Lesson 48: Intro to Spring Boot & Dependency Injection

Spring Boot wires up a Java web application with minimal configuration, and its core idea — dependency injection — means classes declare what they need instead of constructing it themselves.

Real-time example

A @RestController for orders never constructs its own OrderService — Spring injects it, making the controller trivially testable with a mock.

@RestController
@RequestMapping("/orders")
class OrderController {
    private final OrderService service;

    OrderController(OrderService service) { // injected by Spring
        this.service = service;
    }

    @GetMapping("/{id}")
    Order getOrder(@PathVariable String id) {
        return service.findById(id);
    }
}

What's happening

Spring scans for @RestController/@Service classes at startup and wires their constructor dependencies automatically — no new OrderService() anywhere.

Constructor injection (as above) is preferred over field injection — it makes dependencies explicit and required, not hidden.