Keeping main thread running with Thread.sleep vs CountDownLatch

Question

I want my main thread to be running, as I have some listener that will be listening to request/messages in another thread and I don't want my main thread to die.

Which one is better

CountDownLatch

public static void main(String[] args) throws InterruptedException {

    startListener();
    CountDownLatch latch = new CountDownLatch(1);
    latch.await();

}

Or While with sleep

public static void main(String[] args) throws InterruptedException {

    startListener();
    while (true){
        Thread.sleep(1000);
    }

}

What does the listener do exactly? Between the two, countdown latch is better, but there might be constructs that fit even better. — daniu, Jan 20 '19 at 13:37
@daniu : listener will be a JMS listener which will be listening the messages throughout the day, unless we stop the process. — Sujeet, Jan 20 '19 at 13:38
@ernest_k , I don't want to process anything further in main thread. I want it to be alive, so that my listener thread will keep on listening to the messages. — Sujeet, Jan 20 '19 at 13:40
@Sujeet I understand that It's about how the main thread will be notified that your listeners' job is done. Can you show the code of `startListeners()` or whatever code has to finish before the main thread dies? — ernest_k, Jan 20 '19 at 13:42
`CountDownLatch.await` is a more sophisticated mechanism that would wait until you have counted down to 0, while `Thread.sleep` is rarely a good decision, particularly when it's repetitive. Both cases are bad, there is no way to stop either of them (the former requires `latch` to be passed in `startListener`, the latter needs a reference to the main thread to be able to interrupt it). — Andrew Tobilko, Jan 20 '19 at 13:58
What about join those thread in main thread ? Refer to example in this answer: https://stackoverflow.com/a/54022338 — Eric, Jan 20 '19 at 18:49

score 4 · Accepted Answer · answered Jan 20 '19 at 13:44

4

Assuming your listener thread is a non-daemon thread you can let the main thread finish. The JVM will continue to run as long as there are non-daemon threads executing.

CountDownLatch would be a better solution if you plan to implement a graceful shutdown. You can hit the latch and let the main thread finish by executing a cleanup logic. Think how the application will shut down e.g. you get a SIGTERM singal when listener is reading a message from the queue, what then?

answered Jan 20 '19 at 13:44

Karol Dowbecki

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Thanks, Making it non-daemon works ! Is there any impact of making it non-deamon, do we need to take care of anything else ? – Sujeet Jan 20 '19 at 13:52
@Sujeet you can take a look at Spring's [DefaultMessageListenerContainer](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/jms/listener/DefaultMessageListenerContainer.html) to see what are the challenges of reading from JMS. – Karol Dowbecki Jan 20 '19 at 13:55
The only difference is that the JVM will keep running if this thread is alive. – Roee Gavirel Jan 20 '19 at 13:55

score 2 · Answer 2 · answered Jan 20 '19 at 13:44

None.

First of all, if startListener() create a new thread, the application will keep running even if the main thread finishes. That's true as long as there is at least one non-daemon thread running. (This is different from C / C++ where the application is ending with the main. which you might confuse with).

Regarding the options you presented. Both will create an endless loop (without a way to exist), which is probably not the best practice. But if you must choose one of them the latch is better since it won't consume CPU every second.

score 0 · Answer 3 · answered Jan 20 '19 at 14:10

0

Both proposed methods would work, but I suggest to employ the main thread in synchronous processing of incoming messages. Messages then will be processed faster and with less resources (threads). See A Simple Example of Synchronous Message Receives. Listener then is not needed.

answered Jan 20 '19 at 14:10

Alexei Kaigorodov

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Actually I have multiple different kind of listener and they are listening to different different topics in their own thread. So I think a single main thread listening might not be a good idea in this case – Sujeet Jan 20 '19 at 14:17

Keeping main thread running with Thread.sleep vs CountDownLatch

CountDownLatch

Or While with sleep

3 Answers3