Find the Wiring Bug

This application fails to start. Say why, and name the smallest fix.

java

interface ReceiptPrinter { void print(String text); }

@Component
class ThermalReceiptPrinter implements ReceiptPrinter { /* ... */ }

@Component
class EmailReceiptPrinter implements ReceiptPrinter { /* ... */ }

@Service
class CheckoutService {
    CheckoutService(ReceiptPrinter printer) { /* ... */ }
}

See the fix

Two beans implement ReceiptPrinter and nothing tells CheckoutService which one to take — the same shape as the NotificationSender gotcha. If thermal receipts are the default, add @Primary to ThermalReceiptPrinter; if CheckoutService specifically needs email, add @Qualifier("emailReceiptPrinter") to its constructor parameter instead.

Dependency Injection