Semaphore是一个二进制信号量，只有0和1两个值。如果线程想要访问一个共享资源，它必须先获得信号量。如果信号量的内部计数器大于0，那么信号量减1，并允许访问这个资源。否则，如果信号量计数器等于0，线程会等待直至计数器大于0。

所以说计数器大于0，说明有资源可用。计数器等于0，说明没有资源可用。

同时Semaphore提供了一个带有boolean参数的构造方法，true代表公平锁，false代表非公平锁，默认实现是非公平锁。

我们使用Semaphore信号量来重写PrintQueue的例子。

首先实现一个PrintQueue打印队列，有一个Semaphore信号量用来并发访问控制。打印之前使用acquire()方法获取信号量，执行完毕后使用release()方法释放信号量。每次打印等待一个随机时间，模拟打印耗时。

public class PrintQueue {private Semaphore semaphore;public PrintQueue() {//semaphore = new Semaphore(1);   //非公平的semaphore = new Semaphore(1, true); //公平的}public void printJob(Object document) {try {semaphore.acquire();long duration = (long)(Math.random() * 10000);System.out.printf("%s: Print a job duration %d seconds.\n",Thread.currentThread().getName(), duration / 1000);Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();} finally {semaphore.release();}}
}

创建Job线程类，模拟打印请求。

public class Job implements Runnable{private PrintQueue printQueue;public Job(PrintQueue printQueue) {this.printQueue = printQueue;}@Overridepublic void run() {System.out.printf("%s: Going to print a Job.\n", Thread.currentThread().getName());printQueue.printJob(new Object());System.out.printf("%s: The Job has been printed.\n", Thread.currentThread().getName());}
}

主方法类中启动10个打印Job线程。

public class Main {public static void main(String[] args) {PrintQueue printQueue = new PrintQueue();Thread[] threads = new Thread[10];for (int i = 1; i < 10; i++) {threads[i] = new Thread(new Job(printQueue));}for (int i = 1; i < 10; i++) {threads[i].start();}}
}

查看日志，每次只有一个打印Job可以执行打印程序，其他线程处于WAITING状态。

Thread-0: Going to print a Job.
Thread-8: Going to print a Job.
Thread-7: Going to print a Job.
Thread-6: Going to print a Job.
Thread-5: Going to print a Job.
Thread-4: Going to print a Job.
Thread-0: Print a job duration 2 seconds.
Thread-3: Going to print a Job.
Thread-2: Going to print a Job.
Thread-1: Going to print a Job.
Thread-0: The Job has been printed.
Thread-8: Print a job duration 8 seconds.
Thread-8: The Job has been printed.
Thread-7: Print a job duration 8 seconds.
Thread-7: The Job has been printed.
Thread-6: Print a job duration 9 seconds.
Thread-6: The Job has been printed.
Thread-5: Print a job duration 6 seconds.
Thread-5: The Job has been printed.
Thread-4: Print a job duration 7 seconds.
Thread-4: The Job has been printed.
Thread-3: Print a job duration 4 seconds.
Thread-3: The Job has been printed.
Thread-2: Print a job duration 1 seconds.
Thread-2: The Job has been printed.
Thread-1: Print a job duration 1 seconds.
Thread-1: The Job has been printed.