delayqueue

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：

1. 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
2. 对于能够在120分钟内交卷的考生，如何实现这些考生交卷
3. 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
4. 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   [e-mail] */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：

1. 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
2. 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
3. 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 [e-mail] */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;    }    }