Semaphore 可以理解为一个阈值，正在进行的操作数量不能超过此阈值，可以用来限制资源的访问，或者控制某个队列中对象的个数。

public class SemaphoreTest {
    private Semaphore semaphore = new Semaphore(2);
    public void m() {
        try {
            System.out.println(Thread.currentThread().getName()+" started");
            semaphore.acquire();
            Thread.sleep((int) (Math.random() * 2000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            semaphore.release();
        }
        System.out.println(Thread.currentThread().getName()+" ended");
    }
    public static void main(String[] args) {
        SemaphoreTest test = new SemaphoreTest();
        for (int i=0;i<8; i++) {
            new Thread(()->test.m()).start();
        }
    }
}