要保持Spring WebSocket连接的活跃状态，可以使用心跳机制。下面是一个示例代码，演示了如何在Spring WebSocket中实现心跳：

1. 创建一个WebSocket配置类，用于配置WebSocket相关的参数：

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void configureMessageBroker(MessageBrokerRegistry config) {
        config.enableSimpleBroker("/topic");
        config.setApplicationDestinationPrefixes("/app");
    }

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/websocket").withSockJS();
    }

    @Override
    public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
        registration.addDecoratorFactory(webSocketHandlerDecoratorFactory());
    }

    @Bean
    public WebSocketHandlerDecoratorFactory webSocketHandlerDecoratorFactory() {
        return new WebSocketHandlerDecoratorFactory() {
            @Override
            public WebSocketHandler decorate(WebSocketHandler handler) {
                return new CustomWebSocketHandler(handler);
            }
        };
    }
}

2. 创建一个自定义的WebSocketHandler，用于处理WebSocket连接和心跳消息：

public class CustomWebSocketHandler extends WebSocketHandlerDecorator {

    private static final long HEARTBEAT_INTERVAL = 5000; // 心跳间隔时间，单位为毫秒

    public CustomWebSocketHandler(WebSocketHandler delegate) {
        super(delegate);
    }

    @Override
    public void afterConnectionEstablished(WebSocketSession session) throws Exception {
        super.afterConnectionEstablished(session);
        startHeartbeat(session);
    }

    @Override
    public void handleMessage(WebSocketSession session, WebSocketMessage message) throws Exception {
        // 处理收到的消息
        super.handleMessage(session, message);
    }

    @Override
    public void afterConnectionClosed(WebSocketSession session, CloseStatus closeStatus) throws Exception {
        super.afterConnectionClosed(session, closeStatus);
        stopHeartbeat(session);
    }

    private void startHeartbeat(WebSocketSession session) {
        ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
        executor.scheduleAtFixedRate(() -> {
            try {
                session.sendMessage(new TextMessage("heartbeat")); // 发送心跳消息
            } catch (IOException e) {
                e.printStackTrace();
            }
        }, HEARTBEAT_INTERVAL, HEARTBEAT_INTERVAL, TimeUnit.MILLISECONDS);
        session.getAttributes().put("executor", executor);
    }

    private void stopHeartbeat(WebSocketSession session) {
        ScheduledExecutorService executor = (ScheduledExecutorService) session.getAttributes().get("executor");
        executor.shutdown();
    }
}

3. 在控制器中处理客户端发送的消息：

@Controller
public class WebSocketController {

    @MessageMapping("/hello")
    @SendTo("/topic/greetings")
    public String greeting(String message) throws Exception {
        return "Hello, " + message + "!";
    }
}

通过以上的代码，你可以实现Spring WebSocket的心跳机制，保持连接的活跃状态。