Servlet(4)：Session

Session, Cookie及交互

会话（Session）跟踪是Web程序中常用的技术，用来跟踪用户的整个会话。常用的会话跟踪技术是Cookie与Session。

Cookie通过在客户端记录信息确定用户身份，Session通过在服务器端记录信息确定用户身份。

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。

但是仍然有以下三种方式来维持 Web 客户端和 Web 服务器之间的 session 会话：

Cookies：Session创建成功以后，会创建Cookie（key='JSESSIONID', value=Session ID）。通常情况下，一个浏览器的多个Tag可以共享一个Session对象。

隐藏的表单字段（Form适用）：有些浏览器不支持Cookie，可以用表单隐藏域代替。

<input type="hidden" name="sessionid" value="12345">

URL 重写（Link适用）：如果使用URL提交时，可以作为URL参数传递。

http://localhost:8080/ServletTest/login?sessionid=12345

下面是一个使用Cookie来维持Session会话的例子。重点代码讲解在例子中。

 package com.servlettest.session;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.servlet.ServletException;

 import javax.servlet.annotation.WebServlet;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 /**

  * Servlet implementation class Login

  */

 @WebServlet("/default")

 public class Default extends HttpServlet {

     private static final long serialVersionUID = 1L;

     /**

      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         invalidateSession(request);

         response.setContentType("text/html");

         request.setCharacterEncoding("UTF-8");

         response.setCharacterEncoding("UTF-8");

         PrintWriter out = response.getWriter();

         out.println("<!DOCTYPE html>");

         out.println("<html>");

         out.println("<head>");

         out.println("<title>Login Page</title>");

         out.println("</head>");

         out.println("<body>");

         out.println("<form action=\"login\" method=\"POST\">");

         out.println("帐号: <input type=\"text\" name=\"userId\"/><br/>");

         out.println("密码: <input type=\"password\" name=\"password\"/><br/>");

         out.println("<input type=\"submit\" value=\"提交\"/>");

         // HttpServletRequest Attribute和Parameter的区别

         // (1) HttpServletRequest 类有setAttribute()方法，而没有setParameter()方法。

         // (2) 当两个Web组件之间为链接关系时，被链接的组件通过getParameter()方法来获得请求参数，一般通过表单和链接传递的参数使用getParameter。

         // (3) 当两个Web组件之间为转发关系时，转发目标组件通过getAttribute()方法来和转发源组件共享request范围内的数据。

         String message = (String) request.getAttribute("message");

         if (message != null && message != "") {

             out.println("<p>" + message + "</p>");

         }

         out.println("</body>");

         out.println("</html>");

     }

     /**

      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doGet(request, response);

     }

     private static void invalidateSession(HttpServletRequest request) {

         HttpSession session = request.getSession(false);

         if (session != null)

             session.invalidate();

     }

 }

 package com.servlettest.session;

 import java.io.IOException;

 import java.util.HashMap;

 import java.util.Map;

 import javax.servlet.ServletException;

 import javax.servlet.annotation.WebServlet;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 /**

  * Servlet implementation class Login

  */

 @WebServlet("/login")

 public class Login extends HttpServlet {

     private static final long serialVersionUID = 1L;

     private static final Map<String, String[]> USER_INFO = initUserInfo();

     /**

      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("text/html");

         request.setCharacterEncoding("UTF-8");

         response.setCharacterEncoding("UTF-8");

         // 获取用户登录信息

         String userId = request.getParameter("userId");

         String password = request.getParameter("password");

         if ((userId == null || userId == "") || (password == null || password == "")) {

             request.getRequestDispatcher("/default").forward(request, response);

         } else if (USER_INFO.containsKey(userId)) {

             String[] info = USER_INFO.get(userId);

             if (info[1].equals(password)) {

                 // 创建Session

                 setSession(userId, info[0], request, response);

                 request.getRequestDispatcher("/welcome1").forward(request, response);

             } else {

                 request.setAttribute("message", "帐号或密码错误!");

                 request.getRequestDispatcher("/default").forward(request, response);

             }

         } else {

             request.setAttribute("message", "帐号或密码错误!");

             request.getRequestDispatcher("/default").forward(request, response);

         }

     }

     /**

      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doGet(request, response);

     }

     private static Map<String, String[]> initUserInfo() {

         Map<String, String[]> result = new HashMap<>();

         String[] user1 = { "张三", "123" };

         result.put("101", user1);

         String[] user2 = { "李四", "456" };

         result.put("102", user2);

         String[] user3 = { "王五", "789" };

         result.put("103", user3);

         return result;

     }

     private void setSession(String userId, String userName, HttpServletRequest request, HttpServletResponse response) {

         // 如果不存在 session 会话，则创建一个 session 对象

         HttpSession session = request.getSession(true);

         String[] userInfo = { userId, userName };

         session.setAttribute("USER_INFO", userInfo);

     }

 }

 package com.servlettest.session;

 import java.io.IOException;

 import java.io.PrintWriter;

 import java.util.Enumeration;

 import javax.servlet.ServletException;

 import javax.servlet.annotation.WebServlet;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 /**

  * Servlet implementation class Welcome

  */

 @WebServlet("/welcome1")

 public class Welcome1 extends HttpServlet {

     private static final long serialVersionUID = 1L;

     /**

      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("text/html");

         request.setCharacterEncoding("UTF-8");

         response.setCharacterEncoding("UTF-8");

         // 注意request.getSession() 等同于 request.getSession(true)，除非我们确认session一定存在或者sesson不存在时明确有创建session的需要，否则请尽量使用request.getSession(false)

         HttpSession session = request.getSession(false);

         String[] userInfo = (String[]) session.getAttribute("USER_INFO");

         PrintWriter out = response.getWriter();

         out.println("<!DOCTYPE html>");

         out.println("<html>");

         out.println("<head>");

         out.println("<title>Login Page</title>");

         out.println("</head>");

         out.println("<body>");

         out.println("您好 <b>" + userInfo[0] + "-" + userInfo[1] + "</b>, 这是Welcome1画面!");

         out.println("<br/><a href = \"welcome2\">去welcome2</a> | <a href = \"default\">注销</a>");

         out.println("<br/>");

         out.println("<p>SessionID: " + session.getId() + "</p>");

         Enumeration<String> names = session.getAttributeNames();

         while (names.hasMoreElements()) {

             String name = names.nextElement();

             out.println("<p>" + names.nextElement() + ": " + session.getAttribute(name) + "</p>");

         }

         out.println("</table>");

         out.println("</body>");

         out.println("</html>");

         out.close();

     }

     /**

      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doGet(request, response);

     }

 }

 package com.servlettest.session;

 import java.io.IOException;

 import java.io.PrintWriter;

 import java.util.Enumeration;

 import javax.servlet.ServletException;

 import javax.servlet.annotation.WebServlet;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 /**

  * Servlet implementation class Welcome

  */

 @WebServlet("/welcome2")

 public class Welcome2 extends HttpServlet {

     private static final long serialVersionUID = 1L;

     /**

      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("text/html");

         request.setCharacterEncoding("UTF-8");

         response.setCharacterEncoding("UTF-8");

         // 注意request.getSession() 等同于 request.getSession(true)，除非我们确认session一定存在或者sesson不存在时明确有创建session的需要，否则请尽量使用request.getSession(false)

         HttpSession session = request.getSession(false);

         String[] userInfo = (String[]) session.getAttribute("USER_INFO");

         PrintWriter out = response.getWriter();

         out.println("<!DOCTYPE html>");

         out.println("<html>");

         out.println("<head>");

         out.println("<title>Login Page</title>");

         out.println("</head>");

         out.println("<body>");

         out.println("您好 <b>" + userInfo[0] + "-" + userInfo[1] + "</b>, 这是Welcome2画面!");

         out.println("<br/><a href = \"welcome1\">去welcome1</a> | <a href = \"default\">注销</a>");

         out.println("<br/>");

         out.println("<p>SessionID: " + session.getId() + "</p>");

         Enumeration<String> names = session.getAttributeNames();

         while (names.hasMoreElements()) {

             String name = names.nextElement();

             out.println("<p>" + names.nextElement() + ": " + session.getAttribute(name) + "</p>");

         }

         out.println("</body>");

         out.println("</html>");

         out.close();

     }

     /**

      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

      */

     @Override

     protected void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doGet(request, response);

     }

 }

 package com.servlettest.filter;

 import java.io.IOException;

 import java.util.ArrayList;

 import java.util.List;

 import javax.servlet.Filter;

 import javax.servlet.FilterChain;

 import javax.servlet.FilterConfig;

 import javax.servlet.ServletException;

 import javax.servlet.ServletRequest;

 import javax.servlet.ServletResponse;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 import com.servlettest.exception.PermissionException;

 public class PermissionFilter implements Filter {

     private final static List<String> URL = new ArrayList<>();

     @Override

     public void init(FilterConfig arg0) throws ServletException {

         URL.add("/ServletTest/welcome1");

         URL.add("/ServletTest/welcome2");

     }

     @Override

     public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)

             throws IOException, ServletException {

         if (arg0 instanceof HttpServletRequest && arg1 instanceof HttpServletResponse) {

             HttpServletRequest req = (HttpServletRequest) arg0;

             if (URL.contains(req.getRequestURI())) {

                 HttpSession session = req.getSession(true);

                 String[] userInfo = (String[]) session.getAttribute("USER_INFO");

                 if (userInfo == null) {

                     throw new PermissionException();

                 }

             }

         }

         arg2.doFilter(arg0, arg1);

     }

     @Override

     public void destroy() {

     }

 }

 package com.servlettest.exception;

 public class PermissionException extends RuntimeException {

     private static final long serialVersionUID = 1L;

 }

 <!DOCTYPE html>

 <html>

 <head>

 <meta charset="utf-8"/>

 <title>Forbidden</title>

 </head>

 <body>

     <h2>403 禁止访问所请求的页面</h2>

 </body>

 </html>

 <?xml version="1.0" encoding="UTF-8"?>

 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xmlns="http://xmlns.jcp.org/xml/ns/javaee"

     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"

     id="WebApp_ID" version="3.1">

     <filter>

         <filter-name>permissionFilter</filter-name>

         <filter-class>com.servlettest.filter.PermissionFilter</filter-class>

     </filter>

     <filter-mapping>

         <filter-name>permissionFilter</filter-name>

         <url-pattern>/*</url-pattern>

     </filter-mapping>

     <error-page>

         <error-code>403</error-code>

         <location>/html/forbidden.html</location>

     </error-page>

     <error-page>

         <exception-type>com.servlettest.exception.PermissionException</exception-type>

         <location>/html/forbidden.html</location>

     </error-page>

     <welcome-file-list>

         <welcome-file>default</welcome-file>

     </welcome-file-list>

     <session-config>

         <session-timeout>1</session-timeout>

     </session-config>

 </web-app>

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