CountDownLatch了解

CountDownLatch在java.util.concurrent.CountDownLatch包里面，他通常用在多線程中，假如說我們現(xiàn)在有一件事情開了多線程去完成，但是我們需要保證所有的線程都成功運(yùn)行結(jié)束，再去執(zhí)行后面的代碼，這個(gè)時(shí)候CountDownLatch這個(gè)類就派上用場(chǎng)了。

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我們可以把CountDownLatch傳入到每個(gè)線程里面去，在線程執(zhí)行成功時(shí)調(diào)用countDownLatch.countDown();方法，然后在主線程里面執(zhí)行完多線程初始化和分發(fā)執(zhí)行之后執(zhí)行countDownLatch.await();這樣，只有在每個(gè)線程執(zhí)行成功以后，也就是CountDownLatch的計(jì)數(shù)器減到0以后，才會(huì)執(zhí)行countDownLatch.await()后面的代碼。

簡(jiǎn)單的應(yīng)用場(chǎng)景是這樣的，但是具體的怎么設(shè)計(jì)還是需要仔細(xì)斟酌，否則有可能會(huì)出現(xiàn)countDownLatch.countDown();永遠(yuǎn)無法減為0的情況。

下面一個(gè)簡(jiǎn)單的例子來實(shí)現(xiàn)這一應(yīng)用場(chǎng)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; //計(jì)數(shù)器
private String station; //危險(xiǎn)品調(diào)度站
private boolean isOkToSentCar; //檢查結(jié)果，是否滿足發(fā)車條件

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();
}
}
}

/**
* 定義抽象的check方法，用于實(shí)現(xiàn)了DangerCenter的各級(jí)調(diào)度中心來重寫check方法，因?yàn)槊總€(gè)調(diào)度中心自檢的方法可能不一樣
*/
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 = "北京危險(xiǎn)品調(diào)度站";

public BeijingStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}

@Override
public void check() {
System.out.println("北京調(diào)度站正在進(jìn)行車輛自檢..........");

try {
Thread.sleep(2000);
}catch (Exception e) {
e.printStackTrace();
}

System.out.println("北京調(diào)度站車輛自檢完畢，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 = "山東危險(xiǎn)品調(diào)度站";

public ShandongStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}

@Override
public void check() {
System.out.println("山東調(diào)度站正在進(jìn)行車輛自檢..........");

try {
Thread.sleep(1000);
}catch (Exception e) {
e.printStackTrace();
}

System.out.println("山東調(diào)度站車輛自檢完畢，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 = "陜西危險(xiǎn)品調(diào)度站";

public ShanxiStation(CountDownLatch countDownLatch) {
super(countDownLatch,stationName);
}

@Override
public void check() {
System.out.println("陜西調(diào)度站正在進(jìn)行車輛自檢..........");

try {
Thread.sleep(3000);
}catch (Exception e) {
e.printStackTrace();
}

System.out.println("陜西調(diào)度站車輛自檢完畢，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<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("所有站點(diǎn)檢查完畢，可以正常進(jìn)行發(fā)車?。。。。。。。?！");
}
}
}

程序運(yùn)行結(jié)果：

北京調(diào)度站正在進(jìn)行車輛自檢..........

山東調(diào)度站正在進(jìn)行車輛自檢..........

陜西調(diào)度站正在進(jìn)行車輛自檢..........

山東調(diào)度站車輛自檢完畢，一切正常

北京調(diào)度站車輛自檢完畢，一切正常

陜西調(diào)度站車輛自檢完畢，一切正常

所有站點(diǎn)檢查完畢，可以正常進(jìn)行發(fā)車?。。。。。。。。?/p>

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