CyclicBarrier源码解析

1. 先看一下构造函数

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

public CyclicBarrier(int parties) {
    this(parties, null);
}

parties: 官方翻译​​the number of threads that must invoke {@link #await} before the barrier is tripped​​​ 也就是直到调用parties次await()方法，程序才继续执行，否则线程会阻塞在​​await()​​方法这里。

barrierAction：​​the command to execute when the barrier is tripped, or {@code null} if there is no action​​ 程序继续执行时回调的方法

2. 属性

private static class Generation {
        Generation() {}                 // prevent access constructor creation
        boolean broken;                 // initially false
    }

    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    private final int parties;
    /** The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

根据属性可以看出，内部使用lock锁实现的，generation 实现了cyclic功能，每次把count重新赋值为parties，可以认为对这个类做了初始化

3. await()方法

public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

调用了dowait()方法

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;

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

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

            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            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 {
                        // 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();
        }
    }

dowait()方法为整个CyclicBarrier类的核心实现，线程等待数量不满足parties时等待，满足了，唤醒所有线程同时调用​​nextGeneration​​​生产新的generation，这样就可以实现​​cyclic​​的功能了。

在​​nextGeneration()​​​方法中为什么每次都要生产调用​​generation = new Generation()​​ ？

​​Generation​​​类里面有一个状态​​broken​​​，其它在​​await()​​​方法上等待的线程还需要监控这个状态，因为有可能因为某线程等待超时触发​​InterruptedException​​​异常，此时会把​​broken=true​​，并且唤醒所有线程。

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

构造函数中的回调方法由最后一个进来的线程调用run()方法执行，因为只有最后一个进来的线程​​--count​​后才会等于0

      int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

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