ScheduledExecutorService call shutdown for infinite scheduling

Question

I am using a ScheduledExecutorService that would be scheduled to run every minute infinitely

Could some one tell me when to invoke the shutdown on the ScheduledExecutorService

I have already looked into Guava MoreExecutors. This does not work as I need to block the main thread to keep the other threads running continuously

Use case

I have a task that constantly monitors a system to check for latency/failures.

I need to run this task periodically and configure Cloudwatch alarms based on it. Ideally, I would want the task to continue executing even when there is a latency spike, because the alarm would have been triggered

So, I am using a ScheduledExecutorService to periodically schedule the tasks.

I want to know the ideal place where I would call shutdown() on the executor service

Answer 1

Currently, the factory methods in Executors return an instance of ScheduledThreadPoolExecutor ¹ . By default, a ScheduledThreadPoolExecutor will not continue executing periodic tasks after it has been shutdown. You can configure this via setContinueExistingPeriodicTasksAfterShutdownPolicy .

Sets the policy on whether to continue executing existing periodic tasks even when this executor has been shutdown . In this case, executions will continue until shutdownNow or the policy is set to false when already shutdown. This value is by default false .

As it's really an implementation detail what implementation of ScheduledExecutorService the factory methods of Executors return, it's probably better to create a ScheduledThreadPoolExecutor directly.

ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
executor.setContinueExistingPeriodicTasksAfterShutdownPolicy(true);
executor.scheduleAtFixedRate(() -> System.out.println("Hello, World!"), 0L, 1L, TimeUnit.SECONDS);
executor.shutdown();

Note, this currently configures the ScheduledThreadPoolExecutor to use non-daemon threads. Even though shutdown was called, if the periodic tasks aren't cancelled they will keep the JVM alive. Use a custom ThreadFactory if you want to use daemon threads.

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^{1. The newSingleThreadScheduledExecutor methods return a non-configurable wrapper that delegates to a ScheduledThreadPoolExecutor .}