// 1. 定義自定義事件
public class UserRegisteredEvent extends ApplicationEvent {
    private final String username;
    public UserRegisteredEvent(Object source, String username) {
        super(source);
        this.username = username;
    }
    public String getUsername() { return username; }
}

// 2. 定義函數(shù)式事件監(jiān)聽接口
@FunctionalInterface
public interface ApplicationEventListener<T extends ApplicationEvent> {
    void onApplicationEvent(T event);
}

// 3. 在服務(wù)中發(fā)布事件并使用函數(shù)式監(jiān)聽
@Service
public class UserService {
    private final List<ApplicationEventListener<UserRegisteredEvent>> listeners = new CopyOnWriteArrayList<>();
    @Autowired
    private ApplicationEventPublisher eventPublisher;

    // 提供注冊監(jiān)聽器的方法
    public void addRegisterListener(ApplicationEventListener<UserRegisteredEvent> listener) {
        listeners.add(listener);
    }

    public void registerUser(String username, String email) {
        // ... 用戶注冊邏輯 ...
        System.out.println("用戶注冊成功: " + username);
        
        // 發(fā)布事件
        UserRegisteredEvent event = new UserRegisteredEvent(this, username);
        eventPublisher.publishEvent(event);
        
        // 通知所有自定義監(jiān)聽器
        listeners.forEach(listener -> {
            try {
                listener.onApplicationEvent(event);
            } catch (Exception e) {
                // 避免單個(gè)監(jiān)聽器異常影響其他監(jiān)聽器
            }
        });
    }
}

// 4. 在配置或控制器中注冊監(jiān)聽邏輯
@Configuration
public class AppConfig {
    @Autowired
    private UserService userService;
    
    @Bean
    public CommandLineRunner setupListeners() {
        return args -> {
            // 使用Lambda表達(dá)式注冊監(jiān)聽器，代碼非常簡潔
            userService.addRegisterListener(event -> {
                System.out.println("[監(jiān)聽器A] 發(fā)送歡迎郵件給: " + event.getUsername());
            });
            
            userService.addRegisterListener(event -> {
                System.out.println("[監(jiān)聽器B] 記錄用戶注冊日志: " + event.getUsername());
            });
        };
    }
}

// 定義配置校驗(yàn)器接口
@FunctionalInterface
public interface ConfigValidator {
    boolean isValid(String configValue);
}

@Component
public class AppConfigService {
    private final Map<String, String> configMap = new HashMap<>();
    private final List<ConfigValidator> validators = new CopyOnWriteArrayList<>();

    // 注冊校驗(yàn)器
    public void addValidator(ConfigValidator validator) {
        validators.add(validator);
    }

    @PostConstruct
    public void init() {
        // 添加一些內(nèi)置校驗(yàn)器
        addValidator(value -> value != null && !value.trim().isEmpty()); // 非空校驗(yàn)
        addValidator(value -> value.length() >= 8); // 最小長度校驗(yàn)
    }

    public void updateConfig(String key, String value) {
        // 使用Stream API和函數(shù)式接口進(jìn)行所有校驗(yàn)
        boolean allValid = validators.stream()
                                   .allMatch(validator -> validator.isValid(value));
        if (allValid) {
            configMap.put(key, value);
            System.out.println("配置已更新: " + key + " = " + value);
        } else {
            throw new IllegalArgumentException("配置值無效: " + value);
        }
    }
}

// 傳統(tǒng)注解控制器方式
@RestController
@RequestMapping("/api/users")
public class UserController {
    @GetMapping
    public List<User> getUsers() { ... }
}

// 函數(shù)式端點(diǎn)定義方式 (WebFlux)
@Configuration
public class FunctionalRouter {
    @Bean
    public RouterFunction<ServerResponse> userRoutes(UserHandler userHandler) {
        return RouterFunctions.route()
                .GET("/api/users", request -> userHandler.getUsers())
                .build();
    }
}