并发笔记之CyclicBarrier分析

CyclicBarrier是一个同步辅助类，允许一组线程互相等待直到到达一个公共屏障点(common barrier),当涉及到一组固定大小必须相互等待的线程是非常有用的。
他之所以称为cyclic是因为当线程释放，CyclicBarrier可以被重复使用。

主要属性

public class CyclicBarrier {
    /** 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();

    /**
     * 等待的线程数每一代从parties递减到0
     * 当Barrier breaken或者新的一代开始 被重置为parties
     */
    private int count;

构造器

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

其构造其有两个参数：

• parties:表示参与的线程数
• barriesAction：当达到给定数量的线程处于等待状态 启动barrier时执行给定的屏障操作，此操作由最后进入barrier的线程执行。

等待 await

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

await()最终调用的是超时版本的dowait

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;
            //当前generation已损坏 则抛出BrokenBarrierException异常
            if (g.broken)
                throw new BrokenBarrierException();
            //线程中断则终止barrier
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
            //进来一个线程 count自减一
            int index = --count;
            if (index == 0) {  // index==0 表示线程都到位 则执行Runnable任务
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();//出发屏障任务
                    ranAction = true;
                    nextGeneration();//唤醒所有等待任务 并更新generation
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)//非超时等待 调用Condition.await()一直等待
                        trip.await();
                    else if (nanos > 0L)//超时等待
                        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)//generation已更新 返回index
                    return index;

                if (timed && nanos <= 0L) {//等待超时 终止barrier 并抛出异常
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }
    

换代

当线程都到位，并且最后一个进入的线程成功执行了屏障任务，则调用nextGeneration()方法进行换代 并唤醒所有线程

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

终止Barrier

当线程被中断或线程等待超时 则调用breakBarrier()方法终止Barrier

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