CyclicBarrier原理

CyclicBarrier 的字面意思是可循環(huán)使用（Cyclic）的屏障（Barrier）。它要做的事情是，讓一組線程到達(dá)一個屏障（也可以叫同步點）時被阻塞，直到最后一個線程到達(dá)屏障時，屏障才會開門，所有被屏障攔截的線程才會繼續(xù)運行。

當(dāng)某個線程調(diào)用了await方法之后，就會進(jìn)入等待狀態(tài)，并將計數(shù)器+1，直到所有線程調(diào)用await方法使計數(shù)器為CyclicBarrier設(shè)置的值，才可以繼續(xù)執(zhí)行，由于計數(shù)器可以重復(fù)使用，所以我們又叫它循環(huán)屏障。

CyclicBarrier默認(rèn)的構(gòu)造方法是CyclicBarrier(int parties)，其參數(shù)表示屏障攔截的線程數(shù)量，每個線程調(diào)用await方法告訴CyclicBarrier我已經(jīng)到達(dá)了屏障，然后當(dāng)前線程被阻塞。

CyclicBarrier可以用于多線程計算數(shù)據(jù)，最后合并計算結(jié)果的應(yīng)用場景。

源碼分析

    /**
     * 創(chuàng)建一個新的CyclicBarrier當(dāng)給定數(shù)量的參與方(線程)等待它時，它將觸發(fā)，
     * 并且在障礙觸發(fā)時不執(zhí)行預(yù)定義的操作。
     *
     * @param  在barrier被觸發(fā)之前必須調(diào)用await()的線程數(shù)
     * @throws IllegalArgumentException 如果parties小于1拋出異常
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

/**
     * 
     * 當(dāng)前線程調(diào)用await方法的線程告知CyclicBarrier已經(jīng)到達(dá)屏障，然后當(dāng)前線程被阻塞
     *
     * @return 當(dāng)前線程的到達(dá)索引，其中索引為- 1表示第一個到達(dá)的，0表示最后一個到達(dá)的
     * @throws InterruptedException 如果當(dāng)前線程在等待時被中斷
     * @throws BrokenBarrierException 如果另一個線程在當(dāng)前線程等待時被中斷或超時，
     * 或者屏障被重置，或者在調(diào)用await方法時屏障被破壞，或者屏障操作(如果存在)由于異常而失敗
     */
    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()

/**
     * 線程數(shù)量
     */
    private final static int threadCount = 15;
    /**
     * 屏障攔截的線程數(shù)量為5，表示每次屏障會攔截5個線程
     */
    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());
        //每次調(diào)用await方法后計數(shù)器+1，當(dāng)前線程被阻塞
        barrier.await();
        log.info("{} continue", threadNum);
    }

輸出結(jié)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

通過輸出結(jié)果可以知道，每次屏障會阻塞5個線程，5個線程執(zhí)行后計數(shù)器達(dá)到預(yù)設(shè)值，繼續(xù)執(zhí)行后續(xù)操作。

await(long timeout, TimeUnit unit)

/**
     * 線程數(shù)量
     */
    private final static int threadCount = 15;
    /**
     * 屏障攔截的線程數(shù)量為5，表示每次屏障會攔截5個線程
     */
    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());
        //每次調(diào)用await方法后計數(shù)器+1，當(dāng)前線程被阻塞
        //等待2s.為了使在發(fā)生異常的時候，不影響其他線程，一定要catch
        //由于設(shè)置了超時時間后阻塞的線程可能會被中斷，拋出BarrierException異常，如果想繼續(xù)往下執(zhí)行,需要加上try-catch
        try {
            barrier.await(2, TimeUnit.SECONDS);
        }catch (Exception e){
            //查看執(zhí)行異常的線程
            log.info("線程{} 執(zhí)行異常，阻塞被中斷？{}",threadNum,barrier.isBroken());
        }
        log.info("{} continue", threadNum);
    }

輸出結(jié)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 - 線程0 執(zhí)行異常，阻塞被中斷？true
17:06:26.456 [pool-1-thread-3] INFO com.zjq.CyclicBarrier - 線程2 執(zhí)行異常，阻塞被中斷？true
17:06:26.456 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 線程1 執(zhí)行異常，阻塞被中斷？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 - 線程3 執(zhí)行異常，阻塞被中斷？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 - 線程4 執(zhí)行異常，阻塞被中斷？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 - 線程5 執(zhí)行異常，阻塞被中斷？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 - 線程6 執(zhí)行異常，阻塞被中斷？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 - 線程7 執(zhí)行異常，阻塞被中斷？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 - 線程8 執(zhí)行異常，阻塞被中斷？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 - 線程9 執(zhí)行異常，阻塞被中斷？true
17:06:33.436 [pool-1-thread-4] INFO com.zjq.CyclicBarrier - 9 continue

CyclicBarrier(int parties, Runnable barrierAction)

 /**
     * 線程到達(dá)屏障時，優(yōu)先執(zhí)行barrierAction，方便處理更復(fù)雜的業(yè)務(wù)場景
     */
    private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {
        log.info("callback is running");
    });

輸出結(jié)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的區(qū)別

• CountDownLatch的計數(shù)器只能使用一次。而CyclicBarrier的計數(shù)器可以使用reset()方法重置。所以CyclicBarrier能處理更為復(fù)雜的業(yè)務(wù)場景，比如如果計算發(fā)生錯誤，可以重置計數(shù)器，并讓線程們重新執(zhí)行一次。
• CountDownLatch主要用于實現(xiàn)一個或n個線程需要等待其他線程完成某項操作之后，才能繼續(xù)往下執(zhí)行，描述的是一個或n個線程等待其他線程的關(guān)系，而CyclicBarrier是多個線程相互等待，知道滿足條件以后再一起往下執(zhí)行。描述的是多個線程相互等待的場景
• CyclicBarrier還提供其他有用的方法，比如getNumberWaiting方法可以獲得CyclicBarrier阻塞的線程數(shù)量。isBroken方法用來知道阻塞的線程是否被中斷。

以上就是AQS同步組件CyclicBarrier循環(huán)屏障用例剖析的詳細(xì)內(nèi)容，更多關(guān)于AQS同步組件CyclicBarrier的資料請關(guān)注腳本之家其它相關(guān)文章！