Java中的同步——示例程序
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前面说到了Java中的同步问题。下面通过一个小小的实例程序来演示Java中的同步方法。其中对前文提到的Counter类做了稍微的修改。

public class Main {public static void main(String[] args) {Counter c = new Counter();ThreadA a = new ThreadA(c);ThreadB b = new ThreadB(c);Thread threadA = new Thread(a);Thread threadB = new Thread(b);threadA.start();threadB.start();}}

执行上面的代码，可能的结果如下：

before increment, c = 0
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after increment, c = 1
before increment, c = 1
after decrement, c = 2
before decrement, c = 2
after decrement, c = 1
before decrement, c = 1
after increment, c = 0
before increment, c = 0
after increment, c = 1
before increment, c = 1
after decrement, c = 2
before decrement, c = 2
after increment, c = 1
after decrement, c = 1
before decrement, c = 1
after decrement, c = 0

从上面的输出结果中我们不难看出出现了严重的交错现象！ 在increment或者是decrement方法中输出before和after本应该是成对连续出现的。但输出结果却不是如此。

将上面代码的increment()和decrement()方法用synchronized 修饰后，重新运行该程序，输出结果如下：

before increment, c = 0
after increment, c = 1
before increment, c = 1
after increment, c = 2
before decrement, c = 2
after decrement, c = 1
before increment, c = 1
after increment, c = 2
before decrement, c = 2
after decrement, c = 1
before decrement, c = 1
after decrement, c = 0
before decrement, c = 0
after decrement, c = -1
before decrement, c = -1
after decrement, c = -2
before increment, c = -2
after increment, c = -1
before decrement, c = -1
after decrement, c = -2
before decrement, c = -2
after decrement, c = -3
before decrement, c = -3
after decrement, c = -4
before decrement, c = -4
after decrement, c = -5
before increment, c = -5
after increment, c = -4
before decrement, c = -4
after decrement, c = -5
before increment, c = -5
after increment, c = -4
before increment, c = -4
after increment, c = -3
before increment, c = -3
after increment, c = -2
before increment, c = -2
after increment, c = -1
before increment, c = -1
after increment, c = 0

这样输出结果和没有增加synchronized修饰符时的大不相同：单独一次的increment和decrement方法并没有出现交错的现象。只是连续10次的increment()和decrement ()有交错（这个不是synchronized能解决的问题）。

至少，我们从上面的实例程序中可以看到synchronized方法的作用了。

另外，我们还可以使用同步语句（也就是使用Counter对象的同步锁）来代替上面的同步方法，其效果是一样的。有兴趣的网友可以自己尝试一下。