java8 stream map reduce cause a the ConcurrentModificationException, why?

Question

final BigDecimal couponlessAmount = orderItems.stream()
            .filter(item -> !item.getIsUseCoupon())
            .map(item -> item.getTotalAmount().subtract(item.getReduceProductAmount()))
            .reduce(BigDecimal.ZERO, BigDecimal::add);

something causes the java.util.ConcurrentModificationException

java.util.ConcurrentModificationException
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1380) ~[?:1.8.0_77]
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[?:1.8.0_77]
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[?:1.8.0_77]
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_77]
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_77]
    at java.util.stream.ReferencePipeline.reduce(ReferencePipeline.java:474) ~[?:1.8.0_77]
    at com.yijiupi.himalaya.ordercompute.provider.domain.component.computer.CouponUseLimitComputer.getMaxUseAmount(CouponUseLimitComputer.java:103) ~[classes/:?]

sometimes, not everytime, I think it's a bug of JDK, maybe JIT — komite, Aug 04 '16 at 07:45

score 0 · Accepted Answer · answered Aug 08 '16 at 04:13

It's unlikely to be JDK/JIT bug. As usually with ConcurrentModificationException it's a sign that your orderItems is concurrently modified from another thread. You should not use ArrayList concurrently without providing proper external synchronization. Note that wrapping ArrayList into Collections.synchronizedList is not enough here as it does not synchronizes traversals. As an alternative (if modifications are rare) use CopyOnWriteArrayList.

java8 stream map reduce cause a the ConcurrentModificationException, why?

1 Answers1