Introduction to the Spring Framework

Spring is the dominant framework in professional Java web development. Its core idea is Dependency Injection - instead of a class creating the objects it depends on, those dependencies are handed to it from the outside, which makes code far easier to test and reconfigure.

The problem Spring solves

// Without DI - tightly coupled, hard to test
public class OrderService {
    private EmailSender emailSender = new EmailSender(); // hardcoded dependency
}

// With DI - the dependency is handed in, not created internally
public class OrderService {
    private final EmailSender emailSender;

    public OrderService(EmailSender emailSender) {
        this.emailSender = emailSender;
    }
}

With the second version, tests can hand in a fake EmailSender instead of a real one - no actual emails get sent during a test run.

Spring Boot: a REST controller

Spring Boot builds on core Spring to make building web APIs fast, using annotations instead of the manual servlet wiring from the previous lesson:

@RestController
@RequestMapping("/api/students")
public class StudentController {

    private final StudentRepository repository;

    public StudentController(StudentRepository repository) {
        this.repository = repository; // Spring injects this automatically
    }

    @GetMapping("/{id}")
    public Student getStudent(@PathVariable int id) {
        return repository.findById(id);
    }

    @PostMapping
    public Student createStudent(@RequestBody Student student) {
        return repository.save(student);
    }
}

What Spring is doing for you

@RestController tells Spring this class handles web requests and returns data (usually JSON) directly. @GetMapping/@PostMapping map HTTP methods and URL paths to specific methods. Spring automatically creates a StudentController, sees it needs a StudentRepository in its constructor, creates or reuses one, and injects it - all without you writing a single new StudentController(...) anywhere. That automatic wiring is Dependency Injection in practice, and it is the foundation nearly all modern Java web backends are built on.