一，HTTP 是一种"无状态"协议，这意味着每次客户端检索网页时，客户端打开一个单独的连接到 Web 服务器，服务器会自动不保留之前客户端请求的任何记录。所以需要跟踪用户的sessionID来判断是否是同一个用户访问。
1，使用cookie保存sessionID，cookie是将服务器需要保存的信息写入到本地的文件夹里保存。可以将session保存到cookie中，对于客户端的后续请求可以使用接收到的 cookie 来识别。
浏览器不支持 cookie，所以建议不要使用这种方式来维持 session 会话。
例：
package com.test; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.Cookie; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; public class SessionCookies extends HttpServlet { /** * Constructor of the object. */ public SessionCookies() { super(); } /** * Destruction of the servlet. <br> */ public void destroy() { super.destroy(); // Just puts "destroy" string in log // Put your code here } /** * The doGet method of the servlet. <br> * * This method is called when a form has its tag value method equals to get. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //获得session HttpSession ss=request.getSession(); String aa=ss.getId(); PrintWriter out=response.getWriter(); /*out.print(aa);*/ //将session储存到cookie中 Cookie session=new Cookie("session",aa); Cookie cookie=null; Cookie[] cookies=null; //获得cookie中的sessionID cookies=request.getCookies(); for(int i=0;i<cookies.length;i++){ cookie=cookies[i]; out.print(cookie.getName()); } } /** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to post. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">"); out.println("<HTML>"); out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); out.println(" <BODY>"); out.print(" This is "); out.print(this.getClass()); out.println(", using the POST method"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); } /** * Initialization of the servlet. <br> * * @throws ServletException if an error occurs */ public void init() throws ServletException { // Put your code here } }
2，使用HttpSession跟踪
Servlet 还提供了 HttpSession 接口，该接口提供了一种跨多个页面请求或访问网站时识别用户以及存储有关用户信息的方式。
例：
package com.test; import java.io.IOException; import java.io.PrintWriter; 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 SessionUrl extends HttpServlet { /** * Constructor of the object. */ public SessionUrl() { super(); } /** * Destruction of the servlet. <br> */ public void destroy() { super.destroy(); // Just puts "destroy" string in log // Put your code here } /** * The doGet method of the servlet. <br> * * This method is called when a form has its tag value method equals to get. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ Integer typecount=0; public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //获取session。无则创建新的session HttpSession session=request.getSession(true); PrintWriter out=response.getWriter(); //判断是否是新创建的session if(session.isNew()){ out.print("新的session创建成功"); }else{ //如果不是新的session则记录这个session访问页面的次数 typecount++; out.print(typecount); } } /** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to post. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">"); out.println("<HTML>"); out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>"); out.println(" <BODY>"); out.print(" This is "); out.print(this.getClass()); out.println(", using the POST method"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); } /** * Initialization of the servlet. <br> * * @throws ServletException if an error occurs */ public void init() throws ServletException { // Put your code here } }