前言

CountDownLatch和CyclicBarrier兩個(gè)同為java并發(fā)編程的重要工具類，它們?cè)谥T多多線程并發(fā)或并行場(chǎng)景中得到了廣泛的應(yīng)用。但兩者就其內(nèi)部實(shí)現(xiàn)和使用場(chǎng)景而言是各有所側(cè)重的。

內(nèi)部實(shí)現(xiàn)差異

前者更多依賴經(jīng)典的AQS機(jī)制和CAS機(jī)制來(lái)控制器內(nèi)部狀態(tài)的更迭和計(jì)數(shù)器本身的變化，而后者更多依靠可重入Lock等機(jī)制來(lái)控制其內(nèi)部并發(fā)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(shí)戰(zhàn) - 展示各自的使用場(chǎng)景

/** *類說(shuō)明：共5個(gè)初始化子線程，6個(gè)閉鎖扣除點(diǎn)，扣除完畢后，主線程和業(yè)務(wù)線程才能繼續(xù)執(zhí)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(yè)務(wù)線程等待latch的計(jì)數(shù)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(shuō)明：共4個(gè)子線程，他們?nèi)客瓿晒ぷ骱螅怀鲎约航Y(jié)果， *再被統(tǒng)一釋放去做自己的事情，而交出的結(jié)果被另外的線程拿來(lái)拼接字符串 */class UseCyclicBarrier { private static CyclicBarrier barrier = new CyclicBarrier(4,new CollectThread()); //存放子線程工作結(jié)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(wù)*/ 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(jié)

1. Cyclicbarrier結(jié)果匯總的Runable線程可以重復(fù)被執(zhí)行，通過(guò)多次觸發(fā)await()方法，countdownlatch可以調(diào)用await()方法多次；cyclicbarrier若沒(méi)有結(jié)果匯總，則調(diào)用一次await()就夠了；

2. New cyclicbarrier(threadCount)的線程數(shù)必須與實(shí)際的用戶線程數(shù)一致；

3. 協(xié)調(diào)線程同時(shí)運(yùn)行：countDownLatch協(xié)調(diào)工作線程執(zhí)行，是由外面線程協(xié)調(diào);cyclicbarrier是由工作線程之間相互協(xié)調(diào)運(yùn)行；

4. 從構(gòu)造函數(shù)上看出：countDownlatch控制運(yùn)行的計(jì)數(shù)器數(shù)量和線程數(shù)沒(méi)有關(guān)系；cyclicbarrier構(gòu)造中傳入的線程數(shù)等于實(shí)際執(zhí)行線程數(shù);

5. countDownLatch在不能基于執(zhí)行子線程的運(yùn)行結(jié)果做處理，而cyclicbarrier可以;

6. 就使用場(chǎng)景而言，countdownlatch 更適用于框架加載前的一系列初始化工作等場(chǎng)景; cyclicbarrier更適用于需要多個(gè)用戶線程執(zhí)行后，將運(yùn)行結(jié)果匯總再計(jì)算等典型場(chǎng)景;

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