詳解java CountDownLatch和CyclicBarrier在內部實現(xiàn)和場景上的區(qū)別
前言
CountDownLatch和CyclicBarrier兩個同為java并發(fā)編程的重要工具類,它們在諸多多線程并發(fā)或并行場景中得到了廣泛的應用。但兩者就其內部實現(xiàn)和使用場景而言是各有所側重的。
內部實現(xiàn)差異
前者更多依賴經(jīng)典的AQS機制和CAS機制來控制器內部狀態(tài)的更迭和計數(shù)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(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) { // tripped boolean 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(zhàn) - 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(yè)務線程才能繼續(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)務線程等待latch的計數(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(tǒng)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(zhí)行,通過多次觸發(fā)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(shù)必須與實際的用戶線程數(shù)一致;
3. 協(xié)調線程同時運行:countDownLatch協(xié)調工作線程執(zhí)行,是由外面線程協(xié)調;cyclicbarrier是由工作線程之間相互協(xié)調運行;
4. 從構造函數(shù)上看出:countDownlatch控制運行的計數(shù)器數(shù)量和線程數(shù)沒有關系;cyclicbarrier構造中傳入的線程數(shù)等于實際執(zhí)行線程數(shù);
5. countDownLatch在不能基于執(zhí)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(zhí)行后,將運行結果匯總再計算等典型場景;
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