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) {
        //允許最大的登錄數(shù)
        int slots=10;
        ExecutorService executorService = Executors.newFixedThreadPool(slots);
        LoginQueueUsingSemaphore loginQueue = new LoginQueueUsingSemaphore(slots);
        //線程池模擬登錄
        for (int i = 1; i <= slots; i++) {
            final int num=i;
            executorService.execute(()->{
                 if (loginQueue.tryLogin()){
                     System.out.println("用戶:"+num+"登錄成功！");
                 }else {
                     System.out.println("用戶:"+num+"登錄失敗！");
                 }
            });
        }
        executorService.shutdown();
        System.out.println("當前可用許可證數(shù)："+loginQueue.availableSlots());
        //此時已經登錄了10個用戶，再次登錄的時候會返回false
        if (loginQueue.tryLogin()){
            System.out.println("登錄成功！");
        }else {
            System.out.println("系統(tǒng)登錄用戶已滿，登錄失??！");
        }
        //有用戶退出登錄
        loginQueue.logout();
        //再次登錄
        if (loginQueue.tryLogin()){
            System.out.println("登錄成功！");
        }else {
            System.out.println("系統(tǒng)登錄用戶已滿，登錄失??！");
        }
    }

class LoginQueueUsingSemaphore{

    private Semaphore semaphore;

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

    boolean tryLogin() {
        //獲取一個憑證
        return semaphore.tryAcquire();
    }

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

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