用androidpn来实现推送

关于服务器端向Android客户端的推送，主要有三种方式：轮询，应用程序应当阶段性的与服务器进行连接并查询是否有新的消息到达，你必须自己实现与服务器之间的通信，例如消息排队等。而且你还要考虑轮询的频率，如果太慢可能导致某些消息的延迟，如果太快，则会大量消耗网络带宽和电池；SMS（通过发送短信并解析短信内容来获取服务器端的指令），这个出现的问题是很难找到免费的网关来发送短信；最后就是持久连接，主要是Socket通讯，这个解决了性能问题，但是耗电问题依旧没能解决。

在这里，我们主要介绍的是第三种，用持久连接的方式来进行推送。现在比较成熟的及时消息传递协议共有四种，而无疑最为主流就是XMPP协议，它是一种基于XML的传递协议，具有很强的灵活性和可扩展性。它的特点是将复杂性从客户端转移到了服务器端。在网上可以找到很多的XMPP资料，这里就不在赘述了，不然越扯越多。总之，XMPP主要显著的优点主要有以下几个方面：

1、 分布式  任何人都可以运行自己的XMPP服务器，它没有主服务器

2、 安全性很高。使用SASL及TLS等技术的可靠安全性

3、 开发性 它是开源的，易于进行学习和了解

4、 跨平台  毋庸置疑，使用的XML进行传输的

说完优点，我们言归正传，基于XMPP协议的java开发有一个开源框架，那就是smack，它主要封装了一些XMPP的实现。而如果把它直接用在Android上是不行的，因为android缺少了一些java的类库，于是一个改进版的asmack诞生了，它是专门为android而改进的android smack。而另外一个开源框架的诞生，则是对在引用smack的基础上实现和服务器端的持久连接，以实现服务器对客户端的推送，那就是android push notification，简称androidpn。

Androidpn在客户端集成了asmack。这样就可以很容易的简立一个和服务器端的基于xmpp协议的socket连接。Androidpn的客户端中，进行管理连接的类是XmppManager，它主要用来管理连接的信息，比如XMPP的端口、IP、登录的用户名密码，以及对连接的维护。为什么还有用户名和密码？这不得不提到XMPP的具体细节。整个服务器端和客户端的通信是基于一个session（会话）过程，会话开始，首先会指定服务器的端口号，然后把上述提到的信息发送到服务器端，怎么发送消息的呢？以<stream>根节点的方式开始传递，只有在服务器和客户端关闭的时候才会发送它的结束标记</stream>。客户端通过XMPP协议只用做的就是接收消息，而所有其它的操作都交给服务器，比如管理连接、消息保存等等，这样就很大程度的减轻了客户端的负担。那么客户端和服务器端的消息回应是如何实现的？如要通过一个ID来标识，具体细节可以去查看XMPP协议。

一旦注册绑定后，服务器端就和客户端建立了连接，客户端只用负责去接收消息。所以当我们应用Androidpn的时候，客户端会非常的简单。而在服务器端，Androidpn又做了什么呢？

服务器端的展示方面，androidpn主要用到的技术是Spring和Hibernate。主要是用来展示用户状态和发送信息用的，如以下链接：

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