CODEWITHSUNDEEP
javamultithreadingscheduledexecutorservice
Deepanshu Arora
Deepanshu Arora

Reputation: 435

How does ScheduledExecutorService handles terminated threads?

Does ScheduledExecutorService take care of handling terminated thread and generates a new one?

In the example below if any one of my thread terminates due to Error, what happens to thread pool size?

While debugging, I could notice one of the threads created by this service got silently terminated without printing any log statement. On checking Thread dump, I could still see 32 threads were still there and none of them were blocked.

public class CacheManager
{

    private static class CacheRefresher extends Thread
    {
        Cache cache; 

        public CacheRefresher(Cache cache)
        {
            this(cache);
        }

        @Override
        public final void run()
        {

            try {
                LOG.info("cache is getting refreshed for " + cache.type);
                cache.refreshCache();
            } catch (Exception e) {
                String subject = "Cache refresh failed in BMW";
                LOG.log(Level.WARN, subject + ". Exception thrown:", e);
            }
        }       
    }

    public void refreshCaches(List<cache> caches)
    {
        ThreadFactory ourThreadFactory =
                new NamedThreadFactory("CacheHandler", true);

        ScheduledExecutorService scheduleService =
                Executors.newScheduledThreadPool(32, ourThreadFactory);
        initialDelay = 60;
        for (Cache cache : caches) {  
            service.scheduleWithFixedDelay(new CacheRefresher(cache), initialDelay, 20, TimeUnit.SECONDS);
            initialDelay += 2;
            cacheContainers.add(cache);
        }
    }
}

Upvotes: 1

Views: 476

Answers (3)

Hulk
Hulk

Reputation: 6583

While the JavaDocs for Executors.newFixedThreadPool explicitly mention this:

If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.

there is no such strong guarantee about the Executors.newScheduledThreadPool.

It is possible that it behaves the same in this regard, but that is an implementation detail you should not need to care about. The Executor service will provide/create enough threads to perform the given tasks.

Upvotes: 2

michid
michid

Reputation: 10834

Uncaught exceptions in scheduled tasks will not cause scheduler's threads to terminate. However, it will prevent the failing task from being re-scheduled. See the respective documentation for ScheduledThreadPoolExecutor.html#scheduleWithFixedDelay:

The sequence of task executions continues indefinitely until one of the following exceptional completions occur:

  • [...]

  • An execution of the task throws an exception. In this case calling get on the returned future will throw ExecutionException, holding the exception as its cause.

Upvotes: 2

SirFartALot
SirFartALot

Reputation: 1225

As found in Javadoc newScheduledThreadPool(int) there always will be the specified amount of threads. Even if a thread will shutdown, there will be started another one. But in the first place, threads within the ScheduledExecutorService should be reused, even when a exception occurs within the Runnable.run().

And sure the threads are not blocked but waiting for new action to do...

Upvotes: 1

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