一、获取单个许可

信号量设置为3，每次获取1个许可，那么就可以连续获得3个许可。

如下面的信号量Demo

@Slf4j
public class SemaphoreExample1 {

    private static int threadCount = 20;

    public static void main(String[] args) throws InterruptedException {
        ExecutorService exec = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(3);
        for(int i = 0; i < threadCount; i++){
            final int threadNum = i;
            exec.execute(()->{
                try {
                    semaphore.acquire();//获得一个许可
                    test(threadNum);
                    semaphore.release();//释放一个许可
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }

        exec.shutdown();
    }

    private static  void test(int threadNum) throws InterruptedException {

        log.info("{}", threadNum);
        Thread.sleep(1000);

    }
}

　　

　　打印结果如下。信号量设置为3。 差不多每隔1秒钟打印3条数据。

 

二、获得多个许可

线程数为20，信号量为3，每次获得3个许可。相当于单线程。

@Slf4j
public class SemaphoreExample2 {

    private static int threadCount = 20;

    public static void main(String[] args) throws InterruptedException {
        ExecutorService exec = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(3);
        for(int i = 0; i < threadCount; i++){
            final int threadNum = i;
            exec.execute(()->{
                try {
                    semaphore.acquire(3);//获得3个许可， 信号量为3，相当于单线程执行
                    test(threadNum);
                    semaphore.release(3);//释放3个许可
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }

        exec.shutdown();
    }

    private static  void test(int threadNum) throws InterruptedException {

        log.info("{}", threadNum);
        Thread.sleep(1000);

    }
}