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CountDownLatch了解

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。

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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;
}
}

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");
}
}

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");
}
}

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");
}
}

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 stationList = new ArrayList();
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！！！！！！！！！");
}
}
}

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！！！！！！！！！

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