Java多线程系列--“JUC锁”10之 CyclicBarrier原理和示例

概要

本章介绍JUC包中的CyclicBarrier锁。内容包括：
CyclicBarrier简介
CyclicBarrier数据结构
CyclicBarrier源码分析(基于JDK1.7.0_40)
CyclicBarrier示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3533995.html

CyclicBarrier简介

CyclicBarrier是一个同步辅助类，允许一组线程互相等待，直到到达某个公共屏障点 (common barrier point)。因为该 barrier 在释放等待线程后可以重用，所以称它为循环 的 barrier。

注意比较CountDownLatch和CyclicBarrier：
(01) CountDownLatch的作用是允许1或N个线程等待其他线程完成执行；而CyclicBarrier则是允许N个线程相互等待。
(02) CountDownLatch的计数器无法被重置；CyclicBarrier的计数器可以被重置后使用，因此它被称为是循环的barrier。

CyclicBarrier函数列表

CyclicBarrier(int parties)
创建一个新的 CyclicBarrier，它将在给定数量的参与者（线程）处于等待状态时启动，但它不会在启动 barrier 时执行预定义的操作。
CyclicBarrier(int parties, Runnable barrierAction)
创建一个新的 CyclicBarrier，它将在给定数量的参与者（线程）处于等待状态时启动，并在启动 barrier 时执行给定的屏障操作，该操作由最后一个进入 barrier 的线程执行。

int await()
在所有参与者都已经在此 barrier 上调用 await 方法之前，将一直等待。
int await(long timeout, TimeUnit unit)
在所有参与者都已经在此屏障上调用 await 方法之前将一直等待,或者超出了指定的等待时间。
int getNumberWaiting()
返回当前在屏障处等待的参与者数目。
int getParties()
返回要求启动此 barrier 的参与者数目。
boolean isBroken()
查询此屏障是否处于损坏状态。
void reset()
将屏障重置为其初始状态。

CyclicBarrier数据结构

CyclicBarrier的UML类图如下：

CyclicBarrier是包含了"ReentrantLock对象lock"和"Condition对象trip"，它是通过独占锁实现的。下面通过源码去分析到底是如何实现的。

CyclicBarrier源码分析(基于JDK1.7.0_40)

CyclicBarrier完整源码(基于JDK1.7.0_40)

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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/*
 *
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 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * A synchronization aid that allows a set of threads to all wait for
 * each other to reach a common barrier point.  CyclicBarriers are
 * useful in programs involving a fixed sized party of threads that
 * must occasionally wait for each other. The barrier is called
 * <em>cyclic</em> because it can be re-used after the waiting threads
 * are released.
 *
 * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
 * that is run once per barrier point, after the last thread in the party
 * arrives, but before any threads are released.
 * This <em>barrier action</em> is useful
 * for updating shared-state before any of the parties continue.
 *
 * <p><b>Sample usage:</b> Here is an example of
 *  using a barrier in a parallel decomposition design:
 * <pre>
 * class Solver {
 *   final int N;
 *   final float[][] data;
 *   final CyclicBarrier barrier;
 *
 *   class Worker implements Runnable {
 *     int myRow;
 *     Worker(int row) { myRow = row; }
 *     public void run() {
 *       while (!done()) {
 *         processRow(myRow);
 *
 *         try {
 *           barrier.await();
 *         } catch (InterruptedException ex) {
 *           return;
 *         } catch (BrokenBarrierException ex) {
 *           return;
 *         }
 *       }
 *     }
 *   }
 *
 *   public Solver(float[][] matrix) {
 *     data = matrix;
 *     N = matrix.length;
 *     barrier = new CyclicBarrier(N,
 *                                 new Runnable() {
 *                                   public void run() {
 *                                     mergeRows(...);
 *                                   }
 *                                 });
 *     for (int i = 0; i < N; ++i)
 *       new Thread(new Worker(i)).start();
 *
 *     waitUntilDone();
 *   }
 * }
 * </pre>
 * Here, each worker thread processes a row of the matrix then waits at the
 * barrier until all rows have been processed. When all rows are processed
 * the supplied {@link Runnable} barrier action is executed and merges the
 * rows. If the merger
 * determines that a solution has been found then <tt>done()</tt> will return
 * <tt>true</tt> and each worker will terminate.
 *
 * <p>If the barrier action does not rely on the parties being suspended when
 * it is executed, then any of the threads in the party could execute that
 * action when it is released. To facilitate this, each invocation of
 * {@link #await} returns the arrival index of that thread at the barrier.
 * You can then choose which thread should execute the barrier action, for
 * example:
 * <pre>  if (barrier.await() == 0) {
 *     // log the completion of this iteration
 *   }</pre>
 *
 * <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
 * for failed synchronization attempts: If a thread leaves a barrier
 * point prematurely because of interruption, failure, or timeout, all
 * other threads waiting at that barrier point will also leave
 * abnormally via {@link BrokenBarrierException} (or
 * {@link InterruptedException} if they too were interrupted at about
 * the same time).
 *
 * <p>Memory consistency effects: Actions in a thread prior to calling
 * {@code await()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions that are part of the barrier action, which in turn
 * <i>happen-before</i> actions following a successful return from the
 * corresponding {@code await()} in other threads.
 *
 * @since 1.5
 * @see CountDownLatch
 *
 * @author Doug Lea
 */
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 <tt>count</tt> 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;
    }

    /** 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 number of parties */
    private final int parties;
    /* The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    /**
     * Main barrier code, covering the various policies.
     */
    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
               boolean ranAction = false;
               try {
                   final Runnable command = barrierCommand;
                   if (command != null)
                       command.run();
                   ranAction = true;
                   nextGeneration();
                   return 0;
               } finally {
                   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) {
                    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();
        }
    }

    /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @param barrierAction the command to execute when the barrier is
     *        tripped, or {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

    /**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
    public int getParties() {
        return parties;
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * <tt>await</tt> on this barrier.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * <tt>await</tt> is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @return the arrival index of the current thread, where index
     *         <tt>{@link #getParties()} - 1</tt> indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was
     *         broken when {@code await} was called, or the barrier
     *         action (if present) failed due an exception.
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen;
        }
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * <tt>await</tt> on this barrier, or the specified waiting time elapses.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>The specified timeout elapses; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown. If the time is less than or equal to zero, the
     * method will not wait at all.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * <tt>await</tt> is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while
     * waiting, then all other waiting threads will throw {@link
     * BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @param timeout the time to wait for the barrier
     * @param unit the time unit of the timeout parameter
     * @return the arrival index of the current thread, where index
     *         <tt>{@link #getParties()} - 1</tt> indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws TimeoutException if the specified timeout elapses
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was broken
     *         when {@code await} was called, or the barrier action (if
     *         present) failed due an exception
     */
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * Queries if this barrier is in a broken state.
     *
     * @return {@code true} if one or more parties broke out of this
     *         barrier due to interruption or timeout since
     *         construction or the last reset, or a barrier action
     *         failed due to an exception; {@code false} otherwise.
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Resets the barrier to its initial state.  If any parties are
     * currently waiting at the barrier, they will return with a
     * {@link BrokenBarrierException}. Note that resets <em>after</em>
     * a breakage has occurred for other reasons can be complicated to
     * carry out; threads need to re-synchronize in some other way,
     * and choose one to perform the reset.  It may be preferable to
     * instead create a new barrier for subsequent use.
     */
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns the number of parties currently waiting at the barrier.
     * This method is primarily useful for debugging and assertions.
     *
     * @return the number of parties currently blocked in {@link #await}
     */
    public int getNumberWaiting() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }
}

CyclicBarrier是通过ReentrantLock(独占锁)和Condition来实现的。下面，我们分析CyclicBarrier中3个核心函数: 构造函数, await()作出分析。

1. 构造函数

CyclicBarrier的构造函数共2个：CyclicBarrier 和 CyclicBarrier(int parties, Runnable barrierAction)。第1个构造函数是调用第2个构造函数来实现的，下面第2个构造函数的源码。

public CyclicBarrier(int parties, Runnable barrierAction) {
    if (parties <= 0) throw new IllegalArgumentException();
    // parties表示“必须同时到达barrier的线程个数”。
    this.parties = parties;
    // count表示“处在等待状态的线程个数”。
    this.count = parties;
    // barrierCommand表示“parties个线程到达barrier时，会执行的动作”。
    this.barrierCommand = barrierAction;
}

2. 等待函数

CyclicBarrier.java中await()方法如下：

public int await() throws InterruptedException, BrokenBarrierException {
    try {
        return dowait(false, 0L);
    } catch (TimeoutException toe) {
        throw new Error(toe); // cannot happen;
    }
}

说明：await()是通过dowait()实现的。

private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
           TimeoutException {
    final ReentrantLock lock = this.lock;
    // 获取“独占锁(lock)”
    lock.lock();
    try {
        // 保存“当前的generation”
        final Generation g = generation;

        // 若“当前generation已损坏”，则抛出异常。
        if (g.broken)
            throw new BrokenBarrierException();

        // 如果当前线程被中断，则通过breakBarrier()终止CyclicBarrier，唤醒CyclicBarrier中所有等待线程。
        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }

       // 将“count计数器”-1
       int index = --count;
       // 如果index=0，则意味着“有parties个线程到达barrier”。
       if (index == 0) {  // tripped
           boolean ranAction = false;
           try {
               // 如果barrierCommand不为null，则执行该动作。
               final Runnable command = barrierCommand;
               if (command != null)
                   command.run();
               ranAction = true;
               // 唤醒所有等待线程，并更新generation。
               nextGeneration();
               return 0;
           } finally {
               if (!ranAction)
                   breakBarrier();
           }
       }

        // 当前线程一直阻塞，直到“有parties个线程到达barrier” 或 “当前线程被中断” 或 “超时”这3者之一发生，
        // 当前线程才继续执行。
        for (;;) {
            try {
                // 如果不是“超时等待”，则调用awati()进行等待；否则，调用awaitNanos()进行等待。
                if (!timed)
                    trip.await();
                else if (nanos > 0L)
                    nanos = trip.awaitNanos(nanos);
            } catch (InterruptedException ie) {
                // 如果等待过程中，线程被中断，则执行下面的函数。
                if (g == generation && ! g.broken) {
                    breakBarrier();
                    throw ie;
                } else {
                    Thread.currentThread().interrupt();
                }
            }

            // 如果“当前generation已经损坏”，则抛出异常。
            if (g.broken)
                throw new BrokenBarrierException();

            // 如果“generation已经换代”，则返回index。
            if (g != generation)
                return index;

            // 如果是“超时等待”，并且时间已到，则通过breakBarrier()终止CyclicBarrier，唤醒CyclicBarrier中所有等待线程。
            if (timed && nanos <= 0L) {
                breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        // 释放“独占锁(lock)”
        lock.unlock();
    }
}

说明：dowait()的作用就是让当前线程阻塞，直到“有parties个线程到达barrier” 或 “当前线程被中断” 或 “超时”这3者之一发生，当前线程才继续执行。
(01) generation是CyclicBarrier的一个成员遍历，它的定义如下：

private Generation generation = new Generation();

private static class Generation {
    boolean broken = false;
}

在CyclicBarrier中，同一批的线程属于同一代，即同一个Generation；CyclicBarrier中通过generation对象，记录属于哪一代。
当有parties个线程到达barrier，generation就会被更新换代。

(02) 如果当前线程被中断，即Thread.interrupted()为true；则通过breakBarrier()终止CyclicBarrier。breakBarrier()的源码如下：

private void breakBarrier() {
    generation.broken = true;
    count = parties;
    trip.signalAll();
}

breakBarrier()会设置当前中断标记broken为true，意味着“将该Generation中断”；同时，设置count=parties，即重新初始化count；最后，通过signalAll()唤醒CyclicBarrier上所有的等待线程。

(03) 将“count计数器”-1，即--count；然后判断是不是“有parties个线程到达barrier”，即index是不是为0。
当index=0时，如果barrierCommand不为null，则执行该barrierCommand，barrierCommand就是我们创建CyclicBarrier时，传入的Runnable对象。然后，调用nextGeneration()进行换代工作，nextGeneration()的源码如下：

private void nextGeneration() {
    trip.signalAll();
    count = parties;
    generation = new Generation();
}

首先，它会调用signalAll()唤醒CyclicBarrier上所有的等待线程；接着，重新初始化count；最后，更新generation的值。

(04) 在for(;;)循环中。timed是用来表示当前是不是“超时等待”线程。如果不是，则通过trip.await()进行等待；否则，调用awaitNanos()进行超时等待。

 

CyclicBarrier的使用示例

示例1
新建5个线程，这5个线程达到一定的条件时，它们才继续往后运行。

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

public class CyclicBarrierTest1 {

    private static int SIZE = 5;
    private static CyclicBarrier cb;
    public static void main(String[] args) {

        cb = new CyclicBarrier(SIZE);

        // 新建5个任务
        for(int i=0; i<SIZE; i++)
            new InnerThread().start();
    }

    static class InnerThread extends Thread{
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " wait for CyclicBarrier.");

                // 将cb的参与者数量加1
                cb.await();

                // cb的参与者数量等于5时，才继续往后执行
                System.out.println(Thread.currentThread().getName() + " continued.");
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

运行结果：

Thread-1 wait for CyclicBarrier.
Thread-2 wait for CyclicBarrier.
Thread-3 wait for CyclicBarrier.
Thread-4 wait for CyclicBarrier.
Thread-0 wait for CyclicBarrier.
Thread-0 continued.
Thread-4 continued.
Thread-2 continued.
Thread-3 continued.
Thread-1 continued.

结果说明：主线程中新建了5个线程，所有的这些线程都调用cb.await()等待。所有这些线程一直等待，直到cb中所有线程都达到barrier时，这些线程才继续运行！

示例2

新建5个线程，当这5个线程达到一定的条件时，执行某项任务。

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

public class CyclicBarrierTest2 {

    private static int SIZE = 5;
    private static CyclicBarrier cb;
    public static void main(String[] args) {

        cb = new CyclicBarrier(SIZE, new Runnable () {
            public void run() {
                System.out.println("CyclicBarrier's parties is: "+ cb.getParties());
            }
        });

        // 新建5个任务
        for(int i=0; i<SIZE; i++)
            new InnerThread().start();
    }

    static class InnerThread extends Thread{
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " wait for CyclicBarrier.");

                // 将cb的参与者数量加1
                cb.await();

                // cb的参与者数量等于5时，才继续往后执行
                System.out.println(Thread.currentThread().getName() + " continued.");
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

运行结果：

Thread-1 wait for CyclicBarrier.
Thread-2 wait for CyclicBarrier.
Thread-3 wait for CyclicBarrier.
Thread-4 wait for CyclicBarrier.
Thread-0 wait for CyclicBarrier.
CyclicBarrier's parties is: 5
Thread-0 continued.
Thread-4 continued.
Thread-2 continued.
Thread-3 continued.
Thread-1 continued.

结果说明：主线程中新建了5个线程，所有的这些线程都调用cb.await()等待。所有这些线程一直等待，直到cb中所有线程都达到barrier时，执行新建cb时注册的Runnable任务。

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