CyclicBarrier使用与原理

CyclicBarrier当计数减少到0时，会唤醒所有阻塞在同一个Condition上的线程，与CountDownLatch不同的是所有的线程必须同时被唤醒，就好比钓鱼比赛，所有人必须同时开始抛竿一样。CountDownLatch只要求主线程的动作在其他依赖的线程执行完之后执行就OK

测试代码：

package com.jv.parallel.feature;

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/*
 * 处理同一个动作有多个线程同时执行的场景
 */
public class TestCyclicBarrier {
	public static void main(String[] args) {
		//1.得到一个CyclicBarrier实例
		CyclicBarrier cb = new CyclicBarrier(4);
		new Thread(new Fishing(cb),"1").start();
		new Thread(new Fishing(cb),"2").start();
		new Thread(new Fishing(cb),"3").start();
		new Thread(new Fishing(cb),"4").start();
	}
	
	static class Fishing implements Runnable{
		CyclicBarrier cb;
		public Fishing(CyclicBarrier cb) {
			this.cb = cb;
		}

		@Override
		public void run() {
			try {
				cb.await();
				System.out.println("第（" + Thread.currentThread().getName() + "）个人开始钓鱼");
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				e.printStackTrace();
			}
		}
	}
}

原理：

这里没有使用到什么设计模式，CyclicBarrier.await方法调用CyclicBarrier.dowait方法，每次调用await方法都会使计数器-1，当减少到0时就会唤醒所有的线程

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

    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    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();
        }
    }

最核心的部分就是

每次调用await方法都会使内部的计数器临时变量-1，当减少到0时，就会调用nextGeneration方法

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

在这里唤醒所有阻塞的线程

提醒：在声明CyclicBarrier的时候还可以传一个Runnable的实现类，当计数器减少到0时，会执行该实现类