How to send a onetime scheduled task to executorService that has a repeptitive task ongoing

[mike]

Hi,

We have implemented a "KeepAlive service" that we use over TLS.

This service will send a message every 10 seconds to a server and if we get a response then we are ok and connection is considered ok.

This is started using:

KeepAlive keepAlive = new KeepAlive(this);
keepAlive.init();
future = scheduledTaskExecutor.newScheduledTask(keepAlive, 0, properties.getKeepAliveTime(),
TimeUnit.SECONDS);

Note: newScheduledTask() calls ScheduledFuture<?> future = executorService.scheduleAtFixedRate(runnable, delay, period, timeUnit);

However we also have a method where we "instantly" want to find out if we are connected to the server.

This is how it looks like:

public synchronized boolean isConnected() {

KeepAlive keepAlive = new KeepAlive(this);//create another keepAlive for same session.
keepAlive.init();

if (scheduledTaskExecutor!= null) {

ScheduledFuture<?> scheduledTaskOnce = scheduledTaskExecutor.newScheduledTaskOnce(keepAlive, 10,
TimeUnit.MILLISECONDS);
try {
Object object = scheduledTaskOnce.get();
if (object == null) {
logger.debug("Task has finished! {}", object);
}
} catch (InterruptedException | ExecutionException e) {
logger.debug("We could not finished check of connection", e);
}
return keepAlive.isAlive();
}

Note: newScheduledTaskOnce() calls ScheduledFuture<?> future = executorService.schedule(runnable, delay, timeUnit);

Q1: The KeepAlive hangs when doing it as a onetime task in the same executor. Any ideas why?
Q2: How will a onetime task in same scheduledExecutor be handled when we have a repeatedly ongoing?

br,

//mike

[Jeffrey H. Coffield]

Mike,

Without actually running the code or any deep analysis, my first
reaction to this problem would be to create a single method that does
one ping when called and exits with a boolean, possibly with a lock so
only one at a time can run. Then use the scheduler to run it every 10
seconds and set a static variable that other threads can check. Then if
you need an immediate test, just call the original method.

hth,
jeff

[Stanimir Stamenkov]

Thu, 10 Mar 2022 04:32:40 -0800 (PST), /mike/:

> This service will send a message every 10 seconds to a server and if we get a response then we are ok and connection is considered ok.
>

> future = scheduledTaskExecutor.newScheduledTask(keepAlive, 0, properties.getKeepAliveTime(),
> TimeUnit.SECONDS);
>
> Note: newScheduledTask() calls ScheduledFuture<?> future = executorService.scheduleAtFixedRate(runnable, delay, period, timeUnit);
>
> However we also have a method where we "instantly" want to find out if we are connected to the server.
>

> ScheduledFuture<?> scheduledTaskOnce = scheduledTaskExecutor.newScheduledTaskOnce(keepAlive, 10,
> TimeUnit.MILLISECONDS);

> Object object = scheduledTaskOnce.get();

>
> Note: newScheduledTaskOnce() calls ScheduledFuture<?> future = executorService.schedule(runnable, delay, timeUnit);
>
> Q1: The KeepAlive hangs when doing it as a onetime task in the same executor. Any ideas why?
> Q2: How will a onetime task in same scheduledExecutor be handled when we have a repeatedly ongoing?

You haven't specified what is your ScheduledExecutorService
configuration/implementation.

I'm using the following in a real world project:

import java.util.concurrent.*;

public static void main(String[] args) throws Exception {
ScheduledExecutorService executor =
Executors.newSingleThreadScheduledExecutor();

executor.scheduleAtFixedRate(slowTask("Regular", 10),
0, 100, TimeUnit.MILLISECONDS);

ScheduledFuture<?> oneOff = executor
.schedule(slowTask("One-off with delay", 50),
100, TimeUnit.MILLISECONDS);
oneOff.get();

executor.execute(slowTask("Immediate one-off", 100));

Future<?> oneOff2 = executor
.submit(slowTask("Immediate one-off with result", 100));
oneOff2.get();

Thread.sleep(50);

executor.shutdown();
}

static Runnable slowTask(String name, long timeMillis) {
return () -> {
System.out.append(name).print("...");
try {
Thread.sleep(timeMillis);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new IllegalStateException(e);
}
System.out.println(" done.");
};
}

That is a single-thread scheduled executor [1] to serialize both:

* A task scheduled to run at regular intervals; and
* One-off executions.

> Q1: The KeepAlive hangs when doing it as a onetime task in the same executor. Any ideas why?
> Q2: How will a onetime task in same scheduledExecutor be handled when we have a repeatedly ongoing?

One of my uses actually triggers one-off task at the end of the
scheduled task, also – not just externally submitted. Not sure what
"onetime task in the same executor" in you case means.

[1]
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Executors.html#newSingleThreadScheduledExecutor--

--
Stanimir