信号量，用来限制能同时访问共享资源的线程上限。

    1.     public static void main(String[] args) {
    2.         // 1. 创建 semaphore 对象
    3.         Semaphore semaphore = new Semaphore(3);
    5.         // 2. 10个线程同时运行
    6.         for (int i = 0; i < 10; i++) {
    7.             new Thread(() -> {
    8.                 try {
    9.                     semaphore.acquire();
    10.                 } catch (InterruptedException e) {
    11.                     e.printStackTrace();
    12.                 }
    13.                 try {
    14.                     log.debug("running...");
    15.                     sleep(1);
    16.                     log.debug("end...");
    17.                 } finally {
    18.                     semaphore.release();
    19.                 }
    20.             }).start();
    21.         }
    22.     }

    输出

    1. 07:35:15.485 c.TestSemaphore [Thread-2] - running... 
    2. 07:35:15.485 c.TestSemaphore [Thread-1] - running... 
    3. 07:35:15.485 c.TestSemaphore [Thread-0] - running... 
    4. 07:35:16.490 c.TestSemaphore [Thread-2] - end... 
    5. 07:35:16.490 c.TestSemaphore [Thread-0] - end... 
    6. 07:35:16.490 c.TestSemaphore [Thread-1] - end... 
    7. 07:35:16.490 c.TestSemaphore [Thread-3] - running... 
    8. 07:35:16.490 c.TestSemaphore [Thread-5] - running... 
    9. 07:35:16.490 c.TestSemaphore [Thread-4] - running... 
    10. 07:35:17.490 c.TestSemaphore [Thread-5] - end... 
    11. 07:35:17.490 c.TestSemaphore [Thread-4] - end... 
    12. 07:35:17.490 c.TestSemaphore [Thread-3] - end... 
    13. 07:35:17.490 c.TestSemaphore [Thread-6] - running... 
    14. 07:35:17.490 c.TestSemaphore [Thread-7] - running... 
    15. 07:35:17.490 c.TestSemaphore [Thread-9] - running... 
    16. 07:35:18.491 c.TestSemaphore [Thread-6] - end... 
    17. 07:35:18.491 c.TestSemaphore [Thread-7] - end... 
    18. 07:35:18.491 c.TestSemaphore [Thread-9] - end... 
    19. 07:35:18.491 c.TestSemaphore [Thread-8] - running... 
    20. 07:35:19.492 c.TestSemaphore [Thread-8] - end...