Semaphore

semaphore :

英[ˈseməfɔː(r)] 美[ˈseməfɔːr]
n.	旗语; 信号标;
v.	打旗语; (用其他类似的信号系统)发信号;
[例句]Semaphore was widely used at sea, before the advent of electricity.
在电问世以前，旗语在海上运用广泛。
[其他]	第三人称单数：semaphores 复数：semaphores 现在分词：semaphoring 过去式：semaphored 过去分词：semaphored

例子代码和输出

public class Test {final static int CNT = 2;static Semaphore semaphore = new Semaphore(CNT);public static void main(String[] args) throws InterruptedException {Runnable runnable = () -> {try {semaphore.acquire();// 业务处理TimeUnit.SECONDS.sleep(2);System.out.println(Thread.currentThread().getName() + ":" + " finished");} catch (Exception e) {e.printStackTrace();} finally {System.out.println(Thread.currentThread().getName() + ":" + " release");semaphore.release();}};List<Thread> threads = new ArrayList<>(4);for(int i= 0;i<6;i++) {Thread thread = new Thread(runnable);threads.add(thread);}for(Thread thread: threads){thread.start();}}
}

输入如下：可以看到同时最多只有2个线程能得到执行

semaphore.acquire();

CAS操作消耗一个信号量；如果没有信号量可消耗了，就死循环等待了

semaphore.release();

semaphore.acquire()的反向操作。

release增加信号量；acquire减少信号量。