Java的concurrent包里面的CountDownLatch其實(shí)可以把它看作一個(gè)計(jì)數(shù)器，只不過(guò)這個(gè)計(jì)數(shù)器的操作是原子操作，同時(shí)只能有一個(gè)線(xiàn)程去操作這個(gè)計(jì)數(shù)器，也就是同時(shí)只能有一個(gè)線(xiàn)程去減這個(gè)計(jì)數(shù)器里面的值。
你可以向CountDownLatch對(duì)象設(shè)置一個(gè)初始的數(shù)字作為計(jì)數(shù)值，任何調(diào)用這個(gè)對(duì)象上的await()方法都會(huì)阻塞，直到這個(gè)計(jì)數(shù)器的計(jì)數(shù)值被其他的線(xiàn)程減為0為止。
CountDownLatch的一個(gè)非常典型的應(yīng)用場(chǎng)景是：有一個(gè)任務(wù)想要往下執(zhí)行，但必須要等到其他的任務(wù)執(zhí)行完畢后才可以繼續(xù)往下執(zhí)行。假如我們這個(gè)想要繼續(xù)往下執(zhí)行的任務(wù)調(diào)用一個(gè)CountDownLatch對(duì)象的await()方法，其他的任務(wù)執(zhí)行完自己的任務(wù)后調(diào)用同一個(gè)CountDownLatch對(duì)象上的countDown()方法，這個(gè)調(diào)用await()方法的任務(wù)將一直阻塞等待，直到這個(gè)CountDownLatch對(duì)象的計(jì)數(shù)值減到0為止。
舉個(gè)例子，有三個(gè)工人在為老板干活，這個(gè)老板有一個(gè)習(xí)慣，就是當(dāng)三個(gè)工人把一天的活都干完了的時(shí)候，他就來(lái)檢查所有工人所干的活。記住這個(gè)條件：三個(gè)工人先全部干完活，老板才檢查。所以在這里用Java代碼設(shè)計(jì)兩個(gè)類(lèi)，Worker代表工人，Boss代表老板，具體的代碼實(shí)現(xiàn)如下：
- package org.zapldy.concurrent;
-
- import java.util.Random;
- import java.util.concurrent.CountDownLatch;
- import java.util.concurrent.TimeUnit;
-
- public class Worker implements Runnable{
-
- private CountDownLatch downLatch;
- private String name;
-
- public Worker(CountDownLatch downLatch, String name){
- this.downLatch = downLatch;
- this.name = name;
- }
-
- public void run() {
- this.doWork();
- try{
- TimeUnit.SECONDS.sleep(new Random().nextInt(10));
- }catch(InterruptedException ie){
- }
- System.out.println(this.name + "活干完了！");
- this.downLatch.countDown();
-
- }
-
- private void doWork(){
- System.out.println(this.name + "正在干活!");
- }
-
- }
- package org.zapldy.concurrent;
-
- import java.util.concurrent.CountDownLatch;
-
- public class Boss implements Runnable {
-
- private CountDownLatch downLatch;
-
- public Boss(CountDownLatch downLatch){
- this.downLatch = downLatch;
- }
-
- public void run() {
- System.out.println("老板正在等所有的工人干完活......");
- try {
- this.downLatch.await();
- } catch (InterruptedException e) {
- }
- System.out.println("工人活都干完了，老板開(kāi)始檢查了！");
- }
-
- }
- package org.zapldy.concurrent;
-
- import java.util.concurrent.CountDownLatch;
- import java.util.concurrent.ExecutorService;
- import java.util.concurrent.Executors;
-
- public class CountDownLatchDemo {
-
- public static void main(String[] args) {
- ExecutorService executor = Executors.newCachedThreadPool();
-
- CountDownLatch latch = new CountDownLatch(3);
-
- Worker w1 = new Worker(latch,"張三");
- Worker w2 = new Worker(latch,"李四");
- Worker w3 = new Worker(latch,"王二");
-
- Boss boss = new Boss(latch);
-
- executor.execute(w3);
- executor.execute(w2);
- executor.execute(w1);
- executor.execute(boss);
-
- executor.shutdown();
- }
-
- }
當(dāng)你運(yùn)行CountDownLatchDemo這個(gè)對(duì)象的時(shí)候，你會(huì)發(fā)現(xiàn)是等所有的工人都干完了活，老板才來(lái)檢查，下面是我本地機(jī)器上運(yùn)行的一次結(jié)果，可以肯定的每次運(yùn)行的結(jié)果可能與下面不一樣，但老板檢查永遠(yuǎn)是在后面的。
- 王二正在干活!
- 李四正在干活!
- 老板正在等所有的工人干完活......
- 張三正在干活!
- 張三活干完了！
- 王二活干完了！
- 李四活干完了！
- 工人活都干完了，老板開(kāi)始檢查了！
好了，就寫(xiě)到這里，睡覺(jué)去了！
http://zapldy./blog/746458
另一個(gè)比較簡(jiǎn)單的例子
public static void main(String[] args) throws InterruptedException {
int threadNumber = 10;
final CountDownLatch countDownLatch = new CountDownLatch(threadNumber);
for (int i = 0; i < threadNumber; i++) {
final int threadID = i;
new Thread() {
public void run() {
try {
Thread.sleep((long) (Math.random() * 10000));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(String.format("threadID:[%s] finished!!", threadID));
countDownLatch.countDown();
}
}.start();
}
countDownLatch.await();
System.out.println("main thread finished!!");
}
http://www./topic/581476
