AQS同步元件CyclicBarrier迴圈屏障用例剖析

CyclicBarrier原理

CyclicBarrier 的字面意思是可迴圈使用（Cyclic）的屏障（Barrier）。它要做的事情是，讓一組執行緒到達一個屏障（也可以叫同步點）時被阻塞，直到最後一個執行緒到達屏障時，屏障才會開門，所有被屏障攔截的執行緒才會繼續執行。

當某個執行緒呼叫了await方法之後，就會進入等待狀態，並將計數器+1，直到所有執行緒呼叫await方法使計數器為CyclicBarrier設定的值，才可以繼續執行，由於計數器可以重複使用，所以我們又叫它迴圈屏障。

CyclicBarrier預設的構造方法是CyclicBarrier(int parties)，其參數列示屏障攔截的執行緒數量，每個執行緒呼叫await方法告訴CyclicBarrier我已經到達了屏障，然後當前執行緒被阻塞。

CyclicBarrier可以用於多執行緒計算資料，最後合併計算結果的應用場景。

原始碼分析

    /**
     * 建立一個新的CyclicBarrier當給定數量的參與方(執行緒)等待它時，它將觸發，
     * 並且在障礙觸發時不執行預定義的操作。
     *
     * @param  在barrier被觸發之前必須呼叫await()的執行緒數
     * @throws IllegalArgumentException 如果parties小於1丟擲異常
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

/**
     * 
     * 當前執行緒呼叫await方法的執行緒告知CyclicBarrier已經到達屏障，然後當前執行緒被阻塞
     *
     * @return 當前執行緒的到達索引，其中索引為- 1表示第一個到達的，0表示最後一個到達的
     * @throws InterruptedException 如果當前執行緒在等待時被中斷
     * @throws BrokenBarrierException 如果另一個執行緒在當前執行緒等待時被中斷或超時，
     * 或者屏障被重置，或者在呼叫await方法時屏障被破壞，或者屏障操作(如果存在)由於異常而失敗
     */
    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()

/**
     * 執行緒數量
     */
    private final static int threadCount = 15;
    /**
     * 屏障攔截的執行緒數量為5，表示每次屏障會攔截5個執行緒
     */
    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());
        //每次呼叫await方法後計數器+1，當前執行緒被阻塞
        barrier.await();
        log.info("{} continue", threadNum);
    }

輸出結果：

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

通過輸出結果可以知道，每次屏障會阻塞5個執行緒，5個執行緒執行後計數器達到預設值，繼續執行後續操作。

await(long timeout, TimeUnit unit)

/**
     * 執行緒數量
     */
    private final static int threadCount = 15;
    /**
     * 屏障攔截的執行緒數量為5，表示每次屏障會攔截5個執行緒
     */
    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());
        //每次呼叫await方法後計數器+1，當前執行緒被阻塞
        //等待2s.為了使在發生異常的時候，不影響其他執行緒，一定要catch
        //由於設定了超時時間後阻塞的執行緒可能會被中斷，丟擲BarrierException異常，如果想繼續往下執行,需要加上try-catch
        try {
            barrier.await(2, TimeUnit.SECONDS);
        }catch (Exception e){
            //檢視執行異常的執行緒
            log.info("執行緒{} 執行異常，阻塞被中斷？{}",threadNum,barrier.isBroken());
        }
        log.info("{} continue", threadNum);
    }

輸出結果：

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 - 執行緒0 執行異常，阻塞被中斷？true
17:06:26.456 [pool-1-thread-3] INFO com.zjq.CyclicBarrier - 執行緒2 執行異常，阻塞被中斷？true
17:06:26.456 [pool-1-thread-2] INFO com.zjq.CyclicBarrier - 執行緒1 執行異常，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 - 執行緒3 執行異常，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 - 執行緒4 執行異常，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 - 執行緒5 執行異常，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 - 執行緒6 執行異常，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 - 執行緒7 執行異常，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 - 執行緒8 執行異常，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 - 執行緒9 執行異常，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)

 /**
     * 執行緒到達屏障時，優先執行barrierAction，方便處理更復雜的業務場景
     */
    private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {
        log.info("callback is running");
    });

輸出結果：

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的區別

• CountDownLatch的計數器只能使用一次。而CyclicBarrier的計數器可以使用reset()方法重置。所以CyclicBarrier能處理更為複雜的業務場景，比如如果計算髮生錯誤，可以重置計數器，並讓執行緒們重新執行一次。
• CountDownLatch主要用於實現一個或n個執行緒需要等待其他執行緒完成某項操作之後，才能繼續往下執行，描述的是一個或n個執行緒等待其他執行緒的關係，而CyclicBarrier是多個執行緒相互等待，知道滿足條件以後再一起往下執行。描述的是多個執行緒相互等待的場景
• CyclicBarrier還提供其他有用的方法，比如getNumberWaiting方法可以獲得CyclicBarrier阻塞的執行緒數量。isBroken方法用來知道阻塞的執行緒是否被中斷。

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