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CyclicBarrier-源码分析

1 前言

CyclicBarrier是一个同步辅助类,它允许一组线程互相等待,直到到达某个公共屏障点(common barrier point)。在涉及一组固定大小的线程的程序中,这些线程必须不时地互相等待,此时CyclicBarrier很有用。因为该barrier在释放等待线程后可以重用,所以称它为循环的barrier

2 内部类

该内部类用于标记一次Barrier生命周期的状态

  1. true:Barrier被打断,调用await()方法将会抛出BrokenBarrierException异常
  2. false:Barrier有效

一次Barrier生命周期中Barrier的状态不会影响到下一次生命周期

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/**
* 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;
}

3 字段

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/** 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;
  • lock:重入锁
  • trip:条件对象,是AQS-ConditionObject内部类
  • parties:到达barrier的物体数量
  • barrierCommand:为了增加CyclicBarrier的生命周期,当足够多的线程通过屏障时,调用相应的处理逻辑,可由构造方法传入
  • generation:标记本次生命周期的Barrier的状态,如果为true,则Barrier失效。到了next Generation后,又会重新指向一个新的Generation对象
  • count:计数值

4 重要方法

4.1 构造方法

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/**
* 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);
}

/**
* 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;
}

4.2 await

await方法阻塞当前线程直至累计有count(构造方法的参数)个线程阻塞在了await方法上

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/**
* 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
}
}

4.2.1 dowait

dowait方法是实现CyclicBarrier语义的主要方法

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/**
* 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;

//如果是broken状态,则抛出BrokenBarrierException异常。该bool值在breakBarrier方法中被设置为true
if (g.broken)
throw new BrokenBarrierException();

//如果被中断了则设置broken为true,然后抛出InterruptedException异常
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}

//递减count
int index = --count;
//如果index为0,说明有足够多的线程调用了await方法,此时应该放行所有线程
if (index == 0) { //tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
//执行一些额外的逻辑
if (command != null)
command.run();
//如果在执行run方法中抛出了任何异常,则ranAction为false状态
ranAction = true;
//进入下一个生命周期
nextGeneration();
return 0;
} finally {
if (!ranAction)
//将Barrier的当前生命周期的状态标记为不可用
breakBarrier();
}
}

//阻塞当前线程直至被中断,打断或者超时
//loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
//如果不允许超时,将当前线程直接挂起,阻塞在条件对象trip的condition queue(ConditionObject中维护的等待队列)中
if (!timed)
trip.await();
//如果允许超时,阻塞在条件对象trip的condition queue中一段时间
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
//当Barrier处于有效状态
if (g == generation && ! g.broken) {
//设置为打断状态
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();
}
}

//如果Barrier被打断,则抛出BrokenBarrierException异常
if (g.broken)
throw new BrokenBarrierException();

//意味着调用await时处于上一个生命周期,而此时却进入了下一个生命周期中
if (g != generation)
return index;

//允许超时,并且已经超时
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}

4.2.2 nextGeneration

该方法使得CyclicBarrier进入下一次生命周期,唤醒阻塞在trip上的线程,并且重置所有状态

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/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
private void nextGeneration() {
//signal completion of last generation
//唤醒所有阻塞在condition queue中的线程,即那些阻塞在await方法上的线程
trip.signalAll();
//set up next generation
//这就是CyclicBarrier可重用的原因
count = parties;
//重新生成Generation
generation = new Generation();
}

4.3 isBroken

检查当前Barrier生命周期是否有效

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/**
* 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();
}
}

4.4 reset

重置Barrier,使其进入下一个生命周期。对于那些阻塞在上一个生命周期中的线程,会通过nextGeneration方法进行唤醒

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/**
* 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();
}
}

4.5 getNumberWaiting

检查有多少个线程阻塞在await方法的调用中

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/**
* 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();
}
}