ArrrayList为什么线程不安全?怎么将其变为线程安全

package com.weigu.xiaochuang.concurrent;

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

/**
 * Arraylist为什么线程不安全,
 * 通过向其中添加元素进行判断就可以;
 * 那么怎么将他变为线程安全的呢?
 * 利用同步的机制,因为底层仍然是靠synchrozied去实现的
 */
public class ArraylistDemo {
    public static void main(String[] args) {
//        final ArrayList list = new ArrayList<>();
        List list = Collections.synchronizedList(new ArrayList());
        ExecutorService executorService =
                Executors.newFixedThreadPool(100);
        for (int i = 0; i < 10000; i++) {
            executorService.submit(new Runnable() {
                @Override
                public void run() {
                    list.add("5");
                }
            });
        }
        executorService.shutdown();

        while (true) {
            //子线程是否终止
            if (executorService.isTerminated()) {
                System.out.println("所有的子线程都结束了");
                System.out.println(list.size());
                if (list.size() != 10000) {
                    System.out.println("线程不安全");
                } else {
                    System.out.println("线程安全");
                }
                break;
            }
        }
    }
}