Java 并发编程 -- Semaphore

概述

在操作系统中，信号量是个很重要的概念，它在控制进程间的协作方面有着非常重要的作用，通过对信号量的不同操作，可以分别实现进程间的互斥与同步。当然它也可以用于多线程的控制，我们完全可以通过使用信号量来自定义实现类似 Java 中的 synchronized、wait、notify 机制。

Java 并发包中的信号量 Semaphore 实际上是一个功能完毕的计数信号量，从概念上讲，它维护了一个许可集合，对控制一定资源的消费与回收有着很重要的意义。Semaphore 可以控制某个资源被同时访问的任务数，它通过 acquire() 获取一个许可，release() 释放一个许可。如果被同时访问的任务数已满，则其他 acquire 的任务进入等待状态，直到有一个任务被 release 掉，它才能得到许可。
Semaphore 仅仅是对资源的并发访问的任务数进行监控，而不会保证线程安全，因此，在访问的时候，要自己控制线程的安全访问。

使用方法

下面给出一个采用 Semaphore 控制并发访问数量的示例程序：

Semaphore mSemaphore = new Semaphore(2);
public void testSemaphore() {
    Thread thread1 = new Thread(new MyRunnable());
    thread1.setName("Thread1");
    Thread thread2 = new Thread(new MyRunnable());
    thread2.setName("Thread2");
    Thread thread3 = new Thread(new MyRunnable());
    thread3.setName("Thread3");
    Thread thread4 = new Thread(new MyRunnable());
    thread4.setName("Thread4");
    Thread thread5 = new Thread(new MyRunnable());
    thread5.setName("Thread5");

    thread1.start();
    thread2.start();
    thread3.start();
    thread4.start();
    thread5.start();
}

private class MyRunnable implements Runnable{
    @Override
    public void run() {
        try {
            Log.e("Test","Thread "+Thread.currentThread().getName()+" run");
            mSemaphore.acquire();
            Log.e("Test","Thread "+Thread.currentThread().getName()+" acquire");
            Thread.sleep(1000);
            mSemaphore.release();
            Log.e("Test","Thread "+Thread.currentThread().getName()+" release");
            Thread.sleep(1000);
            Log.e("Test","Thread "+Thread.currentThread().getName()+" end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

00:08:15.645 E/Test: Thread Thread1 run
00:08:15.645 E/Test: Thread Thread1 acquire
00:08:15.645 E/Test: Thread Thread5 run
00:08:15.645 E/Test: Thread Thread5 acquire
00:08:15.645 E/Test: Thread Thread3 run
00:08:15.645 E/Test: Thread Thread4 run
00:08:15.645 E/Test: Thread Thread2 run
00:08:16.650 E/Test: Thread Thread1 release
00:08:16.650 E/Test: Thread Thread5 release
00:08:16.650 E/Test: Thread Thread3 acquire
00:08:16.650 E/Test: Thread Thread4 acquire
00:08:17.652 E/Test: Thread Thread5 end
00:08:17.652 E/Test: Thread Thread3 release
00:08:17.652 E/Test: Thread Thread1 end
00:08:17.652 E/Test: Thread Thread4 release
00:08:17.652 E/Test: Thread Thread2 acquire
00:08:18.654 E/Test: Thread Thread3 end
00:08:18.654 E/Test: Thread Thread4 end
00:08:18.655 E/Test: Thread Thread2 release
00:08:19.656 E/Test: Thread Thread2 end

可以看出，Semaphore 允许并发访问 mSemaphore.acquire() 和 mSemaphore.release() 之间资源的任务数一直为 2。