Semaphore 的作用

import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

import static java.util.concurrent.TimeUnit.MICROSECONDS;

@SpringBootTest(classes = GoodsApplication.class)
@RunWith(SpringRunner.class)
public class TestIdWorker {

    @Autowired
    private SpuMapper spuMapper;
    
    //J.U.C并发编程 限制访问线程数量
    private final static Semaphore semaphore = new Semaphore(1);

    @Test
    public void testCyclicBarrier() throws Exception{
        //J.U.C并发编程 栅栏机制
        CyclicBarrier cyclicBarrier = new CyclicBarrier(5);
        for (int i = 0; i < 5; i++) {
            new Thread(()->{
                try {
                    System.out.println("我已经进来啦，查询商品");
                    cyclicBarrier.await(); //等待，栅栏拦住线程，等待栅栏开启，2000线程执行此段代码
                    Spu spu = findById("1335518444044419072");
                    if (spu!=null){
                        System.out.println("已经查到商品啦，商品名"+spu.getName());
                    }
                }catch (Exception e){
                    e.printStackTrace();
                }

            }).start();
        }
        System.in.read(); //不关闭程序
    }

    Spu findById(String id) throws Exception{
        try {
            boolean b = semaphore.tryAcquire(3000, MICROSECONDS); //等待，如果三毫秒内没有拿到令牌，返回false
            if (b == false){
                System.out.println("很遗憾，没拿到令牌");
                return null;
            }
            Spu spu = spuMapper.selectByPrimaryKey(id);
            return spu;
        }finally {
            semaphore.release();
        }
    }
}