一、加鎖實現計數器

public class CounterSync implements Counter {

    volatile int i = 0; // 本質是修改記憶體中某一個變數的值

    public synchronized int incr() {
        return i++;
    }

    public int decr() {
        return i--;
    }

    @Override
    public int get() {
        return i;
    }
}

二、測試

public static void main(String[] args) throws InterruptedException {
        final Counter ct = new CounterSync();

        //模擬多執行緒場景
        CountDownLatch countDownLatch = new CountDownLatch(2);

        for (int i = 0; i < 2; i++) {
            new Thread(() -> {
                long begin = System.nanoTime();
                for (int j = 0; j < 10000; j++) {
                    ct.incr();
                }
                System.out.println("done...運算時間： " + (System.nanoTime() - begin));
                countDownLatch.countDown();
            }).start();
        }

        countDownLatch.await();
        System.out.println("計數器最終結果: " + ct.get());
        // 預期結果應該 --- 20000
    }

三、列印結果

done...運算時間： 1173900
done...運算時間： 803900
計數器最終結果: 20000