jsp程序设计（三）－初试jsp程序设计（2）－Servlet简介
一、什么是Servlet
Servlet 是一个用 Java 编写的程序，此程序在服务器上运行以处理客户端请求。
二、接口和类
1、javax.servlet 包中的接口
2、javax.servlet 包中的类
ServletInputStream 类 用于从客户端读取二进制数据；
ServletOutputStream 类 用于将二进制数据发送到客户端；
HttpServletResponse 接口 提供 HTTP 响应；
三、Servlet 的生命周期
Servlet 的生命周期就是指创建 Servlet 实例之后其存在的时间以及何时消失；
1、生命周期的 3个方法为：
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2、生命周期的各个阶段为：
实例化 Servlet 容器创建 Servlet 的实例
初始化 该容器调用 init() 方法
服 务 如果请求 Servlet，则容器调用 service() 方法
销 毁 销毁实例之前调用 destroy() 方法
不可用 销毁实例并标记为垃圾收集
四、HTTP Servlet 基础知识：
public void doGet(HttpServletRequest req,HttpServletResponse res) public void doPost(HttpServletRequest req,HttpServletResponse res)
doGet() 方法处理客户端作出的 GET 请求。doPost() 方法处理客户端作出的 POST 请求。
五、Servlet 的部署
web.xml文件中配置Servlet：
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HttpServlet 类提供一个抽象类以创建 HTTP Servlet，实现其两个方法：
3、javax.servlet.http 包的接口
HttpServletRequest 接口 提供 HTTP 请求信息；
ServletConfig 接口在初始化的过程中由 Servlet 容器使用；
ServletContext 接口定义 Servlet 用于获取来自其容器的信息的方法；
ServletRequest 接口 向服务器请求信息；
ServletResponse 接口 响应客户端请求；