CountDownLatch 和 CyclicBarrier

如果是一个线程等待一个线程，则可以通过await()和notify()来实现；如果是一个线程等待多个线程，则就可以使用CountDownLatch和CyclicBarrier来实现比较好的控制。

CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

CyclicBarrier : A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.

CountDownLatch : 一个线程(或者多个)， 等待另外N个线程完成某个事情之后才能执行。 CyclicBarrier : N个线程相互等待，任何一个线程完成之前，所有的线程都必须等待。 这样应该就清楚一点了，对于CountDownLatch来说，重点是那个“一个线程”, 是它在等待， 而另外那N的线程在把“某个事情”做完之后可以继续等待，可以终止。而对于CyclicBarrier来说，重点是那N个线程，他们之间任何一个没有完成，所有的线程都必须等待。

CountDownLatch 是计数器, 线程完成一个就记一个, 就像 报数一样, 只不过是递减的.

而CyclicBarrier更像一个水闸, 线程执行就想水流, 在水闸处都会堵住, 等到水满(线程到齐)了, 才开始泄流.

CountDownLatch是通过一个计数器来实现的，计数器的初始值为线程的数量。每当一个线程完成了自己的任务后，计数器的值就会减1。当计数器值到达0时，它表示所有的线程已经完成了任务，然后在闭锁上等待的线程就可以恢复执行任务。

CountDownLatch示例

package com.wlyuan.CountDownLatch; /** * Created by wlyuan on 17-3-12. */ import java.util.concurrent.*; import java.util.*; // Performs some portion of a task: class TaskPortion implements Runnable { private static int counter = 0; private final int id = counter++; private static Random rand = new Random(47); private final CountDownLatch latch; TaskPortion(CountDownLatch latch) { this.latch = latch; } public void run() { try { doWork(); latch.countDown(); } catch(InterruptedException ex) { // Acceptable way to exit } } public void doWork() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000)); System.out.println(this + "completed"); } public String toString() { return String.format("%1$-3d ", id); } } // Waits on the CountDownLatch: class WaitingTask implements Runnable { private static int counter = 0; private final int id = counter++; private final CountDownLatch latch; WaitingTask(CountDownLatch latch) { this.latch = latch; } public void run() { try { latch.await(); System.out.println("Latch barrier passed for " + this); } catch(InterruptedException ex) { System.out.println(this + " interrupted"); } } public String toString() { return String.format("WaitingTask %1$-3d ", id); } } public class CountDownLatchDemo { static final int SIZE = 100; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); // All must share a single CountDownLatch object: CountDownLatch latch = new CountDownLatch(SIZE); for(int i = 0; i < 10; i++) exec.execute(new WaitingTask(latch)); for(int i = 0; i < SIZE; i++) exec.execute(new TaskPortion(latch)); System.out.println("Launched all tasks"); exec.shutdown(); // Quit when all tasks complete } } /* (Execute to see output) *///:~

CyclicBarrier示例

package com.wlyuan.CyclicBarrier; /** * Created by wlyuan on 17-3-12. */ // Using CyclicBarriers. import java.util.concurrent.*; import java.util.*; class Horse implements Runnable { private static int counter = 0; private final int id = counter++; private int strides = 0; private static Random rand = new Random(47); private static CyclicBarrier barrier; public Horse(CyclicBarrier b) { barrier = b; } public synchronized int getStrides() { return strides; } public void run() { try { while(!Thread.interrupted()) { synchronized(this) { strides += rand.nextInt(3); // Produces 0, 1 or 2 } barrier.await(); } } catch(InterruptedException e) { // A legitimate way to exit } catch(BrokenBarrierException e) { // This one we want to know about throw new RuntimeException(e); } } public String toString() { return "Horse " + id + " "; } public String tracks() { StringBuilder s = new StringBuilder(); for(int i = 0; i < getStrides(); i++) s.append("*"); s.append(id); return s.toString(); } } public class HorseRace { static final int FINISH_LINE = 75; private List<Horse> horses = new ArrayList<Horse>(); private ExecutorService exec = Executors.newCachedThreadPool(); private CyclicBarrier barrier; public HorseRace(int nHorses, final int pause) { barrier = new CyclicBarrier(nHorses, new Runnable() { public void run() { StringBuilder s = new StringBuilder(); for(int i = 0; i < FINISH_LINE; i++) s.append("="); // The fence on the racetrack System.out.println(s); for(Horse horse : horses) System.out.println(horse.tracks()); for(Horse horse : horses) if(horse.getStrides() >= FINISH_LINE) { System.out.println(horse + "won!"); exec.shutdownNow(); return; } try { TimeUnit.MILLISECONDS.sleep(pause); } catch(InterruptedException e) { System.out.println("barrier-action sleep interrupted"); } } }); for(int i = 0; i < nHorses; i++) { Horse horse = new Horse(barrier); horses.add(horse); exec.execute(horse); } } public static void main(String[] args) { int nHorses = 7; int pause = 200; if(args.length > 0) { // Optional argument int n = new Integer(args[0]); nHorses = n > 0 ? n : nHorses; } if(args.length > 1) { // Optional argument int p = new Integer(args[1]); pause = p > -1 ? p : pause; } new HorseRace(nHorses, pause); } } /* (Execute to see output) *///:~

运行结果

*******************************************************************0 ************************************************************************1 ********************************************************2 ****************************************************************************3 ************************************************************************4 ***************************************************************************5 **************************************************************6 Horse 3 won!