ReentrantLock 重入鎖，重入體會有多深？

之前寫過這個 http://www.javashuo.com/article/p-hepcauak-ht.html

今天看CyclicBarrier源碼，核心代碼 這一串

/**
     * 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();//獲取鎖
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

           int index = --count;//到達的線程數目 conut-- 
           if (index == 0) {  // tripped 到達數目時
               boolean ranAction = false;
               try {
                   final Runnable command = barrierCommand;
                   if (command != null)
                       command.run();//執行回調線程
                   ranAction = true;
                   nextGeneration();//觸發激活全部線程
                   return 0;
               } finally {
                   if (!ranAction)
                       breakBarrier();
               }
           }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();//等待lock上的條件    定義：private final Condition trip = lock.newCondition();
                    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();
        }
    }

一直疑惑，當一個線程獲取lock.lock()鎖時，何時釋放的，要否則其餘線程進來時,根本進入不了方法，怎麼讓int index = --count;，進而觸發激活線程呢。糊塗好久，最後網上看到trip.await();解釋爲，等待lock上的條件(newCondition())，同時釋放lock鎖。這樣其餘線程才能得到鎖，再進入dowait方法。其實，若是不用鎖上的條件 (newCondition()) ，也就很差體會重入。