1.应用场景，今天生产日志监控到一下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 0
	at 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 0
	at 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
                                        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.

分析：这种情况是多线程并发情况下添加数据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<>()