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。
packagecom.my.base.concurrent.delayQueue;importjava.util.Iterator;importjava.util.Random;importjava.util.concurrent.CountDownLatch;importjava.util.concurrent.DelayQueue;importjava.util.concurrent.Delayed;importjava.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 [email protected]*/publicclassExam {/***
*2014-1-10 下午9:43:48 by 孙振超
*
*@paramargs
*void
*@throwsInterruptedException*/publicstaticvoidmain(String[] args)throwsInterruptedException {//TODO Auto-generated method stubintstudentNumber = 20;
CountDownLatch countDownLatch=newCountDownLatch(studentNumber+1);
DelayQueue< Student> students =newDelayQueue();
Random random=newRandom();for(inti = 0; i < studentNumber; i++) {
students.put(newStudent("student"+(i+1), 30+random.nextInt(120),countDownLatch));
}
Thread teacherThread=newThread(newTeacher(students));
students.put(newEndExam(students, 120,countDownLatch,teacherThread));
teacherThread.start();
countDownLatch.await();
System.out.println(" 考试时间到，全部交卷！");
}
}classStudentimplementsRunnable,Delayed{privateString name;privatelongworkTime;privatelongsubmitTime;privatebooleanisForce =false;privateCountDownLatch countDownLatch;publicStudent(){}publicStudent(String name,longworkTime,CountDownLatch countDownLatch){this.name =name;this.workTime =workTime;this.submitTime = TimeUnit.NANOSECONDS.convert(workTime, TimeUnit.NANOSECONDS)+System.nanoTime();this.countDownLatch =countDownLatch;
}
@OverridepublicintcompareTo(Delayed o) {//TODO Auto-generated method stubif(o ==null|| ! (oinstanceofStudent))return1;if(o ==this)return0;
Student s=(Student)o;if(this.workTime >s.workTime) {return1;
}elseif(this.workTime ==s.workTime) {return0;
}else{return-1;
}
}
@OverridepubliclonggetDelay(TimeUnit unit) {//TODO Auto-generated method stubreturnunit.convert(submitTime -System.nanoTime(), TimeUnit.NANOSECONDS);
}
@Overridepublicvoidrun() {//TODO Auto-generated method stubif(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();
}publicbooleanisForce() {returnisForce;
}publicvoidsetForce(booleanisForce) {this.isForce =isForce;
}
}classEndExamextendsStudent{privateDelayQueuestudents;privateCountDownLatch countDownLatch;privateThread teacherThread;publicEndExam(DelayQueue students,longworkTime, CountDownLatch countDownLatch,Thread teacherThread) {super("强制收卷", workTime,countDownLatch);this.students =students;this.countDownLatch =countDownLatch;this.teacherThread =teacherThread;
}
@Overridepublicvoidrun() {//TODO Auto-generated method stubteacherThread.interrupt();
Student tmpStudent;for(Iterator iterator2 =students.iterator(); iterator2.hasNext();) {
tmpStudent=iterator2.next();
tmpStudent.setForce(true);
tmpStudent.run();
}
countDownLatch.countDown();
}
}classTeacherimplementsRunnable{privateDelayQueuestudents;publicTeacher(DelayQueuestudents){this.students =students;
}
@Overridepublicvoidrun() {//TODO Auto-generated method stubtry{
System.out.println(" test start");while(!Thread.interrupted()){
students.take().run();
}
}catch(Exception e) {//TODO: handle exceptione.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：
packagecom.my.base.concurrent.delayQueue;importjava.util.Random;importjava.util.concurrent.ConcurrentHashMap;importjava.util.concurrent.DelayQueue;importjava.util.concurrent.Delayed;importjava.util.concurrent.TimeUnit;/***Cache.java
*
* Created on 2014-1-11 上午11:30:36 by sunzhenchao [email protected]*/publicclassCache{publicConcurrentHashMap map =newConcurrentHashMap();publicDelayQueue> queue =newDelayQueue>();publicvoidput(K k,V v,longliveTime){
V v2=map.put(k, v);
DelayedItem tmpItem =newDelayedItem(k, liveTime);if(v2 !=null) {
queue.remove(tmpItem);
}
queue.put(tmpItem);
}publicCache(){
Thread t=newThread(){
@Overridepublicvoidrun(){
dameonCheckOverdueKey();
}
};
t.setDaemon(true);
t.start();
}publicvoiddameonCheckOverdueKey(){while(true) {
DelayedItem 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
*@paramargs
* 2014-1-11 上午11:30:36
*@author:孙振超
*@throwsInterruptedException*/publicstaticvoidmain(String[] args)throwsInterruptedException {
Random random=newRandom();intcacheNumber = 10;intliveTime = 0;
Cache cache =newCache();for(inti = 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();
}
}classDelayedItemimplementsDelayed{privateT t;privatelongliveTime ;privatelongremoveTime;publicDelayedItem(T t,longliveTime){this.setT(t);this.liveTime =liveTime;this.removeTime = TimeUnit.NANOSECONDS.convert(liveTime, TimeUnit.NANOSECONDS) +System.nanoTime();
}
@OverridepublicintcompareTo(Delayed o) {if(o ==null)return1;if(o ==this)return0;if(oinstanceofDelayedItem){
DelayedItem tmpDelayedItem = (DelayedItem)o;if(liveTime >tmpDelayedItem.liveTime ) {return1;
}elseif(liveTime ==tmpDelayedItem.liveTime) {return0;
}else{return-1;
}
}longdiff = getDelay(TimeUnit.NANOSECONDS) -o.getDelay(TimeUnit.NANOSECONDS);returndiff > 0 ? 1:diff == 0? 0:-1;
}
@OverridepubliclonggetDelay(TimeUnit unit) {returnunit.convert(removeTime -System.nanoTime(), unit);
}publicT getT() {returnt;
}publicvoidsetT(T t) {this.t =t;
}
@OverridepublicinthashCode(){returnt.hashCode();
}
@Overridepublicbooleanequals(Object object){if(objectinstanceofDelayedItem) {returnobject.hashCode() == hashCode() ?true:false;
}returnfalse;
}
}
