Java forEach lambda 抛出并发修改异常

Question

下面的方法抛出ConcurrentModificationException的原因可能是什么？

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;
    }

堆栈跟踪:

"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)

使用简单的for循环它工作得很好。该方法由多个线程使用其自己的执行对象副本调用。

使用简单 for 循环的解决方案:

 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;
    }

我觉得它与静态方法及其由多个线程访问的局部变量有关，但根据理论，这将是线程局部变量并且不共享。给我一些时间来举一些简单的例子。

Answer 1

你的逻辑可以是这样的:

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

• 您正在 forEach 中使用 forEach(这里我将其替换为 flatMap)
• 然后检查每个元素 alloc.account() 是否为 null(我将其替换为 filter)
• 然后，如果条件正确，则将其添加到集合中(我将其替换为 collect)