CountDownLatch在java.util.concurrent.CountDownLatch包里面，他通常用在多线程中，假如说我们现在有一件事情开了多线程去完成，但是我们需要保证所有的线程都成功运行结束，再去执行后面的代码，这个时候CountDownLatch这个类就派上用场了。
我们可以把CountDownLatch传入到每个线程里面去，在线程执行成功时调用countDownLatch.countDown();方法，然后在主线程里面执行完多线程初始化和分发执行之后执行countDownLatch.await();这样，只有在每个线程执行成功以后，也就是CountDownLatch的计数器减到0以后，才会执行countDownLatch.await()后面的代码。
简单的应用场景是这样的，但是具体的怎么设计还是需要仔细斟酌，否则有可能会出现countDownLatch.countDown();永远无法减为0的情况。
下面一个简单的例子来实现这一应用场景：
package org.hzg.countDownLatch;
import java.util.concurrent.CountDownLatch;
/**
* Created by hzgal on 2019-3-5.
*/
public abstract class DangerCenter implements Runnable {
private CountDownLatch countDownLatch; //计数器
private String station; //危险品调度站
private boolean isOkToSentCar; //检查结果，是否满足发车条件
public DangerCenter(CountDownLatch countDownLatch,String stationName) {
this.countDownLatch = countDownLatch;
this.station = stationName;
this.isOkToSentCar = false;
}
@Override
public void run() {
try {
check();
isOkToSentCar = true;
}catch (Exception e) {
e.printStackTrace();
isOkToSentCar = false;
}finally {
if(countDownLatch != null) {
countDownLatch.countDown();
}
}
}
/**
* 定义抽象的check方法，用于实现了DangerCenter的各级调度中心来重写check方法，因为每个调度中心自检的方法可能不一样
*/
public abstract void check();
public CountDownLatch getCountDownLatch() {
return countDownLatch;
}
public void setCountDownLatch(CountDownLatch countDownLatch) {
this.countDownLatch = countDownLatch;
}
public String getStation() {
return station;
}
public void setStation(String station) {
this.station = station;
}
public boolean isOkToSentCar() {
return isOkToSentCar;
}
public void setOkToSentCar(boolean okToSentCar) {
isOkToSentCar = okToSentCar;
}
}
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package org.hzg.countDownLatch;
import java.util.concurrent.CountDownLatch;
/**
* Created by hzgal on 2019-3-5.
*/
public class BeijingStation extends DangerCenter {
private static String stationName = "北京危险品调度站";
public BeijingStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}
@Override
public void check() {
System.out.println("北京调度站正在进行车辆自检..........");
try {
Thread.sleep(2000);
}catch (Exception e) {
e.printStackTrace();
}
System.out.println("北京调度站车辆自检完毕，一切正常");
}
}
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package org.hzg.countDownLatch;
import java.util.concurrent.CountDownLatch;
/**
* Created by hzgal on 2019-3-5.
*/
public class ShandongStation extends DangerCenter {
private static String stationName = "山东危险品调度站";
public ShandongStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}
@Override
public void check() {
System.out.println("山东调度站正在进行车辆自检..........");
try {
Thread.sleep(1000);
}catch (Exception e) {
e.printStackTrace();
}
System.out.println("山东调度站车辆自检完毕，一切正常");
}
}
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package org.hzg.countDownLatch;
import java.util.concurrent.CountDownLatch;
/**
* Created by hzgal on 2019-3-5.
*/
public class ShanxiStation extends DangerCenter {
private static String stationName = "陕西危险品调度站";
public ShanxiStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}
@Override
public void check() {
System.out.println("陕西调度站正在进行车辆自检..........");
try {
Thread.sleep(3000);
}catch (Exception e) {
e.printStackTrace();
}
System.out.println("陕西调度站车辆自检完毕，一切正常");
}
}
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package org.hzg.countDownLatch;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
/**
* Created by hzgal on 2019-3-5.
*/
public class Demo {
public static void main(String[] args) {
boolean isAllOk = true;
List<DangerCenter> stationList = new ArrayList<DangerCenter>();
CountDownLatch countDownLatch = new CountDownLatch(3);
stationList.add(new BeijingStation(countDownLatch));
stationList.add(new ShandongStation(countDownLatch));
stationList.add(new ShanxiStation(countDownLatch));
Executor stationThreadPool = Executors.newFixedThreadPool(3);
for(DangerCenter stationCenter : stationList) {
stationThreadPool.execute(stationCenter);
}
try {
countDownLatch.await();
}catch (Exception e) {
e.printStackTrace();
}
for(DangerCenter stationCenter : stationList) {
if(!stationCenter.isOkToSentCar()) {
isAllOk = false;
}
}
if (isAllOk) {
System.out.println("所有站点检查完毕，可以正常进行发车！！！！！！！！！");
}
}
}
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| 程序运行结果： 北京调度站正在进行车辆自检.......... 山东调度站正在进行车辆自检.......... 陕西调度站正在进行车辆自检.......... 山东调度站车辆自检完毕，一切正常 北京调度站车辆自检完毕，一切正常 陕西调度站车辆自检完毕，一切正常 所有站点检查完毕，可以正常进行发车！！！！！！！！！ |