Explain Inversion of Control and Dependency Injection, and why Spring uses them.
Answer
- Inversion of Control: your code no longer controls the creation and lifecycle of its collaborators; a container does.
- Dependency Injection is the way IoC is applied: a class declares dependencies (constructor parameters) and the container supplies them.
- Spring scans for beans (
@Component, @Service...), builds the object graph at startup and injects dependencies. - Benefits: loose coupling (depend on interfaces), easy testing with mocks, central configuration, lifecycle management.
- Example:
OrderService(PaymentGateway g); in production Spring gives a Stripe gateway, in tests you pass a mock.
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