在使用CompletableFuture时，发现获取数据时，有时候数据获取不到(值为null)。

代码如下：

package com.example.mavendemo.completablefuture;

import com.google.common.collect.Lists;
import lombok.extern.slf4j.Slf4j;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.*;

/**
 * @ClassName NullTest.java
 * @Description
 * @Author chengjiangbo@xinxuan.net
 * @DateTime 2024/8/15 11:14
 * @Version 1.0
 */
@Slf4j
public class NullTest {

    public static void main(String[] args) {
        new NullTest().test();
    }

    public void test()  {
        //自定义线程池
        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 5, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>(), new ThreadPoolExecutor.CallerRunsPolicy());


        List<Integer> safeList = new ArrayList<>();

        List<Integer> numbers = Arrays.asList(0, 10, 20, 30, 40);

        List<CompletableFuture> completableFutures =  Lists.newArrayList();
        for (int i = 0; i < 5; i++) {
            int finalI = i;
            CompletableFuture completableFuture = CompletableFuture.runAsync(() -> {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Integer value = numbers.get(finalI);
                safeList.add(value * value);
                log.info("异步线程=======>" + Thread.currentThread().getName() + ":" + System.currentTimeMillis());
            }, executor);

            completableFutures.add(completableFuture);
        }

        CompletableFuture<Void> allFuture = CompletableFuture.allOf(completableFutures.toArray(new CompletableFuture[completableFutures.size()]));
        try{
            allFuture.get();
        } catch (Exception e){
            e.printStackTrace();
        }

        safeList.stream().forEach(System.out::println);

        //关闭线程池
        executor.shutdown();
    }
}

获取不正常的结果如下：

经过排查发现是居然是List集合的线程安全的问题导致：

//        List<Integer> safeList = new ArrayList<>();
        //注意，一定要使用线程安全的集合放置值，否组会出现线程安全问题，导致部分数据丢失
        CopyOnWriteArrayList<Integer> safeList = new CopyOnWriteArrayList();

再次运行多遍均获得正确结果：