Java CyclicBarrier 栅栏

使用示例：

import java.io.IOException;
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {
    // 栅栏与闭锁的不同：所有线程必须同时到达栅栏位置，才能继续执行
    // await:阻塞到所有线程都到达栅栏位置，出现异常会抛出BrokenBarrierException，
    // 成功通过栅栏后，await会返回一个唯一可到达的索引号，
    static class Runner implements Runnable {
        // 一个同步辅助类，它允许一组线程互相等待，直到到达某个公共屏障点 (common barrier point)
        private CyclicBarrier barrier;

        private String name;

        public Runner(CyclicBarrier barrier, String name) {
            super();
            this.barrier = barrier;
            this.name = name;
        }

        @Override
        public void run() {
            try {
                Thread.sleep(1000 * (new Random()).nextInt(8));
                System.out.println(name + " 准备好了...");
                // barrier的await方法，在所有参与者都已经在此 barrier 上调用 await 方法之前，将一直等待。
                barrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
            System.out.println(name + " 起跑！");
        }
    }

    public static void main(String[] args) throws IOException, InterruptedException {
        //如果将参数改为4，但是下面只加入了3个选手，这永远等待下去
        //Waits until all parties have invoked await on this barrier.
        CyclicBarrier barrier = new CyclicBarrier(3);

        ExecutorService executor = Executors.newFixedThreadPool(3);
        executor.submit(new Thread(new Runner(barrier, "1号选手")));
        executor.submit(new Thread(new Runner(barrier, "2号选手")));
        executor.submit(new Thread(new Runner(barrier, "3号选手")));

        executor.shutdown();
    }
}

源码分析：

    public CyclicBarrier(int parties) {
            // 构造函数
            this(parties, null);
    }
    // wait 方法，当调用wait个数到达parties时，会同时唤醒所有等待线程
    public int await() throws InterruptedException, BrokenBarrierException {
            return dowait(false, 0L)；
    }
    // 核心方法
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        // 对象内置可重入锁
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 内置静态类
            final Generation g = generation;

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

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

            // count：剩余等待的线程个数
            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    // signalAll
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // 否则自旋阻塞等待
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        // 阻塞等待
                        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();

                if (g != generation)
                    return index;

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

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

generate 和 barrierCommend 理解参考