栅栏Java示例——CyclicBarrier

本文主要是介绍栅栏Java示例——CyclicBarrier，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

一个Barrier的小例子，栅栏数和线程池数量可以随便调整，模拟因栅栏一直等待导致的死锁，当线程数小于栅栏数时会出现死锁情况。

import java.util.Random;
import java.util.concurrent.*;public class BarrierDemo {public static void main(String[] args) {ExecutorService service = Executors.newFixedThreadPool(5);final CyclicBarrier barrier = new CyclicBarrier(4);for (int i = 0; i < 5; i++) {service.execute(new Player("玩家" + i, barrier));}service.shutdown();}
}class Player implements Runnable {private final String name;private final CyclicBarrier barrier;public Player(String name, CyclicBarrier barrier) {this.name = name;this.barrier = barrier;}public void run() {try {TimeUnit.SECONDS.sleep(1 + (new Random().nextInt(3)));System.out.println(name + "已准备,等待其他玩家准备...");barrier.await();TimeUnit.SECONDS.sleep(1 + (new Random().nextInt(3)));System.out.println(name + "已加入游戏");} catch (InterruptedException e) {System.out.println(name + "离开游戏");} catch (BrokenBarrierException e) {System.out.println(name + "离开游戏");}}
}

当前线程数与栅栏数的执行结果：

玩家2已准备,等待其他玩家准备...
玩家1已准备,等待其他玩家准备...
玩家3已准备,等待其他玩家准备...
玩家4已准备,等待其他玩家准备...
玩家0已准备,等待其他玩家准备...
玩家2已加入游戏
玩家1已加入游戏
玩家3已加入游戏
玩家4已加入游戏
#由于还有一个线程0在栅栏中等待，导致这个程序会一直卡在这里。

如果把for循环的循环次数改为4，即只产生4个玩家，栅栏等待4个线程都执行到await那一步后都会同时解锁，继续执行。执行结果：

玩家1已准备,等待其他玩家准备...
玩家2已准备,等待其他玩家准备...
玩家3已准备,等待其他玩家准备...
玩家0已准备,等待其他玩家准备...
玩家1已加入游戏
玩家3已加入游戏
玩家0已加入游戏
玩家2已加入游戏
#程序正常结束

附上CyclicBarrier的源码，其中重要的await()方法可以重点关注，源码解析：await()这个方法调用一次count-1，如果不是最后一个，则循环调用这个方法，，直到所有线程把count减为0，这个方法继续执行。

package java.util.concurrent;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;public class CyclicBarrier {private static class Generation {boolean broken = false;}private final ReentrantLock lock = new ReentrantLock();private final Condition trip = lock.newCondition();private final int parties;private final Runnable barrierCommand;private Generation generation = new Generation();private int count;private void nextGeneration() {// signal completion of last generationtrip.signalAll();// set up next generationcount = parties;generation = new Generation();}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) {  // trippedboolean 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 outfor (;;) {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();}}public CyclicBarrier(int parties, Runnable barrierAction) {if (parties <= 0) throw new IllegalArgumentException();this.parties = parties;this.count = parties;this.barrierCommand = barrierAction;}public CyclicBarrier(int parties) {this(parties, null);}public int getParties() {return parties;}public int await() throws InterruptedException, BrokenBarrierException {try {return dowait(false, 0L);} catch (TimeoutException toe) {throw new Error(toe); // cannot happen}}public int await(long timeout, TimeUnit unit)throws InterruptedException,BrokenBarrierException,TimeoutException {return dowait(true, unit.toNanos(timeout));}public boolean isBroken() {final ReentrantLock lock = this.lock;lock.lock();try {return generation.broken;} finally {lock.unlock();}}public void reset() {final ReentrantLock lock = this.lock;lock.lock();try {breakBarrier();   // break the current generationnextGeneration(); // start a new generation} finally {lock.unlock();}}public int getNumberWaiting() {final ReentrantLock lock = this.lock;lock.lock();try {return parties - count;} finally {lock.unlock();}}
}