Java多线程的Semaphore

public class DemoSemaphore {
    # 1表示同时只允许1个线程访问, 3则表示3个
    private Semaphore semaphore = new Semaphore(3);

    public void exec() {
        try {
            semaphore.acquire();
            long threadId = Thread.currentThread().getId();
            System.out.println(threadId + " acquired");
            long rand = (long)(Math.random() * 1000);
            Thread.sleep(rand);
            System.out.println(threadId + " end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            semaphore.release();
        }
    }

    public static void main(String[] args) {
        DemoSemaphore demo = new DemoSemaphore();
        for (int i = 0; i < 30; i++) {
            new Thread(demo::exec).start();
        }
    }
}