CyclicBarrier和CountDownLatch 都位于java.util.concurrent这个包下，其工作原理的核心要点：
那么接下来给大家分享分析一下JDK1.8的CyclicBarrier的工作原理。
CyclicBarrier从英文字面上理解，循环栅栏，咋一看好像跟同步器没多大关系，而栅栏式一排排的阻拦着，好像也有点同步等待的意思；
CyclicBarrier是也一种同步帮助工具，允许多个线程相互等待，即多个线程到达同步点时被阻塞，直到最后一个线程到达同步点时栅栏才会被打开；
CyclicBarrier内部没有所谓的公平锁\非公平锁的静态内部类，只是利用了ReentrantLock(独占锁)、ConditionObject(条件对象)实现了线程之间相互等待的功能；
CyclicBarrier这个类没有真正的state关键词，它只有parties线程总数量，count还没有进入阻塞的线程数量；
CyclicBarrier的实现是间接利用了ReentrantLock(独占锁)的父类AQS的state变量值；
CountDownLatch，A、B、C组线程同时执行，A先执行完的话就在那里等着，等所有A、B、C线程中执行最久的线程执行完了才开始执行各自的事件；
// 创建给定数值的栅栏总数，也就是支持参与线程的最多数值public CyclicBarrier(int parties)// 创建给定数值的栅栏总数，也就是支持参与线程的最多数值，且当最后一个线程执行完时会回调barrierAction方法public CyclicBarrier(int parties, Runnable barrierAction)// 更新换代，改朝换代，触发唤醒所有在Lock对象上等待的线程，释放所有正在处于阻塞的线程private void nextGeneration()// 打破平衡，并设置打破平衡的标志，然后再唤醒所有被阻塞的线程，private void breakBarrier() // 导致当前线程阻塞，直到其他线程调用trip.signal()或trip.signalAll()方法唤醒该线程public int await()// 比await()多了两个参数，意思就是阻塞等待信号量的最大时长，等待的时间值为timeout，单位为unit；public int await(long timeout, TimeUnit unit)// 阻塞等待的核心方法，如果不需要超时等待信号量的话则nanos参数是没用的，否则就有用private int dowait(boolean timed, long nanos)// 线程之间的等待，这样一个等待的平衡体系是否被打破public boolean isBroken() // 重置为初始状态值，就像初始创建CyclicBarrier该实例对象一样，干干净净的初始状态值public void reset()// 获取目前正在处于阻塞状态的线程数量值public int getNumberWaiting()// 获取线程数量，也就是栅栏数量总数值public int getParties() public int await() throws InterruptedException, BrokenBarrierException { try { return dowait(false, 0L); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen } }比如百米赛跑，我就以赛跑为例生活化阐述该CyclicBarrier原理，场景：百米赛跑十人参赛，终点处有一个裁判计数；
开跑一声枪响，十个人争先恐后的向终点跑去，真的是振奋多秒，令人振奋；
当一个人到达终点，这个人就完成了他的赛跑事情了，就没事一边玩去了，那么裁判则减去一个人；
随着人员陆陆续续的都跑到了终点，最后裁判计数显示还有0个人未到达，意思就是人员都达到了；
然后裁判就拿着登记的成绩屁颠屁颠去输入电脑登记了；
到此打止，这一系列的动作认为是A组线程等待另外其他组线程的操作，直到计数器为零，那么A则再干其他事情；
创建一个给定数值的栅栏总数，也就是支持参与线程的最多数值，但是构造方法二还可以通过传入接口回调，当最后一个阻塞的线程被释放后，它将有机会执行这个被传入的回调接口barrierAction；
/** * 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; }阻塞等待的核心方法，内部会调用trip.await()方法进入Condition等待阻塞队列，一旦栅栏数量为零时则会逐个逐个将Condition等待的队列转移到CLH的等待阻塞队列；
所有线程被唤醒后然后等待dowait方法内部lock.unlock()一个个释放线程等待，阻塞的最后一个线程还有机会执行构造方法传入的接口回调；
/** * 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); // 阻塞的核心方法，重心再次，通过ReentrantLock和Condition组合完成阻塞等待 } catch (TimeoutException toe) { throw new Error(toe); // cannot happen } }dowait(false, 0L); // 阻塞的核心方法，重新再次，通过ReentrantLock和Condition组合完成阻塞等待
/** * Main barrier code, covering the various policies. */ private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException { final ReentrantLock lock = this.lock; // 获取独占锁 lock.lock(); // 通过lock其父类AQS的CLH队列阻塞在此，但是为啥又会继续往下进入临界区执行try方法，其原因就是trip.await()这句代码 try { final Generation g = generation; if (g.broken) // 若平衡被一旦打破，则其他所有的线程都会抛出异常，因为即使这里没遇到抛异常，下面还会有 if (g.broken) 判断 throw new BrokenBarrierException(); if (Thread.interrupted()) { // 检测线程是否在其他地方被中断过，若任何一个线程被中断过 breakBarrier(); // 那么则打破平衡，并设置打破平衡的标志，还原初始状态值，然后再唤醒所有被阻塞的线程， throw new InterruptedException(); } int index = --count; // 执行一个则减1操作，正常情况下count表示还有多少个未进入临界区，即还在lock阻塞队列中 if (index == 0) { // tripped 当count值降为0后，则表明所有线程都执行完了，那么就可以happy的一起改朝换代去做其他事情了 boolean ranAction = false; try { final Runnable command = barrierCommand; // 构造方法传入的接口回调对象 if (command != null) // 当接口不为空时，最后一个执行的线程有机会消费该回调方法 command.run(); ranAction = true; nextGeneration(); // 改朝换代，该执行的都已经执行完了，还原为初始状态值，以便下次可以重复再次使用 return 0; } finally { if (!ranAction) // 若最后一个线程眼看着要完事了，若出现了任何异常的话，也照样打破整体平衡，要么一起生要么一起亡 breakBarrier(); } } // loop until tripped, broken, interrupted, or timed out for (;;) { // 自旋的死循环操作方式 try { if (!timed) // 若不需要使用超时等待信号量的话，那么下面就直接调用trip.await()进入阻塞等待 trip.await(); // 正常情况下，代码执行到此就不动了，该方法内部已经调用了park方法导致线程阻塞等待 else if (nanos > 0L) nanos = trip.awaitNanos(nanos); // 在指定时间内等待信号量 } catch (InterruptedException ie) { // 若在阻塞等待期间由于被中断了 if (g == generation && ! g.broken) { // 如果还没改朝换代，并且平衡标志位还为false的话，则继续打破平衡并且抛出中断异常 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) // 这里也有 if (g.broken) 判断，若平衡被一旦打破，则其他所有的线程都会抛出异常 throw new BrokenBarrierException(); if (g != generation) // 若已经被改朝换代了，那么则直接返回index值 return index; if (timed && nanos <= 0L) { // 若设置了超时标志，并且不管是传入的nanos值也好还是通过等待后返回的nanos也好，只要小于或等于零都会打破平衡 breakBarrier(); throw new TimeoutException(); } } } finally { lock.unlock(); // 释放lock锁 } }打破平衡，并设置打破平衡的标志，然后再唤醒所有被阻塞的线程；
/** * Sets current barrier generation as broken and wakes up everyone. * Called only while holding lock. */ private void breakBarrier() { generation.broken = true; // 设置打破平衡的标志 count = parties; // 重新还原count为初始值 trip.signalAll(); // 发送信号量，唤醒所有Condition中的等待队列 }唤醒所有在Condition中等待的队列，然后还原初始状态值，并且重新换掉generation的引用，改朝换代，为下一轮操作做准备；
/** * 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(); }CyclicBarrier的成员属性 trip( Condition类型 ) 对象的方法：
/** * Implements interruptible condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled or interrupted. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ public final void await() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); // 将当前线程包装一下，然后添加到Condition自己维护的链表队列中 int savedState = fullyRelease(node); // 释放当前线程占有的锁，如果不释放的话，那么在第二次调用lock.lock()的地方； // 如果第一个没执行完的话，那么则会一直阻塞等待，那么也就无法完成栅栏的功能了。 int interruptMode = 0; while (!isOnSyncQueue(node)) { // 是否在AQS的队列中 LockSupport.park(this); // 如果不在AQS队列中的话，则阻塞等待，这里才是最最最核心阻塞的地方 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } // 如果在AQS队列中的话，那么则考虑重入锁，重新竞争锁，重新休息 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); }public CyclicBarrier(int parties, Runnable barrierAction) {}public CyclicBarrier(int parties) {}//挂起当前线程，直至所有线程都到达barrier状态再同时执行后续任务；public int await() throws InterruptedException, BrokenBarrierException { };//让这些线程等待至一定的时间，如果还有线程没有到达barrier状态就直接让到达barrier的线程执行后续任务public int await(long timeout, TimeUnit unit)throws InterruptedException,BrokenBarrierException,TimeoutException { };public class cyclicBarrierTest { public static void main(String[] args) throws InterruptedException { CyclicBarrier cyclicBarrier = new CyclicBarrier(5, new Runnable() { @Override public void run() { System.out.println("线程组执行结束"); } }); for (int i = 0; i < 5; i++) { new Thread(new readNum(i,cyclicBarrier)).start(); } } static class readNum implements Runnable{ private int id; private CyclicBarrier cyc; public readNum(int id,CyclicBarrier cyc){ this.id = id; this.cyc = cyc; } @Override public void run() { synchronized (this){ System.out.println("id:"+id); try { cyc.await(); System.out.println("线程组任务" + id + "结束，其他任务继续"); } catch (Exception e) { e.printStackTrace(); } } } }}id:1id:2id:4id:0id:3线程组执行结束线程组任务3结束，其他任务继续线程组任务1结束，其他任务继续线程组任务4结束，其他任务继续线程组任务0结束，其他任务继续线程组任务2结束，其他任务继续