You can call ui.getInternals().getPushConnection().isConnected() to figure out whether Atmosphere websocket pipe is connected. The question is, is the pipe actually alive and healthy?

WebSocket runs over TCP/IP and never runs over UDP. While TCP/IP guarantees that the packets are not reordered, the actual delivery of the messages is not guaranteed. This is a fundamental limitation of TCP/IP.

The only way to know the state of a TCP/IP connection (and, by extension, the WebSocket pipe), is to send a “ping” request and wait for a reply. Vaadin performs this automatically: the client-side Vaadin javascript sends heartbeats to the server periodically, as described in Application Lifecycle: UI Expiration.

Therefore, to increase the probability of knowing accurately whether websocket pipe is actually alive, you can increase the heartbeat frequency to 1 heartbeat every minute, then checking the value of ui.getInternals().getLastHeartbeatTimestamp(): if the difference from now is less than a minute, the websocket pipe can be considered “up”.

In Vaadin 24+, Atmosphere now by default sends a small message every 1 minute. See the HeartbeatInterceptor class for details. The message only consists of one letter “X”, contrary to what HEARTBEAT_PADDING_CHAR says. You can reconfigure the heartbeat interval via a servlet init parameter:

@WebServlet(name = "myservlet", urlPatterns = {"/*"}, initParams = @WebInitParam(name = ApplicationConfig.HEARTBEAT_INTERVAL_IN_SECONDS, value = "2"))
public class MyServlet extends VaadinServlet {
}

I don’t know whether it’s possible to check that the Atmosphere heartbeat message was delivered successfully.

Conclusion: You can never be 100% sure, but with the heartbeat frequency of 1 minute, you can be reasonably sure.