ScheduledExecutorService Exception handling

Question

I use ScheduledExecutorService to execute a method periodically.

p-code:

ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecu         


        

Answer 1

Old question but the accepted answer doesn't give explanations and provides a poor example and the most upvoted answer is right on some points but finally encourages you to add catch exceptions in every Runnable.run() method.
I disagree because :

• it is not neat : not standard for a task to catch its own exceptions.
• it is not robust : a new Runnable subclass could forget to perform the exception catch and the failover associated.
• it defeats the low coupling promoted by tasks since that couples the tasks to execute with the way of handling the task result.
• it mixes responsibilities : that is not the task responsibility to handle the exception or to communicate the exception to the caller. A task is something to execute.

I think that the exception propagation should be performed by the ExecutorService framework and actually it offers that feature.
Besides, trying to be too clever by trying to short-circuiting the ExecutorService way of working is not a good idea either : the framework may evolve and you want to use it in a standard way.
At last, letting the ExecutorService framework to make its job doesn't mean necessarily halting the subsequent invocations task.
If a scheduled task encounters an issue, that is the caller responsibility to re-schedule or not the task according to the issue cause.
Each layer has its its responsibilities. Keeping these make code both clear and maintainable.

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ScheduledFuture.get() : the right API to catch exceptions and errors occurred in the task

ScheduledExecutorService.scheduleWithFixedDelay()/scheduleAtFixRate() state in their specification :

If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor.

It means that ScheduledFuture.get() doesn't return at each scheduled invocation but that it returns for the last invocation of the task, that is a task cancelation : caused by ScheduledFuture.cancel() or a exception thrown in the task.
So handling the ScheduledFuture return to capture the exception with ScheduledFuture.get() looks right :

  try {
    future.get();

  } catch (InterruptedException e) {
    // ... to handle
  } catch (ExecutionException e) {
    // ... and unwrap the exception OR the error that caused the issue
    Throwable cause = e.getCause();       
  }

---

Example with the default behavior : halting the scheduling if one of the task execution encounters an issue

It executes a task that for the third executions thrown an exception and terminates the scheduling. In some scenarios, we want that.

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

public class ScheduledExecutorServiceWithException {

  public static void main(String[] args) {
    ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);

    // variable used to thrown an error at the 3rd task invocation
    AtomicInteger countBeforeError = new AtomicInteger(3);

    // boolean allowing to leave the client to halt the scheduling task or not after a failure
    Future futureA = executor
        .scheduleWithFixedDelay(new MyRunnable(countBeforeError), 1, 2, TimeUnit.SECONDS);
    try {
      System.out.println("before get()");
      futureA.get(); // will return only if canceled
      System.out.println("after get()");
    } catch (InterruptedException e) {
      // handle that : halt or no
    } catch (ExecutionException e) {
      System.out.println("exception caught :" + e.getCause());
    }

    // shutdown the executorservice
    executor.shutdown();
  }

  private static class MyRunnable implements Runnable {

    private final AtomicInteger invocationDone;

    public MyRunnable(AtomicInteger invocationDone) {
      this.invocationDone = invocationDone;
    }

    @Override
    public void run() {
      System.out.println(Thread.currentThread().getName() + ", execution");
      if (invocationDone.decrementAndGet() == 0) {
        throw new IllegalArgumentException("ohhh an Exception in MyRunnable");
      }
    }
  }
}

Output :

before get()
pool-1-thread-1, execution
pool-1-thread-1, execution
pool-1-thread-1, execution
exception caught :java.lang.IllegalArgumentException: ohhh an Exception in MyRunnable

Example with the possibility to go on the scheduling if one of the task execution encounters an issue

It executes a task that throws an exception at the two first executions and throws an error at the third one. We can see that the client of the tasks can choose to halt or not the scheduling : here I go on in cases of exception and I stop in case of error.

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

public class ScheduledExecutorServiceWithException {

  public static void main(String[] args) {
    ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);

    // variable used to thrown an error at the 3rd task invocation
    AtomicInteger countBeforeError = new AtomicInteger(3);

    // boolean allowing to leave the client to halt the scheduling task or not after a failure
    boolean mustHalt = true;
    do {
      Future futureA = executor
              .scheduleWithFixedDelay(new MyRunnable(countBeforeError), 1, 2, TimeUnit.SECONDS);
      try {
        futureA.get(); // will return only if canceled
      } catch (InterruptedException e) {
        // handle that : halt or not halt
      } catch (ExecutionException e) {
        if (e.getCause() instanceof Error) {
          System.out.println("I halt in case of Error");
          mustHalt = true;
        } else {
          System.out.println("I reschedule in case of Exception");
          mustHalt = false;
        }
      }
    }
    while (!mustHalt);
    // shutdown the executorservice
    executor.shutdown();
  }

  private static class MyRunnable implements Runnable {

    private final AtomicInteger invocationDone;

    public MyRunnable(AtomicInteger invocationDone) {
      this.invocationDone = invocationDone;
    }

    @Override
    public void run() {
      System.out.println(Thread.currentThread().getName() + ", execution");

      if (invocationDone.decrementAndGet() == 0) {
        throw new Error("ohhh an Error in MyRunnable");
      } else {
        throw new IllegalArgumentException("ohhh an Exception in MyRunnable");
      }
    }
  }
}

Output :

pool-1-thread-1, execution
I reschedule in case of Exception
pool-1-thread-1, execution
I reschedule in case of Exception
pool-1-thread-2, execution
I halt in case of Error