CyclicBarrier：（基于Condition）
让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续运行。
原理
在CyclicBarrier类的内部有一个计数器，每个线程在到达屏障点的时候都会调用await方法将自己阻塞，此时计数器会减1，当计数器减为0的时候所有因调用await方法而被阻塞的线程将被唤醒。
成员变量

    private static class Generation {
        boolean broken = false;
    }
    /** The lock for guarding barrier entry 同步操作锁*/
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped 线程拦截器*/
    private final Condition trip = lock.newCondition();
    /** The number of parties 每次拦截的线程数*/
    private final int parties;
    /* The command to run when tripped 换代前执行的任务*/
    private final Runnable barrierCommand;
    /** The current generation 栅栏的当前代*/
    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. 计数器
     */
    private int count;

构造函数

    public CyclicBarrier(int parties) {
        this(parties, null);
    }
    /** barrierAction由最后一个到达栅栏的线程执行*/
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

await()、await(long timeout, TimeUnit unit)

    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));
    }
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;
            // 检查当前屏障是否被打破
            if (g.broken)
                throw new BrokenBarrierException();
            /** 如果当前线程被中断会做以下三件事
            *   1、打破当前屏障
            *   2、唤醒拦截的所有线程
            *   3、抛出中断异常
            */
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
            // 计数器值-1
            int index = --count;
            // 计数器值为0则需唤醒所有线程并转换到下一代
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    // 唤醒所有线程并转化到下一代
                    nextGeneration();
                    return 0;
                } finally {
                    // 确保在任务未成功执行时能将所有线程唤醒
                    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) {
                    // 如果当前线程在等待期间被中断则打破屏障唤醒其他线程
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // 如果在捕获中断异常前已经完成了屏障上的等待，则直接调用线程中断
                        Thread.currentThread().interrupt();
                    }
                }
                // 如果线程因为打破屏障而被唤醒则抛出异常
                if (g.broken)
                    throw new BrokenBarrierException();
                // 如果线程因为换代而被唤醒则返回计数器的值
                if (g != generation)
                    return index;
                // 如果线程等待超时，则唤醒所有等待线程并抛出超时异常
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    // 转化到下一代
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }
    // 打破屏障
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }
    // 重置
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }