这个SalesDemo里面创建了一个Runnable的object，然后start() 两次。如果多跑几次的话，会有机会2个线程一起卖了6件商品，而goodsAmount变成了-1.

这个data racing是因为第56行 `goodsAmount.get()`和第57行`goodsAmount.decrementAndGet()` 两个操作分离开来，没法变成原子化操作，产生了data racing，让商品剩余变成 -1. 虽然 get()和decrementAndGet()都是原子化操作，但需要把他们合起来变成一个更大的原子化操作才能防止 -1 个商品情况出现。

data racing场景：

timeline goodsAmount ThreadA ThreadB

1 1 get(), return 1

2 1 enter if() get(), return 1

3 0 decrement(), 0 enter if()

4 -1 exit if() decrement(), -1

5 -1 -- exit if()

在ThreadA get()完之后还没来得及减一，ThreadB趁此机会get()到了和ThreadA一样的数值，也准备减一，导致剩余-1件商品。

解决办法：将get()和减一捆绑成原子操作。

```

// Other code

private int goodAmount = GOOD_AMOUNT;

/**

* Atomic operation on saling goods. For success sale, return the remain amount, otherwise

* return -1 indicating no goods to sale.

*

* @return remain amount of goods.

*/

private synchronized int saleOneGood() {

if (goodAmount > 0) {

goodAmount--;

return goodAmount ;

}

return -1;

}

// Other code

public void run() {

for (int i = 0; i < APPLE_AMOUNT; i++) {

int goodsRemain = saleOneGood();

if (goodsRemain != -1) {

System.out.println(Thread.currentThread().getName() + " sold a good, remains " + goodsRemain);

}

}

}

```

Best,

JohnstonM

* Atomic operation on saling goods. For success sale, return the remain amount, otherwise

* Atomic operation on saling goods. For success saling, return the remain amount, otherwise

int goodsRemain = saleOneGood();

* Atomic operation on saling goods. For one successful saling, return the remain amount, otherwise

这个data racing是因为第56行 `goodsAmount.get()`和第57行`goodsAmount.decrementAndGet()` 两个操作分离开来，没法变成原子化操作，产生了data racing，让商品剩余变成 -1. 虽然 get()和decrementAndGet()都是原子化操作，但需要把他们合起来变成一个更大的原子化操作才能防止 -1 个商品情况出现。

timeline goodsAmount ThreadA ThreadB

3 0 decrement(), 0 enter if()

4 -1 exit if() decrement(), -1

2 1 enter if() get(), return 1

3 0 decrement(), 1->0 enter if()

4 -1 exit if() decrement(), 0->-1

5 -1 -- exit if()

for (int i = 0; i < APPLE_AMOUNT; i++) {

private synchronized int saleOneGood() {

int goodsRemain = saleOneGood(); // Selling one good atomically