// 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
        }
    }

/**
     * 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;
    }

/**
     * 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
        }
    }

/**
     * 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
        }
    }

/**
     * 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
    }

/**
     * 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();
    }

/**
   * 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:1
id:2
id:4
id:0
id: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
`