详解Java中CountDownLatch异步转同步工具类

使用场景

由于公司业务需求，需要对接socket、MQTT等消息队列。
众所周知 socket 是双向通信，socket的回复是人为定义的，客户端推送消息给服务端，服务端的回复是两条线。无法像http请求有回复。
下发指令给硬件时，需要校验此次数据下发是否成功。
用户体验而言，点击按钮就要知道此次的下发成功或失败。

在这里插入图片描述

如上图模型，

第一种方案使用Tread.sleep
优点:占用资源小，放弃当前cpu资源
缺点: 回复速度快，休眠时间过长，仍然需要等待休眠结束才能返回，响应速度是固定的，无法及时响应第二种方案使用CountDownLatch

package com.lzy.demo.delay;

import java.util.Map;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.DelayQueue;
import java.util.concurrent.Delayed;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class CountDownLatchPool {

    //countDonw池
    private final static Map<Integer, CountDownLatch> countDownLatchMap = new ConcurrentHashMap<>();
    //延迟队列
    private final static DelayQueue<MessageDelayQueueUtil> delayQueue = new DelayQueue<>();

    private volatile static boolean flag =false;
    //单线程池
    private final static ExecutorService t = new ThreadPoolExecutor(1, 1,
        0L, TimeUnit.MILLISECONDS,
        new ArrayBlockingQueue<>(1));

    public static void addCountDownLatch(Integer messageId) {
        CountDownLatch countDownLatch = countDownLatchMap.putIfAbsent(messageId,new CountDownLatch(1) );
        if(countDownLatch == null){
            countDownLatch = countDownLatchMap.get(messageId);
        }
        try {
            addDelayQueue(messageId);
            countDownLatch.await(3L, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("阻塞等待结束~~~~~~");
    }

    public static void removeCountDownLatch(Integer messageId){
        CountDownLatch countDownLatch = countDownLatchMap.get(messageId);
        if(countDownLatch == null)
            return;
        countDownLatch.countDown();
        countDownLatchMap.remove(messageId);
        System.out.println("清除Map数据"+countDownLatchMap);
    }

    private static void addDelayQueue(Integer messageId){
        delayQueue.add(new MessageDelayQueueUtil(messageId));
        clearMessageId();
    }

    private static void clearMessageId(){
        synchronized (CountDownLatchPool.class){
            if(flag){
                return;
            }
            flag = true;
        }
        t.execute(()->{
            while (delayQueue.size() > 0){
                System.out.println("进入线程并开始执行");
                try {
                    MessageDelayQueueUtil take = delayQueue.take();
                    Integer messageId1 = take.getMessageId();
                    removeCountDownLatch(messageId1);
                    System.out.println("清除队列数据"+messageId1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            flag = false;
            System.out.println("结束end----");
        });
    }

    public static void main(String[] args) throws InterruptedException {
        /*
        测试超时清空map
        new Thread(()->addCountDownLatch(1)).start();
        new Thread(()->addCountDownLatch(2)).start();
        new Thread(()->addCountDownLatch(3)).start();
        */
        //提前创建线程，清空countdown
        new Thread(()->{
            try {
                Thread.sleep(500L);
                removeCountDownLatch(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
        //开始阻塞
        addCountDownLatch(1);
    	//通过调整上面的sleep我们发现阻塞市场取决于countDownLatch.countDown()执行时间
    	System.out.println("阻塞结束----");
    }
}
class MessageDelayQueueUtil implements Delayed {

    private Integer messageId;
    private long avaibleTime;

    public Integer getMessageId() {
        return messageId;
    }

    public void setMessageId(Integer messageId) {
        this.messageId = messageId;
    }

    public long getAvaibleTime() {
        return avaibleTime;
    }

    public void setAvaibleTime(long avaibleTime) {
        this.avaibleTime = avaibleTime;
    }

    public MessageDelayQueueUtil(Integer messageId){
        this.messageId = messageId;
        //avaibleTime = 当前时间+ delayTime
        //重试3次，每次3秒+1秒的延迟
        this.avaibleTime=3000*3+1000 + System.currentTimeMillis();
    }

    @Override
    public long getDelay(TimeUnit unit) {
        long diffTime= avaibleTime- System.currentTimeMillis();
        return unit.convert(diffTime,TimeUnit.MILLISECONDS);
    }

    @Override
    public int compareTo(Delayed o) {
        //compareTo用在DelayedUser的排序
        return (int)(this.avaibleTime - ((MessageDelayQueueUtil) o).getAvaibleTime());
    }
}

由于socket并不确定每次都会有数据返回，所以map的数据会越来越大，最终导致内存溢出
需定时清除map内的无效数据。
可以使用DelayedQuene延迟队列来处理，相当于给对象添加一个过期时间

使用方法 addCountDownLatch 等待消息,异步回调消息清空removeCountDownLatch

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