Java中的Semaphore信号量详解

 public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

public static void main(String[] args) {
        //允许最大的登录数
        int slots=10;
        ExecutorService executorService = Executors.newFixedThreadPool(slots);
        LoginQueueUsingSemaphore loginQueue = new LoginQueueUsingSemaphore(slots);
        //线程池模拟登录
        for (int i = 1; i <= slots; i++) {
            final int num=i;
            executorService.execute(()->{
                 if (loginQueue.tryLogin()){
                     System.out.println("用户:"+num+"登录成功！");
                 }else {
                     System.out.println("用户:"+num+"登录失败！");
                 }
            });
        }
        executorService.shutdown();
        System.out.println("当前可用许可证数："+loginQueue.availableSlots());
        //此时已经登录了10个用户，再次登录的时候会返回false
        if (loginQueue.tryLogin()){
            System.out.println("登录成功！");
        }else {
            System.out.println("系统登录用户已满，登录失败！");
        }
        //有用户退出登录
        loginQueue.logout();
        //再次登录
        if (loginQueue.tryLogin()){
            System.out.println("登录成功！");
        }else {
            System.out.println("系统登录用户已满，登录失败！");
        }
    }

class LoginQueueUsingSemaphore{

    private Semaphore semaphore;

    /**
     *
     * @param slotLimit
     */
    public LoginQueueUsingSemaphore(int slotLimit){
        semaphore=new Semaphore(slotLimit);
    }

    boolean tryLogin() {
        //获取一个凭证
        return semaphore.tryAcquire();
    }

    void logout() {
        semaphore.release();
    }

    int availableSlots() {
        return semaphore.availablePermits();
    }
}