在谈到DelayQueue的使用和原理的时候，我们首先介绍一下DelayQueue，DelayQueue是一个无界阻塞队列，只有在延迟期满时才能从中提取元素。该队列的头部是延迟期满后保存时间最长的Delayed 元素。
DelayQueue阻塞队列在我们系统开发中也常常会用到，例如：缓存系统的设计，缓存中的对象，超过了空闲时间，需要从缓存中移出；任务调度系统，能够准确的把握任务的执行时间。我们可能需要通过线程处理很多时间上要求很严格的数据，如果使用普通的线程，我们就需要遍历所有的对象，一个一个的检 查看数据是否过期等，首先这样在执行上的效率不会太高，其次就是这种设计的风格也大大的影响了数据的精度。一个需要12:00点执行的任务可能12:01 才执行,这样对数据要求很高的系统有更大的弊端。由此我们可以使用DelayQueue。
为了具有调用行为，存放到DelayDeque的元素必须继承Delayed接口。Delayed接口使对象成为延迟对象，它使存放在DelayQueue类中的对象具有了激活日期。该接口强制执行下列两个方法。
- CompareTo(Delayed o)：Delayed接口继承了Comparable接口，因此有了这个方法。
- getDelay(TimeUnit unit):这个方法返回到激活日期的剩余时间，时间单位由单位参数指定。
public class DelayEvent implements Delayed {
private Date startDate;
public DelayEvent(Date startDate) {
super();
this.startDate = startDate;
}
@Override
public int compareTo(Delayed o) {
long result = this.getDelay(TimeUnit.NANOSECONDS)
- o.getDelay(TimeUnit.NANOSECONDS);
if (result < 0) {
return -1;
} else if (result > 0) {
return 1;
} else {
return 0;
}
}
@Override
public long getDelay(TimeUnit unit) {
Date now = new Date();
long diff = startDate.getTime() - now.getTime();
return unit.convert(diff, TimeUnit.MILLISECONDS);
}
}public class DelayTask implements Runnable {
private int id;
private DelayQueue<DelayEvent> queue;
public DelayTask(int id, DelayQueue<DelayEvent> queue) {
super();
this.id = id;
this.queue = queue;
}
@Override
public void run() {
Date now = new Date();
Date delay = new Date();
delay.setTime(now.getTime() + id * 1000);
System.out.println("Thread " + id + " " + delay);
for (int i = 0; i < 100; i++) {
DelayEvent delayEvent = new DelayEvent(delay);
queue.add(delayEvent);
}
}
}public class DelayDequeMain {
public static void main(String[] args) throws Exception {
DelayQueue<DelayEvent> queue = new DelayQueue<DelayEvent>();
Thread threads[] = new Thread[5];
for (int i = 0; i < threads.length; i++) {
DelayTask task = new DelayTask(i + 1, queue);
threads[i] = new Thread(task);
}
for (int i = 0; i < threads.length; i++) {
threads[i].start();
}
for (int i = 0; i < threads.length; i++) {
try {
threads[i].join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
do {
int counter = 0;
DelayEvent delayEvent;
do {
delayEvent = queue.poll();
if (delayEvent != null) {
counter++;
}
} while (delayEvent != null);
System.out.println("At " + new Date() + " you have read " + counter+ " event");
TimeUnit.MILLISECONDS.sleep(500);
} while (queue.size() > 0);
}
}Thread 3 Fri May 06 11:00:20 CST 2016
Thread 1 Fri May 06 11:00:18 CST 2016
Thread 5 Fri May 06 11:00:22 CST 2016
Thread 4 Fri May 06 11:00:21 CST 2016
Thread 2 Fri May 06 11:00:19 CST 2016
At Fri May 06 11:00:17 CST 2016 you have read 0 event
At Fri May 06 11:00:18 CST 2016 you have read 0 event
At Fri May 06 11:00:18 CST 2016 you have read 100 event
At Fri May 06 11:00:19 CST 2016 you have read 0 event
At Fri May 06 11:00:19 CST 2016 you have read 100 event
At Fri May 06 11:00:20 CST 2016 you have read 0 event
At Fri May 06 11:00:20 CST 2016 you have read 100 event
At Fri May 06 11:00:21 CST 2016 you have read 0 event
At Fri May 06 11:00:21 CST 2016 you have read 100 event
At Fri May 06 11:00:22 CST 2016 you have read 0 event
At Fri May 06 11:00:22 CST 2016 you have read 100 event