Dependency injection in a web application means components declare what they need rather than constructing it themselves, letting the framework wire collaborators together and decouple classes from each other's concrete implementations.
Key Points: • Testability improves significantly since dependencies can be swapped for mocks or stubs in unit tests without touching production wiring. • Loose coupling between layers (controllers, services, repositories) means implementations can be swapped without ripping through calling code. • Boilerplate object-construction code disappears from business logic, since the container handles instantiation and lifecycle. • Configuration is centralized, making it easier to see and reason about how the application is assembled. • It supports scalability and maintainability as the codebase grows, since new implementations or cross-cutting concerns (like caching or transactions) can be introduced without invasive changes.
Example: A UserController that depends on a UserService interface, rather than a concrete class, can be tested with a mock UserService and, separately, can have its real implementation swapped for a caching-decorated version without any change to the controller.
Interview Tip: A concise interview answer is:
"Dependency injection makes web applications easier to test, since you can substitute mocks for real dependencies, and easier to change, since components depend on abstractions rather than concrete classes. It also keeps construction logic out of business code and centralizes configuration, which pays off as the application grows."