JUC包下CyclicBarrier学习笔记

CyclicBarrier,一个同步辅助类,在API中是这么介绍的:

它允许一组线程互相等待,直到到达某个公共屏障点 (common barrier point)。在涉及一组固定大小的线程的程序中,这些线程必须不时地互相等待,此时 CyclicBarrier 很有用。因为该 barrier 在释放等待线程后可以重用,所以称它为循环 的 barrier。

通俗点讲就是:让一组线程到达一个屏障时被阻塞,直到最后一个线程到达屏障时,屏障才会开门,所有被屏障拦截的线程才会继续干活。

先可以看下测试结果:

等待线程进来之后并没有立即去执行相关业务逻辑,当线程达到你设计的要求之后才回去执行相关业务逻辑数据,测试代码如下

package com.cxy.cyclicBarrier;

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

/**
 * Created by Administrator on 2017/4/10.
 */
public class CyclicBarrierDemo {
    private  static CyclicBarrier cyclicBarrier=new CyclicBarrier(10);

    public static void main(String[] args) throws InterruptedException {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (int i =0;i< 10;i++){
            System.out.println("创建线程"+i);
            final int threadNum =i;
            Thread.sleep(1000);
            executorService.execute( ()->{
                try {
                    cxyDemo(threadNum);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
            });
        }
        executorService.shutdown();

    }
    private static void  cxyDemo(int threadNum) throws InterruptedException, BrokenBarrierException {
        Thread.sleep(1000);
        System.out.println(Thread.currentThread().getName() + "开始等待其他线程");
        cyclicBarrier.await();
        System.out.println(threadNum);

    }
}

其中利用到lamba表达式,

利用到可缓存的线程池,可以其中有个注释不是很对

  public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * 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);
    }

两个构造方法:

第一个需要传入线程数,还有就是Runnable 

第二个 需要传入得线程数

这个参数定义的线程数,当线程数到设置的值,那么就回去执行awit方法后面的逻辑

在我上面案例中使用的是第一种构造方法:

下面将使用第二种案例:

package com.cxy.cyclicBarrier;

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

/**
 * Created by Administrator on 2017/4/10.
 */
public class CyclicBarrierDemo {
    /*  private  static CyclicBarrier cyclicBarrier=new CyclicBarrier(10);

      public static void main(String[] args) throws InterruptedException {
          ExecutorService executorService = Executors.newCachedThreadPool();
          for (int i =0;i< 10;i++){
              System.out.println("创建线程"+i);
              final int threadNum =i;
              Thread.sleep(1000);
              executorService.execute( ()->{
                  try {
                      cxyDemo(threadNum);
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  } catch (BrokenBarrierException e) {
                      e.printStackTrace();
                  }
              });
          }
          executorService.shutdown();

      }
      private static void  cxyDemo(int threadNum) throws InterruptedException, BrokenBarrierException {
          Thread.sleep(1000);
          System.out.println(Thread.currentThread().getName() + "开始等待其他线程");
          cyclicBarrier.await();
          System.out.println(threadNum);

      }*/
    private static CyclicBarrier cyclicBarrier;

    static class CyclicBarrierThread extends Thread {
        public void run() {
            System.out.println(Thread.currentThread().getName() + "到了"); //等待
            try {
                cyclicBarrier.await();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        cyclicBarrier = new CyclicBarrier(5, new Runnable() {
            @Override
            public void run() {
                System.out.println("人到齐了,开始吧....");
            }
        });
        for (int i = 0; i < 5; i++) {
            new CyclicBarrierThread().start();
        }
    }
}

执行结果:

那么await方法到底干了什么呢:

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

最终调用的dowait方法: /**

     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {

      //实现锁,安全 final ReentrantLock
lock = this.lock; lock.lock(); try { final Generation g = generation;         //分代被破坏,异常抛出 if (g.broken) throw new BrokenBarrierException(); if (Thread.interrupted()) {
          //线程被中断, breakBarrier();
throw new InterruptedException(); }         //线程进来,减少数据量 int index = --count; if (index == 0) { // tripped
          // 达到指定的数量时候,执行
boolean ranAction = false; try { final Runnable command = barrierCommand; if (command != null) command.run();
            //唤醒线程 ranAction
= true;
            //更新分代 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 {
        //最终释放锁
lock.unlock(); } }
  • 最后一个线程到达,即index == 0
  • 超出了指定时间(超时等待)
  • 其他的某个线程中断当前线程
  • 其他的某个线程中断另一个等待的线程
  • 其他的某个线程在等待barrier超时
  • 其他的某个线程在此barrier调用reset()方法。reset()方法用于将屏障重置为初始状态   

应用场景;开会,还有就是例如现金很火的游戏,只有等待大家准备好了,才开始执行进入房间

posted @ 2019-04-10 10:07  菩提树下的丁春秋  阅读(238)  评论(0编辑  收藏  举报