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();
}
}
}