What is Spring Boot's externalized configuration with @Value and @ConfigurationProperties?

# application.yml:
app:
  payment:
    gateway-url: https://payment.example.com/api
    timeout-seconds: 30
    retry-attempts: 3
    supported-currencies: [USD, EUR, GBP, JPY]

api:
  rate-limit: 1000

// Method 1: @Value for simple individual values
@Service
public class RateLimiter {

    @Value("${api.rate-limit}")
    private int rateLimit;

    @Value("${api.rate-limit:500}")  // default value if not configured
    private int rateLimitWithDefault;

    @Value("${MISSING_PROP:#{null}}")  // null default
    private String optionalProp;

    @Value("${spring.application.name}")
    private String appName;
}

// Method 2: @ConfigurationProperties for grouped values (recommended)
@ConfigurationProperties(prefix = "app.payment")
@Validated
public record PaymentProperties(
    @NotBlank URL gatewayUrl,
    @Positive int timeoutSeconds,
    @Min(1) @Max(10) int retryAttempts,
    @NotEmpty List<String> supportedCurrencies
) {}

// Register and inject:
@EnableConfigurationProperties(PaymentProperties.class)
@SpringBootApplication
public class App { ... }

@Service
@RequiredArgsConstructor
public class PaymentService {
    private final PaymentProperties config;  // type-safe injection
    public void processPayment(String currency) {
        if (!config.supportedCurrencies().contains(currency)) {
            throw new UnsupportedCurrencyException(currency);
        }
    }
}