Spring Boot Principles

Spring Boot is the framework most Java developers reach for to build web applications and APIs. It takes the powerful-but-complex Spring framework and makes it delightfully easy to start. This lesson covers the core ideas and walks through building a REST API.

Inversion of Control & Dependency Injection

Spring's heart is an IoC container: it creates and wires your objects (called beans) for you. Instead of new-ing dependencies, you declare what you need and Spring injects it.

@Service
class OrderService {
    private final PaymentGateway gateway;

    OrderService(PaymentGateway gateway) {   // Spring injects this automatically
        this.gateway = gateway;
    }
}

You met Dependency Injection as a principle earlier - Spring makes it automatic. Annotate a class as a component (@Service, @Repository, @Component) and Spring manages it.

Auto-configuration & starters

Spring Boot's superpower is auto-configuration: it inspects your dependencies and configures sensible defaults automatically. Add a starter dependency and things just work.

spring-boot-starter-web   → embedded web server + REST support, ready to go
spring-boot-starter-data-jpa → Hibernate + a datasource, pre-wired

Building a REST API

Here's a complete, working REST controller:

@RestController
@RequestMapping("/api/products")
class ProductController {

    private final ProductRepository repo;

    ProductController(ProductRepository repo) {
        this.repo = repo;
    }

    @GetMapping
    List<Product> all() {
        return repo.findAll();
    }

    @GetMapping("/{id}")
    Product one(@PathVariable Long id) {
        return repo.findById(id)
                   .orElseThrow(() -> new NotFoundException(id));
    }

    @PostMapping
    Product create(@RequestBody Product product) {
        return repo.save(product);
    }
}

• @RestController - this class handles HTTP requests and returns JSON.
• @GetMapping / @PostMapping - map HTTP methods to your Java methods.
• @PathVariable / @RequestBody - bind URL parts and JSON bodies to parameters.

Spring converts objects to and from JSON automatically. That's a full CRUD API in under 30 lines.

The main application

@SpringBootApplication
public class ShopApplication {
    public static void main(String[] args) {
        SpringApplication.run(ShopApplication.class, args);
    }
}

Run it, and an embedded web server starts on port 8080 - no separate server to install.

Profiles & configuration

application.properties (or .yml) holds configuration, and profiles let you vary it per environment (dev, test, prod):

spring.datasource.url=jdbc:postgresql://localhost/shop
server.port=8080

Actuator: production-ready features

Spring Boot Actuator adds ready-made endpoints for health checks, metrics, and info - essential for running in production:

GET /actuator/health   → {"status":"UP"}

Quick check

An application isn't just code - it's a system. Next, we zoom out to system design.