1. CountDownLatch

1.1 说明

一种同步辅助工具，允许一个或多个线程等待其他线程执行的一组操作完成。

给定一个计数值。当每个线程完成后，调用{@link countDown}方法给计数值减一。在当前计数达到零之前，调用{@link#await await}方法的线程将一直阻塞，直到计数值为0后将释放所有等待线程。

1.2 源码

public class CountDownLatch {
    /**
     * 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) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * 初始化
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * 导致当前线程等待，直到count为0
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }
}

1.3 示例

public class CountDownLatchDemo {
    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(15);
        for (int i = 0; i < 15; i++) {
            new Thread(() -> {
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                countDownLatch.countDown();

            }, ""+ i).start();
        }

        try {
            countDownLatch.await();
            System.out.println("main thread");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

结果：

2. CyclicBarrier

2.1 说明

一种同步辅助工具，允许一组线程全部等待对方到达一个公共屏障点。CyclicBarrier在涉及固定大小的线程方的程序中非常有用，这些线程偶尔必须相互等待。这个屏障被称为循环的，因为它可以在等待的线程被释放后重新使用。

CyclicBarrier支持可选的可运行命令，该命令在参与方中的最后一个线程到达之后，但在释放任何线程之前，在每个屏障点运行一次。此屏障操作有助于在任何一方继续之前更新共享状态。

2.2 源码

public class CyclicBarrier {
 
    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();
    //每次拦截的线程数，在构造时进行赋值
    private final int parties;
   
    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; //内部计数器与parties相等

    /**
     * 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(); //若线程中断，唤醒所有线程
                throw new InterruptedException();
            }

            int index = --count; //计数减一
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();  //执行指定任务
                    ranAction = true;
                    nextGeneration(); //唤醒所有线程，转到下一代
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // 如果计数器不为0则执行此循环
            for (;;) {
                try {
                    //根据传入的参数来决定是定时等待还是非定时等待
                    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();
        }
    }

    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);
    }

    public int getParties() {
        return parties;
    }

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

    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    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();
        }
    }
}

应用场景：多个线程计算最后汇总
CyclicBarrier 和CountDownLatch 区别
1.CountDownLatch 只能使用一次，CyclicBarrier 的计数器可以使用reset()方法重置。所以CyclicBarrier 可以处理更复杂的逻辑。
例如，计算发生错误可以重置计数器，并让线程重新执行一次。
2.CyclicBarrier 还提供其他方法，如getNumberWaiting方法可以获得被阻塞的线程数等