JAVA并发编程之CyclicBarrier

一.概述

   使用JAVA编写并发程序的时候，我们需要仔细去思考一下并发流程的控制，如何让各个线程之间协作完成某项工作。有时候，我们启动N个线程去做一件事情，只有当这N个线程都达到某一个临界点的时候，我们才能继续下面的工作，就是说如果这N个线程中的某一个线程先到达预先定义好的临界点，它必须等待其他N-1线程也到达这个临界点，接下来的工作才能继续，只要这N个线程中有1个线程没有到达所谓的临界点，其他线程就算抢先到达了临界点，也只能等待，只有所有这N个线程都到达临界点后，接下来的事情才能继续。
    在日常工作中，我们经常和别人一起合作做一些项目，项目中有一些关键点，比如说代码联调，假如有M个开发人员，要想进行联调，这M个开发人员都必须到达这个临界点——把自己的功能模块完成，否则就无法进行联调。

二.CyclicBarrier的图示


关于上面的图片，首先A,B,C,D四个线程跑起来之后，发现向前推进的速度不一样，在某一时刻发现A,B,C已经到达临界点，但是D还没有到达临界点，此时A,B,C只能等待，当D到达临界点之后，A,B,C,D才能一起越过屏障，进行下面的工作。

三.分析一段CyclicBarrier的代码

import java.util.concurrent.CyclicBarrier;

class PerformTest {

    private int           threadCount;      // 线程数目
    private CyclicBarrier cycliBarrier;
    private int           loopCount = 10;

    public PerformTest(int threadCount){
        this.threadCount = threadCount;
        // 创建一个新的屏障，只有所有线程达到临界点之后，这里的匿名线程对象才会被执行
        this.cycliBarrier = new CyclicBarrier(this.threadCount, new Runnable() {

            /**
             * <pre>
             * 当所有线程都达到某个点的时候，这个方法就会被调用
             * </pre>
             */
            @Override
            public void run() {
                collectTestResult();
            }
        });

        for (int i = 0; i < this.threadCount; ++i) {
            Thread t = new Thread("working thread " + i) {

                @Override
                public void run() {
                    for (int j = 0; j < loopCount; ++j) {
                        doTest();
                        try {
                            /* 在这里如果发现计数值不是0的话，就会发生线程的阻塞，注意体会这里对于
                             * 对于cycliBarrier的循环利用
                             */
                            cycliBarrier.await();
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }
            };

            t.start();
        }
    }

    private void collectTestResult() {
        System.out.println("All Threads have been tested, and the result has been collected!");
    }

    private void doTest() {
        System.out.println(Thread.currentThread().getName() + " is tested!");
    }
}

public class CyclicBarrierTest1 {

    public static void main(String[] args) {
        new PerformTest(2);
    }
}

输出结果如下:
working thread 0 is tested!
working thread 1 is tested!
All Threads have been tested, and the result has been collected!
working thread 1 is tested!
working thread 0 is tested!
All Threads have been tested, and the result has been collected!
working thread 0 is tested!
working thread 1 is tested!
All Threads have been tested, and the result has been collected!
working thread 1 is tested!
working thread 0 is tested!
All Threads have been tested, and the result has been collected!
working thread 0 is tested!
working thread 1 is tested!
All Threads have been tested, and the result has been collected!
working thread 1 is tested!
working thread 0 is tested!
All Threads have been tested, and the result has been collected!
working thread 0 is tested!
working thread 1 is tested!
All Threads have been tested, and the result has been collected!
working thread 1 is tested!
working thread 0 is tested!
All Threads have been tested, and the result has been collected!
working thread 0 is tested!
working thread 1 is tested!
All Threads have been tested, and the result has been collected!
working thread 1 is tested!
working thread 0 is tested!
All Threads have been tested, and the result has been collected!


下面我们来分析一下上述这段代码的输出结果：
在程序中我们创建的两个线程，working thread 1和working thread 2。这两个线程内部都有10次for循环，cycliBarrier的总数是线程数2。线程1运行起来，在进行一次for循环之后，线程就处于等待状态，此时cycliBarrier中等待的数目变为1，由于线程处于等待状态，此时线程1无法继续向下进行，过一会working thread 2开始运行，在第一次for循环中cycliBarrier中的等待数目变为0，此时线程working thread 1被唤醒，working thread 1继续第二次循环，在第二次循环开始时，cycliBarrier就开始重复利用了，当cycliBarrier的等待数目变为0后，cycliBarrier就会恢复初始状态，进行再一次的并发流程控制。后面的运行过程是一样的，至于cycliBarrier的等待计数为0后，working thread 1和working thread 2那个先开始运行，要看具体的调度机制了。

四.CycliBarrier和CountDownLatch的区别

CycliBarrier和CountDownLatch都可以用来控制并发执行的流程，保证所有线程都做完自己的工作后在继续一些共性的工作。


1.CycliBarrier可以重复利用，但是CountDownLatch不可以重复利用。


2.CountDownLatch中计数值的递减需要自己手动去触发，但是CycliBarrier中计数值的减小是自动减小的。