Does channel.close() actually stop scheduled tasks by channel.eventLoop().scheduleAtFixedRate()? - tcp

Question

I am developing a device control program (Tcp Server) using Netty.
I have to send status request message every 1 second, Therefor I am using the method channel.eventLoop().scheduleAtFixedRate() and it works fine.
channel.eventLoop().scheduleAtFixedRate( () -> {
channel.writeAndFlush("REQ MSG");
System.out.println("REQ MSG");
}, 1000, 1000, TimeUnit.MILLISECONDS);
and when I call channel.close(), the above scheduled task is automatically stopped.
But when I eliminate channel.writeAndFlush() and call channel.close(), the above scheduled task is not stopped.
channel.eventLoop().scheduleAtFixedRate( () -> {
// channel.writeAndFlush("REQ MSG");
System.out.println("REQ MSG");
}, 1000, 1000, TimeUnit.MILLISECONDS);
What's the difference between the two?

Answer 1

There shouldn't be any difference... In both cases you need to explicit cancel the task via future.cancel() once you want to stop it.

Answer 2

I couldn't find the exact cause in my case, and as I advised, I fixed it as follows.
private final CopyOnWriteArrayList<ScheduledFuture<?>> userTaskFutures = new CopyOnWriteArrayList<>();
public void scheduleAtFixedRate(Runnable task, long initialDelay, long period, TimeUnit unit) {
ScheduledFuture<?> future = channel.eventLoop().scheduleAtFixedRate(task, initialDelay, period, unit);
userTaskFutures.add(future);
}
public void disconnect() {
...
if (!userTaskFutures.isEmpty()) {
userTaskFutures.forEach(future -> future.cancel(true));
userTaskFutures.clear();
}
}

Answer 3

Had same situation, my setup: websocket server. When new connection created I've creating ttl handler like this
channel.eventLoop().scheduleWithFixedDelay(
TTLHandler(channel), 0, 10, TimeUnit.SECONDS
)
The code inside TTLHandler make internal logic and then try to drop expireded channel :
channel.deregister()
channel.disconnect()
channel.close()
None of this closing methods stopped eventLoop() and TTLHandler keep going.
My investigation lead me into ScheduledFutureTask.java where DefaultPromise checks cancellation by isCancelled() method for CancellationException
private static boolean isCancelled0(Object result) {
return result instanceof CauseHolder && ((CauseHolder) result).cause instanceof CancellationException;
}
so I directly added into my disconnect section additional
throw CancellationException()
and it works