Java并发编程中的CyclicBarrier线程屏障详解

 /** The lock for guarding barrier entry */
    // 可重入锁
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    // 条件队列，具体看AQS
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    // 参与的线程数量
    private final int parties;
    /* The command to run when tripped */
    // 由最后一个进入 barrier 的线程执行的操作
    private final Runnable barrierCommand;
    /** The current generation */
    // 当前代
    private Generation generation = new Generation();
    // 正在等待进入屏障的线程数量
    private int count;

public CyclicBarrier(int parties, Runnable barrierAction) {
        // 参与的线程数量小于等于0，抛出异常
        if (parties <= 0) throw new IllegalArgumentException();
        // 设置parties
        this.parties = parties;
        // 设置count
        this.count = parties;
        // 设置barrierCommand
        this.barrierCommand = barrierAction;
    }

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        // 保存当前锁
        final ReentrantLock lock = this.lock;
        // 锁定
        lock.lock();
        try {
            // 保存当前代
            final Generation g = generation;
            if (g.broken) // 屏障被破坏，抛出异常
                throw new BrokenBarrierException();
            if (Thread.interrupted()) { // 线程被中断
                // 损坏当前屏障，并且唤醒所有的线程，只有拥有锁的时候才会调用
                breakBarrier();
                // 抛出异常
                throw new InterruptedException();
            }
            // 减少正在等待进入屏障的线程数量
            int index = --count;
            if (index == 0) {  // 正在等待进入屏障的线程数量为0，所有线程都已经进入
                // 运行的动作标识
                boolean ranAction = false;
                try {
                    // 保存运行动作
                    final Runnable command = barrierCommand;
                    if (command != null) // 动作不为空
                        // 运行
                        command.run();
                    // 设置ranAction状态
                    ranAction = true;
                    // 进入下一代
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction) // 没有运行的动作
                        // 损坏当前屏障
                        breakBarrier();
                }
            }
            // loop until tripped, broken, interrupted, or timed out
            // 无限循环
            for (;;) {
                try {
                    if (!timed) // 没有设置等待时间
                        // 等待
                        trip.await(); 
                    else if (nanos > 0L) // 设置了等待时间，并且等待时间大于0
                        // 等待指定时长
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) { 
                    if (g == generation && ! g.broken) { // 等于当前代并且屏障没有被损坏
                        // 损坏当前屏障
                        breakBarrier();
                        // 抛出异常
                        throw ie;
                    } else { // 不等于当前带后者是屏障被损坏
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        // 中断当前线程
                        Thread.currentThread().interrupt();
                    }
                }
                if (g.broken) // 屏障被损坏，抛出异常
                    throw new BrokenBarrierException();
                if (g != generation) // 不等于当前代
                    // 返回索引
                    return index;
                if (timed && nanos <= 0L) { // 设置了等待时间，并且等待时间小于0
                    // 损坏屏障
                    breakBarrier();
                    // 抛出异常
                    throw new TimeoutException();
                }
            }
        } finally {
            // 释放锁
            lock.unlock();
        }
    }

private void nextGeneration() {
        // signal completion of last generation
        // 唤醒所有线程
        trip.signalAll();
        // set up next generation
        // 恢复正在等待进入屏障的线程数量
        count = parties;
        // 新生一代
        generation = new Generation();
    }

 
    static class MyThread extends Thread {
        private CyclicBarrier cb;
        public MyThread(String name, CyclicBarrier cb) {
            super(name);
            this.cb = cb;
        }
        public void run() {
            System.out.println(Thread.currentThread().getName() + " going to await");
            try {
                cb.await();
                System.out.println(Thread.currentThread().getName() + " continue");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        CyclicBarrier cb = new CyclicBarrier(3, new Thread("barrierAction") {
            public void run() {
                System.out.println(Thread.currentThread().getName() + " barrier action");
            }
        });
        MyThread t1 = new MyThread("t1", cb);
        MyThread t2 = new MyThread("t2", cb);
        t1.start();
        t2.start();
        System.out.println(Thread.currentThread().getName() + " going to await");
        cb.await();
        System.out.println(Thread.currentThread().getName() + " continue");
    }