CountDownLatch了解

發布時間：2020-05-19 08:22:20 來源：網絡閱讀：571 作者：dreamyoungman 欄目：編程語言

CountDownLatch在java.util.concurrent.CountDownLatch包里面，他通常用在多線程中，假如說我們現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行結束，再去執行后面的代碼，這個時候CountDownLatch這個類就派上用場了。

我們可以把CountDownLatch傳入到每個線程里面去，在線程執行成功時調用countDownLatch.countDown();方法，然后在主線程里面執行完多線程初始化和分發執行之后執行countDownLatch.await();這樣，只有在每個線程執行成功以后，也就是CountDownLatch的計數器減到0以后，才會執行countDownLatch.await()后面的代碼。

簡單的應用場景是這樣的，但是具體的怎么設計還是需要仔細斟酌，否則有可能會出現countDownLatch.countDown();永遠無法減為0的情況。

下面一個簡單的例子來實現這一應用場景：

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; //計數器
private String station; //危險品調度站
private boolean isOkToSentCar; //檢查結果，是否滿足發車條件

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，用于實現了DangerCenter的各級調度中心來重寫check方法，因為每個調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 = "北京危險品調度站";

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

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

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

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

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

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

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

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

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

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

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

System.out.println("陜西調度站車輛自檢完畢，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("所有站點檢查完畢，可以正常進行發車！！！！！！！！！");
}
}
}

程序運行結果：

北京調度站正在進行車輛自檢..........

山東調度站正在進行車輛自檢..........

陜西調度站正在進行車輛自檢..........

山東調度站車輛自檢完畢，一切正常

北京調度站車輛自檢完畢，一切正常

陜西調度站車輛自檢完畢，一切正常

所有站點檢查完畢，可以正常進行發車！！！！！！！！！

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