CyclicBarrier学习

概述

字面意思回环栅栏，通过它可以实现让一组线程等待至某个状态之后再全部同时执行。叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。我们暂且把这个状态就叫做barrier，当调用await()方法之后，线程就处于barrier了。

package com.business.thread;

import java.util.Map;
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CyclicBarrier;

public class UseCyclicBarrier {
	static CyclicBarrier barrier = new CyclicBarrier(5, new collectThread());
	private static ConcurrentHashMap<String, Long> resultMap = new ConcurrentHashMap<String, Long>();// 存放子线程工作结果的容器

	public static void main(String[] args) {
		for (int i = 0; i <= 4; i++) {
			Thread thread = new Thread(new SubThread());
			thread.start();
		}
	}

	// 汇总线程
	private static class collectThread implements Runnable {

		public void run() {
			StringBuilder result = new StringBuilder();
			for (Map.Entry<String, Long> workResult : resultMap.entrySet()) {
				result.append("[" + workResult.getValue() + "]");
			}
			System.out.println("result :" + result);
			System.out.println("do other business");
		}
	}

	// 工作线程
	private static class SubThread implements Runnable {

		public void run() {
			long id = Thread.currentThread().getId();
			resultMap.put(id + "", id);
			Random r = new Random();

			if (r.nextBoolean()) {
				try {
					Thread.sleep((int) (1000 + id));
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				System.out.println("Thread_" + id + "......do sth");
			}
			try {
				//等待collectThread线程完成
				barrier.await();
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (BrokenBarrierException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			try {
				Thread.sleep((int) (1000 + id));
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			System.out.println("Thread_" + id + "............do business");

		}
	}
}

执行结果：

Thread_13......do sth
result :[11][12][13][14][10]
do other business
Thread_10............do business
Thread_11............do business
Thread_12............do business
Thread_13............do business
Thread_14............do business

 

SubThread线程需要等待collectThread线程完成之后，才能继续下面的任务。

CyclicBarrier与CountDownLatch比较

　　1）CountDownLatch:一个线程(或者多个)，等待另外N个线程完成某个事情之后才能执行；CyclicBarrier:N个线程相互等待，任何一个线程完成之前，所有的线程都必须等待。

　　2）CountDownLatch:一次性的；CyclicBarrier:可以重复使用。

　　3）CountDownLatch基于AQS；CyclicBarrier基于锁和Condition。本质上都是依赖于volatile和CAS实现的。