Java多线程之concurrent包（四）——Semaphore信号量

Semaphore当前在多线程环境下被扩放使用，操作系统的信号量是个很重要的概念，在进程控制方面都有应用。Java 并发库 的Semaphore 可以很轻松完成信号量控制，Semaphore可以控制某个资源可被同时访问的个数，通过 acquire() 获取一个许可，如果没有就等待，而 release() 释放一个许可。

public class SemaphoreDemo {    public static void main(String[] args)    {        Runnable limitedCall = new Runnable() {            final Random rand = new Random();            //一共只有三个坑，没有释放的就等待            final Semaphore available = new Semaphore(3);            public void run()            {                int time = rand.nextInt(15);                try                {                    available.acquire();                    System.out.println("Executing " +                         "long-running action for " +                         time + " seconds... #" + Thread.currentThread().getName());                    Thread.sleep(time * 1000);                    System.out.println("Done with #" +                             Thread.currentThread().getName() + "!");                    available.release();                }                catch (InterruptedException intEx)                {                    intEx.printStackTrace();                }            }        };        for (int i=0; i<10; i++)            new Thread(limitedCall).start();    }}

执行结果如下：

Executing long-running action for 7 seconds... #Thread-1Executing long-running action for 2 seconds... #Thread-2Executing long-running action for 14 seconds... #Thread-0Done with #Thread-2!Executing long-running action for 1 seconds... #Thread-3Done with #Thread-3!Executing long-running action for 4 seconds... #Thread-7Done with #Thread-1!Executing long-running action for 10 seconds... #Thread-9Done with #Thread-7!Executing long-running action for 1 seconds... #Thread-6Done with #Thread-6!Executing long-running action for 9 seconds... #Thread-5Done with #Thread-0!Executing long-running action for 6 seconds... #Thread-4Done with #Thread-9!Executing long-running action for 12 seconds... #Thread-8Done with #Thread-5!Done with #Thread-4!Done with #Thread-8!

可以从结果当中看出，线程1，2，0三个线程限制性。后续线程需要执行必须等到释放掉该信号量。

补充：
计数信号的一种特殊情况是互斥，或者互斥信号。互斥就是具有单一许可权的计数信号，意味着在给定时间仅一个线程可以具有许可权（也称为二进制信号）。互斥可以用于管理对共享资源的独占访问。

虽然互斥许多地方与锁定一样，但互斥还有一个锁定通常没有的其他功能，就是互斥可以由具有许可权的线程之外的其他线程来释放。这在死锁恢复时会非常有用。