示例代码

package com.example.demo.service;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class UnSafeCDTest {

    ExecutorService executorService = Executors.newFixedThreadPool(5);

    public static void main(String[] args) throws InterruptedException {
        for(int i = 0; i < 20; i++){
            UnSafeCDTest unSafeCDTest = new UnSafeCDTest();
            unSafeCDTest.test();
        }

    }


    public void test() throws InterruptedException {
        List<String> list = new ArrayList<>();
      //  List<String> list = new CopyOnWriteArrayList<>();
        CountDownLatch countDownLatch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            executorService.submit(() -> {
                // TODO
                list.add("1" );
                // TODO
                // cd
                countDownLatch.countDown();

            });
        }
        countDownLatch.await();
        System.out.println( list.size());

    }
}

输出结果

可以看到有一个结果为4；
修改ArrayList为线程安全的List,CopyOnWriteArrayList;
验证结果

CopyOnWriteArrayList为什么是线程安全的
查看源码可以看到通过了加锁实现了线程安全

/**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }