subList引起的java.util.ConcurrentModificationException原因解析

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我是靠谱客的博主还单身帆布鞋，最近开发中收集的这篇文章主要介绍subList引起的java.util.ConcurrentModificationException原因解析，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

最近需要处理一份挺大的数据，因此想到用多线程，将文件数据读到list，然后切割成n份处理，结果报了ConcurrentModificationException，大概的代码如下：

 public void execute(ExecutorContext executorContext) {

        List<String> lists = Utils.readFile2List("list.txt");
        lists = lists.stream().map(a -> a.replace(""", "")).collect(Collectors.toList());
        // 分成n份数据来跑
        int size = lists.size();
        int count = size / threadNum;
        int index = 0;
        poolExecutor = new ThreadPoolExecutor(threadNum, threadNum,
                0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(threadNum),
                new ThreadPoolExecutor.CallerRunsPolicy());
        List<Future<Map<String, Set<String>>>> futures = new ArrayList<>();
        while (index < size) {
            // 需新建一个list，否则操作sublist修改数据后会导致和原list modCount不相等，从而报错
            List<String> subList = lists.subList(index, index + count > size? size: index+count);
            futures.add(poolExecutor.submit(new DealListCall(subList, index)));
            index+=count;
        }
        for(Future<Map<String, Set<String>>> future: futures) {
            try {
                Map<String, Set<String>> stringSetMap = future.get();
                writefileinfo("list1", stringSetMap.get("set1"));
                logger.info("index{}, list1{}", JsonUtils.objectToJson(stringSetMap.get("index")), JsonUtils
                        .objectToJson(stringSetMap.get("set1")));
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }

    }

    private class DealListCall implements Callable<Map<String, Set<String>>> {

        private List<String> lists;
        private int index;

        public DealListCall(List<String> lists, int index) {
            this.lists = lists;
            this.index = index;
        }
        @Override
        public Map<String, Set<String>> call() throws Exception {
            Map<String, Set<String>> map = new HashMap<>();
            Set<String> set1 = new HashSet<>();
            for (int i = 0; i < 64; i++) {
                int size = lists.size();
                int index = 0;
                // 用set来保证去重
                Set<String> removelists = new HashSet<>();
                Set<String> result = new HashSet<>();
                while (index < size) {
                    long start = System.currentTimeMillis();
                    ...
                    index+=1000;
                    logger.info("当前i{}, index{}, 耗费时间为{}", i, index, System.currentTimeMillis() - start);
                }
                set1.addAll(result);
                lists.removeAll(removelists);
            }
            map.put("set1", set1);
            map.put("index", set);
            Thread.sleep(2000+index);
            return map;
        }
    }

    public Boolean writefileinfo(String path, Set<String> set) {
        FileSystemResource resource = new FileSystemResource(path);
        try {
            FileWriter fileWriter = (new FileWriter(resource.getFile(), true));
            for (String str : set) {
                fileWriter.write(str + "n");
            }
            fileWriter.close();
        } catch (IOException e) {
            return false;
        }
        return true;
    }

运行后报了ConcurrentModificationException，然后在网上找了一下，原因是切割list时使用了subList方法，而subList方法返回的是原list中某段数据的视图，然后ArrayList$SubList.size方法,如果原List和SubList的modCount不相等就会报错，查看源代码如下：

public E set(int index, E e) {
    rangeCheck(index);
    checkForComodification();
    E oldValue = ArrayList.this.elementData(offset + index);
    ArrayList.this.elementData[offset + index] = e;
    return oldValue;
}
public E get(int index) {
    rangeCheck(index);
    checkForComodification();
    return ArrayList.this.elementData(offset + index);
}

private void checkForComodification() {
     if (ArrayList.this.modCount != this.modCount)
         throw new ConcurrentModificationException();
     }

解决办法是新建一个list对象来包装原来的list，即new ArrayList(list1.subList(0, list1.size()))