Using Delayed queues in practice
来源:互联网 发布:朱丹 知乎 编辑:程序博客网 时间:2024/06/01 09:46
Often there are use cases when you have some kind of work or job queue and there is a need not to handle each work item or job immediately but with some delay. For example, if user clicks a button which triggers some work to be done, and one second later user realizes he / she was mistaken and job shouldn’t start at all. Or, f.e. there could be a use case when some work elements in a queue should be removed after some delay (expiration).
There are a lot of implementations out there, but one I would like to describe is using pure JDK concurrent framework classes: DelayedQueue and Delayed interface.
Let me start with simple (and empty) interface which defines the work item. I am skipping the implementation details like properties and methods as those are not important.
package com.example.delayed;public interface WorkItem { // Some properties and methods here}
The next class in our model will represent the postponed work item and implement Delayed interface. There are just few basic concepts to take into account: the delay itself and the actual time the respective work item has been submitted. This is how expiration would be calculated. So let’s do that by introducing PostponedWorkItem class.
package com.example.delayed;import java.util.concurrent.Delayed;import java.util.concurrent.TimeUnit;public class PostponedWorkItem implements Delayed {private final long origin;private final long delay;private final WorkItem workItem;public PostponedWorkItem(final WorkItem workItem, final long delay) {this.origin = System.currentTimeMillis();this.workItem = workItem;this.delay = delay;}@Overridepublic long getDelay(TimeUnit unit) {return unit.convert(delay - (System.currentTimeMillis() - origin),TimeUnit.MILLISECONDS);}@Overridepublic int compareTo(Delayed delayed) {if (delayed == this) {return 0;}if (delayed instanceof PostponedWorkItem) {long diff = delay - ((PostponedWorkItem) delayed).delay;return ((diff == 0) ? 0 : ((diff < 0) ? -1 : 1));}long d = (getDelay(TimeUnit.MILLISECONDS) - delayed.getDelay(TimeUnit.MILLISECONDS));return ((d == 0) ? 0 : ((d < 0) ? -1 : 1));}}
As you can see, we create new instance of the class and save the current system time in internal origin property. The getDelayed method calculates the actual time left before work item gets expired. The delay is external setting which comes as constructor parameter. The mandatory implementation of Comparable<Delayed> is required as Delayed extends this interface.
Now, we are mostly done! To complete the example, let’s make sure that same work item won’t be submitted twice to the work queue by implementing equals and hashCode (implemenation is pretty trivial and should not require any comments).
package com.example.delayed;import java.util.concurrent.Delayed;import java.util.concurrent.TimeUnit;public class PostponedWorkItem implements Delayed {private final long origin;private final long delay;private final WorkItem workItem;public PostponedWorkItem(final WorkItem workItem, final long delay) {this.origin = System.currentTimeMillis();this.workItem = workItem;this.delay = delay;}@Overridepublic long getDelay(TimeUnit unit) {return unit.convert(delay - (System.currentTimeMillis() - origin),TimeUnit.MILLISECONDS);}@Overridepublic int compareTo(Delayed delayed) {if (delayed == this) {return 0;}if (delayed instanceof PostponedWorkItem) {long diff = delay - ((PostponedWorkItem) delayed).delay;return ((diff == 0) ? 0 : ((diff < 0) ? -1 : 1));}long d = (getDelay(TimeUnit.MILLISECONDS) - delayed.getDelay(TimeUnit.MILLISECONDS));return ((d == 0) ? 0 : ((d < 0) ? -1 : 1));}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result+ ((workItem == null) ? 0 : workItem.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj) {return true;}if (obj == null) {return false;}if (!(obj instanceof PostponedWorkItem)) {return false;}final PostponedWorkItem other = (PostponedWorkItem) obj;if (workItem == null) {if (other.workItem != null) {return false;}} else if (!workItem.equals(other.workItem)) {return false;}return true;}}
The last step is to introduce some kind of manager which will scheduled work items and periodically polls out expired ones: meet WorkItemScheduler class.
package com.example.delayed;import java.util.ArrayList;import java.util.Collection;import java.util.concurrent.BlockingQueue;import java.util.concurrent.DelayQueue;public class WorkItemScheduler { private final long delay = 2000; // 2 seconds private final BlockingQueue< PostponedWorkItem > delayed = new DelayQueue< PostponedWorkItem >(); public void addWorkItem( final WorkItem workItem ) { final PostponedWorkItem postponed = new PostponedWorkItem( workItem, delay ); if( !delayed.contains( postponed )) { delayed.offer( postponed ); } } public void process() { final Collection< PostponedWorkItem > expired = new ArrayList< PostponedWorkItem >(); delayed.drainTo( expired ); for( final PostponedWorkItem postponed: expired ) { // Do some real work here with postponed.getWorkItem() } }}
Usage of BlockingQueue guarantees thread safety and high level of concurrency. The process method should be run periodically in order to drain work items queue. It could be annotated by @ Scheduled annotation from Spring Framework or by EJB’s @Scheduleannotation from JEE 6.
- Using Delayed queues in practice
- SQLite Practice Using C# in Windows
- Using template in practice(实际运用)
- Using Timer Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- leetcode_Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- Implement Stack using Queues
- 编程之美:最大公约数问题
- PHP 字符串函数 substr 用法
- android的开发环境配置
- 辨别程序是C编译的还是C++编译的方式
- SQL LIKE的基本用法
- Using Delayed queues in practice
- Hprose for php(二)——服务器
- 百度测试开发电面
- [蓝桥杯历届试题] 国庆星期日
- Android程序架构目录结构介绍
- java基础知识回顾(三)
- GDAL Data Model(&转)
- GDAL 的安装介绍及使用
- C++内存分配秘籍—new,malloc,GlobalAlloc详解(Zhuan)