J.U.C并发框架源码阅读（七）CyclicBarrier
基于版本jdk1.7.0_80
java.util.concurrent.CyclicBarrier
代码如下
/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ 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 * <em>cyclic</em> because it can be re-used after the waiting threads * are released. * * <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 <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> * 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. * * <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> if (barrier.await() == 0) { * // 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). * * <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 <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. */ 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 */ private final int parties; /* The command to run when tripped */ 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. */ private int count; /** * 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(); } /** * Sets current barrier generation as broken and wakes up everyone. * Called only while holding lock. */ private void breakBarrier() { 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 { final ReentrantLock lock = this.lock; lock.lock(); 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(); ranAction = true; nextGeneration(); return 0; } finally { if (!ranAction) breakBarrier(); } } // 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 <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. * * @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); } /** * 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 * <tt>await</tt> 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 * <tt>await</tt> 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 * <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 <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 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 * <tt>await</tt> 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 * <tt>await</tt> 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 * <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 TimeoutException if the specified timeout elapses * @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 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(); } } }
0. CyclicBarrier简介
CyclicBarrier 的字面意思是可循环使用（Cyclic）的屏障（Barrier）。它要做的事情是，让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续干活。CyclicBarrier默认的构造方法是CyclicBarrier(int parties)，其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier我已经到达了屏障，然后当前线程被阻塞。
CyclicBarrier初始时还可带一个Runnable的参数， 此Runnable任务在CyclicBarrier的数目达到后，所有其它线程被唤醒前被执行。
1. CyclicBarrier和CountDownLatch的区别
CountDownLatch的计数器只能使用一次。而CyclicBarrier的计数器可以使用reset() 方法重置。所以CyclicBarrier能处理更为复杂的业务场景，比如如果计算发生错误，可以重置计数器，并让线程们重新执行一次。
CyclicBarrier还提供其他有用的方法，比如getNumberWaiting方法可以获得CyclicBarrier阻塞的线程数量。isBroken方法用来知道阻塞的线程是否被中断。
2. CyclicBarrier原理概述
跟之前讲解的几个并发容器不同，CyclicBarrier没有用到AQS，而是直接使用了ReentrantLock作为核心同步工具。
CyclicBarrier内部维护了一个ReentrantLock对象lock，当工作线程调用CyclicBarrier.await方法时，会检查已经到达的线程数是否已经满足需求，如果没有，则在lock的一个Condition上等待。如果满足了需求，则向所有在这个Condition上等待的线程发送唤醒信号。然后重置CyclicBarrier。
3. CyclicBarrier.await方法的调用轨迹
public int await() throws InterruptedException, BrokenBarrierException { try { 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();//加锁 try { final Generation g = generation; if (g.broken)//这一代的CyclicBarrier已经损坏 throw new BrokenBarrierException(); if (Thread.interrupted()) {//如果工作线程被中断，则CyclicBarrier损坏 breakBarrier(); throw new InterruptedException(); } int index = --count;//由于方法一开始就加锁，所以这里可以安全的执行自减操作，无需cas if (index == 0) { // tripped//如果计数减到0了，那么执行收尾工作：执行barrierCommand，重置CyclicBarrier boolean ranAction = false; try { final Runnable command = barrierCommand; if (command != null) command.run();//如果有barrierCommand则执行 ranAction = true; nextGeneration();//重置CyclicBarrier return 0; } finally { if (!ranAction)//如果barrierCommand抛出异常了，CyclicBarrier不会被重置，其他的等待线程也不会被唤醒 breakBarrier(); } } // loop until tripped, broken, interrupted, or timed out for (;;) { try {//工作线程在lock的Condition上等待 if (!timed) trip.await(); else if (nanos > 0L) nanos = trip.awaitNanos(nanos);//实现超时语义 } 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(); } }
逻辑很简单，主要就是利用内部维护的ReentrantLock对象lock，以及lock关联的Condition对象trip，完成等待/超时等待/响应中断的语义
4. 一些辅助方法
/** * Sets current barrier generation as broken and wakes up everyone. * Called only while holding lock. */ private void breakBarrier() { generation.broken = true; count = parties; trip.signalAll();//唤醒所有等待线程 } public boolean isBroken() { final ReentrantLock lock = this.lock; lock.lock(); try { return generation.broken; } finally { lock.unlock(); } } /** * Updates state on barrier trip and wakes up everyone. * Called only while holding lock. */ private void nextGeneration() { // signal completion of last generation trip.signalAll();//唤醒所有等待线程 // set up next generation count = parties; generation = new Generation(); } public void reset() { final ReentrantLock lock = this.lock; lock.lock(); try { breakBarrier(); // break the current generation nextGeneration(); // start a new generation } finally { lock.unlock(); } }
总的来说逻辑不算复杂，就不多解释了。
