用于同步的CyclicBarrier与CountDownLatch

Question

我已经阅读了Bloch的第69条(有效的Java)，有一个简单的定时执行框架的例子，它使用3个CountDownLatch来同步线程。另外，Bloch说这个例子可以用1个CyclicBarrier重写。我已经尝试过这样做，并获得了下一个代码：

     public static long time(ExecutorService exec, int count) throws InterruptedException, BrokenBarrierException {
            CyclicBarrier cyclicBarrier = new CyclicBarrier(count+1);
            for (int i = 0; i < count; i++) {
                exec.submit(new Worker(cyclicBarrier, i));
            }
            System.out.println("Start processing");
            cyclicBarrier.await();
            long time = System.nanoTime();
            cyclicBarrier.await();
            long elapsedTime = (System.nanoTime() - time)/1000;
            System.out.println("FIN. ");
            return elapsedTime;

        }
private static class Worker implements Runnable {

        final CyclicBarrier cyclicBarrier;
        int workerNumber;

        Worker(CyclicBarrier cyclicBarrier, int workerCount) {
            this.cyclicBarrier = cyclicBarrier;
            this.workerNumber = workerCount;
        }

        public void run() {
            try {
                cyclicBarrier.await();
                work();
                cyclicBarrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
        }
        private void work() {
            System.out.println("Worker " + workerNumber + " starts his job");
            System.out.println("Worker " + workerNumber + " finishes his job");
        }

    }

但我认为这里(在time方法中)有一个问题:在第一个await之后，我尝试测量线程开始执行的时间，所以我想在所有其他线程开始之前测量这个时间。不能保证此(long time = System.nanoTime();)指令将在另一个线程启动之前执行。

如何使用CyclicBarrier实现这样的功能？有什么建议吗？

Answer 1

CyclicBarrier允许您定义要在所有线程达到屏障之后、恢复执行之前执行的屏障操作。使用CyclicBarrier(int, Runnable)来定义它。

显然，在您的情况下，需要区分执行屏障操作之前和完成它之后，就像这样：

class TimingAction implements Runnable {
    private boolean running;
    private long start;
    private long elapsed;

    public void run() {
        if (running) {
            elapsed = System.nanoTime() - start;
            running = false;
        } else {
            start = System.nanoTime();
            running = true;
        }
    }
    ...
}

TimingAction timer = new TimingAction();
CyclicBarrier cyclicBarrier = new CyclicBarrier(count + 1, timer);
...
long elapsed = timer.getElapsed();