Java高并发专题之16、JUC中的CountDownLatch
本篇内容
- 介绍CountDownLatch及使用场景
- 提供几个使用示例介绍CountDownLatch的使用
- 手写一个并行处理任务的工具类
思考一个需求
假如有这样一个需求,当我们需要解析一个Excel里多个sheet的数据时,可以考虑使用多线程,每个线程解析一个sheet里的数据,等到所有的sheet都解析完之后,程序需要统计解析总耗时。分析一下:解析每个sheet耗时可能不一样,总耗时就是最长耗时的那个操作。
我们能够想到的最简单的做法是使用join,代码如下:
package com.itsoku.chat13;
import java.util.concurrent.TimeUnit;
/**
* 微信公众号:javacode2018,获取年薪50万课程
*/
public class Demo1 {
public static class T extends Thread {
//休眠时间(秒)
int sleepSeconds;
public T(String name, int sleepSeconds) {
super(name);
this.sleepSeconds = sleepSeconds;
}
@Override
public void run() {
Thread ct = Thread.currentThread();
long startTime = System.currentTimeMillis();
System.out.println(startTime + "," + ct.getName() + ",开始处理!");
try {
//模拟耗时操作,休眠sleepSeconds秒
TimeUnit.SECONDS.sleep(this.sleepSeconds);
} catch (InterruptedException e) {
e.printStackTrace();
}
long endTime = System.currentTimeMillis();
System.out.println(endTime + "," + ct.getName() + ",处理完毕,耗时:" + (endTime - startTime));
}
}
public static void main(String[] args) throws InterruptedException {
long starTime = System.currentTimeMillis();
T t1 = new T("解析sheet1线程", 2);
t1.start();
T t2 = new T("解析sheet2线程", 5);
t2.start();
t1.join();
t2.join();
long endTime = System.currentTimeMillis();
System.out.println("总耗时:" + (endTime - starTime));
}
}
输出:
1563767560271,解析sheet1线程,开始处理!
1563767560272,解析sheet2线程,开始处理!
1563767562273,解析sheet1线程,处理完毕,耗时:2002
1563767565274,解析sheet2线程,处理完毕,耗时:5002
总耗时:5005
代码中启动了2个解析sheet的线程,第一个耗时2秒,第二个耗时5秒,最终结果中总耗时:5秒。上面的关键技术点是线程的join()
方法,此方法会让当前线程等待被调用的线程完成之后才能继续。可以看一下join的源码,内部其实是在synchronized方法中调用了线程的wait方法,最后被调用的线程执行完毕之后,由jvm自动调用其notifyAll()方法,唤醒所有等待中的线程。这个notifyAll()方法是由jvm内部自动调用的,jdk源码中是看不到的,需要看jvm源码,有兴趣的同学可以去查一下。所以JDK不推荐在线程上调用wait、notify、notifyAll方法。
而在JDK1.5之后的并发包中提供的CountDownLatch也可以实现join的这个功能。
CountDownLatch介绍
CountDownLatch称之为闭锁,它可以使一个或一批线程在闭锁上等待,等到其他线程执行完相应操作后,闭锁打开,这些等待的线程才可以继续执行。确切的说,闭锁在内部维护了一个倒计数器。通过该计数器的值来决定闭锁的状态,从而决定是否允许等待的线程继续执行。
常用方法:
public CountDownLatch(int count):构造方法,count表示计数器的值,不能小于0,否者会报异常。
public void await() throws InterruptedException:调用await()会让当前线程等待,直到计数器为0的时候,方法才会返回,此方法会响应线程中断操作。
public boolean await(long timeout, TimeUnit unit) throws InterruptedException:限时等待,在超时之前,计数器变为了0,方法返回true,否者直到超时,返回false,此方法会响应线程中断操作。
public void countDown():让计数器减1
CountDownLatch使用步骤:
- 创建CountDownLatch对象
- 调用其实例方法
await()
,让当前线程等待 - 调用
countDown()
方法,让计数器减1 - 当计数器变为0的时候,
await()
方法会返回
示例1:一个简单的示例
我们使用CountDownLatch来完成上面示例中使用join实现的功能,代码如下:
package com.itsoku.chat13;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* 微信公众号:javacode2018,获取年薪50万课程
*/
public class Demo2 {
public static class T extends Thread {
//休眠时间(秒)
int sleepSeconds;
CountDownLatch countDownLatch;
public T(String name, int sleepSeconds, CountDownLatch countDownLatch) {
super(name);
this.sleepSeconds = sleepSeconds;
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
Thread ct = Thread.currentThread();
long startTime = System.currentTimeMillis();
System.out.println(startTime + "," + ct.getName() + ",开始处理!");
try {
//模拟耗时操作,休眠sleepSeconds秒
TimeUnit.SECONDS.sleep(this.sleepSeconds);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
countDownLatch.countDown();
}
long endTime = System.currentTimeMillis();
System.out.println(endTime + "," + ct.getName() + ",处理完毕,耗时:" + (endTime - startTime));
}
}
public static void main(String[] args) throws InterruptedException {
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "线程 start!");
CountDownLatch countDownLatch = new CountDownLatch(2);
long starTime = System.currentTimeMillis();
T t1 = new T("解析sheet1线程", 2, countDownLatch);
t1.start();
T t2 = new T("解析sheet2线程", 5, countDownLatch);
t2.start();
countDownLatch.await();
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "线程 end!");
long endTime = System.currentTimeMillis();
System.out.println("总耗时:" + (endTime - starTime));
}
}
输出:
1563767580511,main线程 start!
1563767580513,解析sheet1线程,开始处理!
1563767580513,解析sheet2线程,开始处理!
1563767582515,解析sheet1线程,处理完毕,耗时:2002
1563767585515,解析sheet2线程,处理完毕,耗时:5002
1563767585515,main线程 end!
总耗时:5003
从结果中看出,效果和join实现的效果一样,代码中创建了计数器为2的CountDownLatch
,主线程中调用countDownLatch.await();
会让主线程等待,t1、t2线程中模拟执行耗时操作,最终在finally中调用了countDownLatch.countDown();
,此方法每调用一次,CountDownLatch内部计数器会减1,当计数器变为0的时候,主线程中的await()会返回,然后继续执行。注意:上面的countDown()
这个是必须要执行的方法,所以放在finally中执行。
示例2:等待指定的时间
还是上面的示例,2个线程解析2个sheet,主线程等待2个sheet解析完成。主线程说,我等待2秒,你们还是无法处理完成,就不等待了,直接返回。如下代码:
package com.itsoku.chat13;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* 微信公众号:javacode2018,获取年薪50万课程
*/
public class Demo3 {
public static class T extends Thread {
//休眠时间(秒)
int sleepSeconds;
CountDownLatch countDownLatch;
public T(String name, int sleepSeconds, CountDownLatch countDownLatch) {
super(name);
this.sleepSeconds = sleepSeconds;
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
Thread ct = Thread.currentThread();
long startTime = System.currentTimeMillis();
System.out.println(startTime + "," + ct.getName() + ",开始处理!");
try {
//模拟耗时操作,休眠sleepSeconds秒
TimeUnit.SECONDS.sleep(this.sleepSeconds);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
countDownLatch.countDown();
}
long endTime = System.currentTimeMillis();
System.out.println(endTime + "," + ct.getName() + ",处理完毕,耗时:" + (endTime - startTime));
}
}
public static void main(String[] args) throws InterruptedException {
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "线程 start!");
CountDownLatch countDownLatch = new CountDownLatch(2);
long starTime = System.currentTimeMillis();
T t1 = new T("解析sheet1线程", 2, countDownLatch);
t1.start();
T t2 = new T("解析sheet2线程", 5, countDownLatch);
t2.start();
boolean result = countDownLatch.await(2, TimeUnit.SECONDS);
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "线程 end!");
long endTime = System.currentTimeMillis();
System.out.println("主线程耗时:" + (endTime - starTime) + ",result:" + result);
}
}
输出:
1563767637316,main线程 start!
1563767637320,解析sheet1线程,开始处理!
1563767637320,解析sheet2线程,开始处理!
1563767639321,解析sheet1线程,处理完毕,耗时:2001
1563767639322,main线程 end!
主线程耗时:2004,result:false
1563767642322,解析sheet2线程,处理完毕,耗时:5002
从输出结果中可以看出,线程2耗时了5秒,主线程耗时了2秒,主线程中调用countDownLatch.await(2, TimeUnit.SECONDS);
,表示最多等2秒,不管计数器是否为0,await方法都会返回,若等待时间内,计数器变为0了,立即返回true,否则超时后返回false。
示例3:2个CountDown结合使用的示例
有3个人参见跑步比赛,需要先等指令员发指令枪后才能开跑,所有人都跑完之后,指令员喊一声,大家跑完了。
示例代码:
package com.itsoku.chat13;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* 微信公众号:javacode2018,获取年薪50万课程
*/
public class Demo4 {
public static class T extends Thread {
//跑步耗时(秒)
int runCostSeconds;
CountDownLatch commanderCd;
CountDownLatch countDown;
public T(String name, int runCostSeconds, CountDownLatch commanderCd, CountDownLatch countDown) {
super(name);
this.runCostSeconds = runCostSeconds;
this.commanderCd = commanderCd;
this.countDown = countDown;
}
@Override
public void run() {
//等待指令员枪响
try {
commanderCd.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
Thread ct = Thread.currentThread();
long startTime = System.currentTimeMillis();
System.out.println(startTime + "," + ct.getName() + ",开始跑!");
try {
//模拟耗时操作,休眠runCostSeconds秒
TimeUnit.SECONDS.sleep(this.runCostSeconds);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
countDown.countDown();
}
long endTime = System.currentTimeMillis();
System.out.println(endTime + "," + ct.getName() + ",跑步结束,耗时:" + (endTime - startTime));
}
}
public static void main(String[] args) throws InterruptedException {
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "线程 start!");
CountDownLatch commanderCd = new CountDownLatch(1);
CountDownLatch countDownLatch = new CountDownLatch(3);
long starTime = System.currentTimeMillis();
T t1 = new T("小张", 2, commanderCd, countDownLatch);
t1.start();
T t2 = new T("小李", 5, commanderCd, countDownLatch);
t2.start();
T t3 = new T("路人甲", 10, commanderCd, countDownLatch);
t3.start();
//主线程休眠5秒,模拟指令员准备发枪耗时操作
TimeUnit.SECONDS.sleep(5);
System.out.println(System.currentTimeMillis() + ",枪响了,大家开始跑");
commanderCd.countDown();
countDownLatch.await();
long endTime = System.currentTimeMillis();
System.out.println(System.currentTimeMillis() + "," + Thread.currentThread().getName() + "所有人跑完了,主线程耗时:" + (endTime - starTime));
}
}
输出:
1563767691087,main线程 start!
1563767696092,枪响了,大家开始跑
1563767696092,小张,开始跑!
1563767696092,小李,开始跑!
1563767696092,路人甲,开始跑!
1563767698093,小张,跑步结束,耗时:2001
1563767701093,小李,跑步结束,耗时:5001
1563767706093,路人甲,跑步结束,耗时:10001
1563767706093,main所有人跑完了,主线程耗时:15004
代码中,t1、t2、t3启动之后,都阻塞在commanderCd.await();
,主线程模拟发枪准备操作耗时5秒,然后调用commanderCd.countDown();
模拟发枪操作,此方法被调用以后,阻塞在commanderCd.await();
的3个线程会向下执行。主线程调用countDownLatch.await();
之后进行等待,每个人跑完之后,调用countDown.countDown();
通知一下countDownLatch
让计数器减1,最后3个人都跑完了,主线程从countDownLatch.await();
返回继续向下执行。
手写一个并行处理任务的工具类
package com.itsoku.chat13;
import org.springframework.util.CollectionUtils;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.function.Consumer;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
* 微信公众号:javacode2018,获取年薪50万课程
*/
public class TaskDisposeUtils {
//并行线程数
public static final int POOL_SIZE;
static {
POOL_SIZE = Integer.max(Runtime.getRuntime().availableProcessors(), 5);
}
/**
* 并行处理,并等待结束
*
* @param taskList 任务列表
* @param consumer 消费者
* @param <T>
* @throws InterruptedException
*/
public static <T> void dispose(List<T> taskList, Consumer<T> consumer) throws InterruptedException {
dispose(true, POOL_SIZE, taskList, consumer);
}
/**
* 并行处理,并等待结束
*
* @param moreThread 是否多线程执行
* @param poolSize 线程池大小
* @param taskList 任务列表
* @param consumer 消费者
* @param <T>
* @throws InterruptedException
*/
public static <T> void dispose(boolean moreThread, int poolSize, List<T> taskList, Consumer<T> consumer) throws InterruptedException {
if (CollectionUtils.isEmpty(taskList)) {
return;
}
if (moreThread && poolSize > 1) {
poolSize = Math.min(poolSize, taskList.size());
ExecutorService executorService = null;
try {
executorService = Executors.newFixedThreadPool(poolSize);
CountDownLatch countDownLatch = new CountDownLatch(taskList.size());
for (T item : taskList) {
executorService.execute(() -> {
try {
consumer.accept(item);
} finally {
countDownLatch.countDown();
}
});
}
countDownLatch.await();
} finally {
if (executorService != null) {
executorService.shutdown();
}
}
} else {
for (T item : taskList) {
consumer.accept(item);
}
}
}
public static void main(String[] args) throws InterruptedException {
//生成1-10的10个数字,放在list中,相当于10个任务
List<Integer> list = Stream.iterate(1, a -> a + 1).limit(10).collect(Collectors.toList());
//启动多线程处理list中的数据,每个任务休眠时间为list中的数值
TaskDisposeUtils.dispose(list, item -> {
try {
long startTime = System.currentTimeMillis();
TimeUnit.SECONDS.sleep(item);
long endTime = System.currentTimeMillis();
System.out.println(System.currentTimeMillis() + ",任务" + item + "执行完毕,耗时:" + (endTime - startTime));
} catch (InterruptedException e) {
e.printStackTrace();
}
});
//上面所有任务处理完毕完毕之后,程序才能继续
System.out.println(list + "中的任务都处理完毕!");
}
}
运行代码输出:
1563769828130,任务1执行完毕,耗时:1000
1563769829130,任务2执行完毕,耗时:2000
1563769830131,任务3执行完毕,耗时:3001
1563769831131,任务4执行完毕,耗时:4001
1563769832131,任务5执行完毕,耗时:5001
1563769833130,任务6执行完毕,耗时:6000
1563769834131,任务7执行完毕,耗时:7001
1563769835131,任务8执行完毕,耗时:8001
1563769837131,任务9执行完毕,耗时:9001
1563769839131,任务10执行完毕,耗时:10001
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]中的任务都处理完毕!
TaskDisposeUtils是一个并行处理的工具类,可以传入n个任务内部使用线程池进行处理,等待所有任务都处理完成之后,方法才会返回。比如我们发送短信,系统中有1万条短信,我们使用上面的工具,每次取100条并行发送,待100个都处理完毕之后,再取一批按照同样的逻辑发送。
来源:http://itsoku.com/course/1/16