CyclicBarrier使用與原理

CyclicBarrier當計數減小到0時，會喚醒全部阻塞在同一個Condition上的線程，與CountDownLatch不一樣的是全部的線程必須同時被喚醒，就比如釣魚比賽，全部人必須同時開始拋竿同樣。CountDownLatch只要求主線程的動做在其餘依賴的線程執行完以後執行就OK

測試代碼：

package com.jv.parallel.feature;

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

/*
 * 處理同一個動做有多個線程同時執行的場景
 */
public class TestCyclicBarrier {
	public static void main(String[] args) {
		//1.獲得一個CyclicBarrier實例
		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() + "）我的開始釣魚");
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				e.printStackTrace();
			}
		}
	}
}

 

原理：

這裏沒有使用到什麼設計模式，CyclicBarrier.await方法調用CyclicBarrier.dowait方法，每次調用await方法都會使計數器-1，當減小到0時就會喚醒全部的線程

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

每次調用await方法都會使內部的計數器臨時變量-1，當減小到0時，就會調用nextGeneration方法

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

在這裏喚醒全部阻塞的線程

 

提醒：在聲明CyclicBarrier的時候還能夠傳一個Runnable的實現類，當計數器減小到0時，會執行該實現類