Java的CyclicBarrier循环屏障解析

    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);
    }

public static void main(String[] args) {
		CyclicBarrier c = new CyclicBarrier(5,new Runnable() {
			@Override
			public void run() {
				System.out.println("开始");
			}
		});
		ExecutorService es = Executors.newCachedThreadPool();
		for (int i = 0; i < 5; i++) {
			es.execute(new Task(i, c));
		}
		es.shutdown();
	}
	static public class Task implements Runnable {
		private int i;
		private CyclicBarrier cyclicBarrier;
		public Task(int i,CyclicBarrier cyclicBarrier) {
			this.i = i;
			this.cyclicBarrier = cyclicBarrier;
		}
		@Override
		public void run() {
			System.out.println(i+"准备完毕");
			try {
				cyclicBarrier.await();
				Thread.sleep(100);
				System.out.println(i+"加入了");
			} catch (InterruptedException e) {	
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				e.printStackTrace();
			}
		}
	}

    /**
     * 如果当前线程不是最后一个达到的它就会休眠直到出现下面的任意一种情况：
     *  1. 最后一个线程达到
     *  2. 一些其他的线程中断了当前线程
     *  3. 一些其他的线程中断了正在等待的线程
     *  4. 一些其他线程在等待过程中超时了
     *  5. 一些其他的线程在barrier被重置
     *  当前线程在等待的过程中如果被中断直接会抛出异常并且当前线程的中断状态会被清除
     *  如果在等待的过程中一些线程被中断,其他的等待线程就会抛出异常,barrier就会被设置成broken为true的状态
     *  如果当前线程是最后一个达到的话,如果构造方法中有runnable任务,直接由最后一个线程去执行,其他线程继续等待
     *  在barrier action中,如果出现异常会由当前线程传播并且barrier被设置成broken为true的状态
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // 永远不会进入catch
        }
    }

    /**
     * Main barrier code, covering the various policies.
     */
    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();
        }
    }

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

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