使用HttpSession接口开发的步骤：
1.获取HttpSession对象
2.对HttpSession对象进行读写
3.手工终止HttpSession，或者自动终止
常用方法：
getId():返回包含分配给这个会话的唯一表示的字符串。在使用URL改写以及标识会话时比较有用
setAttribute():使用指定的名称将对象绑定到这个会话。（setValue()方法）
getAttribution():返回绑定到此会话的对象。(getValue()方法)
invalide():终止当前会话
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//获取会话对象
HttpSession session = request.getSession(true);
response.setContentType("text/html;charset=GB2312");
response.setCharacterEncoding("gb2312");//确保参数信息以汉字编码方式提取
PrintWriter out = response.getWriter();
//从会话中获取属性
int count =1;
Integer i = (Integer)session.getAttribute(COUNTER_KEY);
//如果存在以前的数值
if(i!=null){
count=i.intValue()+1;
}
//将属性信息存入会话
session.setAttribute(COUNTER_KEY, new Integer(count));
Date lastAccessed = new Date(session.getLastAccessedTime());
Date sessionCreated = new Date(session.getCreationTime());
DateFormat formatter = DateFormat.getDateTimeInstance(DateFormat.MEDIUM,DateFormat.MEDIUM);
//输出对话信息
out.println("<html>");
out.println("<head>");
out.println("<title>会话计数器</title>");
out.println("</head>");
out.println("<body>");
out.println("你的会话ID:<b>"+session.getId()+"<br>");
out.println("会话创建时间："+formatter.format(sessionCreated)+"<br>");
out.println("会话上次访问时间："+formatter.format(lastAccessed)+"<br>");
out.println("</b>会话期间你向页面发起<b>"+count+"</b>次请求");
out.println("<form method=GET action=""+request.getRequestURI()+"">");
out.println("<input type='submit'"+"value="再次点击...">");
out.println("</form>");
out.println("</body>");
out.println("</html>");
}
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request,response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request,response);
}