摘要：序包里有幾個能幫助人們管理相互合作的線程集的類，為多線程常見的應(yīng)用場景預(yù)置了抽象好的類庫。如果沒報錯就更新屏障狀態(tài)并喚醒所有線程繼續(xù)執(zhí)行。如果還有未到達(dá)的線程，就進(jìn)入一個死循環(huán)，直到超時線程中斷屏障失效全部完成等情況下退出。

我的博客   轉(zhuǎn)載請注明原創(chuàng)出處。

java.util.concurrent包里有幾個能幫助人們管理相互合作的線程集的類，為多線程常見的應(yīng)用場景預(yù)置了抽象好的類庫。在遇到這些應(yīng)用場景時應(yīng)該直接重用合適的庫類而不要試圖提供手工的鎖與條件的集合。

官方定義上文已經(jīng)給出，人話版是等待特定數(shù)量的線程都到達(dá)同步屏障后各線程才繼續(xù)執(zhí)行。

同步屏障有兩個構(gòu)造函數(shù)，第一個構(gòu)造函數(shù)只需要指定需要等待的線程數(shù)量，第二構(gòu)造函數(shù)多了一個在特定數(shù)量的線程都到達(dá)同步屏障時優(yōu)先執(zhí)行的Runnable。

例子：

public class CyclicBarrierTest {

    // 等待4個線程到達(dá)同步屏障，全部到達(dá)后優(yōu)先執(zhí)行一個 Runnable
    private static CyclicBarrier cyclicBarrier = new CyclicBarrier(4,
    () -> System.out.println("全部到達(dá)同步屏障" + LocalDateTime.now()));

    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        Runnable runnable = () -> {
            System.out.println("到達(dá)同步屏障" + LocalDateTime.now());
            try {
                cyclicBarrier.await();
            } catch (InterruptedException | BrokenBarrierException e) {
                e.printStackTrace();
            }
            System.out.println("繼續(xù)執(zhí)行");
        };
        List list = Arrays.asList(runnable, runnable, runnable);
        list.forEach(runnable1 -> new Thread(runnable1).start());
        Thread.sleep(1000);
        System.out.println("最后一個線程到達(dá)同步屏障");
        cyclicBarrier.await();
    }

}

輸出：

到達(dá)同步屏障2018-08-12T14:33:16.769
到達(dá)同步屏障2018-08-12T14:33:16.769
到達(dá)同步屏障2018-08-12T14:33:16.769
最后一個線程到達(dá)同步屏障
全部到達(dá)同步屏障2018-08-12T14:33:17.746
繼續(xù)執(zhí)行
繼續(xù)執(zhí)行
繼續(xù)執(zhí)行

Process finished with exit code 0

同步屏障的應(yīng)用場景是那種多線程執(zhí)行任務(wù)，在全部任務(wù)執(zhí)行完成后需要進(jìn)行一些操作的場景。比如對每個用戶進(jìn)行充值統(tǒng)計，最后匯總返回。

CyclicBarrier的方法如上，分別是

getParties()  返回需要到達(dá)同步屏障的線程數(shù)量
await() 等待所有線程到達(dá)
await(long, TimeUnit) 帶時間限制的await()
isBroken() 判斷阻塞的線程是否被中斷
reset() 重置計數(shù)器
getNumberWaiting() 當(dāng)前被阻塞的線程數(shù)量，該方法主要用于調(diào)試和斷言

那么CyclicBarrier是怎么實現(xiàn)這個效果的呢？我們從最常用的await()方法入手。

可以看到await()方法主要是調(diào)用了CyclicBarrier私有的dowait()方法

如注釋所言，dowait()方法就是實現(xiàn)功能的主要方法了。

首先拿到可重入的鎖

然后通過內(nèi)部類Generation判斷阻塞的線程是否被中斷或該屏障已經(jīng)失效。

如果線程沒有被中斷，那么就獲取還沒有到達(dá)的線程數(shù)量并減一。如果已經(jīng)沒有需要等待的線程了，就判斷是否有需要執(zhí)行的Runnable。如果沒報錯就更新屏障狀態(tài)并喚醒所有線程繼續(xù)執(zhí)行。Runnable執(zhí)行報錯的話執(zhí)行breakBarrier()方法。

如果還有未到達(dá)的線程，就進(jìn)入一個死循環(huán)，直到超時、線程中斷、屏障失效、全部完成等情況下退出。

完整的代碼：

/**
     * Each use of the barrier is represented as a generation instance.
     * The generation changes whenever the barrier is tripped, or
     * is reset. There can be many generations associated with threads
     * using the barrier - due to the non-deterministic way the lock
     * may be allocated to waiting threads - but only one of these
     * can be active at a time (the one to which {@code count} applies)
     * and all the rest are either broken or tripped.
     * There need not be an active generation if there has been a break
     * but no subsequent reset.
     */
    private static class Generation {
        boolean broken = false;
    }

    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    private final int parties;
    /* The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    /**
     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // We"re about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        Thread.currentThread().interrupt();
                    }
                }

                if (g.broken)
                    throw new BrokenBarrierException();

                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }