CyclicBarrier详解

AtomicInteger解析：http://donald-draper.iteye.com/blog/2359555
锁持有者管理器AbstractOwnableSynchronizer：http://donald-draper.iteye.com/blog/2360109
AQS线程挂起辅助类LockSupport：http://donald-draper.iteye.com/blog/2360206
AQS详解-CLH队列，线程等待状态：http://donald-draper.iteye.com/blog/2360256
AQS-Condition详解：http://donald-draper.iteye.com/blog/2360381
可重入锁ReentrantLock详解：http://donald-draper.iteye.com/blog/2360411
CountDownLatch使用场景：http://donald-draper.iteye.com/blog/2348106
CountDownLatch详解：http://donald-draper.iteye.com/blog/2360597

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * 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
 * [i]cyclic[/i] because it can be re-used after the waiting threads
 * are released.
 *
 同步工具CyclicBarrier，一个集合线程，等待每一个线程达到共同的屏障点。
CyclicBarriers对一个复杂的线程集合必须互相等待完成任务，场景非常有用。
同步工具的屏障可以循环利用，因为在所有等待线程释放锁时，他可以被重新使用。


 * <p>A <tt>CyclicBarrier</tt> 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 [i]barrier action[/i] is useful
 * for updating shared-state before any of the parties continue.
 *
CyclicBarrier的构造函数中，有一个带Runnable，在所有线程到达屏障点，并且共享锁没有完全释放，
这个功能，对于在其他线程继续执行任务前，更新共享状态非常有用。

 * <p><b>Sample usage:</b> Here is an example of
 *  using a barrier in a parallel decomposition design:
 * <pre>
 简单的一个实例用，在并行的分解任务中，使用barrier
 * 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;
 *     barrier = new CyclicBarrier(N,
 *                                 new Runnable() {
 *                                   public void run() {
 *                                     mergeRows(...);
 *                                   }
 *                                 });
 *     for (int i = 0; i < N; ++i)
 *       new Thread(new Worker(i)).start();
 *
 *     waitUntilDone();
 *   }
 * }
 * </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 <tt>done()</tt> will return
 * <tt>true</tt> and each worker will terminate.
 上述实例，描述的每个线程处理矩阵的每一行数据，当线程处理完一行数据时，等待其他线程处理完各自
 的一行数据。当所有的线程处理完各自行数据时，屏障点线程Runnable，执行合并矩阵的行数据。
 当屏障点线程Runnable，决定执行合并是，每个线程的done函数返回true，结束每个线程工作。

 * <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:
屏障点action动作线程的执行，不能依赖于组线程中将要暂定的线程，分组中的每一个线程，都可以
执行action，在共享锁被释放之前。为了优化action的执行，我们可以利用，在每个线程调用await方法时，
返回线程到达屏障点的index，来决定，那个线程执行屏障动作。
 * <pre>  if (barrier.await() == 0) {
         //最后一个到达屏障点的线程，执行屏障action
 *     // log the completion of this iteration
 *   }</pre>
 *
 * <p>The <tt>CyclicBarrier</tt> 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).
 *
CyclicBarrier对于失败同步的尝试，用all-or-none breakage model：
如果一个线程，因为中断，失败，超时，永久的离开屏障点，那么其他在屏障点等待的线程，
通过BrokenBarrierException，abnormally离开。

 * <p>Memory consistency effects: Actions in a thread prior to calling
 * {@code await()}
 * [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
 * 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.
 *
 内存一致性：actions优先call await函数，这个基于内存可见机制-happen-before法则。
 屏障点的分组线程，返回happen-before，协调分组线程工作的线程，await的成功返回。
 * @since 1.5
 * @see CountDownLatch
 *
 * @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 <tt>count</tt> 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.
     */
    每次屏障点，表示一代实例。当屏障点被打开或者重置时，generation将会改变。
    由于锁以不确定的方式，分配给等待线程，线程可以多代屏障点的方式，使用barrier。
    如果线程组存在break，并且没有reset，则不需要激活一代。
    Generation可以这么理解，当有线程有多个分组，一个分组执行完，执行下一组；每一组
    我们可以理解为Generation，当线程组出现break，且没有reset，则Generation不会被激活。
    private static class Generation {
        boolean broken = false;
	
    }

    /** The lock for guarding barrier entry */
    屏障点保护锁
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    条件等待，直到所有的线程打开锁，
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    共享锁数量
    private final int parties;
    /* The command to run when tripped */
    障碍点执行的命令
    private final Runnable barrierCommand;
    /** The current generation */
    当前代
    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.
     */
    表示分组中，还有多少个在等待。在每一代，count从parties to 0。
    在每一次创建新生代中或broken时，count重置为parties
    private int count;
 }

先看构造：
 

/**
     * Creates a new <tt>CyclicBarrier</tt> 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.
     *常见一个屏障点，当所有parties线程在等待时，将会打开，同时最后一个进入
     屏障点的线程，将会执行barrierAction。
     * @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 <tt>CyclicBarrier</tt> 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);
    }

线程代broken处理
     

 /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    当线程持有锁，设置当前线程代broken，唤醒当前代线程
    private void breakBarrier() {
        //
        generation.broken = true;
	//重置共享锁状态
        count = parties;
	//唤醒所有在屏障点，等待的线程
        trip.signalAll();
    }

创建下一代
   

  /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
     线程持有锁，更新屏障点状态，唤醒所有等待，线程
    private void nextGeneration() {
        // signal completion of last generation
	//唤醒上一代，完成的线程
        trip.signalAll();
        // set up next generation
	//重置共享锁状态
        count = parties;
	//创建下一代
        generation = new Generation();
    }
}

我们来看屏障等待

/**
     * Waits until all {@linkplain #getParties parties} have invoked
     * <tt>await</tt> on this barrier.
     * 等待所享有的线程到达屏障点
     * <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:
     当线程不是最后一个到达屏障点，线程将会不会被调度，直到以下情况发生
     * [list]
     * <li>The last thread arrives; or最后一个线程到达屏障点
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or其他线程中断当前线程
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or其他等待线程，被中断
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * [/list]
     *一些线程等待屏障点超时，或其他以下线程调用reset
     * <p>If the current thread:
     * [list]
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * [/list]
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     当前线程带着中断状态，在等待屏障点，当中断异常抛出时，当前线程中断消除。
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * <tt>await</tt> is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *当其他线程在等待，如果屏障点被重置，或broke，则抛出BrokenBarrierException
     * <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.
     *在等待的过程中，如果其他线程中断，则抛出BrokenBarrierException，屏障点
     设置为broken状态。
     * <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.
     如果当前线程，是最后一个到达屏障点的，如果屏障点动作线程不为null，
     则执行action，在下一代线程组执行任务前。
     * 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.
     *如果在执行action的过程中，出现异常，则当前线程将会抛出异常，屏障点处于破位状态
     * @return the arrival index of the current thread, where index
     *         <tt>{@link #getParties()} - 1</tt> 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 [i]another[/i] 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 an exception.
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
	    //委托给dowait
            return dowait(false, 0L);
        } 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;
        lock.lock();
        try {
	    //获取线程代
            final Generation g = generation;
            //如果屏障点破位，则抛出BrokenBarrierException
            if (g.broken)
                throw new BrokenBarrierException();
            //如果线程中断，则设置屏障点破位，重置count为parties，
	    //唤醒所有在屏障点，等待的线程,抛出中断异常
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
          //共享锁数量，自减
           int index = --count;
           if (index == 0) {  // tripped
               boolean ranAction = false;
               try {
                   final Runnable command = barrierCommand;
                   if (command != null)
		       //如果所有线程达到屏障点，则执行action
                       command.run();
                   ranAction = true;
		   //创建一下代
                   nextGeneration();
		   //返回0，屏障点解除
                   return 0;
               } finally {
                   if (!ranAction)
                       breakBarrier();
               }
           }
            
            // loop until tripped, broken, interrupted, or timed out
	    //自旋，直到所有线程到达屏障点，当前代broken，中断，或超时
            for (;;) {
                try {
		    //非超时等待await，否则awaitNanos
                    if (!timed)
                        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) {
		   //如果超时，解除屏障点
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

小节:
线程到达屏障点时，首先检查线程代，有没有broken，如果broken，
则抛出BrokenBarrierException，如果线程中断，则当前代broken，
重置共享锁状态，唤醒所有等待线程。如果上述条件不满足，则释放
count，判断是否当前代线程，是否都到达屏障点，如果是，判断action
是否为null，不为null，则执行action；当释放count，当前代线程，仍有在执行的，
自旋等待屏障点条件trip，如果是超时等待，则判断时间是否超时，超时则breakBarrier。

再看

 public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

与await基本相同，都是委托给dowait

/**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
  public int getParties() {
        return parties;
    }
   /**
     * 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 [i]after[/i]
     * 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.
     *返回在屏障点等待线程数
     * @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();
        }
    }

总结：
屏障点思想，当每个线程完成任务时，自旋等待条件Condition trip，释放共享锁，count减1；当线程代的最后一个线程到达屏障点时，唤醒线程代中所有等待的线程，
如果有action，执行action，然后创建下一代线程。如果在线程代未结束之前，有等待线程中断或超时，则结束当前代，唤醒所有等待线程，重置count为parties。