我們有些場景，是需要使用 多線各一起執行某些操作的，比如進行併發測試，比如進行多執行緒資料彙總。

　　自然，我們可以使用 CountDownLatch, CyclicBarrier, 以及多個 Thread.join()。 雖然最終的效果都差不多，但實際卻各有千秋。我們此處主要看 CyclicBarrier .

　　概要: CyclicBarrier 使用 n 個 permit 進行初始化，當n個執行緒都到達後進行放行，然後進入下一個迴圈週期。在放行的同時，還可以設定一個執行方法，即相當於回撥操作。

一、CyclicBarrier 具體實現

　　主迴圈等待！

    // CyclicBarrier
    /**
     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        // 使用一個 互斥鎖，保證進行排隊等待的安全性
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 使用的一 Generation 代表一生迴圈週期，當週期到達後，替換此值
            final Generation g = generation;

            // 針對異常情況，直接丟擲異常，一般是用於多執行緒之間通訊
            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                // breakBarrier 是針對其他執行緒的，而 丟擲的 InterruptedException 是針對當前執行緒的
                // 從而達到中斷標誌全域性可見的效果
                breakBarrier();
                throw new InterruptedException();
            }

            // 以下邏輯為進入了等待區域， count-1， 當減到0之後，就代表需要進行放行了
            int index = --count;
            // 放行
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    // 如果設定了回撥，則立即執行回撥，在當前執行緒中
                    if (command != null)
                        command.run();
                    ranAction = true;
                    // 迴圈週期迭代，此操作後，其他所有等待執行緒都將被返回，進入下一輪週期
                    nextGeneration();
                    return 0;
                } finally {
                    // 未知異常，撤銷當前的等待
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    // 一直在此處進行等待，直到被喚醒，被喚醒時，則意味著有事件發生了
                    // 等待中將會釋放鎖，從而讓其他執行緒進入
                    // 此處的 await() 是一個複雜的故事，因為它要保證在 notify 時的鎖競爭問題
                    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();
                    }
                }

                // 此情況為發生了異常，被喚醒，則直接丟擲異常退出
                if (g.broken)
                    throw new BrokenBarrierException();

                // 生命週期被迭代，可以放行了
                if (g != generation)
                    return index;

                // 如果是等待超時，則丟擲超時異常
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

　　可以看到，主要邏輯就是在於 生命週期的迭代操作，但是這個生命週期的標誌異常的簡單:

    // 只有一個標識位， broken 為 true 時，發生了異常，整體退出
    private static class Generation {
        boolean broken = false;
    }

　　而到達的執行緒數足夠之後，需要進行週期迭代，只是 Generation 更換一個變數，另外就是要起到通知所有等待執行緒的作用:

    // CyclicBarrier
    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        // 先通知等待執行緒，但此時當前執行緒仍然持有鎖，所以其他執行緒仍然處理等待狀態
        // 然後再設定下一週期，直到本執行緒當前同步塊退出之後，其他執行緒才可以進行工作
        // 此處依賴於 ReentrantLock
        // 此處體現 wait/notify 的鎖作用域問題
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

　　而呼叫 入口 僅是呼叫 dowait() 方法而已.

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

　　CyclicBarrier 本身的等待邏輯是簡單巧妙的，使用 ReentrantLock 的目的是為了實現帶超時等待的效果，否則就是一個 wait/notify 機制的實現。當然 wait/notify 的邏輯還是很關鍵很複雜的，後續如有必要再寫一文說明。

　　完整程式碼如下:

public class CyclicBarrier {
    /**
     * 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();
        }
    }

    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @param barrierAction the command to execute when the barrier is
     *        tripped, or {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

    /**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
    public int getParties() {
        return parties;
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} on this barrier.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * {@code await} is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @return the arrival index of the current thread, where index
     *         {@code getParties() - 1} indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was
     *         broken when {@code await} was called, or the barrier
     *         action (if present) failed due to an exception
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} on this barrier, or the specified waiting time elapses.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>The specified timeout elapses; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown. If the time is less than or equal to zero, the
     * method will not wait at all.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * {@code await} is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while
     * waiting, then all other waiting threads will throw {@link
     * BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @param timeout the time to wait for the barrier
     * @param unit the time unit of the timeout parameter
     * @return the arrival index of the current thread, where index
     *         {@code getParties() - 1} indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws TimeoutException if the specified timeout elapses.
     *         In this case the barrier will be broken.
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was broken
     *         when {@code await} was called, or the barrier action (if
     *         present) failed due to an exception
     */
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * Queries if this barrier is in a broken state.
     *
     * @return {@code true} if one or more parties broke out of this
     *         barrier due to interruption or timeout since
     *         construction or the last reset, or a barrier action
     *         failed due to an exception; {@code false} otherwise.
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Resets the barrier to its initial state.  If any parties are
     * currently waiting at the barrier, they will return with a
     * {@link BrokenBarrierException}. Note that resets <em>after</em>
     * a breakage has occurred for other reasons can be complicated to
     * carry out; threads need to re-synchronize in some other way,
     * and choose one to perform the reset.  It may be preferable to
     * instead create a new barrier for subsequent use.
     */
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns the number of parties currently waiting at the barrier.
     * This method is primarily useful for debugging and assertions.
     *
     * @return the number of parties currently blocked in {@link #await}
     */
    public int getNumberWaiting() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }
}

二、簡單看一下 CountDownLatch 的同時等待實現

　　CountDownLatch 會在初始化時，申請 n 個 permit, 呼叫 await() 進行阻塞， 直到 permit=0 時，await() 才進行返回。每呼叫一次 countDown(); permit 都會減1直到為0止;

    // CountDownLatch.await()  等待
    public void await() throws InterruptedException {
        // 僅是去嘗試獲取一個而已
        sync.acquireSharedInterruptibly(1);
    }
    
    // CountDownLatch.countDown() 釋放鎖, 當 permit=0 後，放行 await() 
    public void countDown() {
        // 此處僅是委託給了 AQS 進行釋放、通知處理
        sync.releaseShared(1);
    }
    
    // CountDownLatch 內部鎖實現的是否可以持有鎖的邏輯
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            // 只要 state=0, 都可以放行
            return (getState() == 0) ? 1 : -1;
        }

        // 釋放鎖 countDown 邏輯, 做減1操作
        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                // 如果已經被釋放，則直接返回
                if (c == 0)
                    return false;
                // 忽略傳入值 releases, 只做減1操作, 所以 state 必定有等於0的時候
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    // 只有等於0, 才能進行真正的釋放通知操作
                    return nextc == 0;
            }
        }
    }

　　可以看出, CountDownLatch 的同時等待實現更加簡單，幾乎都是依賴於 AQS 進行實現。同樣，從實際效果來說，也是一個 wait/notify 的實現。只是此處的 notify 執行完之後就釋放了鎖，即無法保證 notify 之後的執行緒安全性。

嘮叨: 論 wait/notify 機制的安全