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(() -> {// TODOlist.add("1" );// TODO// cdcountDownLatch.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();}}