper Override

As it turns out, the problem often lies in the way generics and inheritance are structured. Our implementation must be appropriately aligned for EJB to recognize the overridden methods.

Check Your Interfaces

Initially, the interfaces may have been declared incorrectly. It’s crucial that FooRepository extends the Repository<Foo>, like so:

public interface FooRepository extends Repository<Foo> {
   //other methods
}

This minor tweak clarifies that FooRepository is specifically tied to the Foo entity.

Main Method Implementation

To ensure the functionality, we can implement a main method as follows:

public static void main(String[] args) {
    FooRepository fooRepository = new FooRepositoryImpl();
    fooRepository.delete(new Foo("Bar"));
}

public class Foo {
    private String value;

    public Foo(String inValue) {
        super();
        value = inValue;
    }
    public String toString() {
        return value;
    }
}

public class AbstractRepository<E> implements Repository<E> {
    public void delete(E entity) {
        System.out.println("Delete-" + entity.toString());
    }
}

public class FooRepositoryImpl extends AbstractRepository<Foo> implements FooRepository {
    @Override
    public void delete(Foo entity) {
        System.out.println("something before");
        super.delete(entity);
    }
}

The Result

Running this main method will print:

something before
Delete-Bar

This shows that the overridden delete method in FooRepositoryImpl executes correctly, followed by the inherited implementation from AbstractRepository.

Conclusion

In summary, when working with EJB 3 session beans and generics, ensure that your interfaces and inheritance are correctly structured. By following these guidelines and the provided examples, developers can successfully override session bean methods while leveraging the power and flexibility of generics in Java.

If you encounter issues, always verify that the generic types are properly aligned and that you are indeed overriding the intended method.

By resolving generics and EJB issues effectively, developers can take full advantage of Java EE’s capabilities for building enterprise-level applications.