Java并发编程 - AQS 之 CyclicBarrier

简介

CyclicBarrier（可重用屏障/栅栏）类似于 CountDownLatch（倒计数闭锁），它能阻塞一组线程直到某个事件的发生。
- 与闭锁的关键区别在于，所有的线程必须同时到达屏障位置，才能继续执行。
- 闭锁用于等待事件，而屏障用于等待其他线程。
- CyclicBarrier 可以使一定数量的线程反复地在屏障位置处汇集。当线程到达屏障位置时将调用 await() 方法，这个方法将阻塞直到所有线程都到达屏障位置。如果所有线程都到达屏障位置，那么屏障将打开，此时所有的线程都将被释放，而屏障将被重置以便下次使用。
CyclicBarrier 是 JDK 1.5 的 java.util.concurrent 并发包中提供的一个并发工具类。
- 所谓 Cyclic 即循环的意思，所谓 Barrier 即屏障的意思。
- CyclicBarrier 是一个同步辅助类，它允许一组线程相互等待直到所有线程都到达一个公共的屏障点。
- 在程序中有固定数量的线程，这些线程有时候必须等待彼此，这种情况下，使用 CyclicBarrier 很有帮助。
- 这个屏障之所以用循环修饰，是因为在所有的线程释放彼此之后，这个屏障是可以 重新使用 的。

5df9653f0001250619201080.jpg (1920×1080)

CyclicBarrier允许一组线程相互等待，直到到达某个公共的屏障点，通过CyclicBarrier，可以实现多个线程间相互等待，直到所有的线程都准备好，等待条件可以重用，又称为循环屏障，可以用于多线程计算数据，最终汇总计算结果的场景。

应用场景

CyclicBarrier 常用于多线程分组计算。
比如一个大型的任务，常常需要分配好多子任务去执行，只有当所有子任务都执行完成时候，才能执行主任务，这时候，就可以选择 CyclicBarrier。

CountDownLatch和CyclicBarrier区别

CountDownLatch 是一个线程(或者多个)，等待另外 N 个线程完成某个事情之后才能执行；CyclicBarrier 是 N 个线程相互等待，任何一个线程完成之前，所有的线程都必须等待。
CountDownLatch 的计数器只能使用一次。而 CyclicBarrier 的计数器可以使用 reset() 方法重置；CyclicBarrier 能处理更为复杂的业务场景，比如如果计算发生错误，可以重置计数器，并让线程们重新执行一次。
CountDownLatch 采用减计数方式；CyclicBarrier 采用加计数方式。

CyclicBarrier 原理

CyclicBarrier 内部使用了 ReentrantLock 和 Condition 两个类。

案例一

package com.mmall.concurrency.example.aqs;

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class CyclicBarrierExample1 {

    private static CyclicBarrier barrier = new CyclicBarrier(5);

    public static void main(String[] args) throws Exception {

        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            executor.execute(() -> {
                try {
                    race(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

    private static void race(int threadNum) throws Exception {
        Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        barrier.await();
        log.info("{} continue", threadNum);
    }
}

// 输出
21:36:05.124 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 is ready
21:36:06.123 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 is ready
21:36:07.123 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 is ready
21:36:08.123 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 is ready
21:36:09.124 [pool-1-thread-5] INFO  c.m.concurrency.example.aqs.test9 - 4 is ready
21:36:09.124 [pool-1-thread-5] INFO  c.m.concurrency.example.aqs.test9 - 4 continue
21:36:09.124 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 continue
21:36:09.124 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 continue
21:36:09.124 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 continue
21:36:09.124 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 continue
21:36:10.124 [pool-1-thread-6] INFO  c.m.concurrency.example.aqs.test9 - 5 is ready
21:36:11.125 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 6 is ready
21:36:12.125 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 7 is ready
21:36:13.126 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 8 is ready
21:36:14.127 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 9 is ready
21:36:14.127 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 9 continue
21:36:14.127 [pool-1-thread-6] INFO  c.m.concurrency.example.aqs.test9 - 5 continue
21:36:14.127 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 6 continue
21:36:14.127 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 8 continue
21:36:14.127 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 7 continue

Process finished with exit code 0

分析 await 方法

在 CyclicBarrier 上进行阻塞等待，直到发生以下情形之一。
- 在 CyclicBarrier 上等待的线程数量达到 parties，则所有线程被释放，继续执行。
- 当前线程被中断，则抛出 InterruptedException 异常，并停止等待，继续执行。
- 其他等待的线程被中断，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。
- 其他等待的线程超时，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。
- 其他线程调用 CyclicBarrier.reset() 方法，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。
线程调用 await() 表示自己已经到达栅栏。
BrokenBarrierException 表示栅栏已经被破坏，破坏的原因可能是其中一个线程
await() 时被中断或者超时。

案例二

package com.mmall.concurrency.example.aqs;

import lombok.extern.slf4j.Slf4j;

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

@Slf4j
public class CyclicBarrierExample2 {

    private static CyclicBarrier barrier = new CyclicBarrier(5);

    public static void main(String[] args) throws Exception {

        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            executor.execute(() -> {
                try {
                    race(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

    private static void race(int threadNum) throws Exception {
        Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        try {
            barrier.await(2000, TimeUnit.MILLISECONDS);
        } catch (Exception e) {
            log.warn("Exception", e);
        }
        log.info("{} continue", threadNum);
    }
}

// 输出
21:43:03.378 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 is ready
21:43:04.376 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 is ready
21:43:05.376 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 is ready
21:43:05.382 [pool-1-thread-1] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:05.382 [pool-1-thread-2] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:05.382 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 continue
21:43:05.382 [pool-1-thread-3] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:05.382 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 continue
21:43:05.382 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 continue
21:43:06.377 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 is ready
21:43:06.377 [pool-1-thread-4] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:06.377 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 continue
21:43:07.377 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 4 is ready
21:43:07.377 [pool-1-thread-3] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:07.377 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 4 continue
21:43:08.378 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 5 is ready
21:43:08.378 [pool-1-thread-4] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:08.378 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 5 continue
21:43:09.378 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 6 is ready
21:43:09.378 [pool-1-thread-3] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:09.378 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 6 continue
21:43:10.379 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 7 is ready
21:43:10.379 [pool-1-thread-4] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:10.379 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 7 continue
21:43:11.380 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 8 is ready
21:43:11.380 [pool-1-thread-3] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:11.380 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 8 continue
21:43:12.380 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 9 is ready
21:43:12.380 [pool-1-thread-4] WARN  c.m.concurrency.example.aqs.test9 - Exception
21:43:12.380 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 9 continue

Process finished with exit code 0

分析 await(timeout,TimeUnit) 方法

在 CyclicBarrier 上进行限时的阻塞等待，直到发生以下情形之一。
- 在 CyclicBarrier 上等待的线程数量达到 parties，则所有线程被释放，继续执行。
- 当前线程被中断，则抛出 InterruptedException 异常，并停止等待，继续执行。
- 当前线程等待超时，则抛出 TimeoutException 异常，并停止等待，继续执行。
- 其他等待的线程被中断，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。
- 其他等待的线程超时，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。
- 其他线程调用 CyclicBarrier.reset() 方法，则当前线程抛出 BrokenBarrierException 异常，并停止等待，继续执行。

案例三

package com.mmall.concurrency.example.aqs;

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class CyclicBarrierExample3 {

    private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {
        log.info("callback is running");
    });

    public static void main(String[] args) throws Exception {

        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            Thread.sleep(1000);
            executor.execute(() -> {
                try {
                    race(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

    private static void race(int threadNum) throws Exception {
        Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        barrier.await();
        log.info("{} continue", threadNum);
    }
}

// 输出
21:47:26.683 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 is ready
21:47:27.682 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 is ready
21:47:28.684 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 is ready
21:47:29.683 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 is ready
21:47:30.683 [pool-1-thread-5] INFO  c.m.concurrency.example.aqs.test9 - 4 is ready
21:47:30.683 [pool-1-thread-5] INFO  c.m.concurrency.example.aqs.test9 - callback is running
21:47:30.683 [pool-1-thread-5] INFO  c.m.concurrency.example.aqs.test9 - 4 continue
21:47:30.683 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 0 continue
21:47:30.683 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 1 continue
21:47:30.683 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 2 continue
21:47:30.683 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 3 continue
21:47:31.684 [pool-1-thread-6] INFO  c.m.concurrency.example.aqs.test9 - 5 is ready
21:47:32.684 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 6 is ready
21:47:33.685 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 7 is ready
21:47:34.685 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 8 is ready
21:47:35.685 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 9 is ready
21:47:35.685 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - callback is running
21:47:35.685 [pool-1-thread-1] INFO  c.m.concurrency.example.aqs.test9 - 9 continue
21:47:35.686 [pool-1-thread-6] INFO  c.m.concurrency.example.aqs.test9 - 5 continue
21:47:35.686 [pool-1-thread-4] INFO  c.m.concurrency.example.aqs.test9 - 6 continue
21:47:35.686 [pool-1-thread-3] INFO  c.m.concurrency.example.aqs.test9 - 7 continue
21:47:35.686 [pool-1-thread-2] INFO  c.m.concurrency.example.aqs.test9 - 8 continue

Process finished with exit code 0

在案例一的基础上，无非新增了一个回调函数功能：在线程达到屏障的时候，优先执行该回调函数先。

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Java并发编程 - AQS 之 CyclicBarrier

简介

应用场景

CountDownLatch和CyclicBarrier区别

CyclicBarrier 原理

案例一

案例二

案例三

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