What are @Retryable and @ConcurrencyLimit in Spring Boot 4 and how do they work?

// Spring Boot 3 (required separate dependency + @EnableRetry):
// pom.xml: spring-retry dependency
// Config class: @EnableRetry
@Service
public class StockService {
    @Retryable(value = {NetworkException.class}, maxAttempts = 2)
    public void updateStock() { ... }
}

// Spring Boot 4: works out of the box with spring-boot-starter-resilience
// No @EnableRetry needed!
@Service
public class StockService {

    // Retry with exponential backoff and jitter
    @Retryable(
        includes  = NetworkException.class,  // exception types to retry on
        maxRetries = 3,                       // number of retry attempts
        backoff    = @Backoff(
            delay      = 500,                // initial delay in ms
            multiplier = 2.0,                // exponential multiplier
            random     = true                // add jitter
        )
    )
    public Order fetchOrderFromRemote(String orderId) {
        return remoteService.fetch(orderId); // automatically retried on NetworkException
    }

    // Bulkhead pattern: limit concurrent callers
    @ConcurrencyLimit(limit = 5)  // max 5 concurrent executions
    public void processPayment(Payment payment) {
        // Only 5 threads can execute this simultaneously
        // 6th caller blocks until one slot frees up
        paymentGateway.process(payment);
    }

    // Combining both:
    @Retryable(includes = NetworkException.class, maxRetries = 3)
    @ConcurrencyLimit(limit = 10)
    public void updateStock() {
        externalInventoryApi.update();
    }
}