CyclicBarrier 是如何做到等待多线程到达一起执行的？

　　我们有些场景，是需要使用 多线各一起执行某些操作的，比如进行并发测试，比如进行多线程数据汇总。

　　自然，我们可以使用 CountDownLatch, CyclicBarrier, 以及多个 Thread.join()。 虽然最终的效果都差不多，但实际却各有千秋。我们此处主要看 CyclicBarrier .

　　概要: CyclicBarrier 使用 n 个 permit 进行初始化，当n个线程都到达后进行放行，然后进入下一个循环周期。在放行的同时，还可以设置一个执行方法，即相当于回调操作。

一、CyclicBarrier 具体实现

　　主循环等待！

// CyclicBarrier
/**
* Main barrier code, covering the various policies.
*/
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
// 使用一个 互斥锁，保证进行排队等待的安全性
final ReentrantLock lock = this.lock;
lock.lock();
try {
// 使用的一 Generation 代表一生循环周期，当周期到达后，替换此值
final Generation g = generation;

// 针对异常情况，直接抛出异常，一般是用于多线程之间通信
if (g.broken)
throw new BrokenBarrierException();

if (Thread.interrupted()) {
// breakBarrier 是针对其他线程的，而 抛出的 InterruptedException 是针对当前线程的
// 从而达到中断标志全局可见的效果
breakBarrier();
throw new InterruptedException();
}

// 以下逻辑为进入了等待区域， count-1， 当减到0之后，就代表需要进行放行了
int index = --count;
// 放行
if (index == 0) {  // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
// 如果设置了回调，则立即执行回调，在当前线程中
if (command != null)
command.run();
ranAction = true;
// 循环周期迭代，此操作后，其他所有等待线程都将被返回，进入下一轮周期
nextGeneration();
return 0;
} finally {
// 未知异常，撤销当前的等待
if (!ranAction)
breakBarrier();
}
}

// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
// 一直在此处进行等待，直到被唤醒，被唤醒时，则意味着有事件发生了
// 等待中将会释放锁，从而让其他线程进入
// 此处的 await() 是一个复杂的故事，因为它要保证在 notify 时的锁竞争问题
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();
}
}

// 此情况为发生了异常，被唤醒，则直接抛出异常退出
if (g.broken)
throw new BrokenBarrierException();

// 生命周期被迭代，可以放行了
if (g != generation)
return index;

// 如果是等待超时，则抛出超时异常
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}

　　可以看到，主要逻辑就是在于 生命周期的迭代操作，但是这个生命周期的标志异常的简单:

// 只有一个标识位， broken 为 true 时，发生了异常，整体退出
private static c
1062
lass Generation {
boolean broken = false;
}

　　而到达的线程数足够之后，需要进行周期迭代，只是 Generation 更换一个变量，另外就是要起到通知所有等待线程的作用:

// CyclicBarrier
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
private void nextGeneration() {
// signal completion of last generation
// 先通知等待线程，但此时当前线程仍然持有锁，所以其他线程仍然处理等待状态
// 然后再设置下一周期，直到本线程当前同步块退出之后，其他线程才可以进行工作
// 此处依赖于 ReentrantLock
// 此处体现 wait/notify 的锁作用域问题
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}

　　而调用 入口 仅是调用 dowait() 方法而已.

// CyclicBarrier
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}

　　CyclicBarrier 本身的等待逻辑是简单巧妙的，使用 ReentrantLock 的目的是为了实现带超时等待的效果，否则就是一个 wait/notify 机制的实现。当然 wait/notify 的逻辑还是很关键很复杂的，后续如有必要再写一文说明。

　　完整代码如下:

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;
}

/** 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 {@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
aec
, 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.

2ba8
*
* <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();
}
}
}

二、简单看一下 CountDownLatch 的同时等待实现

　　CountDownLatch 会在初始化时，申请 n 个 permit, 调用 await() 进行阻塞， 直到 permit=0 时，await() 才进行返回。每调用一次 countDown(); permit 都会减1直到为0止;

// CountDownLatch.await()  等待
public void await() throws InterruptedException {
// 仅是去尝试获取一个而已
sync.acquireSharedInterruptibly(1);
}

// CountDownLatch.countDown() 释放锁, 当 permit=0 后，放行 await()
public void countDown() {
// 此处仅是委托给了 AQS 进行释放、通知处理
sync.releaseShared(1);
}

// CountDownLatch 内部锁实现的是否可以持有锁的逻辑
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;

Sync(int count) {
setState(count);
}

int getCount() {
return getState();
}

protected int tryAcquireShared(int acquires) {
// 只要 state=0, 都可以放行
return (getState() == 0) ? 1 : -1;
}

// 释放锁 countDown 逻辑, 做减1操作
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
// 如果已经被释放，则直接返回
if (c == 0)
return false;
// 忽略传入值 releases, 只做减1操作, 所以 state 必定有等于0的时候
int nextc = c-1;
if (compareAndSetState(c, nextc))
// 只有等于0, 才能进行真正的释放通知操作
return nextc == 0;
}
}
}

　　可以看出, CountDownLatch 的同时等待实现更加简单，几乎都是依赖于 AQS 进行实现。同样，从实际效果来说，也是一个 wait/notify 的实现。只是此处的 notify 执行完之后就释放了锁，即无法保证 notify 之后的线程安全性。

唠叨: 论 wait/notify 机制的安全性！