Should ScheduledExecutorService.scheduleAt* methods re-schedule tasks if the task throws RuntimeException/Error?

Question

The other day I was implementing an important service in my application, that should continue to run no matter what. So I used the following construct:

ScheduledExecutorService ses =
Executors.newSingleThreadScheduledExecutor();

//If the thread dies, another will take over
ses.scheduleAtFixedRate(importantPeriodicTask, 1, 1, TimeUnit.NANOSECONDS);

...only to find out that when importantPeriodicTask acutually throws a RuntimeException or Error, the ScheduledExecutorService will stop executing this task (they will cease to be scheduled).

This is of course exactly what the javadoc says:

If any execution of the task encounters an exception, subsequent executions are suppressed.

So shame on me, but I couldn't understand why the authors implemented ScheduledExecutorService like this.

Granted, a RuntimeException or Error should generally not be caught, especially Error. But in reality, especially in case of RuntimeException, truth is that they are thrown quite common in production deployment, and I feel it is almost always desirable that while that particular operation should fail, the app itself should not fail because of that isolated error.

It is true that a suppression of one periodic task does not affect other kinds of periodic tasks. But given the nature of most periodic tasks, shouldn't these tasks perceived as a "service", rather than isolated tasks?

In other words, shouldn't only that one instance of importantPeriodicTask fail, and the task itself be continued to be rescheduled?

Answer 1

In my opinion the current behavior is reasonable. RuntimeExceptions usually refer to bugs. They can practically occur anywhere in the task's code. If the task is stateful for example, it may leave its state inconsistent, and subsequent executions will have an unexpected behavior. In general, I don't like code that tries to recover from its own bugs, but that's my opinion.

If you wish to change the behavior of ScheduledExecutorService, take a look at the following generic solution:

http://www.javaspecialists.eu/archive/Issue154.html