CyclicBarrier源碼解析

1. 先看一下構造函數

public CyclicBarrier(int parties, Runnable barrierAction) {
    if (parties <= 0) throw new IllegalArgumentException();
    this.parties = parties;
    this.count = parties;
    this.barrierCommand = barrierAction;
}

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

parties: 官方翻譯the number of threads that must invoke {@link #await} before the barrier is tripped 也就是直到調用parties次await()方法，程序才繼續執行，否則線程會阻塞在await()方法這裏。

barrierAction：the command to execute when the barrier is tripped, or {@code null} if there is no action 程序繼續執行時回調的方法

2. 屬性

    private static class Generation {
        Generation() {}                 // prevent access constructor creation
        boolean broken;                 // initially 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();
    }

根據屬性可以看出，內部使用lock鎖實現的，generation 實現了cyclic功能，每次把count重新賦值爲parties，可以認爲對這個類做了初始化

3. await()方法

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

調用了dowait()方法

    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();
        }
    }

dowait()方法爲整個CyclicBarrier類的核心實現，線程等待數量不滿足parties時等待，滿足了，喚醒所有線程同時調用nextGeneration生產新的generation，這樣就可以實現cyclic的功能了。

在nextGeneration()方法中爲什麼每次都要生產調用generation = new Generation() ？

Generation類裏面有一個狀態broken，其它在await()方法上等待的線程還需要監控這個狀態，因爲有可能因爲某線程等待超時觸發InterruptedException異常，此時會把broken=true，並且喚醒所有線程。

    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

構造函數中的回調方法由最後一個進來的線程調用run()方法執行，因爲只有最後一個進來的線程--count後纔會等於0

			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();
                }
            }

實時內容請關注微信公衆號，公衆號與博客同時更新：程序員星星