map ConcurrentModificationException in for header

问题

I have the following code:

private Multimap<Object, ComplexCalcStrategy> strategies = HashMultimap.create();
....
Collection<ComplexCalcStrategy> strategiesThatNeedUpdating = strategies.get(mktDataChange.getOptionId());
for (ComplexCalcStrategy strategy : strategiesThatNeedUpdating) {  //row number 88
     updateMarketData(strategy.getStrategy(), mktDataChange);
}

and in logs I see following trace:

java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
    at java.util.HashMap$KeyIterator.next(HashMap.java:828)
    at com.google.common.collect.AbstractMapBasedMultimap$WrappedCollection$WrappedIterator.next(AbstractMapBasedMultimap.java:486)
    at package.ClassName.processMarketDataChange(ClassName.java:88)

I don't understand how does this code can produce CME.

Please, share your ideas.

P.S.

map fills here:

@Override
public void registerStrategy(final ComplexCalcStrategy strategy) {
    for (Integer optionId : strategy.getOptions()) {
        strategies.put(optionId, strategy);

    }
    if (strategy.getStrategy().tiedToStock) {
        strategies.put(strategy.getUnderlying(), strategy);
    }

    ....
}

This method invokes in another thread and looks like sometime iteration and map filling happens simultaneously and it is root cause.

I understand that I can use synchronized version of multimap.

Will it fix issue?

Is there nicer way?

回答1:

ConcurrentModificationException:

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.

If you modify the keys/values in a Multimap during iteration of the keys/values then you cannot continue iterating after the modification (hence the exception).

Consider using Multimaps.transformValues(Multimap, Function) instead.

来源：https://stackoverflow.com/questions/36228084/map-concurrentmodificationexception-in-for-header