CyclicBarrier源码解析

CyclicBarrier类似一个栅栏，只有规定的线程数量到了这个栅栏之后才会放行

首先来看CyclicBarrier的构造方法

public CyclicBarrier(int parties) {
        this(parties, null);
    }

public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

CyclicBarrier有两个构造函数，第二个构造函数会传入一个Runnable对象，功能就是CyclicBarrier会在规定的线程到达之后执行这个Runnbale方法

CyclicBarrier的await（）方法

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

 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();
                throw new InterruptedException();
            }

            int index = --count;      //每个线程调用一次await（）方法，count的就减1      
            if (index == 0) {  // tripped  //如果是最后一个线程到达了的话，index为0
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();  //如果构造函数传入了Runnable的话，就会执行run方法
                    ranAction = true;
                    nextGeneration();   //会把在Condition里的所有同步队列放到aqs中的同步队列中去，并把waitStatus从Condition改成SINGAL，并把Count的值
                    return 0;           重新改为原来的值,实现重用
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) { //进入到这里说明线程不是最后一个到达的线程
                try {
                    if (!timed)
                        trip.await(); //线程会被阻塞，并放入到Condition的等待队列里去
                    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();
        }
    }

private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }