JUC源码阅读之CyclicBarrier-AQS的典型实现（五）_berrier aqs实现

1、类注释摘抄

对于失败的同步尝试，CyclicBarrier 使用了一种要么全部要么全不 (all-or-none) 的破坏模式：如果因为中断、失败或者超时等原因，导致线程过早地离开了屏障点，那么在该屏障点等待的其他所有线程也将通过 BrokenBarrierException（如果它们几乎同时被中断，则用 InterruptedException）以反常的方式离开。

2、源码中的主要方法

    /**
     * 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.
	 *每个barrier都有一个Generation。当barrier被tripped或者重置时，Generation会变化
	 *使用barrier的线程会和许多Generation相关，因为不确定的方式，锁可能被分配给等待线程。
	 *但是每次只有一个在活动，其余的都会被破坏或者tripped。如果有中断但是没有后继重置，不需要激活新的一代
     */
    private static class Generation {
        boolean broken = false;
    }
 
    /** The lock for guarding barrier entry */
	//barrier的入口锁
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
	//Condition等待tripped
    private final Condition trip = lock.newCondition();
    /** The number of parties */
	//通过构造器传入的参数，表示总的等待线程的数量。
    private final int parties;
    /* The command to run when tripped */
	//当屏障正常打开后运行的程序，通过最后一个调用await的线程来执行。
    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.
	 *还在等待的线程数量。每一代的Conuts都会从总的等待数量下降到0
	 每启动新的一代，或者broken时会重置为总等待数量。
     */
    private int count;
 
    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
	 *当barrier trip的时候更新state。唤醒每一个
	 *只有当拥有锁的时候调用。
     */
    private void nextGeneration() {
        // signal completion of last generation
		//通知上一代完成，
        trip.signalAll();
        // set up next generation设置新的一代。
        count = parties;
        generation = new Generation();
    }
 
    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
	 *把当前barrier的一代设置为broken，唤醒每一个。
	 *拥有锁的时候调用。
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }
 
    /**
     * Main barrier code, covering the various policies.
	 *主要的barrier代码，覆盖了各种策略
     */
    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)//如果是broken状态，则抛出异常
                throw new BrokenBarrierException();
 
            if (Thread.interrupted()) {//如果线程是中断状态，则唤醒所有，generation的broken为true，抛出异常
                breakBarrier();
                throw new InterruptedException();
            }
 
            int index = --count;//减少当前count
            if (index == 0) {  // tripped如果count当前为0，则证明已经是trripped状态，运行之前输入的Runnable
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();//通知所有线程，继续下一代的设置。
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }
 
            // loop until tripped, broken, interrupted, or timed out
			//除非tripped，中断或者超时，否则循环
            for (;;) {
                try {
                    if (!timed)//如果没设置超时状态，则等待
                        trip.await();//await()会释放锁，这样下一个线程就可以继续dowait()
                    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();
				//因为别的线程是最后一个await的，也就是会调用nextGeneration();生成一个新的generation
				//所以正常情况下g会不等于generation，执行返回index
                if (g != generation)
                    return index;
 
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }