CyclicBarrier

java并发编程之美 学习笔记

CyclicBarrier

CyclicBarrier（回环屏障），它可以让一组线程全部达到一个状态后,再全部同时执行。
这里之所以叫作回环是因为当所有等待线程执行完毕，并重置CyclicBarrier的状态后它可以被重用。

demo

场景：旅行团有两个人A和B, 旅行路线中有1,2,3三个站点， 必须所有人到达站点时，才可以向下一个站点触发.

public class CyclicBarrierTest {
	private static CyclicBarrier cyclicBarrier = new CyclicBarrier(2);

	public static void main(String[] args) {
		ExecutorService executorService = new ThreadPoolExecutor(2, 2, 0l, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
		executorService.submit(() -> {
			try {
				System.out.println("A arrived ----- Step1");
				cyclicBarrier.await();
				System.out.println("A arrived ----- Step2");
				cyclicBarrier.await();
				System.out.println("A arrived ----- Step3");
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				e.printStackTrace();
			}
		});

		executorService.submit(() -> {
			try {
				System.out.println("B arrived ----- Step1");
				//B to buy something ...
				Thread.sleep(30);
				cyclicBarrier.await();
				System.out.println("B arrived ----- Step2");
				//B to  pee...
				Thread.sleep(70);
				cyclicBarrier.await();
				System.out.println("B arrived ----- Step3");
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				e.printStackTrace();
			}
		});

		executorService.shutdown();
	}
}

执行结果：

源码分析

结构
CyclicBarrier内部是基于独占锁实现的，本质上还是基于AQS锁实现。

public class CyclicBarrier {

  	//记录线程数： 当parties个线程调用await()后，所有的线程会会冲破屏障，向后执行
    private final int parties;

    //初始值为parties
    //每当一个线程执行await()后，count--，当count == 0时表示所有线程到达屏障点，继而向后执行
    private int count;

    //当所有线程到达某一个屏障点时执行。
    private final Runnable barrierCommand;



    private static class Generation {
   		//记录屏障是否被打破 --- 并未申明为volatile,因为它是在lock()内部使用的，所以无需什么
        boolean broken = false;
    }

	//独占锁
    private final ReentrantLock lock = new ReentrantLock();

    //Condition等待屏障被打破
    private final Condition trip = lock.newCondition();

    //当到达下一个屏障点时，generation 会重新new
    private Generation generation = new Generation();
}

初始化

    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);
    }

为什么会同时定义parties ，count 两个属性来记录线程数呢?
因为当count-- 到0时，会重新将parties 赋值给count,从而重复使用.

await()

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

    //带有超时时间的await()
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

await()的执行逻辑，最终还是委托给dowait()来实现的。

dowait()

/**
     * @param  timed                 是否配置超时时间
     * @param  nanos                超时时间,单位为nanos
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        //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) { //当index == 0, 表示所有线程到达屏障点

                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run(); //执行初始化的barrierCommand
                    ranAction = true;

                    //激活执行await()而阻塞的线程，并重置count,和new Generation()
                    nextGeneration();

                    //返回 0
                    return 0;
                } finally {
                    if (!ranAction) 
                    	//如果ranAction ==false
                    	//breakBarrier()： 激活执行await()而阻塞的线程，并重置count, 设置generation为broken
                    	//与nextGeneration()的区别为：：： 设置：generation.broken = true;
                        breakBarrier();
                }
            }

            //index != 0
            // 循环直到： tripped, broken, interrupted, or timed out 等事件发生;
            for (;;) {
                try {
                    if (!timed)
                    	//等待trip.singal()或者singalAll()触发
                        trip.await();
                    else if (nanos > 0L)
                    	//带有超时时间的等待
                        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)
                    return index;

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

nextGeneration()
当正常打破屏障点时，执行nextGeneration().

    private void nextGeneration() {
       //激活执行await()而阻塞的线程
        trip.signalAll();
        //重置count
        count = parties;
        //new Generation()
        generation = new Generation();
    }

breakBarrier()
当非正常打破屏障点时，执行breakBarrier().

  private void breakBarrier() {
  	  //设置broken为true
       generation.broken = true;
       count = parties;//重置count
       trip.signalAll(); //激活执行await()而阻塞的线程
   }