Spring - Difference Between Inversion of Control and Dependency Injection

Inversion of Control (IoC) and Dependency Injection (DI) are both fundamental concepts in the Spring framework, and while they are closely related and often used interchangeably, they are not quite the same. Let's explore the differences:

Inversion of Control (IoC)

Concept: IoC is a design principle where the control over the flow of an application is inverted. Instead of the application controlling its flow and dependencies, an external entity (like a framework or container) manages it.
Purpose: IoC is used to decouple components and layers in the system. This makes systems more modular and easier to maintain, test, and scale.
Implementation in Spring: In the Spring framework, IoC is achieved using the Spring IoC container. The container manages the lifecycle of beans, their creation, wiring, and destruction.
Variants: Dependency Injection is just one form of IoC. Other forms of IoC include Dependency Lookup, Method Invocation, etc.

Dependency Injection (DI)

Concept: DI is a subtype of IoC where dependencies are injected into a class from the outside, rather than the class creating or looking up its dependencies. This can be done through constructors, setter methods, or fields.
Purpose: The main aim of DI is to achieve Inversion of Control, reducing the coupling between classes. It promotes the SOLID principles, especially the Dependency Inversion Principle.
Implementation in Spring: In Spring, DI is achieved by the IoC container injecting dependencies into beans when they are created. This can be done via XML configuration, annotations, or Java config.
Types:
- Constructor Injection: Dependencies are injected through the bean's constructor.
- Setter Injection: Dependencies are injected through setter methods.
- Field Injection: Dependencies are injected directly into fields using annotations.

In Summary:

IoC is a broader principle where control is handed over to an external entity (like a framework or container). It's about who initiates the call. If your code initiates a call, it's traditional; if the container/system/framework calls your code, it's IoC.
DI is a form of IoC where dependencies are provided from the outside. It's about how components acquire their dependencies.

In Spring's context, when people talk about IoC, they often refer to the mechanism where the Spring container controls the beans and their lifecycle. When they discuss DI, they're typically referring to how beans receive their dependencies from the Spring container.