Java并发系列—工具类：CyclicBarrier

CyclicBarrier的字面意思是循环屏障。允许一组线程达到一个屏障（或者同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被拦截的线程才会继续运行。

使用实例

public class CyclicBarrierDemo {

    public static void main(String[] args) {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(2);
        new Thread(new Action(cyclicBarrier)).start();
        new Thread(new Action(cyclicBarrier)).start();
    }


   static class Action implements Runnable{

        private CyclicBarrier cyclicBarrier;

        public Action(CyclicBarrier cyclicBarrier){
            this.cyclicBarrier = cyclicBarrier;
        }


        @Override
        public void run() {
            System.out.println(Thread.currentThread()+" get ready");
            try {
                cyclicBarrier.await();
            } catch (InterruptedException | BrokenBarrierException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread()+" work done");
        }
    }
}

实现原理

通过查看源码可知await()方法由dowait()方法完成的，源码如下：

private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
           TimeoutException {
    // 1、获取锁并加锁
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        // 2、获取代表屏障使用的generation实例
        final Generation g = generation;
        if (g.broken)
            throw new BrokenBarrierException();

        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }
        // 3、被拦截的线程数的副本值 - 1
        int index = --count;
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run();
                ranAction = true;
                // 5、唤醒被拦截的线程和初始化下一次屏障使用
                nextGeneration();
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        // loop until tripped, broken, interrupted, or timed out
        // 4、当前线程进入自旋过程，释放锁等待被唤醒
        for (;;) {
            try {
                if (!timed)
                    trip.await();
                else if (nanos > 0L)
                    nanos = trip.awaitNanos(nanos);
            } catch (InterruptedException ie) {
                if (g == generation && ! g.broken) {
                    breakBarrier();
                    throw ie;
                } else {
                 Thread.currentThread().interrupt();
                }
            }

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

            if (g != generation)
                return index;

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

dowait方法的执行流程如下：

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