package cn.com.pep;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
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
* <em>cyclic</em> because it can be re-used after the waiting threads
* are released.
*
* <p>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 <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> {@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<Thread> threads = new ArrayList<Thread>(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();
* }
* }}</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 {@code done()} will return
* {@code true} 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> {@code
* if (barrier.await() == 0) {
* // log the completion of this iteration
* }}</pre>
*
* <p>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).
*
* <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 CountDownLatch2
*
* @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.
*/
/**
* @Description:CyclicBarrier的类似于CountDownLatch，实现了一组线程之间的相互等待，直到所有线程都到齐了，再向下执行。
* 而与CountDownLatch不同的是，其实现采用的是条件队列，而非共享锁，并且是可以重用的。
* 那如何理解Generation这个“代”呢?
* 打个比方：游乐场里的过山车有10个座位，每次都凑满了10个之后，管理员才打开门，让10个游客过去，开动过山车。然后再关门，其余的游客再继续等待，等再凑够“下一波”的10个人，
* 这个模型中，前面已经通过的10个游客，就是“上一代”，而这个大门就是“barrier”，可以重复开启，后面继续等待排队的下一波游客，就是下一代。
* @version: V1.0
* @author: wwh
* @date: 2023年4月23日 上午9:52:52
*/
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 */
//需要一起通过barrier的线程数
private final int parties;
/* The command to run when tripped */
//当所有线程通过barrier之前，需要做的工作
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.
*/
//此barrier需要等待的线程数，初始值为parties，在同一个generation中，每调用一次await()，自减1
private int count;
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
//开启下一代
private void nextGeneration() {
// signal completion of last generation
//唤醒条件队列trip中所有等待的线程
trip.signalAll();
// set up next generation
//初始化count
count = parties;
//开启新一代，即将“代”的打破标记broken设置为false
generation = new Generation();
}
/**
* Sets current barrier generation as broken and wakes up everyone.
* Called only while holding lock.
*/
//打破当前barrier，这个怎么理解呢？某一时段景区的人比较少，很久都凑不够10个人，这时候管理员没办法，就打开门（barrier）,将这一拨人（generation）放进去，乘坐过山车，此时，管理员的这个行为就是一个breakBarrier。
private void breakBarrier() {
//将当前generation打破标记broken设置为true
generation.broken = true;
//重置count
count = parties;
//唤醒条件队列condition中正在等待的线程
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;
/**
* 只有调用breakBarrier()，才能将当前generation的broken标记置为true，如果当前线程在执行await()是，发现当前generation已经被打破了，
* 则直接抛出BrokenBarrierException异常。
*/
if (g.broken)
throw new BrokenBarrierException();
/**
* 此时，假如检测到当前线程被“中断”了，那么就应该将当前barrier打破,这是因为处于同一个barrier中的所有线程是相互等待的。既然当前线程已经被中断了，
* 那么其它线程肯定等不到当前线程执行完成了，所以执行breakBarrier()方法，将当前barrier打破，唤醒其他线程，其他线程被唤醒后，发现当前的generation的
* broken标记已经为true，则这些被唤醒的线程也都抛出BrokenBarrierExceptoin异常。
* CyclicBarrier实现中采用了“all-or-none breakage model”模型，即全损或者全无损模型。
*/
if (Thread.interrupted()) {
//检测到当前线程中断，打破当前线程所处的barrier，并且抛出InterruptedException异常。
breakBarrier();
throw new InterruptedException();
}
//用来表示当前线程进入到barrier的顺序，index=parties-1,表示第一个进入到barrier的线程，index=parties-2表示第二个，依次类推，知道index =0表示最后一个进入barrier的线程
int index = --count;
//index == 0，所有线程都已经就绪了，准备通过barrier，这个条件只有barrier中的最后一个进来线程才能达到。
if (index == 0) { // tripped
boolean ranAction = false;
try {
//所有的线程通过barrier之前，执行完通过barrier之前的准备工作（由barrier中的最后一个线程来执行），然后所有的线程再通过barrier。
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
//唤醒当前代中处于等待状态的线程，开启新的一代
nextGeneration();
return 0;
} finally {
//说明最后一个进入barrier的线程在执行通过barrier的准备时候发生了异常，则打破当前barrier
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
//如果index数不为0，就表示还有需要等待的其他线程，所以将当前线程挂起，等待所有线程都到齐、或者超时、被其他线程中断
for (;;) {
try {
//阻塞等待，等待被其他线程中断唤醒或者sigal()唤醒
if (!timed)
trip.await();
else if (nanos > 0L)
//带超时时间的阻塞，等到超时时间到
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
//等待的过程中发生了中断异常
if (g == generation && ! g.broken) {
//当前线程还处于这个“代”中，并且这个barrier还没有被打破，那么需要将这个barrier打破，结束这个barrier中其他线程的无异议的等待
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.
/**
* 走到这里来了，应该有两种情况：
* 1、说明新的代已经产生了，当前线程已经不在这个代中了，那么只是重置先线程的中断标记，不需要做额外处理了；
* 2、说明中断发生时候，barrier已经被打破了，所以也只是记录下线程的中断标记；
* 3、
*/
Thread.currentThread().interrupt();
}
}
/**
* 能走到这里来说明，线程已经被唤醒了，检查一下broken的状态，如果为true则直接抛出BrokenBarrierException异常，
* 能使broken的状态变为true的只有breakBarrier()方法，对应以下几种情况：
* 1、其他在执行await()方法线程被挂起之前，就已经被中断了，导致触发了breakBarrier（）方法；
* 2、其他在执行await()方法线程在被挂起等待的过程中，被中断唤醒了，导致触发了breakBarrier()方法；
* 3、最后一个进入barrier的线程在执行barrierCommand过程中发生了异常，导致breakBarrier()方法被调用；
* 4、显示调用reset()方法；
*/
if (g.broken)
throw new BrokenBarrierException();
//如果线程被正常唤醒，下一代已经被开启了，则返回线程进入barrier的顺序
if (g != generation)
return index;
//允许超时，并且超时时间已到，则打破当前barrier，并抛出TimeOutException异常；
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
*/
//需要通过这个barrier的线程数
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, 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
*/
//不带超时时间的等待
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.
*
* <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();
}
}
}
标签：thread,barrier,lock,broken,waiting,源码,线程,AQS,CyclicBarrier From： https://www.cnblogs.com/wha6239/p/17346295.html