一、定义
CyclicBarrier 字面意思回环栅栏（循环屏障），通过它可以实现让一组线程等待至某个状态（屏障点）之后再全部同时执行。叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。
CyclicBarrier 作用是让一组线程相互等待，当达到一个共同点时，所有之前等待的线程再继续执行，且 CyclicBarrier 功能可重复使用。
CyclicBarrier 可以 让一组线程到达一个屏障时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会释放。
注意,被阻塞的线程 ,不是同时 被释放的。但时间间隔几乎 可以说没有。
二、使用
package com.util;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.logging.Logger;
/**
* @author : lssffy
* @Description : 循环屏障
* @date : 2023/12/17 19:47
*/
public class CyclicBarrierTest {
private static final Logger logger = Logger.getLogger("CyclicBarrierTest");
public static void main(String[] args) {
//屏障的线程数量：5
CyclicBarrier cb = new CyclicBarrier(5);
for (int i = 0; i < 5; i++) {
int flag = i;
new Thread(()->{
try {
//最后一个线程休眠3秒，看看所有到达屏障的线程 也是休眠3秒之后再执行
if(flag%5==4){
Thread.sleep(5000);
}
//阻塞在这里，直到到达屏障的线程数量=5才会被释放
cb.await();
System.out.println(Thread.currentThread().getName());
}catch (InterruptedException e) {
logger.info("Thread interrupted" +e.getMessage());
}catch (BrokenBarrierException e){
logger.info("BrokenBarrierException" + e.getMessage());
}
}).start();
}
System.out.println("线程执行完毕");
}
}
最终执行结果，是最后到达屏障的最先执行，其次才是之前到达屏障阻塞的所有线程，其他线程被释放的顺序，也不一定是按照await方法的调用顺序来定的
3、方法
构造方法
一个有参构造，一个无参构造,有参构造相对于无参构造来说 多传入了一个Runnable实例, 当一个周期的屏障被 释放后, 可以执行 Runnable实例的 run方法逻辑。
/**
* 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;
}
parties表示屏障拦截的线程数量，每个线程调用 await 方法告诉 CyclicBarrier 我已经到达了屏障，然后当前线程被阻塞。
用于在线程到达屏障时，优先执行 barrierAction，方便处理更复杂的业务场景(该线程的执行时机是在到达屏障之后再执行)
重要方法
/**
* 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
}
}
屏障 指定数量的线程全部调用await()方法时，这些线程不再阻塞
BrokenBarrierException 表示栅栏已经被破坏，破坏的原因可能是其中一个线程 await() 时被中断或者超时
/**
* 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));
}
循环 通过reset()方法可以进行重置
/**
* 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、原理
CyclicBarrier 是利用 ReentrantLock锁 和 ReentrantLock里的条件队列 来实现 所有线程 在屏障处 阻塞的效果的。
dowait逻辑
/**
* 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(); //加锁
try {
//判断count 需要到达屏障的线程数
int index = --count;
if (index == 0) { // tripped
boolean ranAction = false;
try {
//这个就是有参构造传进来的第二个参数Runnable实例
final Runnable command = barrierCommand;
if (command != null)
//最后一个到达屏障的线程 同步调用
command.run();
ranAction = true;
//释放所有
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
if (!timed)
// 释放,进入条件队列阻塞
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 {
// 释放锁
lock.unlock();
}
}
CyclicBarrier 流程
主要是的流程：
1、获取锁 如果 count != 0 就进入阻塞；
2、进入阻塞之前，首先需要进入条件队列，然后释放锁，最后阻塞；
3、如果 count != 0 会进行一个唤醒，将所有的条件队列中的节点转换为阻塞队列；
4、被唤醒过后会进行锁的获取，如果锁获取失败，会进入 lock 的阻塞队列；
5、如果锁获取成功，进行锁的释放，以及唤醒，同步队列中的线程。
CyclicBarrier 与 CountDownLatch的区别
1、CountDownLatch的计数器只能使用一次，而CyclicBarrier的计数器可以使用reset() 方法重置。所以CyclicBarrier能处理更为复杂的业务场景，比如如果计算发生错误，可以重置计数器，并让线程们重新执行一次
2、CyclicBarrier还提供getNumberWaiting(可以获得CyclicBarrier阻塞的线程数量)、isBroken(用来知道阻塞的线程是否被中断)等方法。
3、CountDownLatch会阻塞主线程，CyclicBarrier不会阻塞主线程，只会阻塞子线程。
4、CountDownLatch和CyclicBarrier都能够实现线程之间的等待，只不过它们侧重点不同。 5、CountDownLatch一般用于一个或多个线程，等待其他线程执行完任务后，再执行。 6、CyclicBarrier一般用于一组线程互相等待至某个状态，然后这一组线程再同时执行。
7、CyclicBarrier 还可以提供一个 barrierAction，合并多线程计算结果。
8、CyclicBarrier是通过ReentrantLock的"独占锁"和Conditon来实现一组线程的阻塞唤醒的，而CountDownLatch则是通过AQS的“共享锁”实现
