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I have the following code that is throwing a ConcurrentModificationException for some reason. It's thrown when I run it in a debugger with a breakpoint set in one of threads, but apparently not otherwise. I've confirmed that the class in question is not shared between threads by setting the breakpoint (using IntelliJ) to suspend at the thread level and checking that each class instance belongs to unique threads.

public class EventAccumulator implements Observer {
  private AtomicInteger inputCount = new AtomicInteger();

  private ArrayDeque<Integer> countsList;

  ...

  private void updatePeriodCount() {
    countsList.addFirst(inputCount.get());
    inputCount.set(0);
    while (countsList.size() >= 30) {
      countsList.pollLast();
    }
    int finalCount = 0;
    for (int count : countsList) { //<--ConcurrentModificationException
      finalCount += count;
    }
    ...
  }

Exception:

Exception in thread "Thread-23" java.util.ConcurrentModificationException
    at java.util.ArrayDeque$DeqIterator.next(ArrayDeque.java:648)
    at com.abc.dept.featurex.aspectx.a.EventAccumulator.updatePeriodCount(EventAccumulator.java:64)
    at com.abc.dept.featurex.aspectx.a.EventAccumulator.access$300(EventAccumulator.java:20)
    at com.abc.dept.featurex.aspectx.a.EventAccumulator$EventAccumulatorMaintenanceThread.run(EventAccumulator.java:99)

If the class instance is not shared between threads why is ConcurrentModificationException thrown? Is this safe to use in non-debug mode (i.e. production) assuming the debug mode is using background threads (i.e. through some optimization) that are not being reported in the IDE?

awgtek
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  • This Exception just means, that your List was updated since the iterator was created. It doesn't say anything about on which Thread that modification happened. Are you abolutely shure, that you don't modify the content of your List in the for-loop? – keuleJ Dec 03 '18 at 15:39
  • `countsList` is private and only modifed in the code listed via `pollLast()` as shown and not in the for loop shown. – awgtek Dec 03 '18 at 15:43
  • Any other methods that change countsList or make countsList public (e.g. getter)? – keuleJ Dec 03 '18 at 16:20
  • no, the only place in the code where `countsList` is used (other than in a `PostConstruct` initialization) is in the `updatePeriodCount()` method shown. There are no getters or other places where `countsList` is modified. – awgtek Dec 03 '18 at 16:32

1 Answers1

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No, this class is almost certainly not safe to use in production. Debuggers don't normally change the content of a collection. And that's what happened here. The content of countsList was changed, while you were iterating over it.

Try to find the source of the modification with the debugger or with log. Maybe you want to set breakpoints in certain methods of ArrayDequeue or override it and place some log in it.

keuleJ
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  • Ok, I thought it was the debugger, thanks for clearing that up. I followed your suggestion on the breakpoint and discovered the init thread wasn't setting a flag causing a second thread to be created on update, and that was the cause. Thanks. – awgtek Dec 03 '18 at 18:25