/** The lock for guarding barrier entry */
    // 可重入鎖
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    // 條件隊(duì)列，具體看AQS
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    // 參與的線程數(shù)量
    private final int parties;
    /* The command to run when tripped */
    // 由最后一個(gè)進(jìn)入 barrier 的線程執(zhí)行的操作
    private final Runnable barrierCommand;
    /** The current generation */
    // 當(dāng)前代
    private Generation generation = new Generation();
    // 正在等待進(jìn)入屏障的線程數(shù)量
    private int count;

public CyclicBarrier(int parties, Runnable barrierAction) {
        // 參與的線程數(shù)量小于等于0，拋出異常
        if (parties <= 0) throw new IllegalArgumentException();
        // 設(shè)置parties
        this.parties = parties;
        // 設(shè)置count
        this.count = parties;
        // 設(shè)置barrierCommand
        this.barrierCommand = barrierAction;
    }

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        // 保存當(dāng)前鎖
        final ReentrantLock lock = this.lock;
        // 鎖定
        lock.lock();
        try {
            // 保存當(dāng)前代
            final Generation g = generation;
            if (g.broken) // 屏障被破壞，拋出異常
                throw new BrokenBarrierException();
            if (Thread.interrupted()) { // 線程被中斷
                // 損壞當(dāng)前屏障，并且喚醒所有的線程，只有擁有鎖的時(shí)候才會(huì)調(diào)用
                breakBarrier();
                // 拋出異常
                throw new InterruptedException();
            }
            // 減少正在等待進(jìn)入屏障的線程數(shù)量
            int index = --count;
            if (index == 0) {  // 正在等待進(jìn)入屏障的線程數(shù)量為0，所有線程都已經(jīng)進(jìn)入
                // 運(yùn)行的動(dòng)作標(biāo)識(shí)
                boolean ranAction = false;
                try {
                    // 保存運(yùn)行動(dòng)作
                    final Runnable command = barrierCommand;
                    if (command != null) // 動(dòng)作不為空
                        // 運(yùn)行
                        command.run();
                    // 設(shè)置ranAction狀態(tài)
                    ranAction = true;
                    // 進(jìn)入下一代
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction) // 沒有運(yùn)行的動(dòng)作
                        // 損壞當(dāng)前屏障
                        breakBarrier();
                }
            }
            // loop until tripped, broken, interrupted, or timed out
            // 無限循環(huán)
            for (;;) {
                try {
                    if (!timed) // 沒有設(shè)置等待時(shí)間
                        // 等待
                        trip.await(); 
                    else if (nanos > 0L) // 設(shè)置了等待時(shí)間，并且等待時(shí)間大于0
                        // 等待指定時(shí)長(zhǎng)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) { 
                    if (g == generation && ! g.broken) { // 等于當(dāng)前代并且屏障沒有被損壞
                        // 損壞當(dāng)前屏障
                        breakBarrier();
                        // 拋出異常
                        throw ie;
                    } else { // 不等于當(dāng)前帶后者是屏障被損壞
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        // 中斷當(dāng)前線程
                        Thread.currentThread().interrupt();
                    }
                }
                if (g.broken) // 屏障被損壞，拋出異常
                    throw new BrokenBarrierException();
                if (g != generation) // 不等于當(dāng)前代
                    // 返回索引
                    return index;
                if (timed && nanos <= 0L) { // 設(shè)置了等待時(shí)間，并且等待時(shí)間小于0
                    // 損壞屏障
                    breakBarrier();
                    // 拋出異常
                    throw new TimeoutException();
                }
            }
        } finally {
            // 釋放鎖
            lock.unlock();
        }
    }

private void nextGeneration() {
        // signal completion of last generation
        // 喚醒所有線程
        trip.signalAll();
        // set up next generation
        // 恢復(fù)正在等待進(jìn)入屏障的線程數(shù)量
        count = parties;
        // 新生一代
        generation = new Generation();
    }

 
    static class MyThread extends Thread {
        private CyclicBarrier cb;
        public MyThread(String name, CyclicBarrier cb) {
            super(name);
            this.cb = cb;
        }
        public void run() {
            System.out.println(Thread.currentThread().getName() + " going to await");
            try {
                cb.await();
                System.out.println(Thread.currentThread().getName() + " continue");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        CyclicBarrier cb = new CyclicBarrier(3, new Thread("barrierAction") {
            public void run() {
                System.out.println(Thread.currentThread().getName() + " barrier action");
            }
        });
        MyThread t1 = new MyThread("t1", cb);
        MyThread t2 = new MyThread("t2", cb);
        t1.start();
        t2.start();
        System.out.println(Thread.currentThread().getName() + " going to await");
        cb.await();
        System.out.println(Thread.currentThread().getName() + " continue");
    }