CyclicBarrier学习一

一、定义
CyclicBarrier 字面意思回环栅栏（循环屏障），通过它可以实现让一组线程等待至某个状态（屏障点）之后再全部同时执行。叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。
CyclicBarrier 作用是让一组线程相互等待，当达到一个共同点时，所有之前等待的线程再继续执行，且 CyclicBarrier 功能可重复使用。
CyclicBarrier 可以 让一组线程到达一个屏障时被阻塞，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释放。

注意,被阻塞的线程 ,不是同时 被释放的。但时间间隔几乎 可以说没有。

二、使用

package com.util;

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

/**
 * @author : lssffy
 * @Description : 循环屏障
 * @date : 2023/12/17 19:47
 */
public class CyclicBarrierTest {

    private static final Logger logger = Logger.getLogger("CyclicBarrierTest");

    public static void main(String[] args) {
        //屏障的线程数量：5
        CyclicBarrier cb = new CyclicBarrier(5);
        for (int i = 0; i < 5; i++) {
            int flag = i;
            new Thread(()->{
                try {
                    //最后一个线程休眠3秒，看看所有到达屏障的线程 也是休眠3秒之后再执行
                    if(flag%5==4){
                        Thread.sleep(5000);
                    }
                    //阻塞在这里，直到到达屏障的线程数量=5才会被释放
                    cb.await();
                    System.out.println(Thread.currentThread().getName());
                }catch (InterruptedException e) {
                    logger.info("Thread interrupted" +e.getMessage());
                }catch (BrokenBarrierException e){
                    logger.info("BrokenBarrierException" + e.getMessage());
                }
            }).start();
        }
        System.out.println("线程执行完毕");
    }
}

最终执行结果，是最后到达屏障的最先执行，其次才是之前到达屏障阻塞的所有线程，其他线程被释放的顺序，也不一定是按照await方法的调用顺序来定的

3、方法
构造方法
一个有参构造，一个无参构造,有参构造相对于无参构造来说 多传入了一个Runnable实例, 当一个周期的屏障被 释放后, 可以执行 Runnable实例的 run方法逻辑。

    /**
     * Creates a new {@code CyclicBarrier} 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);
    }

    /**
     * Creates a new {@code CyclicBarrier} 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;
    }

parties表示屏障拦截的线程数量，每个线程调用 await 方法告诉 CyclicBarrier 我已经到达了屏障，然后当前线程被阻塞。
用于在线程到达屏障时，优先执行 barrierAction，方便处理更复杂的业务场景(该线程的执行时机是在到达屏障之后再执行)
重要方法

/**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} 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
     * {@code await} 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
     *         {@code getParties() - 1} 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 to an exception
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

屏障 指定数量的线程全部调用await()方法时，这些线程不再阻塞
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

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} 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
     * {@code await} 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
     *         {@code getParties() - 1} 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.
     *         In this case the barrier will be broken.
     * @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 to an exception
     */
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

循环 通过reset()方法可以进行重置

    /**
     * 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();
        }
    }

4、原理
CyclicBarrier 是利用 ReentrantLock锁 和 ReentrantLock里的条件队列 来实现 所有线程 在屏障处 阻塞的效果的。
dowait逻辑

    /**
     * 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(); //加锁
        try {
        	//判断count 需要到达屏障的线程数
            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                	//这个就是有参构造传进来的第二个参数Runnable实例
                    final Runnable command = barrierCommand;
                    if (command != null)
                    	//最后一个到达屏障的线程 同步调用
                        command.run();
                    ranAction = true;
                    //释放所有
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                    	// 释放,进入条件队列阻塞
                        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 {
        	// 释放锁
            lock.unlock();
        }
    }

CyclicBarrier 流程
主要是的流程：
1、获取锁 如果 count != 0 就进入阻塞；
2、进入阻塞之前，首先需要进入条件队列，然后释放锁，最后阻塞；
3、如果 count != 0 会进行一个唤醒，将所有的条件队列中的节点转换为阻塞队列；
4、被唤醒过后会进行锁的获取，如果锁获取失败，会进入 lock 的阻塞队列；
5、如果锁获取成功，进行锁的释放，以及唤醒，同步队列中的线程。

CyclicBarrier 与 CountDownLatch的区别
1、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。所以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
2、CyclicBarrier还提供getNumberWaiting(可以获得CyclicBarrier阻塞的线程数量)、isBroken(用来知道阻塞的线程是否被中断)等方法。
3、CountDownLatch会阻塞主线程，CyclicBarrier不会阻塞主线程，只会阻塞子线程。
4、CountDownLatch和CyclicBarrier都能够实现线程之间的等待，只不过它们侧重点不同。 5、CountDownLatch一般用于一个或多个线程，等待其他线程执行完任务后，再执行。 6、CyclicBarrier一般用于一组线程互相等待至某个状态，然后这一组线程再同时执行。
7、CyclicBarrier 还可以提供一个 barrierAction，合并多线程计算结果。
8、CyclicBarrier是通过ReentrantLock的"独占锁"和Conditon来实现一组线程的阻塞唤醒的，而CountDownLatch则是通过AQS的“共享锁”实现