AQS中的CyclicBarrier源码解析

package cn.com.pep;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * 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 {@code CyclicBarrier} 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> {@code
 * 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;
 *     Runnable barrierAction =
 *       new Runnable() { public void run() { mergeRows(...); }};
 *     barrier = new CyclicBarrier(N, barrierAction);
 *
 *     List<Thread> threads = new ArrayList<Thread>(N);
 *     for (int i = 0; i < N; i++) {
 *       Thread thread = new Thread(new Worker(i));
 *       threads.add(thread);
 *       thread.start();
 *     }
 *
 *     // wait until done
 *     for (Thread thread : threads)
 *       thread.join();
 *   }
 * }}</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 {@code done()} will return
 * {@code true} 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> {@code
 * if (barrier.await() == 0) {
 *   // log the completion of this iteration
 * }}</pre>
 *
 * <p>The {@code CyclicBarrier} 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 CountDownLatch2
 *
 * @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 {@code count} 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.
     */
    /**
     * @Description:CyclicBarrier的类似于CountDownLatch，实现了一组线程之间的相互等待，直到所有线程都到齐了，再向下执行。
     * 而与CountDownLatch不同的是，其实现采用的是条件队列，而非共享锁，并且是可以重用的。
     * 那如何理解Generation这个“代”呢?
     * 打个比方：游乐场里的过山车有10个座位，每次都凑满了10个之后，管理员才打开门，让10个游客过去，开动过山车。然后再关门，其余的游客再继续等待，等再凑够“下一波”的10个人，
     * 这个模型中，前面已经通过的10个游客，就是“上一代”，而这个大门就是“barrier”，可以重复开启，后面继续等待排队的下一波游客，就是下一代。
     * @version: V1.0
     * @author: wwh
     * @date: 2023年4月23日  上午9:52:52
     */
    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 */
    //需要一起通过barrier的线程数
    private final int parties;
    /* The command to run when tripped */
    //当所有线程通过barrier之前，需要做的工作
    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.
     */
    //此barrier需要等待的线程数，初始值为parties，在同一个generation中，每调用一次await()，自减1
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    //开启下一代
    private void nextGeneration() {
        // signal completion of last generation
        //唤醒条件队列trip中所有等待的线程
        trip.signalAll();
        // set up next generation
        //初始化count
        count = parties;
        //开启新一代，即将“代”的打破标记broken设置为false
        generation = new Generation();
    }

    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    //打破当前barrier，这个怎么理解呢？某一时段景区的人比较少，很久都凑不够10个人，这时候管理员没办法，就打开门（barrier）,将这一拨人（generation）放进去，乘坐过山车，此时，管理员的这个行为就是一个breakBarrier。
    private void breakBarrier() {
        //将当前generation打破标记broken设置为true
        generation.broken = true;
        //重置count
        count = parties;
        //唤醒条件队列condition中正在等待的线程
        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;
            /**
             * 只有调用breakBarrier()，才能将当前generation的broken标记置为true，如果当前线程在执行dowait()时候，发现当前的generation已经被标记为true，
             * 则直接抛出BrokenBarrierException异常。
             */
            if (g.broken)
                throw new BrokenBarrierException();
            
            /**
             * 此时，假如检测到当前线程被“中断”了，那么就应该将当前barrier打破,这是因为处于同一个barrier中的所有线程是相互等待的。既然当前线程已经被中断了，
             * 那么其它线程肯定等不到当前线程执行完成了，所以执行breakBarrier()方法，将当前barrier打破，唤醒其他线程，其他线程被唤醒后，发现当前的generation的
             * broken标记已经为true，则这些被唤醒的线程也都抛出BrokenBarrierExceptoin异常。
             * CyclicBarrier实现中采用了“all-or-none breakage model”模型，即全损或者全无损模型。
             */
            if (Thread.interrupted()) {
                //检测到当前线程中断，打破当前线程所处的barrier，并且抛出InterruptedException异常。
                breakBarrier();
                throw new InterruptedException();
            }
            
            //用来表示当前线程进入到barrier的顺序，index=parties-1,表示第一个进入到barrier的线程，index=parties-2表示第二个，依次类推，知道index =0表示最后一个进入barrier的线程
            int index = --count;
            //index == 0，所有线程都已经就绪了，准备通过barrier，这个条件只有barrier中的最后一个进来线程才能达到。
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    //所有的线程通过barrier之前，执行完通过barrier之前的准备工作（由barrier中的最后一个线程来执行），然后所有的线程再通过barrier。
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    //唤醒当前代中处于等待状态的线程，开启新的一代
                    nextGeneration();
                    return 0;
                } finally {
                    //说明最后一个进入barrier的线程在执行通过barrier的准备时候发生了异常，则打破当前barrier
                    if (!ranAction)
                        breakBarrier();
                }
            }
            
            // loop until tripped, broken, interrupted, or timed out
            //如果index数不为0，就表示还有需要等待的其他线程，所以将当前线程挂起，等待所有线程都到齐、或者超时、被其他线程中断
            for (;;) {
                try {
                    //阻塞等待，等待被其他线程中断唤醒或者sigal()唤醒
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        //带超时时间的阻塞，等到超时时间到
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    //等待的过程中发生了中断异常
                    if (g == generation && ! g.broken) {
                        //当前线程还处于这个“代”中，并且这个barrier还没有被打破，那么需要将这个barrier打破，结束这个barrier中其他线程的无异议的等待
                        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.
                        /**
                         * 走到这里来了，应该有两种情况：
                         * 1、说明新的代已经产生了，当前线程已经不在这个代中了，那么只是重置先线程的中断标记，不需要做额外处理了；
                         * 2、说明中断发生时候，barrier已经被打破了，所以也只是记录下线程的中断标记；
                         * 3、
                         */
                        Thread.currentThread().interrupt();
                    }
                }
                
                /**
                 * 能走到这里来说明，线程已经被唤醒了，检查一下broken的状态，如果为true则直接抛出BrokenBarrierException异常，
                 * 能使broken的状态变为true的只有breakBarrier()方法，对应以下几种情况：
                 * 1、其他在执行await()方法线程被挂起之前，就已经被中断了，导致触发了breakBarrier（）方法；
                 * 2、其他在执行await()方法线程在被挂起等待的过程中，被中断唤醒了，导致触发了breakBarrier()方法；
                 * 3、最后一个进入barrier的线程在执行barrierCommand过程中发生了异常，导致breakBarrier()方法被调用；
                 * 4、显示调用reset()方法；
                 */
                if (g.broken)
                    throw new BrokenBarrierException();
                
                //如果线程被正常唤醒，下一代已经被开启了，则返回线程进入barrier的顺序
                if (g != generation)
                    return index;
                //允许超时，并且超时时间已到，则打破当前barrier，并抛出TimeOutException异常；
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * 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;
    }

    /**
     * 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);
    }

    /**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
    //需要通过这个barrier的线程数
    public int getParties() {
        return parties;
    }

    /**
     * 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
     */
    //不带超时时间的等待
    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
     * {@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));
    }

    /**
     * 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();
        }
    }
}