How does Spring Boot 4 handle async processing with @Async?

// Enable async processing:
@SpringBootApplication
@EnableAsync
public class App { ... }

// Async service:
@Service
public class EmailService {

    // Fire-and-forget (no return value):
    @Async
    public void sendWelcomeEmail(String email) {
        // Runs in separate thread; caller continues immediately
        emailGateway.send(email, "Welcome!", buildWelcomeBody(email));
    }

    // Async with result:
    @Async
    public CompletableFuture<EmailStats> getEmailStats(String userId) {
        EmailStats stats = emailGateway.fetchStats(userId);  // slow I/O
        return CompletableFuture.completedFuture(stats);
    }
}

// Caller:
@Service
public class RegistrationService {

    public User register(RegisterRequest req) {
        User user = userRepo.save(new User(req));
        emailService.sendWelcomeEmail(user.email()); // returns immediately
        return user;  // registration complete; email sends in background
    }

    // Parallel async calls:
    public DashboardData loadDashboard(String userId) throws Exception {
        CompletableFuture<OrderStats>  orders  = orderService.getStats(userId);
        CompletableFuture<EmailStats>  emails  = emailService.getEmailStats(userId);
        CompletableFuture<PaymentInfo> payment = paymentService.getInfo(userId);

        // Wait for all 3 to complete (run in parallel)
        CompletableFuture.allOf(orders, emails, payment).join();
        return new DashboardData(orders.get(), emails.get(), payment.get());
    }
}

# Custom async executor thread pool:
spring:
  task:
    execution:
      pool:
        core-size: 5
        max-size: 20
        queue-capacity: 100
      thread-name-prefix: async-