前言

CountDownLatch和CyclicBarrier兩個同為java并發編程的重要工具類，它們在諸多多線程并發或并行場景中得到了廣泛的應用。但兩者就其內部實現和使用場景而言是各有所側重的。

內部實現差異

前者更多依賴經典的AQS機制和CAS機制來控制器內部狀態的更迭和計數器本身的變化，而后者更多依靠可重入Lock等機制來控制其內部并發安全性和一致性。

public class { //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) { return (getState() == 0) ? 1 : -1; } protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) {int c = getState();if (c == 0) return false;int nextc = c-1;if (compareAndSetState(c, nextc)) return nextc == 0; } } } private final Sync sync; ... ...// }

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) { // 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 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(); } } ... ... // }

實戰 - 展示各自的使用場景

/** *類說明：共5個初始化子線程，6個閉鎖扣除點，扣除完畢后，主線程和業務線程才能繼續執行 */public class UseCountDownLatch { static CountDownLatch latch = new CountDownLatch(6); /*初始化線程*/ private static class InitThread implements Runnable{ public void run() { System.out.println('Thread_'+Thread.currentThread().getId() +' ready init work......'); latch.countDown(); for(int i =0;i<2;i++) {System.out.println('Thread_'+Thread.currentThread().getId() +' ........continue do its work'); } } } /*業務線程等待latch的計數器為0完成*/ private static class BusiThread implements Runnable{ public void run() { try {latch.await(); } catch (InterruptedException e) {e.printStackTrace(); } for(int i =0;i<3;i++) {System.out.println('BusiThread_'+Thread.currentThread().getId() +' do business-----'); } } } public static void main(String[] args) throws InterruptedException { new Thread(new Runnable() { public void run() {SleepTools.ms(1);System.out.println('Thread_'+Thread.currentThread().getId() +' ready init work step 1st......');latch.countDown();System.out.println('begin step 2nd.......');SleepTools.ms(1);System.out.println('Thread_'+Thread.currentThread().getId() +' ready init work step 2nd......');latch.countDown(); } }).start(); new Thread(new BusiThread()).start(); for(int i=0;i<=3;i++){ Thread thread = new Thread(new InitThread()); thread.start(); } latch.await(); System.out.println('Main do ites work........'); }}

/** *類說明：共4個子線程，他們全部完成工作后，交出自己結果， *再被統一釋放去做自己的事情，而交出的結果被另外的線程拿來拼接字符串 */class UseCyclicBarrier { private static CyclicBarrier barrier = new CyclicBarrier(4,new CollectThread()); //存放子線程工作結果的容器 private static ConcurrentHashMap<String,Long> resultMap = new ConcurrentHashMap<String,Long>(); public static void main(String[] args) { for(int i=0;i<4;i++){ Thread thread = new Thread(new SubThread()); thread.start(); } } /*匯總的任務*/ private static class CollectThread implements Runnable{ @Override public void run() { StringBuilder result = new StringBuilder(); for(Map.Entry<String,Long> workResult:resultMap.entrySet()){result.append('['+workResult.getValue()+']'); } System.out.println(' the result = '+ result); System.out.println('do other business........'); } } /*相互等待的子線程*/ private static class SubThread implements Runnable{ @Override public void run() { long id = Thread.currentThread().getId(); resultMap.put(Thread.currentThread().getId()+'',id); try { Thread.sleep(1000+id); System.out.println('Thread_'+id+' ....do something ');barrier.await();Thread.sleep(1000+id);System.out.println('Thread_'+id+' ....do its business ');barrier.await(); } catch (Exception e) {e.printStackTrace(); } } }}

兩者總結

1. Cyclicbarrier結果匯總的Runable線程可以重復被執行，通過多次觸發await()方法，countdownlatch可以調用await()方法多次；cyclicbarrier若沒有結果匯總，則調用一次await()就夠了；

2. New cyclicbarrier(threadCount)的線程數必須與實際的用戶線程數一致；

3. 協調線程同時運行：countDownLatch協調工作線程執行，是由外面線程協調;cyclicbarrier是由工作線程之間相互協調運行；

4. 從構造函數上看出：countDownlatch控制運行的計數器數量和線程數沒有關系；cyclicbarrier構造中傳入的線程數等于實際執行線程數;

5. countDownLatch在不能基于執行子線程的運行結果做處理，而cyclicbarrier可以;

6. 就使用場景而言，countdownlatch 更適用于框架加載前的一系列初始化工作等場景; cyclicbarrier更適用于需要多個用戶線程執行后，將運行結果匯總再計算等典型場景;

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