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