Servlet之ServletContext、Session、Cookie

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ServletContext、Session、Cookie都可以用于存储数据，不过三者存储数据的位置及作用域不同。

这里写图片描述

由上图我们可以看出
ServletContext存储于服务端，每个客户端都可以访问到，数据共享。
Session存储于服务端，每个客户端有自己独立的数据区域。
Cookie存储于客户端。

下面通过代码来看一下三者的用法

package com.gujin.servlet;import java.io.IOException;import java.io.PrintWriter;import java.text.MessageFormat;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ContextServlet extends HttpServlet{   @Override   protected void doGet(HttpServletRequest req, HttpServletResponse response)         throws ServletException, IOException   {      ServletContext context = this.getServletConfig()               .getServletContext();      Integer count = (Integer) context.getAttribute("visit_count");      if (count == null)      {         count = 1;      }      else      {         count++;      }      context.setAttribute("visit_count", count);      // 设置响应内容类型      response.setContentType("text/html;charset=UTF-8");      PrintWriter writer = response.getWriter();      // 想输出流写的内容就是客户端接收到的内容      writer.print("<html>");      writer.print("<head>");      writer.print("<meta charset='UTF-8'>");      writer.print("</head>");      writer.print("<body>");      writer.print(MessageFormat.format("<h1>访问次数：{0}</h1>", count));      writer.print("</body>");      writer.print("<html>");      writer.flush();      writer.close();   }}

package com.gujin.servlet;import java.io.IOException;import java.io.PrintWriter;import java.text.MessageFormat;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class SessionServlet extends HttpServlet{   @Override   protected void doGet(HttpServletRequest request, HttpServletResponse response)         throws ServletException, IOException   {      String flag = request.getParameter("flag");      HttpSession session = request.getSession();      if ("login".equals(flag))      {         session.setAttribute("login", "login");      }      else if ("logout".equals(flag))      {         session.removeAttribute("login");      }      PrintWriter writer = response.getWriter();      // 想输出流写的内容就是客户端接收到的内容      writer.print("<html>");      writer.print("<head>");      writer.print("<meta charset='UTF-8'>");      writer.print("</head>");      writer.print("<body>");      writer.print(MessageFormat.format("<h1>{0}</h1>",            session.getAttribute("login") == null ? "未登录" : "已登录"));      writer.print("</body>");      writer.print("<html>");      writer.flush();      writer.close();   }}

package com.gujin.servlet;import java.io.IOException;import java.io.PrintWriter;import java.text.MessageFormat;import javax.servlet.ServletException;import javax.servlet.http.Cookie;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class CookieServlet extends HttpServlet{   @Override   protected void doGet(HttpServletRequest request, HttpServletResponse response)         throws ServletException, IOException   {      // 设置响应内容类型      response.setContentType("text/html;charset=UTF-8");      PrintWriter writer = response.getWriter();      // 想输出流写的内容就是客户端接收到的内容      writer.print("<html>");      writer.print("<head>");      writer.print("<meta charset='UTF-8'>");      writer.print("</head>");      writer.print("<body>");      writer.print("<ul>");      // 获得所有Cookie      Cookie[] cookies = request.getCookies();      if (cookies != null)      {         for (Cookie cookie : cookies)         {            writer.print(MessageFormat.format("<li>{0}:{1}</li>",                  cookie.getName(), cookie.getValue()));         }      }      Cookie cookie = new Cookie("CookieServlet", System.currentTimeMillis() + "");      //添加Cookie      response.addCookie(cookie);      writer.print("</ul>");      writer.print("</body>");      writer.print("<html>");      writer.flush();      writer.close();   }}

然后在web.xml中进行配置

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">  <display-name>Servlet</display-name>  <servlet>    <servlet-name>context</servlet-name>    <servlet-class>com.gujin.servlet.ContextServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>context</servlet-name>    <url-pattern>/context</url-pattern>  </servlet-mapping>  <servlet>    <servlet-name>session</servlet-name>    <servlet-class>com.gujin.servlet.SessionServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>session</servlet-name>    <url-pattern>/session</url-pattern>  </servlet-mapping>  <servlet>    <servlet-name>cookie</servlet-name>    <servlet-class>com.gujin.servlet.CookieServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>cookie</servlet-name>    <url-pattern>/cookie</url-pattern>  </servlet-mapping></web-app>

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