AQS同步组件CyclicBarrier

CyclicBarrier

CyclicBarrier 也是一个同步辅助类，它允许一组线程相互等待直到到达某个工作屏障点，通过他可以完成多线程之间的相互等待。每个线程都就绪之后才能执行后面的操作。和 CountLatch 有相似的地方都是通过计数器来实现的。当某个线程执行了 await（）方法后就进入等待状态，计数器进行加 1 操作，当增加后的值达到我们设定的值后，线程被唤醒，继续执行后续操作。CyclicBarrier 是可重用的计数器，CyclicBarrier 的使用场景和 CountDownLatch 的使用场景很相似，可以用于多线程计算数据最后总计结果。

CyclicBarrier 和 CountDownLatch 的使用区别:

1. CountDownLatch 的计数器只能使用一次，CyclicBarrier 可以用 reset 方法重置。
2. CountDownLatch 是一个线程等待其他线程完成某个操作后才能继续执行。也就是一个或个多线程等待其他的关系，而 CyclicBarrier 是实现了多个线程之间的相互等待，所有线程都满足了条件之后才能继续使用。

演示代码

@Slf4j
public class CyclicBarrierExample1 {private static CyclicBarrier barrier = new CyclicBarrier(5);

    public static void main(String[] args) throws Exception {ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            executor.execute(() -> {
                try {race(threadNum);
                } catch (Exception e) {log.error("exception", e);
                }
            });
        }
        executor.shutdown();}

    private static void race(int threadNum) throws Exception {Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        barrier.await();
        log.info("{} continue", threadNum);
    }
}

输出结果如下：

20:43:46.324 [pool-1-thread-1] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 0 is ready
20:43:47.322 [pool-1-thread-2] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 1 is ready
20:43:48.323 [pool-1-thread-3] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 2 is ready
20:43:49.323 [pool-1-thread-4] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 3 is ready
20:43:50.325 [pool-1-thread-5] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 4 is ready
20:43:50.325 [pool-1-thread-5] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 4 continue
20:43:50.325 [pool-1-thread-1] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 0 continue
20:43:50.325 [pool-1-thread-2] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 1 continue
20:43:50.325 [pool-1-thread-4] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 3 continue
20:43:50.325 [pool-1-thread-3] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 2 continue
20:43:51.325 [pool-1-thread-6] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 5 is ready
20:43:52.326 [pool-1-thread-1] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 6 is ready
20:43:53.326 [pool-1-thread-2] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 7 is ready
20:43:54.326 [pool-1-thread-4] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 8 is ready
20:43:55.327 [pool-1-thread-3] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 9 is ready
20:43:55.327 [pool-1-thread-3] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 9 continue
20:43:55.327 [pool-1-thread-6] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 5 continue
20:43:55.327 [pool-1-thread-1] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 6 continue
20:43:55.327 [pool-1-thread-2] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 7 continue
20:43:55.327 [pool-1-thread-4] INFO com.concurrency.example.aqs.CyclicBarrierExample1 - 8 continue

我们定义了 private static CyclicBarrier barrier = new CyclicBarrier(5)，每次调用 await 后加 1 知道等于 5 就一起执行接下来的操作。

我们在创建 CyclicBarrier 的时候是可以传入一段 runable 的，下面看一下代码

@Slf4j
public class CyclicBarrierExample3 {private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {log.info("callback is running");
    });

    public static void main(String[] args) throws Exception {ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            executor.execute(() -> {
                try {race(threadNum);
                } catch (Exception e) {log.error("exception", e);
                }
            });
        }
        executor.shutdown();}

    private static void race(int threadNum) throws Exception {Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        barrier.await();
        log.info("{} continue", threadNum);
    }
}

在线程到达 = 执行屏障时，线程会优先执行这个 runable

private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {log.info("callback is running");
    });