java.util.concurrent.CyclicBarrier源码分析

[diaosichengxuyuan]

CyclicBarrier做的事情是，让一组线程到达一个屏障（也可以叫同步点-Barrier）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干活。而之所以叫Cyclic，是因为所有等待线程被释放后，CyclicBarrier可以被重用。

public class CyclicBarrier {

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

    private final ReentrantLock lock = new ReentrantLock();
    
    private final Condition trip = lock.newCondition();
    
    private final int parties;
    
    private final Runnable barrierCommand;
    
    private Generation generation = new Generation();

    private int count;

    private static class Generation {
        boolean broken = false;
    }

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

    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    //栅栏的主方法
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        //加锁，只有一个线程进入处理
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;
            
            //栅栏可以跳出时
            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
            
            //当最后一个线程到达时
            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    //执行回调函数
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    //重置栅栏
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }


            for (;;) {
                try {
                    //线程死循环在此等待
                    if (!timed)
                        trip.await();
                    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();

                //等待的线程一般都会在此跳出
                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }
}