关于Java CyclicBarrier reset的理解

1、在做leetcode中H2O 生成题目时用到了CyclicBarrier，于是尝试写了以下代码：

java 复制代码

package utils;

import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class H2O {
    private CyclicBarrier cb = new CyclicBarrier(3, new Runnable() {
        @Override public void run() {
            System.out.println("|");
            cb.reset();
        }
    });

    private int count = 5;

    public void pintH2O(){
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        executorService.execute(new Runnable() {
            @Override public void run() {
                for(int i = 0; i < count; i++){
                    System.out.print('H');
                    try{
                        cb.await();
                    } catch (Exception e){
                        System.out.print(" 1error " + e.toString());
                    }
                }
            }
        });

        executorService.execute(new Runnable() {
            @Override public void run() {
                for(int i = 0; i < count; i++){
                    System.out.print('H');
                    try{
                        cb.await();
                    } catch (Exception e){
                        System.out.print(" 2error " + e.toString());
                    }
                }
            }
        });

        executorService.execute(new Runnable() {
            @Override public void run() {
                for(int i = 0; i < count; i++){
                    System.out.print('O');
                    try{
                        cb.await();
                    } catch (Exception e){
                        System.out.print(" 3error " + e.toString());
                    }
                }
            }
        });

        executorService.shutdown();

    }
}

2、执行后产生了如下结果

java 复制代码

HHO|
O 1error java.util.concurrent.BrokenBarrierExceptionH 2error java.util.concurrent.BrokenBarrierExceptionH|
H 3error java.util.concurrent.BrokenBarrierExceptionO 1error java.util.concurrent.BrokenBarrierExceptionH|
H 3error java.util.concurrent.BrokenBarrierExceptionO 1error java.util.concurrent.BrokenBarrierExceptionH|
H 2error java.util.concurrent.BrokenBarrierExceptionH 3error java.util.concurrent.BrokenBarrierExceptionO|
 1error java.util.concurrent.BrokenBarrierException 2error java.util.concurrent.BrokenBarrierException
Process finished with exit code 0

3、在stackoverflow中发现了相同的问题Why CyclicBarrier reset() method is throwing BrokenBarrierException

原因如下：

（1）执行reset时，如果有线程正在等待屏障，则此线程会抛出BrokenBarrierException

（2）CyclicBarrier初始化时配置的Runnable(barrierAction)，会在所有线程突破屏障后，由最后一个到达的线程去执行，这就解释了为什么第一次没有线程抛BrokenBarrierException，而后每次都另外2个线程抛出异常

（3）直观看来，reset做了2件事情：1、突破当前屏障；2、生成一个新屏障

（4）在查看以下源码后发现：无论是reset还是屏障被突破，都会生成一个新的generation instance，所以在循环中时不必调用reset，一般情况下也用不到此方法，上述代码把reset去掉即可正常执行

java 复制代码

public class CyclicBarrier {
    /**
     * Each use of the barrier is represented as a generation instance.
     * The generation changes whenever the barrier is tripped, or
     * is reset. There can be many generations associated with threads
     * using the barrier - due to the non-deterministic way the lock
     * may be allocated to waiting threads - but only one of these
     * can be active at a time (the one to which {@code count} applies)
     * and all the rest are either broken or tripped.
     * There need not be an active generation if there has been a break
     * but no subsequent reset.
     */
    private static class Generation {
        boolean broken = false;
    }
....

参考：

CyclicBarrier的克星---BrokenBarrierException