对资源的许可访问 Semaphore用法小例
Semaphore 通常用于限制可以访问某些资源（物理或逻辑的）的线程数目
主要方法
???public Semaphore(int permits)? //构造一个指定许可数目的信号量
?? public void acquire()throws InterruptedException? // 获取信号量，没有则阻塞当前线程
???public void release()? //释放一个信号量
?? 另外还有相应的重载方法
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例子
以下代码描述了这样一个应用，有2台pc，5个玩家，同一时刻只能有2个玩家使用pc（因为只有2台pc嘛），同一pc同一时刻只能归于一个玩家使用
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输出
Player 1 play game with PC-I start at time 2011-04-14 12:59:49
Player 2 play game with PC-II start at time 2011-04-14 12:59:49
Player 2 play game with PC-II over at time 2011-04-14 12:59:50
Player 3 play game with PC-II start at time 2011-04-14 12:59:50
Player 1 play game with PC-I over at time 2011-04-14 12:59:50
Player 4 play game with PC-I start at time 2011-04-14 12:59:50
Player 4 play game with PC-I over at time 2011-04-14 12:59:51
Player 3 play game with PC-II over at time 2011-04-14 12:59:51
Player 5 play game with PC-I start at time 2011-04-14 12:59:51
Player 5 play game with PC-I over at time 2011-04-14 12:59:52
