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：5CyclicBarrier 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 generationnextGeneration(); // 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) {  // trippedboolean 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 outfor (;;) {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