Spring Dependency Injection Püf Noktaları

@Service
public class OrderService {
    private final PaymentClient paymentClient;
    public OrderService(PaymentClient paymentClient) {
        this.paymentClient = paymentClient;
    }
}

public NotificationService(Optional<SmsGateway> smsGateway) {
    this.smsGateway = smsGateway;
}

public ReportGenerator(ObjectProvider<PdfExporter> provider) {
    this.exporter = provider.getIfAvailable(DefaultPdfExporter::new);
}

@Order(1) @Component class CreditCardValidator implements PaymentValidator {}
@Order(2) @Component class FraudCheckValidator implements PaymentValidator {}

@Service
public class PaymentPipeline {
    private final List<PaymentValidator> validators;
    public PaymentPipeline(List<PaymentValidator> validators) {
        this.validators = validators; // Spring @Order sırasına göre enjekte eder
    }
}

public CacheRouter(Map<String, CacheManager> cacheManagers) {
    this.redisCache = cacheManagers.get("redisCacheManager");
}

@Service
public class ShippingService {
    public ShippingService(@Qualifier("fedexClient") CourierClient client) { ... }
}

// ❌ Kaçının
@Autowired(required = false)
private MetricsCollector metricsCollector;

// ✅ Tercih edin
public MyService(Optional<MetricsCollector> metricsCollector) { ... }

private final InventoryClient inventoryClient; // final => thread-safe, test edilebilir

// Geçici çözüm (önerilmez, sadece son çare):
public ServiceA(@Lazy ServiceB serviceB) { this.serviceB = serviceB; }

// Doğru çözüm: ortak mantığı üçüncü bir servise çıkarın
public ServiceA(SharedLogic sharedLogic) { ... }

@ConfigurationProperties(prefix = "payment")
public record PaymentProperties(String apiKey, Duration timeout, boolean sandboxMode) {}

public @Nullable Customer findByEmail(String email) { ... } // dönebilir veya null olabilir

@Bean
@ConditionalOnProperty(name = "features.sms.enabled", havingValue = "true")
public SmsGateway smsGateway() { return new TwilioSmsGateway(); }

public BatchJobRunner(ObjectProvider<JobContext> jobContextProvider) {
    this.jobContextProvider = jobContextProvider;
}
public void run() {
    JobContext ctx = jobContextProvider.getObject(); // her çağrıda yeni instance
}

public sealed interface DiscountPolicy permits PercentageDiscount, FlatDiscount {}
public record PercentageDiscount(double rate) implements DiscountPolicy {}
public record FlatDiscount(BigDecimal amount) implements DiscountPolicy {}

BigDecimal apply(DiscountPolicy policy, BigDecimal price) {
    return switch (policy) {
        case PercentageDiscount p -> price.multiply(BigDecimal.valueOf(1 - p.rate()));
        case FlatDiscount f -> price.subtract(f.amount());
    };
}

@Bean
public AsyncTaskExecutor applicationTaskExecutor() {
    return new VirtualThreadTaskExecutor("app-vt-");
}

public JobDispatcher(Optional<AsyncTaskExecutor> executor) {
    this.executor = executor.orElseGet(SyncTaskExecutor::new);
}

@Test
void discountAppliedCorrectly() {
    OrderService service = new OrderService(new FakePaymentClient()); // Spring gerekmiyor
    assertEquals(..., service.process(order));
}

// ❌ Kaçının
PaymentClient client = new StripePaymentClient(); // @Transactional, proxy vs. çalışmaz

// ✅ DI'a bırakın
public OrderService(PaymentClient client) { this.client = client; }

@ArchTest
static final ArchRule noFieldInjection = noFields()
    .that().areDeclaredInClassesThat().resideInAPackage("..service..")
    .should().beAnnotatedWith(Autowired.class);

@Primary
@Service
public class DefaultPricingEngine implements PricingEngine { ... }

public class DashboardService {
    private final OrdersClient ordersClient;
    private final InventoryClient inventoryClient;

    public DashboardService(OrdersClient ordersClient, InventoryClient inventoryClient) {
        this.ordersClient = ordersClient;
        this.inventoryClient = inventoryClient;
    }

    public Dashboard load() throws Exception {
        try (var scope = StructuredTaskScope.open()) {
            var orders = scope.fork(ordersClient::fetchAll);
            var stock = scope.fork(inventoryClient::fetchAll);
            scope.join();
            return new Dashboard(orders.get(), stock.get());
        }
    }
}

public record OrderSummary(String orderId, BigDecimal total, Instant createdAt) {}

public OrderSummary summarize(Order order) { // servis enjekte edilmiş, dönüş tipi record
    return new OrderSummary(order.id(), order.total(), order.createdAt());
}

// Sadece framework zorunlu kılıyorsa:
@Autowired
public void setLegacyHandler(LegacyHandler handler) { this.handler = handler; }