Dependency Injection in Spring

What Is Dependency Injection?

DI is a design pattern where an object receives its dependencies from an external source rather than creating them itself.

Without DI (Tightly Coupled)

class OrderService {
    private OrderRepository repo = new JdbcOrderRepository();  // Hardcoded!
    private NotificationService notifier = new EmailNotificationService();

    public void createOrder(Order order) {
        repo.save(order);
        notifier.send(order);
    }
}

With DI (Loosely Coupled)

@Service
class OrderService {
    private final OrderRepository repo;
    private final NotificationService notifier;

    public OrderService(OrderRepository repo, NotificationService notifier) {
        this.repo = repo;
        this.notifier = notifier;
    }

    public void createOrder(Order order) {
        repo.save(order);
        notifier.send(order);
    }
}

Types of DI

Type	How It Works	Spring Support
Constructor	Inject via constructor	@Autowired (preferred)
Setter	Inject via setter method	@Autowired
Field	Inject directly into field	@Autowired (avoid)

DI Container

Spring IoC Container
    |
    +--> Creates beans
    +--> Manages lifecycle
    +--> Injects dependencies
    +--> Handles scopes (singleton, prototype)

Benefits

1. Loose coupling — Change implementations easily
2. Testability — Inject mocks for testing
3. Reusability — Same bean in different contexts
4. Single responsibility — Container manages wiring

Configuration

• Use properties over YAML for simple configs
• Externalize configuration
• Use profiles for environments
• Validate on startup

Bean Management

• Prefer constructor injection
• Use appropriate scope
• Implement lazy initialization
• Clean up resources

Security

• Use Spring Security
• Implement CSRF protection
• Use method-level security
• Log security events

Key Points

• Understanding Dependency Injection is essential for production systems
• Always consider scalability and maintainability
• Test thoroughly before deploying to production
• Monitor performance and set up alerting

Common Patterns

1. Validation: Always validate input at the boundary
2. Error Handling: Use structured error responses
3. Logging: Log key events for debugging
4. Testing: Unit, integration, and load tests
5. Documentation: Keep docs updated with code changes