线程可以理解为下载的通道,一个线程就是一个文件的下载通道,多线程也就是同时开起好几个下载通道.当服务器提供下载服务时,使用下载者是共享带宽的,在优先级相同的情况下,总服务器会对总下载线程进行平均分配.线程越多,下载速度越快.当前的下载软件都支持多线程技术.
通常服务器同时与多个用户连接，用户之间共享带宽。如果N个用户的优先级都相同，那么每个用户连接到该服务器上的实际带宽就是服务器带宽的N分之一。如果户数目较多，则每个用户只能占有可怜的一点带宽，下载将会是个漫长的过程。
具体研究内容：下载功能实现，包括单线程下载功能，多线程下载功能，多任务下载功能，删除任务的实现。断点续传等功能包括，下载过程中，暂停下载，承接上次未完成的下载任务。
关键词：多线程；线程安全；断点续传

Abstract
The line Cheng can understand the passage for time be loaded with , a line Cheng is a document time be loaded with passage , multiwire Cheng is just to open up several time be loaded with passage at the same time. Being put into use downloading person is to share bandwidth's while the server provides time be loaded with service, the average the general server meeting is carried out on general time be loaded with line Cheng distributes under preferential step is identical situation. The line Cheng is getting quicker as downloading speed much more. The popular time be loaded with software all supports multiwire Cheng. The simultaneous and many consumer of server links up generally , shares bandwidth between the consumer. If consumer's N preferential step is all identical,every consumer links N one of mark being server bandwidth to owing upper reality of server bandwidth so. Fruit the consumer number is more, every consumer can only occupy pitiful little bandwidth then , downloading the meeting is endless process. Study content concretely: The time be loaded with function realizes , includes the single line Cheng time be loaded with function, multi-thread time be loaded with function, the multitasking downloads a function , delete the mission realization. Breaking point adds biography wait for a function to suspend time be loaded with in including , downloading process, carries on not be completed time be loaded with last time mission.

Keywords: Multithreading; Thread security; Broken/Resume

目 录
1 引言 1
1.1 网络下载技术 3
1.2 网络基本构架 3
1.3 多线程技术 3
2.1 课题的研究背景与意义 3
2.1.1 课题的研究背景 3
2.1.2 课题的研究意义 4
2.1.3 多线程下载的现状及发展趋势 4
2.2 可行性分析 5
2.2.1 技术可行性 5
2.2.2 操作可行性 6
3 相关基础知识以 6
3.1 JAVA中的多线程与线程安全 6
3.1.1 Java中的多线程 6
3.1.2 Java中的线程安全 7
3.2 HTTP协议简介 8
3.3 断点续传原理 8
4 需求分析 10
4.1用户需求分析 10
4.2 业务流分析 11
5. 整体设计 11
5.1 系统设计要点 11
5.2 系统总体功能结构 12
5.3 开发环境：myeclipse + SWT 15
5.4 URL类和URLConnection类的使用 15
6.系统实现 16
6.1用户界面实现 16
6.2 下载任务实现 20
6.2.1 下载任务类图 20
6.2.2 下载任务顺序图 20
6.2.3 下载任务具体实现 21
6.3 监控下载信息设计 25
6.3.1 监控下载信息类图 25
6.3.2 监控下载信息顺序图 26
6.3.3 监控下载信息实现 26
结 论 27
参考文献 28
致 谢 29