JDK1.8源码阅读与翻译:CyclicBarrier

/*
 * 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.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
 * cyclic because it can be re-used after the waiting threads
 * are released.
 * CyclicBarrier是一个同步辅助类,允许一组线程全部等待彼此到达一个公共屏障点。
 * CyclicBarriers在涉及固定数量的一批线程必须偶尔等待彼此的程序中很有用。被称
 * 为cyclic(循环)barrier(屏障)是因为它可以在释放等待线程后继续重复使用。
 *
 * 

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 barrier action is useful * for updating shared-state before any of the parties continue. * CyclicBarrier支持一个可选的命令(Runnable),该命令在每个屏障点运行一次, * 该命令执行的时机在在同批次的最后一个线程到达屏障点之后,在任意线程释放之前。 * 这个屏障操作对于任何一方(线程)继续之前,对于更新共享状态有用。 * *

Sample usage: Here is an example of using a barrier in a * parallel decomposition design: * *

 {@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 threads = new ArrayList(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();
 *   }
 * }}
* * 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. * 以上代码中,每个worker线程处理二维数组中的一行,然后在屏障处等待直到所有行均被处理。当所 * 有行都被处理后,将执行屏障操作Runnable,合并这些行。如果这个Runnable线程确定问题解决, * 则done()将返回true,从而每个worker线程退出while循环,执行结束。 * *

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: * 如果屏障操作执行时不依赖被挂起的任何线程,则任何线程都可以在释放后执行这个屏障操作。为了 * 实现这一点,每次await()调用都会返回该线程到达屏障的序号。你可以选择具体哪个线程来执行屏 * 障操作。例如下边是让序号为0的线程执行指定操作: *

 {@code
 * if (barrier.await() == 0) {
 *   // log the completion of this iteration
 * }}
* *

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). * CyclicBarrier为失败的同步尝试应用俱荣俱损模型:如果一个线程由于中断,错误或超时离开屏障, * 所有其他在屏障处等待的线程都将由于BrokenBarrierException异常离开屏障点(或是由于 * InterruptedException被同时中断) * *

Memory consistency effects: Actions in a thread prior to calling * {@code await()} * happen-before * actions that are part of the barrier action, which in turn * happen-before actions following a successful return from the * corresponding {@code await()} in other threads. * 内存一致效果:先前调用await()的线程的动作,happen-before(先于发生)屏障操作的动作, * 而屏障操作的动作先于发生其他线程从await()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 {@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. * 每次(CyclicBarrier可重用,指每一轮的使用)对屏障的使用用一个Generation实例 * 来表示。每当屏障被跳过或重置时都会替换这个generation属性。使用屏障的线程可以有 * 很多代 - 由于锁是用非确定方式分配给等待线程的 - 但是同一时间只有一个generation * (一代)会被激活(应用当前count的),其他所有的要么破坏要么跳过屏障。如果有中断 * 但没有后续的重置,则不需要活动的generation。 */ private static class Generation { boolean broken = false; } /** The lock for guarding barrier entry */ /** 守护屏障入口的锁 */ private final ReentrantLock lock = new ReentrantLock(); /** Condition to wait on until tripped */ /** 屏障跳过前用来让线程进入等待的对象 */ private final Condition trip = lock.newCondition(); /** The number of parties */ /** 参与屏障阻拦的线程数 */ private final int parties; /* The command to run when tripped */ /* 屏障被跳过时执行的逻辑 */ private final Runnable barrierCommand; /** The current generation */ /** 标识CyclicBarrier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. * 未到达屏障的线程数量。每一代都会从parties的值减到0。在生成新一代或被破坏时 * 重置为parties的值。 */ private int count; /** * Updates state on barrier trip and wakes up everyone. * Called only while holding lock. * 越过屏障时更新状态并唤醒所有线程。 * 只在持有锁后才调用。 */ private void nextGeneration() { // signal completion of last generation // 唤醒当前一代的所有线程 trip.signalAll(); // set up next generation // 设置新一代的值 count = parties; generation = new Generation(); } /** * Sets current barrier generation as broken and wakes up everyone. * Called only while holding lock. * 将当前的屏障设置为被破坏并唤醒所有线程。 * 只在持有锁后才调用。 */ private void breakBarrier() { generation.broken = true; count = parties; trip.signalAll(); } /** * Main barrier code, covering the various policies. * 屏障的主要代码,涵盖各种策略。 */ private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException { // CyclicBarrier比CountDownLatch复杂,这里引入了可重入锁来保证线程安全。 // 其实本类直接利用了ReentrantLock的非公平锁来实现。 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;// 对count做递减 if (index == 0) { // tripped 减到0,越过屏障了 boolean ranAction = false; try { final Runnable command = barrierCommand; if (command != null) // 虽然是Runnable,但并未起新线程,这里直接执行的run command.run(); ranAction = true; nextGeneration(); return 0; } finally { if (!ranAction) // 异常情况,将本代标记为被破坏,唤醒所有线程 breakBarrier(); } } // loop until tripped, broken, interrupted, or timed out // 持续循环,直到屏障被越过、破坏、线程中断或超时 for (;;) { try { if (!timed) // 不限时等待,AQS里面调用了LockSupport.park trip.await(); else if (nanos > 0L) // 限时等待,AQS里面调用了LockSupport.parkNanos 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. // 即使我们没有被中断也要结束等待,所以这个中断可以认为是后续动作。 Thread.currentThread().interrupt(); } } if (g.broken) throw new BrokenBarrierException(); if (g != generation)// 当前线程不属于当前代了也不会异常 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. * 创建一个新的CyclicBarrier,屏障会在等待线程数达到指定的parties数值时跳过, * 跳过时会执行指定的屏障操作,该操作被最后一个到达屏障的线程执行(count减到0时)。 * * @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. * 创建一个新的CyclicBarrier,屏障会在等待线程数达到指定的parties数值时跳过, * 跳过时不执行屏障操作。 * * @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. * 返回越过屏障需要的线程数。 * * @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. * 一直等待,直到指定数量(parties)的线程调用过await。 * *

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: * 如果当前线程不是最后一个调用await的,那它将禁止再进行线程调度并在以下任意情况 * 发生之前都处于休眠状态: *

    *
  • The last thread arrives; or *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or *
  • Some other thread {@linkplain Thread#interrupt interrupts} * one of the other waiting threads; or *
  • Some other thread times out while waiting for barrier; or *
  • Some other thread invokes {@link #reset} on this barrier. *
* 最后一个线程调用了await;其他线程中断了当前线程;其他线程中断了已处于等待状态 * 的线程;等待中的线程超时;其他线程调用了屏障的reset * *

If the current thread: *

    *
  • has its interrupted status set on entry to this method; or *
  • is {@linkplain Thread#interrupt interrupted} while waiting *
* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * 如果当前线程在进入本方法之前就设置了中断状态,或在等待时被中断,那么将会抛出 * InterruptedException并清理掉当前线程的中断状态。 * *

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. * 如果在任何线程等待过程中调用了屏障的reset方法,或者await在调用时或任意线程等待 * 中屏障的isBroken为true,则会抛出BrokenBarrierException * *

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. * 如果任意线程在等待时被中断,则其他所有的等待线程都将抛出BrokenBarrierException, * 且屏障的状态被置为被破坏。 * *

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. * 如果当前线程是最后一个调用await的,并且屏障创建时传入了非null的屏障操作,那么当前 * 线程将会在唤醒其他线程前执行这个操作。如果在屏障操作执行中发生了异常,这个异常将会抛 * 出到当前线程且屏障状态被置为被破坏。 * * @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 another 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. * *

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: *

    *
  • The last thread arrives; or *
  • The specified timeout elapses; or *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or *
  • Some other thread {@linkplain Thread#interrupt interrupts} * one of the other waiting threads; or *
  • Some other thread times out while waiting for barrier; or *
  • Some other thread invokes {@link #reset} on this barrier. *
* *

If the current thread: *

    *
  • has its interrupted status set on entry to this method; or *
  • is {@linkplain Thread#interrupt interrupted} while waiting *
* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

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. * *

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. * *

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. * *

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 another 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 after * 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. * 重置屏障到初始状态。如果任意线程正在屏障处等待,他们将抛出BrokenBarrierException。 * 注意在屏障由于其他原因被破坏后进行重置可能会很复杂;这些线程需要用其他方式重新同步, * 并选择其中一个执行reset。为后续使用创建一个新的屏障可能是最优的选择。 */ 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. * 返回正在屏障处等待的线程数量。本方法主要用于调试和断言。 * * @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(); } } }

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