CyclicBarrier 源码阅读

package java.util.concurrent;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 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 {@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.     */    private static class Generation {        boolean broken = false;//是否被中断或异常情况发生    }    /** The lock for guarding barrier entry */    private final ReentrantLock lock = new ReentrantLock();//锁对象    /** Condition to wait on until tripped */    private final Condition trip = lock.newCondition();//条件对象    /** The number of parties */    private final int parties;//线程数量    /* The command to run when tripped */    private final Runnable barrierCommand;//完成后运行的代码    /** 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.     */    private int count;    /**     * Updates state on barrier trip and wakes up everyone.     * Called only while holding lock.     */    private void nextGeneration() {//下一次迭代初始化        // signal completion of last generation        trip.signalAll();        // set up next generation        count = parties;        generation = new Generation();    }    /**     * Sets current barrier generation as broken and wakes up everyone.     * Called only while holding lock.     */    private void breakBarrier() {//破坏栅栏        generation.broken = true;        count = parties;        trip.signalAll();    }    /**     * Main barrier code, covering the various policies.     */    private int dowait(boolean timed, long nanos)        throws InterruptedException, BrokenBarrierException,               TimeoutException {//主方法        final ReentrantLock lock = this.lock;        lock.lock();//获取锁，互斥，当调用trip.await();时释放锁        try {            final Generation g = generation;            if (g.broken)                throw new BrokenBarrierException();            if (Thread.interrupted()) {                breakBarrier();                throw new InterruptedException();            }            int index = --count;            if (index == 0) {  // tripped                boolean ranAction = false;                try {                    final Runnable command = barrierCommand;                    if (command != null)                        command.run();//所有线程运行await方法后运行这里                    ranAction = true;                    nextGeneration();//初始化下一次迭代，trip.signalAll();会唤醒所有阻塞线程                    return 0;                } finally {                    if (!ranAction)                        breakBarrier();//如果结果任务运行异常则破坏栅栏                }            }            // loop until tripped, broken, interrupted, or timed out            for (;;) {                try {                    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) {                    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     */    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     */    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();        }    }}