在學習Java 多執行緒併發開發過程中，瞭解到DelayQueue類的主要作用：是一個無界的BlockingQueue，用於放置實現了Delayed介面的物件，其中的物件只能在其到期時才能從佇列中取走。這種佇列是有序的，即隊頭物件的延遲到期時間最長。注意：不能將null元素放置到這種佇列中。

Delayed，一種混合風格的介面，用來標記那些應該在給定延遲時間之後執行的物件。此介面的實現必須定義一個 compareTo 方法，該方法提供與此介面的 getDelay 方法一致的排序。

在網上也看到兩個示例，但這兩個示例個人在實際執行時均沒有達到滿足業務場景的效果，因而對其進行了修改，供大家參考討論。

業務場景一：多考生考試

這個場景中幾個點需要注意：

1. 考試時間為120分鐘，30分鐘後才可交卷，初始化考生完成試卷時間最小應為30分鐘
2. 對於能夠在120分鐘內交卷的考生，如何實現這些考生交卷
3. 對於120分鐘內沒有完成考試的考生，在120分鐘考試時間到後需要讓他們強制交卷
4. 在所有的考生都交完卷後，需要將控制執行緒關閉

實現思想：用DelayQueue儲存考生（Student類），每一個考生都有自己的名字和完成試卷的時間，Teacher執行緒對DelayQueue進行監控，收取完成試卷小於120分鐘的學生的試卷。當考試時間120分鐘到時，先關閉Teacher執行緒，然後強制DelayQueue中還存在的考生交卷。每一個考生交卷都會進行一次countDownLatch.countDown()，當countDownLatch.await()不再阻塞說明所有考生都交完捲了，而後結束考試。

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 孫振超   [email protected]
 */

public class Exam {

    /**
     *
     *2014-1-10 下午9:43:48 by 孫振超
     *
     *@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(" 考試時間到，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, 希望用時" + workTime + "分鐘"+" ,實際用時 120分鐘" );
        }else {
            System.out.println(name + " 交卷, 希望用時" + workTime + "分鐘"+" ,實際用時 "+workTime +" 分鐘");  
        }
        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("強制收卷", 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();
        }
    }
    
}

業務場景二：具有過期時間的快取

這個場景中幾個點需要注意：

1. 當向快取中新增key-value對時，如果這個key在快取中存在並且還沒有過期，需要用這個key對應的新過期時間
2. 為了能夠讓DelayQueue將其已儲存的key刪除，需要重寫實現Delayed介面新增到DelayQueue的DelayedItem的hashCode函式和equals函式
3. 當快取關閉，監控程式也應關閉，因而監控執行緒應當用守護執行緒

具體實現如下：

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 [email protected]
 */
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:孫振超
     * @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;
    }
    
}