CountDownLatch简单介绍

来源：互联网发布：unity3d教程编辑：程序博客网时间：2024/05/24 00:54

本文摘自Java编程思想（第4版），写下来是便于能够随时复习理解。

它是一个同步辅助类，被用来同步一个或者多个任务，强制他们等待其它一组任务执行完成。

可以向CountDownLatch对象设置一个初始计数值，任何调用该对象await()的方法都将被阻塞，直到该计数值为0；其它任务结束时可调用该对象的

countDown()方法来减小这个计数值。CountDownLatch被设计为只触发一次，计数值不能被重置。如果你需要能够重置计数值的版本，可使用CyclicBarrier。简单的来说程序会先调用countDown()方法（调用await()的方法会被阻塞）会使计数器的值不断减小，当计数器的值为0时，调用await()的方法才会执行。

CountDownLatch的典型用法是将一个程序分为n个互相独立的任务。

TaskPortion:调用countDown()方法，将计数值不断减小直至0，模拟系统的初始部分。.

WaitingTask：表示系统中必须等待的部分，调用await()方法，当前线程被阻塞，只有当计数值为0时（即初始部分完成）才会被执行。

import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.TimeUnit;public class TaskPortion implements Runnable{private static int counter = 0;private final int id = counter ++;private static Random random = new Random(47);private final CountDownLatch latch;public TaskPortion(CountDownLatch latch){this.latch = latch;}@Overridepublic void run() {try {doWork();latch.countDown();} catch (InterruptedException e) {e.printStackTrace();}}private void doWork() throws InterruptedException {TimeUnit.MILLISECONDS.sleep(random.nextInt(2000));System.out.println(this+"completed");}public String toString(){return String.format("%1$-3d", id);}}

import java.util.concurrent.CountDownLatch;public class WaitingTask implements Runnable{private static int counter = 0;private final int id = counter ++;private final CountDownLatch latch;public WaitingTask(CountDownLatch latch){this.latch = latch;}@Overridepublic void run() {try {latch.await();System.out.println("latch barrier pass for");} catch (InterruptedException e) {System.out.println( "interrupted");}}public String toString(){return String.format("%1$-3d", id);}}

import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CountDownLatchDemo {static final int SIZE = 10;public static void main(String[] args) {ExecutorService exes = Executors.newCachedThreadPool();CountDownLatch latch = new CountDownLatch(SIZE);for(int i=0;i<5;i++)exes.execute(new WaitingTask(latch));for(int i=0;i<SIZE;i++)exes.execute(new TaskPortion(latch));System.out.println(" latch all tasks");exes.shutdown();}}

运行结果： latch all tasks
9 completed
7 completed
5 completed
8 completed
1 completed
2 completed
6 completed
4 completed
0 completed
3 completed
latch barrier pass for
latch barrier pass for
latch barrier pass for
latch barrier pass for
latch barrier pass for

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