volatile在高并发环境下能保证可见性和指令重拍吗？

Question

package fangfa;

import java.util.concurrent.CountDownLatch;

public class Ljsdjj24324 {
    public static void main(String[] args) throws InterruptedException {
        Hasdy hasdy = new Hasdy();
        CountDownLatch countDownLatch = new CountDownLatch(9999);
        for (int i = 0; i < 9999; i++) {
            new Thread(()->{
                for (int i1 = 0; i1 < 100; i1++) {
                    hasdy.setaaa();
                }
                countDownLatch.countDown();
            }).start();
        }
        countDownLatch.await();
        Thread.sleep(5000);
        System.out.println(hasdy.getasd());
    }
}
class Hasdy{
    volatile int a = 1;
    void setaaa(){
        a+=1;
    }
    int getasd(){
        return a;
    }
}

volatile并没有保证多线程安全
为什么会这样。
根据Happens-Before规则，先开启的线程对数据的修改对后续线程应该是可见的啊
难道是我理解错了吗？