Semaphore

可以理解为一个限流器。个人觉得他能实现的东西，Condition接口也能实现，不过condition接口的实现没如此灵活。

使用方法如下：

public class SemaphoreTest {

    Semaphore semaphore = new Semaphore(2);

    public static void main(String[] args) {
        SemaphoreTest semaphoreTest = new SemaphoreTest();
        for (int i = 0; i < 20; i++) {
            new Thread(() -> {
                try {
                    semaphoreTest.enterRoom();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }, "Thread-" + i).start();
        }
    }

    public void enterRoom() throws InterruptedException {
        if (semaphore.availablePermits() == 0) {
            System.out.println(Thread.currentThread().getName() + " : " + "被禁止访问了...");
        }

        semaphore.acquire();
        System.out.println(Thread.currentThread().getName() + " : " + "成功进入房间，开始嗨皮。");
        Thread.sleep(1000);
        System.out.println(Thread.currentThread().getName() + " : " + "离开了房间。");
        semaphore.release();
    }
}