AQS同步组件CyclicBarrier循环屏障用例剖析

CyclicBarrier原理

CyclicBarrier 的字面意思是可循环使用（Cyclic）的屏障（Barrier）。它要做的事情是，让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续运行。

当某个线程调用了await方法之后，就会进入等待状态，并将计数器+1，直到所有线程调用await方法使计数器为CyclicBarrier设置的值，才可以继续执行，由于计数器可以重复使用，所以我们又叫它循环屏障。

CyclicBarrier默认的构造方法是CyclicBarrier(int parties)，其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier我已经到达了屏障，然后当前线程被阻塞。

CyclicBarrier可以用于多线程计算数据，最后合并计算结果的应用场景。

源码分析

    
    public CyclicBarrier(int parties) {
        this(parties, null);
    }


    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

使用案例

await()

    private final static int threadCount = 15;
    
    private static CyclicBarrier barrier = new CyclicBarrier(5);

    public static void main(String[] args) throws Exception {

        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < threadCount; 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.getNumberWaiting());
        //每次调用await方法后计数器+1，当前线程被阻塞
        barrier.await();
        log.info("{} continue", threadNum);
    }

输出结果：

16:16:40.245 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 0 is ready 0
16:16:41.244 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 1 is ready 1
16:16:42.244 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 2 is ready 2
16:16:43.244 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 3 is ready 3
16:16:44.245 [pool-1-thread-5] INFO com.zjq.aqs.CyclicBarrier - 4 is ready 4
16:16:44.245 [pool-1-thread-5] INFO com.zjq.aqs.CyclicBarrier - 4 continue
16:16:44.245 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 0 continue
16:16:44.245 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 1 continue
16:16:44.245 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 2 continue
16:16:44.245 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 3 continue
16:16:45.245 [pool-1-thread-6] INFO com.zjq.aqs.CyclicBarrier - 5 is ready 0
16:16:46.245 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 6 is ready 1
16:16:47.246 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 7 is ready 2
16:16:48.246 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 8 is ready 3
16:16:49.246 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 9 is ready 4
16:16:49.246 [pool-1-thread-6] INFO com.zjq.aqs.CyclicBarrier - 5 continue
16:16:49.246 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 9 continue
16:16:49.246 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 6 continue
16:16:49.246 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 8 continue
16:16:49.246 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 7 continue
16:16:50.247 [pool-1-thread-5] INFO com.zjq.aqs.CyclicBarrier - 10 is ready 0
16:16:51.247 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 11 is ready 1
16:16:52.247 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 12 is ready 2
16:16:53.248 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 13 is ready 3
16:16:54.248 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 14 is ready 4
16:16:54.248 [pool-1-thread-4] INFO com.zjq.aqs.CyclicBarrier - 14 continue
16:16:54.248 [pool-1-thread-5] INFO com.zjq.aqs.CyclicBarrier - 10 continue
16:16:54.248 [pool-1-thread-3] INFO com.zjq.aqs.CyclicBarrier - 12 continue
16:16:54.248 [pool-1-thread-2] INFO com.zjq.aqs.CyclicBarrier - 11 continue
16:16:54.248 [pool-1-thread-1] INFO com.zjq.aqs.CyclicBarrier - 13 continue

通过输出结果可以知道，每次屏障会阻塞5个线程，5个线程执行后计数器达到预设值，继续执行后续操作。

await(long timeout, TimeUnit unit)

    private final static int threadCount = 15;
    
    private static CyclicBarrier barrier = new CyclicBarrier(5);

    public static void main(String[] args) throws Exception {

        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < threadCount; 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.getNumberWaiting());
        //每次调用await方法后计数器+1，当前线程被阻塞
        //等待2s.为了使在发生异常的时候，不影响其他线程，一定要catch
        //由于设置了超时时间后阻塞的线程可能会被中断，抛出BarrierException异常，如果想继续往下执行,需要加上try-catch
        try {
            barrier.await(2, TimeUnit.SECONDS);
        }catch (Exception e){
            //查看执行异常的线程
            log.info("线程{} 执行异常，阻塞被中断？{}",threadNum,barrier.isBroken());
        }
        log.info("{} continue", threadNum);
    }

输出结果：

17:06:24.440 [pool-1-thread-1] INFO com.zjq.CyclicBarrier - 0 is ready0
17:06:25.435 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 1 is ready1
17:06:26.435 [pool-1-thread-3] INFO com.zjq.CyclicBarrier - 2 is ready2
17:06:26.455 [pool-1-thread-1] INFO com.zjq.CyclicBarrier - 线程0 执行异常，阻塞被中断？true
17:06:26.456 [pool-1-thread-3] INFO com.zjq.CyclicBarrier - 线程2 执行异常，阻塞被中断？true
17:06:26.456 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 线程1 执行异常，阻塞被中断？true
17:06:26.456 [pool-1-thread-1] INFO com.zjq.CyclicBarrier - 0 continue
17:06:26.456 [pool-1-thread-3] INFO com.zjq.CyclicBarrier - 2 continue
17:06:26.456 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 1 continue
17:06:27.434 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 3 is ready0
17:06:27.434 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 线程3 执行异常，阻塞被中断？true
17:06:27.434 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 3 continue
17:06:28.435 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 4 is ready0
17:06:28.435 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 线程4 执行异常，阻塞被中断？true
17:06:28.435 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 4 continue
17:06:29.435 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 5 is ready0
17:06:29.435 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 线程5 执行异常，阻塞被中断？true
17:06:29.435 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 5 continue
17:06:30.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 6 is ready0
17:06:30.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 线程6 执行异常，阻塞被中断？true
17:06:30.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 6 continue
17:06:31.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 7 is ready0
17:06:31.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 线程7 执行异常，阻塞被中断？true
17:06:31.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 7 continue
17:06:32.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 8 is ready0
17:06:32.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 线程8 执行异常，阻塞被中断？true
17:06:32.436 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 8 continue
17:06:33.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 9 is ready0
17:06:33.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 线程9 执行异常，阻塞被中断？true
17:06:33.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 9 continue

CyclicBarrier(int parties, Runnable barrierAction)

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

输出结果：

17:11:38.867 [pool-1-thread-1] INFO com.zjq.CyclicBarrier3 - 0 is ready
17:11:38.966 [pool-1-thread-2] INFO com.zjq.CyclicBarrier3 - 1 is ready
17:11:39.067 [pool-1-thread-3] INFO com.zjq.CyclicBarrier3 - 2 is ready
17:11:39.167 [pool-1-thread-4] INFO com.zjq.CyclicBarrier3 - 3 is ready
17:11:39.268 [pool-1-thread-5] INFO com.zjq.CyclicBarrier3 - 4 is ready
17:11:39.268 [pool-1-thread-5] INFO com.zjq.CyclicBarrier3 - callback is running
17:11:39.268 [pool-1-thread-5] INFO com.zjq.CyclicBarrier3 - 4 continue
17:11:39.268 [pool-1-thread-1] INFO com.zjq.CyclicBarrier3 - 0 continue
17:11:39.268 [pool-1-thread-2] INFO com.zjq.CyclicBarrier3 - 1 continue
17:11:39.268 [pool-1-thread-3] INFO com.zjq.CyclicBarrier3 - 2 continue
17:11:39.268 [pool-1-thread-4] INFO com.zjq.CyclicBarrier3 - 3 continue
17:11:39.369 [pool-1-thread-6] INFO com.zjq.CyclicBarrier3 - 5 is ready
17:11:39.470 [pool-1-thread-7] INFO com.zjq.CyclicBarrier3 - 6 is ready
17:11:39.570 [pool-1-thread-8] INFO com.zjq.CyclicBarrier3 - 7 is ready
17:11:39.671 [pool-1-thread-9] INFO com.zjq.CyclicBarrier3 - 8 is ready
17:11:39.772 [pool-1-thread-10] INFO com.zjq.CyclicBarrier3 - 9 is ready
17:11:39.772 [pool-1-thread-10] INFO com.zjq.CyclicBarrier3 - callback is running
17:11:39.772 [pool-1-thread-10] INFO com.zjq.CyclicBarrier3 - 9 continue
17:11:39.772 [pool-1-thread-6] INFO com.zjq.CyclicBarrier3 - 5 continue
17:11:39.772 [pool-1-thread-9] INFO com.zjq.CyclicBarrier3 - 8 continue
17:11:39.772 [pool-1-thread-7] INFO com.zjq.CyclicBarrier3 - 6 continue
17:11:39.772 [pool-1-thread-8] INFO com.zjq.CyclicBarrier3 - 7 continue

CyclicBarrier和CountDownLatch的区别

• CountDownLatch的计数器只能使用一次。而CyclicBarrier的计数器可以使用reset()方法重置。所以CyclicBarrier能处理更为复杂的业务场景，比如如果计算发生错误，可以重置计数器，并让线程们重新执行一次。
• CountDownLatch主要用于实现一个或n个线程需要等待其他线程完成某项操作之后，才能继续往下执行，描述的是一个或n个线程等待其他线程的关系，而CyclicBarrier是多个线程相互等待，知道满足条件以后再一起往下执行。描述的是多个线程相互等待的场景
• CyclicBarrier还提供其他有用的方法，比如getNumberWaiting方法可以获得CyclicBarrier阻塞的线程数量。isBroken方法用来知道阻塞的线程是否被中断。

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