如何解决hashset在多线程下不安全？

	## 两种反射  推荐使用第二种  效率较高
	Set set = Collections.synchronizedSet(new HashSet<>());
    Set set = new CopyOnWriteArraySet<>();//写入时复制

package unsafe;

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;

public class SetTest {
    public static void main(String[] args) {
//        Set set = new HashSet<>();
//        Set set = Collections.synchronizedSet(new HashSet<>());
        Set<String> set = new CopyOnWriteArraySet<>();//写入时复制
        for (int i = 0; i < 100; i++) {
            new Thread(()->{
                set.add(UUID.randomUUID().toString().substring(0,5));
                System.out.println(set);
            },String.valueOf(i)).start();
        }
    }
}

hashset的底层是什么？