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java高并发系列 - 第16天:JUC中等待多线程完成的工具类CountDownLatch,必备技能

这是java高并发系列第16篇文章。

本篇内容

  1. 介绍CountDownLatch及使用场景

  2. 提供几个示例介绍CountDownLatch的使用

  3. 手写一个并行处理任务的工具类

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;

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:

  1. 创建CountDownLatch对象

  2. 调用其实例方法 await(),让当前线程等待

  3. 调用 countDown()方法,让计数器减1

  4. 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;

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;

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;

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;

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。


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