Spring - When to Use Factory Design Pattern Instead of Dependency Injection

Dependency Injection (DI) and the Factory Design Pattern are both used to create objects in an application, but they serve different purposes and scenarios. Understanding when to use one over the other is crucial for building maintainable and scalable software. Here's a comparison of the two and some guidelines on when to use which:

Dependency Injection (DI):

Purpose: DI is all about inverting the control of creating and managing objects. Instead of the object managing its own dependencies, an external entity (the container, in the case of Spring) provides those dependencies.
Use Cases:
- When you need inversion of control where an external system (e.g., the Spring container) automatically wires up dependencies.
- For loose coupling between components. It becomes easier to switch out implementations or mock objects for testing.
- When the lifecycle of the dependencies needs to be managed by a container, e.g., Singletons, Prototype beans in Spring.
- When you need aspect-oriented features like transactions, logging, security, etc. DI frameworks like Spring make it easy to wrap or proxy objects to add additional behavior.

Factory Design Pattern:

Purpose: The Factory pattern is used to create objects based on certain criteria, without specifying the exact class of the object that will be created. It abstracts the instantiation process.
Use Cases:
- When the creation logic of an object is complex or requires some conditional operations.
- When the system needs to be extensible and new types or classes can be added later without changing the existing code (Open/Closed Principle).
- When you want to hide the logic of creating objects from the client and just provide a common interface to create a new object.
- When the exact type of the object might be determined at runtime.

When to use Factory over DI:

Dynamic Creation: If the type of object you need to create can change dynamically based on runtime parameters or conditions, a Factory might be more suitable.
Complex Creation Logic: If there's a lot of logic involved in creating an instance of an object, encapsulating that logic inside a Factory makes sense.
Outside of DI Container's Control: Sometimes, you might be working in a context where the DI container doesn't have control, and you need to produce objects. In this scenario, a Factory could be more appropriate.
Local Object Management: If the lifecycle and management of the object are local to a specific part of your application and you don't want it to be globally managed by a DI container, a Factory might be a better choice.
Integration with Legacy Code: If you're integrating with legacy systems that rely on the Factory pattern, you might want to continue using a Factory for consistency.

Conclusion:

While DI and the Factory pattern might seem to overlap, they serve distinct purposes. DI is great for automating dependency management and promoting decoupled designs, while the Factory pattern is more about abstracting and encapsulating object creation based on specific logic or criteria.

In many real-world scenarios, these two can also complement each other. For instance, you might inject a Factory into a class using DI, allowing that class to produce objects dynamically based on its logic while still benefiting from the advantages of dependency injection.