详解Java中CountDownLatch异步转同步工具类

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。

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mapnew Thread(()->addCountDownLatch(1)).start();new Thread(()->addCountDownLatch(2)).start();new Thread(()->addCountDownLatch(3)).start();*///提前创建线程，清空countdownnew 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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