1.1 案例三：记录网站的登录成功的人数.1.1.1 需求:

• 用来存取数据:
  
• 用来向ServletContext中存入数据.
  
• 用来从ServletContext中获取数据.
  
• 用来从ServletContext中移除数据.
• 
  1.1.3 代码实现:

/*** 登录代码的Servlet*/public class UserCountServlet extends HttpServlet {        private static final long serialVersionUID = 1L;        @Override        public void init() throws ServletException {                // 初始化一个变量count的值为0.                int count = 0;                // 将这个值存入到ServletContext中.                this.getServletContext().setAttribute("count", count);        }        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {                try {                        response.setContentType("text/html;charset=UTF-8");                        // 1.接收表单提交的参数.                        String username = request.getParameter("username");                        String password = request.getParameter("password");                        // 2.封装到实体对象中.                        User user = new User();                        user.setUsername(username);                        user.setPassword(password);                        // 3.调用业务层处理数据.                        UserService userService = new UserService();                        User existUser = userService.login(user);                        // 4.根据处理结果显示信息(页面跳转).                        if(existUser == null){                                // 登录失败                                response.getWriter().println("
登录失败：用户名或密码错误!
");                        }else{                                // 登录成功

// 记录次数:                            int count = (int) this.getServletContext().getAttribute("count");                            count++;                            this.getServletContext().setAttribute("count", count);                            response.getWriter().println("
登录成功:您好:"+existUser.getNickname()+"
");                            response.getWriter().println("
页面将在5秒后跳转!
");                            response.setHeader("Refresh", "5;url=/day09/CountServlet");                    }            } catch (Exception e) {                    e.printStackTrace();            }    }    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {            doGet(request, response);    }

}

public class CountServlet extends HttpServlet {        private static final long serialVersionUID = 1L;        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {                // 获得Count的值。                response.setContentType("text/html;charset=UTF-8");                int count = (int) this.getServletContext().getAttribute("count");                response.getWriter().println("
您是第"+count+"位登录成功的用户！
");        }        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {                doGet(request, response);        }}1.1.4 总结:1.1.4.1 ServletConfig:了解.获得Servlet的配置信息.* String getServletName();                ---获得Servlet在web.xml中配置的name的值.* String getInitParameter(String name);        ---获得Servlet的初始化参数的.* Enumeration getInitParameterNames();                ---获得所有Servlet的初始化参数的名称.1.1.4.2 ServletContext:重要ServletContext的作用:* 1.用来获得全局初始化参数.* 2.用来获得文件的MIME的类型.* 3.作为域对象存取数据.ServletContext是一个域对象.    * 作用范围:整个web工程.    * 创建:服务器启动的时候,tomcat服务器为每个web项目创建一个单独ServletContext对象.    * 销毁:服务器关闭的时候,或者项目从服务器中移除的时候.* 4.用来读取web项目下的文件.