Java forEach lambda throws concurrentModificationException

Question

What could be the reason that below method throws ConcurrentModificationException?

static Set<String> setOfAllocAccountsIn(final @NotNull Execution execution) {
        final Set<String> allocAccounts = new HashSet<>();
        execution.legs().forEach(leg -> leg.allocs().forEach(alloc -> {
            if (alloc.account() != null) {
                allocAccounts.add(alloc.account());
            }
        }));
        return allocAccounts;
    }

Stack Trace:

"java.util.ConcurrentModificationException:
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1382)
    at com.client.stp.util.DealsBuilderUtil.setOfAllocAccountsIn(DealsBuilderUtil.java:146)
    at com.client.stp.builder.GridDealsObjectBuilder.build(GridDealsObjectBuilder.java:47)
    at com.client.stp.processor.DealToPDXMLTransformer.transform(DealToPDXMLTransformer.java:29)
    at com.client.stp.processor.WorkPartitionerEngine.lambda$onEventSubmit$0(WorkPartitionerEngine.java:40)
    at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:514)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
    at java.base/java.lang.Thread.run(Thread.java:844)

With simple for-loop it works fine. The method is called by multiple threads with its own copy of execution object.

Solution which worked with simple for loop:

 static Set<String> setOfAllocAccountsIn(final @NotNull Execution execution) {
        final Set<String> allocAccounts = new HashSet<>();
        for (final ExecutionLeg executionLeg : execution.legs()) {
            for (final ExecutionAlloc executionAlloc : executionLeg.allocs()) {
                if (executionAlloc.account() != null) {
                    allocAccounts.add(executionAlloc.account());
                }
            }
        }
        return allocAccounts;
    }

I feel that it has something to do with static method and its local variable accessed by multiple threads but per theory that will be thread local variable and are not shared. Give me some time to put down some simple example.

Please post a minimal reproducible example. Is it possible that one of the methods modifies one of the collections involved (eg maybe executionAlloc.account() modifies on of the collections?) — Mark Rotteveel
– Mark Rotteveel, Commented May 18, 2018 at 9:33

Youcef LAIDANI · Accepted Answer · 2018-05-18 11:42:39Z

2

Your logic can be like this :

return execution.legs().stream()
            .flatMap(leg -> leg.allocs().stream())
            .map(executionAlloc -> executionAlloc.account())
            .filter(Objects::nonNull)
            .collect(Collectors.toSet());

You are using forEach inside forEach (Here I replace it with flatMap)
then you check for each element if alloc.account() is null or not (I replace it with filter)
then if the condition is correct you add it to the Set (I replace it with a collect)

edited May 18, 2018 at 11:42

answered May 18, 2018 at 8:41

Youcef LAIDANI

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4 Comments

Eran Over a year ago

Yes, this code is cleaner than the OP's code, but how would making this change solve the ConcurrentModificationException?

Youcef LAIDANI Over a year ago

@Eran honestly I don't see where the OP can get ConcurrentModificationException

Holger Over a year ago

Since the result of leg.allocs() has a forEach method, it is not an array but likely a collection, so it should be leg.allocs().stream() instead of Stream.of(leg.allocs()). Further, you gain a lot, if you do .map(executionAlloc -> executionAlloc.account()) before the filter, as then, the predicate becomes a simple null-check (filter(Objects::nonNull)) instead of needing to repeat the account() call.

Youcef LAIDANI Over a year ago

this is correct @Holger thank you for the details, I edit my answer

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