volatile 不能保证原子性，所以在并发的情况下，会存在自增不一致的情况。例如

有问题的代码

public class JavaDemo {

    private static Integer num = 0;
    public static void main(String[] args) throws InterruptedException {

        CountDownLatch downLatch = new CountDownLatch(1000);
        for (int i = 1; i <= 1000; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    num++;
                    downLatch.countDown();
                }
            }).start();
        }

        downLatch.await();

        System.out.println(num);
    }

} 

没问题的代码 ， 使用并发包下面的方法进行自增

public class JavaDemo {

    private static AtomicInteger num = new AtomicInteger(0);

     
    public static void main(String[] args) throws InterruptedException {

        CountDownLatch downLatch = new CountDownLatch(1000);
        for (int i = 1; i <= 1000; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    num.getAndIncrement();
                    downLatch.countDown();
                }
            }).start();
        }

        downLatch.await();

        System.out.println(num);
    }

}