@Test

     public void test1() {

         final Semaphore semaphore = new Semaphore(2);

         ExecutorService executorService = Executors.newCachedThreadPool();

         final AtomicInteger executeNums = new AtomicInteger();

         for(int num = 0; num < 20; num++) {

             executorService.execute(new Runnable() {

                 @Override

                 public void run() {

                     try {

                         semaphore.acquire();

                         System.err.println("thread-" + Thread.currentThread().getId());

                         Thread.sleep(1000);

                         executeNums.incrementAndGet();

                         semaphore.release();

                     } catch (InterruptedException e) {

                         e.printStackTrace();

                     }

                 }

             });

         }

         while (executeNums.get() < 20) {

             try {

                 Thread.sleep(1000);

             } catch (InterruptedException e) {

                 e.printStackTrace();

             }

         }

         executorService.shutdown();

     }