java 之DelayQueue实际运用示例
在学习Java 多线程并发开发过程中，了解到DelayQueue类的主要作用：是一个无界的BlockingQueue，用于放置实现了Delayed接口的对象，其中的对象只能在其到期时才能从队列中取走。这种队列是有序的，即队头对象的延迟到期时间最长。注意：不能将null元素放置到这种队列中。
Delayed，一种混合风格的接口，用来标记那些应该在给定延迟时间之后执行的对象。此接口的实现必须定义一个 compareTo 方法，该方法提供与此接口的 getDelay 方法一致的排序。
在网上也看到两个示例，但这两个示例个人在实际运行时均没有达到满足业务场景的效果，因而对其进行了修改，供大家参考讨论。
业务场景一：多考生考试
该场景来自于http://ideasforjava.iteye.com/blog/657384，模拟一个考试的日子，考试时间为120分钟，30分钟后才可交卷，当时间到了，或学生都交完卷了考试结束。
这个场景中几个点需要注意：
- 考试时间为120分钟，30分钟后才可交卷，初始化考生完成试卷时间最小应为30分钟
- 对于能够在120分钟内交卷的考生，如何实现这些考生交卷
- 对于120分钟内没有完成考试的考生，在120分钟考试时间到后需要让他们强制交卷
- 在所有的考生都交完卷后，需要将控制线程关闭
实现思想：用DelayQueue存储考生（Student类），每一个考生都有自己的名字和完成试卷的时间，Teacher线程对DelayQueue进行监控，收取完成试卷小于120分钟的学生的试卷。当考试时间120分钟到时，先关闭Teacher线程，然后强制DelayQueue中还存在的考生交卷。每一个考生交卷都会进行一次countDownLatch.countDown()，当countDownLatch.await()不再阻塞说明所有考生都交完卷了，而后结束考试。
package com.my.base.concurrent.delayQueue; import java.util.Iterator; import java.util.Random; import java.util.concurrent.CountDownLatch; import java.util.concurrent.DelayQueue; import java.util.concurrent.Delayed; import java.util.concurrent.TimeUnit; /** *this project is created for my partactice. *In the project I will write the mybatis by myself * *2014-1-10 下午9:43:48 *@author 孙振超 mychaoyue2011@163.com */ public class Exam { /** * *2014-1-10 下午9:43:48 by 孙振超 * *@param args *void * @throws InterruptedException */ public static void main(String[] args) throws InterruptedException { // TODO Auto-generated method stub int studentNumber = 20; CountDownLatch countDownLatch = new CountDownLatch(studentNumber+1); DelayQueue< Student> students = new DelayQueue<Student>(); Random random = new Random(); for (int i = 0; i < studentNumber; i++) { students.put(new Student("student"+(i+1), 30+random.nextInt(120),countDownLatch)); } Thread teacherThread =new Thread(new Teacher(students)); students.put(new EndExam(students, 120,countDownLatch,teacherThread)); teacherThread.start(); countDownLatch.await(); System.out.println(" 考试时间到，全部交卷！"); } } class Student implements Runnable,Delayed{ private String name; private long workTime; private long submitTime; private boolean isForce = false; private CountDownLatch countDownLatch; public Student(){} public Student(String name,long workTime,CountDownLatch countDownLatch){ this.name = name; this.workTime = workTime; this.submitTime = TimeUnit.NANOSECONDS.convert(workTime, TimeUnit.NANOSECONDS)+System.nanoTime(); this.countDownLatch = countDownLatch; } @Override public int compareTo(Delayed o) { // TODO Auto-generated method stub if(o == null || ! (o instanceof Student)) return 1; if(o == this) return 0; Student s = (Student)o; if (this.workTime > s.workTime) { return 1; }else if (this.workTime == s.workTime) { return 0; }else { return -1; } } @Override public long getDelay(TimeUnit unit) { // TODO Auto-generated method stub return unit.convert(submitTime - System.nanoTime(), TimeUnit.NANOSECONDS); } @Override public void run() { // TODO Auto-generated method stub if (isForce) { System.out.println(name + " 交卷, 希望用时" + workTime + "分钟"+" ,实际用时 120分钟" ); }else { System.out.println(name + " 交卷, 希望用时" + workTime + "分钟"+" ,实际用时 "+workTime +" 分钟"); } countDownLatch.countDown(); } public boolean isForce() { return isForce; } public void setForce(boolean isForce) { this.isForce = isForce; } } class EndExam extends Student{ private DelayQueue<Student> students; private CountDownLatch countDownLatch; private Thread teacherThread; public EndExam(DelayQueue<Student> students, long workTime, CountDownLatch countDownLatch,Thread teacherThread) { super("强制收卷", workTime,countDownLatch); this.students = students; this.countDownLatch = countDownLatch; this.teacherThread = teacherThread; } @Override public void run() { // TODO Auto-generated method stub teacherThread.interrupt(); Student tmpStudent; for (Iterator<Student> iterator2 = students.iterator(); iterator2.hasNext();) { tmpStudent = iterator2.next(); tmpStudent.setForce(true); tmpStudent.run(); } countDownLatch.countDown(); } } class Teacher implements Runnable{ private DelayQueue<Student> students; public Teacher(DelayQueue<Student> students){ this.students = students; } @Override public void run() { // TODO Auto-generated method stub try { System.out.println(" test start"); while(!Thread.interrupted()){ students.take().run(); } } catch (Exception e) { // TODO: handle exception e.printStackTrace(); } } }
业务场景二：具有过期时间的缓存
该场景来自于http://www.cnblogs.com/jobs/archive/2007/04/27/730255.html，向缓存添加内容时，给每一个key设定过期时间，系统自动将超过过期时间的key清除。
这个场景中几个点需要注意：
- 当向缓存中添加key-value对时，如果这个key在缓存中存在并且还没有过期，需要用这个key对应的新过期时间
- 为了能够让DelayQueue将其已保存的key删除，需要重写实现Delayed接口添加到DelayQueue的DelayedItem的hashCode函数和equals函数
- 当缓存关闭，监控程序也应关闭，因而监控线程应当用守护线程
具体实现如下：
package com.my.base.concurrent.delayQueue; import java.util.Random; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.DelayQueue; import java.util.concurrent.Delayed; import java.util.concurrent.TimeUnit; /** *Cache.java * * Created on 2014-1-11 上午11:30:36 by sunzhenchao mychaoyue2011@163.com */ public class Cache<K, V> { public ConcurrentHashMap<K, V> map = new ConcurrentHashMap<K, V>(); public DelayQueue<DelayedItem<K>> queue = new DelayQueue<DelayedItem<K>>(); public void put(K k,V v,long liveTime){ V v2 = map.put(k, v); DelayedItem<K> tmpItem = new DelayedItem<K>(k, liveTime); if (v2 != null) { queue.remove(tmpItem); } queue.put(tmpItem); } public Cache(){ Thread t = new Thread(){ @Override public void run(){ dameonCheckOverdueKey(); } }; t.setDaemon(true); t.start(); } public void dameonCheckOverdueKey(){ while (true) { DelayedItem<K> delayedItem = queue.poll(); if (delayedItem != null) { map.remove(delayedItem.getT()); System.out.println(System.nanoTime()+" remove "+delayedItem.getT() +" from cache"); } try { Thread.sleep(300); } catch (Exception e) { // TODO: handle exception } } } /** * TODO * @param args * 2014-1-11 上午11:30:36 * @author:孙振超 * @throws InterruptedException */ public static void main(String[] args) throws InterruptedException { Random random = new Random(); int cacheNumber = 10; int liveTime = 0; Cache<String, Integer> cache = new Cache<String, Integer>(); for (int i = 0; i < cacheNumber; i++) { liveTime = random.nextInt(3000); System.out.println(i+" "+liveTime); cache.put(i+"", i, random.nextInt(liveTime)); if (random.nextInt(cacheNumber) > 7) { liveTime = random.nextInt(3000); System.out.println(i+" "+liveTime); cache.put(i+"", i, random.nextInt(liveTime)); } } Thread.sleep(3000); System.out.println(); } } class DelayedItem<T> implements Delayed{ private T t; private long liveTime ; private long removeTime; public DelayedItem(T t,long liveTime){ this.setT(t); this.liveTime = liveTime; this.removeTime = TimeUnit.NANOSECONDS.convert(liveTime, TimeUnit.NANOSECONDS) + System.nanoTime(); } @Override public int compareTo(Delayed o) { if (o == null) return 1; if (o == this) return 0; if (o instanceof DelayedItem){ DelayedItem<T> tmpDelayedItem = (DelayedItem<T>)o; if (liveTime > tmpDelayedItem.liveTime ) { return 1; }else if (liveTime == tmpDelayedItem.liveTime) { return 0; }else { return -1; } } long diff = getDelay(TimeUnit.NANOSECONDS) - o.getDelay(TimeUnit.NANOSECONDS); return diff > 0 ? 1:diff == 0? 0:-1; } @Override public long getDelay(TimeUnit unit) { return unit.convert(removeTime - System.nanoTime(), unit); } public T getT() { return t; } public void setT(T t) { this.t = t; } @Override public int hashCode(){ return t.hashCode(); } @Override public boolean equals(Object object){ if (object instanceof DelayedItem) { return object.hashCode() == hashCode() ?true:false; } return false; } }
