转载：http://www.cnblogs.com/nullzx/p/5270233.html

在Java的并发包中，Semaphore类表示信号量。Semaphore内部主要通过AQS（AbstractQueuedSynchronizer）实现线程的管理。Semaphore有两个构造函数，参数permits表示许可数，它最后传递给了AQS的state值。线程在运行时首先获取许可，如果成功，许可数就减1，线程运行，当线程运行结束就释放许可，许可数就加1。如果许可数为0，则获取失败，线程位于AQS的等待队列中，它会被其它释放许可的线程唤醒。在创建Semaphore对象的时候还可以指定它的公平性。一般常用非公平的信号量，非公平信号量是指在获取许可时先尝试获取许可，而不必关心是否已有需要获取许可的线程位于等待队列中，如果获取失败，才会入列。而公平的信号量在获取许可时首先要查看等待队列中是否已有线程，如果有则入列。

构造函数源代码

//非公平的构造函数
public Semaphore(int permits) {
    sync = new NonfairSync(permits);
}

//通过fair参数决定公平性
public Semaphore(int permits, boolean fair) {
    sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}

acquire源代码

public void acquire() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

final int nonfairTryAcquireShared(int acquires) {
    for (;;) {
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
            compareAndSetState(available, remaining))
            return remaining;
    }
}

可以看出，如果remaining <0 即获取许可后，许可数小于0，则获取失败，在doAcquireSharedInterruptibly方法中线程会将自身阻塞，然后入列。

release源代码

public void release() {
    sync.releaseShared(1);
}

public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

protected final boolean tryReleaseShared(int releases) {
    for (;;) {
        int current = getState();
        int next = current + releases;
        if (next < current) // overflow
           throw new Error("Maximum permit count exceeded");
        if (compareAndSetState(current, next))
            return true;
    }
}

可以看出释放许可就是将AQS中state的值加1。然后通过doReleaseShared唤醒等待队列的第一个节点。可以看出Semaphore使用的是AQS的共享模式，等待队列中的第一个节点，如果第一个节点成功获取许可，又会唤醒下一个节点，以此类推。

使用示例

package javalearning;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class SemaphoreDemo {
	private Semaphore smp = new Semaphore(3); 
	private Random rnd = new Random();
	
	class TaskDemo implements Runnable{
		private String id;
		TaskDemo(String id){
			this.id = id;
		}
		@Override
		public void run(){
			try {
				smp.acquire();
				System.out.println("Thread " + id + " is working");
				Thread.sleep(rnd.nextInt(1000));
				smp.release();
				System.out.println("Thread " + id + " is over");
			} catch (InterruptedException e) {
			}
		}
	}
	
	public static void main(String[] args){
		SemaphoreDemo semaphoreDemo = new SemaphoreDemo();
		//注意我创建的线程池类型，
		ExecutorService se = Executors.newCachedThreadPool();
		se.submit(semaphoreDemo.new TaskDemo("a"));
		se.submit(semaphoreDemo.new TaskDemo("b"));
		se.submit(semaphoreDemo.new TaskDemo("c"));
		se.submit(semaphoreDemo.new TaskDemo("d"));
		se.submit(semaphoreDemo.new TaskDemo("e"));
		se.submit(semaphoreDemo.new TaskDemo("f"));
		se.shutdown();
	}
}

运行结果

Thread c is working

Thread b is working

Thread a is working

Thread c is over

Thread d is working

Thread b is over

Thread e is working

Thread a is over

Thread f is working

Thread d is over

Thread e is over

Thread f is over

可以看出，最多同时有三个线程并发执行，也可以认为有三个公共资源（比如计算机的三个串口）。