Analyzing the Lost Exception Issue in ScheduledThreadPoolExecutor and Its Impact on Periodic Task Scheduling

In a previous blog post we highlighted a pitfall of ScheduledThreadPoolExecutor : when a scheduled task throws an exception, the exception information is lost and the task is no longer rescheduled.

When a periodic task is submitted, it is first placed into the delayed queue DelayedWorkQueue via the method ScheduledThreadPoolExecutor#scheduleAtFixedRate .

When a core thread of the pool retrieves the task, it executes ScheduledThreadPoolExecutor.ScheduledFutureTask#run , which internally calls super.runAndReset() .

If runAndReset() returns true , the task’s next execution time is updated and the task is re‑queued for the next cycle. If it returns false , the task is not rescheduled.

When the scheduled task throws an exception, runAndReset() ultimately returns false . The executor captures the exception internally, does not re‑throw it, and never calls Future#get() , so the exception information is discarded and the periodic task stops being scheduled.

In summary, when using ScheduledThreadPoolExecutor for periodic tasks, you must ensure that the task does not throw uncaught exceptions; otherwise, the exception is silently lost and the task will no longer be scheduled.