1.应用场景，今天生产日志监控到一组new ArrayList() 进行add 异常，具体日志如下：

eptionHandler.handler(178): TXXYBUSSINESS|执行异常
java.util.concurrent.CompletionException: java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 0at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315) ~[?:?]at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320) ~[?:?]at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1807) ~[?:?]at com.txxy.common.support.ThreadContextTaskDecorator.lambda$decorate$0(ThreadContextTaskDecorator.java:20) ~[txxy-common.jar:10.3.10-SNAPSHOT]at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]at java.lang.Thread.run(Thread.java:833) ~[?:?]
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 0at java.util.ArrayList.add(ArrayList.java:455) ~[?:?]at java.util.ArrayList.add(ArrayList.java:467) ~[?:?]at com.txxy.api.support.PersonalRiskReportSupport.lambda$riskType4$28(PersonalRiskReportSupport.java:528) ~[classes/:?]at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1804) ~[?:?]

具体代码：

List<DTO> riskSummaryList = new ArrayList<>();// 10条记录批量查询Lists.partition(ids, Paging.DEF_PAGE_SIZE).forEach(arrIds -> {List<CompletableFuture<Void>> list =arrIds.stream().map(id -> CompletableFuture.runAsync(() -> {try {// ....代码省略了http请求 得到的responseDetail 结果集if (responseDetail.isSuccess()) {riskSummaryList.add(new DTO() );}} catch (TxxyBusinessException e) {}}, apiTaskExecutor)).collect(Collectors.toList());list.stream().map(CompletableFuture::join).collect(Collectors.toList());});result.setRiskSummaryList(riskSummaryList);

上面代码是对ids 分片按照10条记录查询远程接口，并将结果集放到list riskSummaryList 对象中 riskSummaryList.add(new DTO() )最后输出结果，代码比较简单.

分析：这种情况属于多线程并发产生的，ArrayList是一个非线程安全对象，在并发操作同一个对象时就可能出现上面的异常 情况，ArrayList 源码中ensureCapacity 这个方法是非线程安全的导致计算值扩容不够抛出的异常。
查看ArrayList源码：

解决方法：
1.使用官网提供的方法Collections.synchronizedList

  List list = Collections.synchronizedList(new ArrayList(...));

2.new一个对象CopyOnWriteArrayList方法

new CopyOnWriteArrayList<>()

最后希望能帮助到你解决问题，或是评论区留言问题进行讨论~