前言

CyclicBarrier要做的事情是，让一组线程到达一个屏障（也可以叫同步点）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运行. 简单地说就是人到齐了后才可以让每个人继续去做自己的事情.

CycliBarrier是通过ReentrantLock和Condition实现的一个数据结构.

本文代码: 代码下载

例子1

先通过一个简单的例子了解一下CyclicBarrier.

package com.sourcecode.concurrencytools_CyclicBarrier;

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class CyclicBarrierTest4 {
    static CyclicBarrier c = new CyclicBarrier(5);
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        for (int i = 0; i < 5; i++) {
            Thread thread = new MyThread();
            thread.start();
        }
    }

    static class MyThread extends Thread {
        @Override
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " tries to wait!");
                c.await();
            } catch (Exception e) {
                System.out.println(e);
                //System.out.println(Thread.currentThread().getName() + "------>" + c.isBroken() + ", interrupted status:" + Thread.currentThread().isInterrupted());
            } finally {
                System.out.println(Thread.currentThread().getName() + " finishes!");
            }
        }
    }
}

运行结果如下: 初始化CyclicBarrier的时候参数是5,表示需要等待5个线程达到后才可以打开屏障,正如结果所示,thread-0到thread-3在等待,到最后一个线程thread-4到达屏障时,此时屏障打开,每个线程执行各自接下来的模块.

如果初始化参数大于5,比如6,此程序将一直阻塞,因为没有第6个线程到达该屏障.

Thread-0 tries to wait!
Thread-1 tries to wait!
Thread-2 tries to wait!
Thread-3 tries to wait!
Thread-4 tries to wait!
Thread-4 finishes!
Thread-1 finishes!
Thread-0 finishes!
Thread-2 finishes!
Thread-3 finishes!

实现思路分析

cyclicbarrier.png

    private static class Generation {
      boolean broken = false;
    }
    /** 重入锁 */
    private final ReentrantLock lock = new ReentrantLock();
    /** 一个lock对象的Condition实例 */
    private final Condition trip = lock.newCondition();
    /** 拦截线程的总个数 */
    private final int parties;
    /** The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();
    /** 拦截线程的剩余需要数量 */
    private int count;

从该图可以看出CyclicBarrier有一个重入锁的变量lock并且持有一个该锁的Condition实例trip,就可以大概知道该CyclicBarrier会让线程尝试获取锁并且在拿到锁后将屏障个数减减操作,然后根据count的数量来决定是否调用trip.await()操作,比如count==0表示最后一个到达屏障的线程,那么就不需要调用trip的方法了.

构造方法

    public CyclicBarrier(int parties) {
      this(parties, null);
    }

    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

第二个参数Runnable barrierAction表示的是当最后一个到达屏障的线程先执行完该barrierAction的run方法后再执行唤醒其他线程的操作.简单地说当到达屏障时,先执行barrierAction的业务再执行其他线程的业务.

await方法

await方法有两个,分别为await()和await(long timeout, TimeUnit unit)方法,一个没有超时返回,另外一个有超时返回,但是两者都是调用dowait(boolean timed, long nanos),该方法是整个CyclicBarrier的核心实现.

public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }
public int await(long timeout, TimeUnit unit)
            throws InterruptedException,
            BrokenBarrierException,
            TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

所以接下来的看看该方法dowait是如何实现的.

/**
     * @param timed 是否需要超时
     * @param nanos 时长
     * @return 返回还需要等待多少个线程才可以到达屏障
     * @throws InterruptedException 当前线程中断
     * @throws BrokenBarrierException 有其他线程中断或者其他线程超时
     * @throws TimeoutException 当前线程等待超时
     */
    private int dowait(boolean timed, long nanos)
            throws InterruptedException, BrokenBarrierException,
            TimeoutException {
        // 获取重入锁
        final ReentrantLock lock = this.lock;
        // 尝试获取锁
        lock.lock();
        try {

            //System.out.println(Thread.currentThread().getName() + " get locks.");

            // 获得当前代
            final Generation g = generation;

            // 如果有线程中断或者超时
            if (g.broken)
                throw new BrokenBarrierException();

            // 如果当前线程被中断
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            int index = --count;
            //System.out.format("index=%d\n", index);
            if (index == 0) {  // 最后一个到达屏障的线程
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration(); //更新下一代
                    return 0;
                } finally {
                    // 如果执行command.run发生异常,则breakBarrier
                    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) {
                    // 如果等待过程中有被线程中断
                    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();
                    }
                }

                // 如果当代的broken为true,表明有线程被中断
                if (g.broken)
                    throw new BrokenBarrierException();

                // 如果换代了 表示可以返回了
                if (g != generation)
                    return index;

                // 如果超时则先break the current generation
                // 再抛出超时异常
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            // 释放锁
            //System.out.println(Thread.currentThread().getName() + " release locks.");
            lock.unlock();
        }
    }
/**
     *  break the current generation
     *  1. broken设置为true
     *  2. count 重新设置为parties
     *  3. 唤醒所有线程
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    /**
     *  start a new generation
     *  1. 唤醒所有等待中的线程
     *  2. count 重新设置为parties
     *  3. generation 设置成一个新的Generation对象
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

该方法的流程大概如下:
1. 尝试获取锁
2. 如果不是最后一个到达屏障的线程,则进入for循环中一直等待(此时该线程会释放锁)直到被最后一个线程唤醒或者被某个线程中断后调用breakBarrier方法唤醒. 唤醒后需要竞争再次获得锁后才可以继续执行.
3. 如果是最后一个到达屏障的线程,如果barrierCommand不为空,则需要先执行barrierCommand.run()方法,然后通过nextGeneration唤醒等待的线程.
4. 在所有异常退出或者正常退出都需要释放锁.
流程图如下

dowait.png

例子2

设置线程屏障为3,启动两个线程2秒超时等待,让最后一个线程3秒后才到达屏障.

package com.sourcecode.concurrencytools_CyclicBarrier;
import java.util.concurrent.TimeUnit;
public class CyclicBarrierTest5 {
    static CyclicBarrier c = new CyclicBarrier(3);
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        for (int i = 0; i < 2; i++) {
            Thread thread = new MyThread();
            thread.start();
        }
        TimeUnit.SECONDS.sleep(3);
        System.out.println(Thread.currentThread().getName() + "------>" + "tries to wait!");
        c.await();
        System.out.println(Thread.currentThread().getName() + "------>" + "finishes!");
    }

    static class MyThread extends Thread {
        @Override
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " tries to wait!");
                c.await(1, TimeUnit.SECONDS);
                //c.await();
            } catch (Exception e) {
                System.out.println(Thread.currentThread().getName() + "---->" + e);
                //System.out.println(Thread.currentThread().getName() + "------>" + c.isBroken() + ", interrupted status:" + Thread.currentThread().isInterrupted());
            } finally {
                System.out.println(Thread.currentThread().getName() + " finishes!");
            }
        }
    }
}

结果如下: 可以看到第一个线程出现超时异常后,表示该线程已经调用了breakBarrier方法,所以可以看到后续的两个线程都是抛出BrokenBarrierException异常.

Thread-0 tries to wait!
Thread-1 tries to wait!
Thread-1---->java.util.concurrent.TimeoutException
Thread-0---->com.sourcecode.concurrencytools_CyclicBarrier.BrokenBarrierException
Thread-0 finishes!
Thread-1 finishes!
main------>tries to wait!
Exception in thread "main" com.sourcecode.concurrencytools_CyclicBarrier.BrokenBarrierException
    at com.sourcecode.concurrencytools_CyclicBarrier.CyclicBarrier.dowait(CyclicBarrier.java:69)
    at com.sourcecode.concurrencytools_CyclicBarrier.CyclicBarrier.await(CyclicBarrier.java:39)
    at com.sourcecode.concurrencytools_CyclicBarrier.CyclicBarrierTest5.main(CyclicBarrierTest5.java:14)

isBroken方法和reset方法

/**
     * @return 当前代是否被破坏, 被破坏的两种情况, 某个线程中断或者等待超时
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }
public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

reset留作遇到好的例子后再分析

参考

1. Java并发编程的艺术