[Java知识库]Java 之 CyclicBarrier 原理篇

引言

CyclicBarrier 就像就组长召唤大家开会一样，首先是预约一个时间，小组成员提前做好准备，待开会组长发言完，小组成员再做出响应；

测试示例

import java.util.concurrent.CyclicBarrier;

public class CyclicBarrierTest {

    public static void main(String[] args) {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3, () -> {
            System.out.println(Thread.currentThread().getName() + " 组长: 明天组团去旅游");
        });
        System.out.println("组长: 大家一会开个小会");
        for (int i = 0; i < 3; i++) {
            new Thread(() -> {
                try {
                    System.out.println(Thread.currentThread().getName() + " 准备");
                    cyclicBarrier.await(); // 等待组长发话
                    System.out.println(Thread.currentThread().getName() + " OK");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "t" + i).start();
        }
    }
}

输出结果：

CyclicBarrier 可以在构造的时候就预设一个命令，待子线程做完前置处理后执行这个预设命令，最后再执行子线程的后置处理；

CyclicBarrier 源码结构

public class CyclicBarrier {
	
    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();

}

CyclicBarrier 是直接使用 ReentrantLock 和 Condition 来实现，不像CountDownLatch 和 Semaphore 直接搞一个实现了 AbstractQueuedSynchronizer 同步器的 Sync 内部类；

await 等待构造器的预设命令执行

private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
           TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        final Generation g = generation;

        if (g.broken)
            throw new BrokenBarrierException();

        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }

        int index = --count;
        if (index == 0) {  // tripped 如果是最后一个子线程执行，则执行构造函数预设的命令
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run();
                ranAction = true;
                nextGeneration();  // 唤醒其他阻塞的子线程
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        // loop until tripped, broken, interrupted, or timed out
        for (;;) {
            try {
                if (!timed)
               		// 对子线程构造一个链表，并睡眠
                    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 {
        lock.unlock();
    }
}

遍历链表唤醒每个子线程 ↓

总结

CyclicBarrier 会对非最后一个子线程的其他子线程维护成一个链表的形式，如果是最后一个子线程，则用该线程先去执行构造时预设的命令，再唤醒其他睡眠的线程，最后一个线程不会加入到链表；
CyclicBarrier 和 CountDownLatch、Semaphore的不同点是 CyclicBarrier 使用到了Lock锁，CountDownLatch、Semaphore 实现是全程无锁的
相同点是都使用到了 AbstractQueuedSynchronizer 和 LockSupport来阻塞唤醒线程