同步屏障CyclicBarrier

CyclicBarrier,根据字面意思理解:循环屏障。屏障的意思:CyclicBarrier可以让一组线程到达某个屏障点时被阻塞,直到最后一个线程到达屏障点时,屏障才会放开,所有被屏障阻塞的线程才开始执行任务;循环:当所有线程被释放后,这个CyclicBarrier可以被重用。
有两个很常用的构造方法:
第一个:
    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }
这个会创建一个同步屏障,参数表示屏障拦截的线程数量,这些线程需要调用await()方法告诉CyclicBarrier自己到达了屏障点,然后阻塞住,等待其他线程,直到最后一个线程到达屏障点。
第二个:
    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @param barrierAction the command to execute when the barrier is
     *        tripped, or {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }
这个和上面的方法的区别是,当所有线程释放后,会随机选择一个线程来执行barrierAction。
简单示例:
package com.java4all.mypoint;

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

/**
 * Author: yunqing
 * Date: 2018/7/23
 * Description:
 */
public class MyRunnable implements Runnable{

    public CyclicBarrier cyclicBarrier;

    @Override
    public void run() {
        try {
            Thread.sleep(20000);
            System.out.println("子线程正在执行任务,当前线程为:"+Thread.currentThread().getName()+"      时间:"+ LocalTime.now().toString());
            cyclicBarrier.await();
        }catch (BrokenBarrierException bbnex){
            bbnex.printStackTrace();
        }catch (InterruptedException inex){
            inex.printStackTrace();
        }
    }


    public CyclicBarrier getCyclicBarrier() {
        return cyclicBarrier;
    }

    public void setCyclicBarrier(CyclicBarrier cyclicBarrier) {
        this.cyclicBarrier = cyclicBarrier;
    }
}
package com.java4all.mypoint;

import java.time.LocalTime;
import java.util.concurrent.CyclicBarrier;

/**
 * Author: yunqing
 * Date: 2018/7/18
 * Description:线程测试
 * 测试点:一组线程执行完后再执行某任务
 */
public class ThreadTest {

    private static CyclicBarrier cyclicBarrier;

    public static void main(String[] args)throws Exception{
        System.out.println("主线程正在执行前:"+Thread.currentThread().getName()+"      时间:"+ LocalTime.now().toString());
        cyclicBarrier = new CyclicBarrier(10, new Runnable() {
            @Override
            public void run() {
                System.out.println("------子线程都执行完了吧!执行任务A!当前线程为:"+Thread.currentThread().getName()+"      时间:"+ LocalTime.now().toString());
            }
        });

        for(int i = 1;i <= 10;i++){
            MyRunnable myRunnable = new MyRunnable();
            myRunnable.setCyclicBarrier(cyclicBarrier);
            new  Thread(myRunnable).start();
        }
        System.out.println("主线程正在执行后:"+Thread.currentThread().getName()+"      时间:"+ LocalTime.now().toString());
    }

}
执行结果为:
主线程正在执行前:main      时间:17:30:47.103
主线程正在执行后:main      时间:17:30:47.105
子线程正在执行任务,当前线程为:Thread-5      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-4      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-6      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-0      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-1      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-2      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-3      时间:17:31:07.105
子线程正在执行任务,当前线程为:Thread-9      时间:17:31:07.106
子线程正在执行任务,当前线程为:Thread-7      时间:17:31:07.106
子线程正在执行任务,当前线程为:Thread-8      时间:17:31:07.106
------子线程都执行完了吧!执行任务A!当前线程为:Thread-4      时间:17:31:07.107

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