Servlets 简明教程

Servlets - Session Tracking

HTTP 是一种“无状态”协议，这意味着每次客户端检索网页时，客户端都会打开与 Web 服务器的独立连接，该服务器不会自动保留以前客户端请求的任何记录。

HTTP is a "stateless" protocol which means each time a client retrieves a Web page, the client opens a separate connection to the Web server and the server automatically does not keep any record of previous client request.

仍然有以下三种方法来维持 Web 客户端与 Web 服务器之间的会话：

Still there are following three ways to maintain session between web client and web server −

Cookies

Web 服务器可以为每个网络客户端分配一个作为 cookie 的唯一会话 ID，并且在接收到 cookie 后，可用于识别客户的后续请求。

A webserver can assign a unique session ID as a cookie to each web client and for subsequent requests from the client they can be recognized using the recieved cookie.

这可能不是一个有效的方法，因为很多时候浏览器不支持 cookie，因此我建议不要使用这些步骤来维护会话。

This may not be an effective way because many time browser does not support a cookie, so I would not recommend to use this procedure to maintain the sessions.

Hidden Form Fields

Web 服务器可以结合唯一会话 ID 发送隐藏的 HTML 表单字段，如下所示：

A web server can send a hidden HTML form field along with a unique session ID as follows −

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

此条目表示，提交表单时，指定的名称和值将自动包含在 GET 或 POST 数据中。每次 Web 浏览器发送请求时，都可以使用 session_id 值来跟踪不同的 Web 浏览器。

This entry means that, when the form is submitted, the specified name and value are automatically included in the GET or POST data. Each time when web browser sends request back, then session_id value can be used to keep the track of different web browsers.

这可能是跟踪会话的有效方法，但单击常规（<A HREF…​>）超文本链接不会导致表单提交，因此隐藏表单字段也不能支持常规会话跟踪。

This could be an effective way of keeping track of the session but clicking on a regular (<A HREF…​>) hypertext link does not result in a form submission, so hidden form fields also cannot support general session tracking.

URL Rewriting

你可以在每个 URL 的末尾附加一些额外的数据来标识会话，并且服务器可以将该会话标识符与它存储的有关该会话的数据相关联。

You can append some extra data on the end of each URL that identifies the session, and the server can associate that session identifier with data it has stored about that session.

例如，对于 [role="bare"] [role="bare"]http://tutorialspoint.com/file.htm;sessionid = 12345，会话标识符附加为 sessionid = 12345，可从 Web 服务器访问以识别客户端。

For example, with [role="bare"]http://tutorialspoint.com/file.htm;sessionid = 12345, the session identifier is attached as sessionid = 12345 which can be accessed at the web server to identify the client.

URL 重写是维护会话的更好方法，即使浏览器不支持 cookie，它也能发挥作用。URL 重写的缺点是即使对于简单的静态 HTML 页面，你也必须动态生成每个 URL 以分配会话 ID。

URL rewriting is a better way to maintain sessions and it works even when browsers don’t support cookies. The drawback of URL re-writing is that you would have to generate every URL dynamically to assign a session ID, even in case of a simple static HTML page.

The HttpSession Object

除了上述三种方法外，servlet 还提供了 HttpSession Interface，它提供了一种跨多个页面请求或访问网站识别用户并存储有关该用户信息的方法。

Apart from the above mentioned three ways, servlet provides HttpSession Interface which provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user.

servlet 容器使用此接口来创建 HTTP 客户端与 HTTP 服务器之间的会话。会话会在一个指定的时间段内保持，跨多个来自用户的连接或页面请求。

The servlet container uses this interface to create a session between an HTTP client and an HTTP server. The session persists for a specified time period, across more than one connection or page request from the user.

你可以通过调用 HttpServletRequest 的公共方法 getSession() 来获取 HttpSession 对象，如下所示：

You would get HttpSession object by calling the public method getSession() of HttpServletRequest, as below −

HttpSession session = request.getSession();

在向客户端发送任何文档内容之前，你需要调用 request.getSession()。以下是通过 HttpSession 对象可用的重要方法的摘要：

You need to call request.getSession() before you send any document content to the client. Here is a summary of the important methods available through HttpSession object −

Sr.No.

Method & Description

1

*public Object getAttribute(String name) * This method returns the object bound with the specified name in this session, or null if no object is bound under the name.

2

public Enumeration getAttributeNames() This method returns an Enumeration of String objects containing the names of all the objects bound to this session.

3

*public long getCreationTime() * This method returns the time when this session was created, measured in milliseconds since midnight January 1, 1970 GMT.

4

*public String getId() * This method returns a string containing the unique identifier assigned to this session.

5

*public long getLastAccessedTime() * This method returns the last accessed time of the session, in the format of milliseconds since midnight January 1, 1970 GMT

6

*public int getMaxInactiveInterval() * This method returns the maximum time interval (seconds), that the servlet container will keep the session open between client accesses.

7

*public void invalidate() * This method invalidates this session and unbinds any objects bound to it.

8

*public boolean isNew( * This method returns true if the client does not yet know about the session or if the client chooses not to join the session.

9

*public void removeAttribute(String name) * This method removes the object bound with the specified name from this session.

10

*public void setAttribute(String name, Object value) * This method binds an object to this session, using the name specified.

11

*public void setMaxInactiveInterval(int interval) * This method specifies the time, in seconds, between client requests before the servlet container will invalidate this session.

Session Tracking Example

此示例描述了如何使用 HttpSession 对象找出会话的创建时间和上次访问时间。如果尚未存在会话，我们将为请求关联一个新会话。

This example describes how to use the HttpSession object to find out the creation time and the last-accessed time for a session. We would associate a new session with the request if one does not already exist.

// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// Extend HttpServlet class
public class SessionTrack extends HttpServlet {

   public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

      // Create a session object if it is already not  created.
      HttpSession session = request.getSession(true);

      // Get session creation time.
      Date createTime = new Date(session.getCreationTime());

      // Get last access time of this web page.
      Date lastAccessTime = new Date(session.getLastAccessedTime());

      String title = "Welcome Back to my website";
      Integer visitCount = new Integer(0);
      String visitCountKey = new String("visitCount");
      String userIDKey = new String("userID");
      String userID = new String("ABCD");

      // Check if this is new comer on your web page.
      if (session.isNew()) {
         title = "Welcome to my website";
         session.setAttribute(userIDKey, userID);
      } else {
         visitCount = (Integer)session.getAttribute(visitCountKey);
         visitCount = visitCount + 1;
         userID = (String)session.getAttribute(userIDKey);
      }
      session.setAttribute(visitCountKey,  visitCount);

      // Set response content type
      response.setContentType("text/html");
      PrintWriter out = response.getWriter();

      String docType =
         "<!doctype html public \"-//w3c//dtd html 4.0 " +
         "transitional//en\">\n";

      out.println(docType +
         "<html>\n" +
            "<head><title>" + title + "</title></head>\n" +

            "<body bgcolor = \"#f0f0f0\">\n" +
               "<h1 align = \"center\">" + title + "</h1>\n" +
               "<h2 align = \"center\">Session Infomation</h2>\n" +
               "<table border = \"1\" align = \"center\">\n" +

                  "<tr bgcolor = \"#949494\">\n" +
                     "  <th>Session info</th><th>value</th>
                  </tr>\n" +

                  "<tr>\n" +
                     "  <td>id</td>\n" +
                     "  <td>" + session.getId() + "</td>
                  </tr>\n" +

                  "<tr>\n" +
                     "  <td>Creation Time</td>\n" +
                     "  <td>" + createTime + "  </td>
                  </tr>\n" +

                  "<tr>\n" +
                     "  <td>Time of Last Access</td>\n" +
                     "  <td>" + lastAccessTime + "  </td>
                  </tr>\n" +

                  "<tr>\n" +
                     "  <td>User ID</td>\n" +
                     "  <td>" + userID + "  </td>
                  </tr>\n" +

                  "<tr>\n" +
                     "  <td>Number of visits</td>\n" +
                     "  <td>" + visitCount + "</td>
                  </tr>\n" +
               "</table>\n" +
            "</body>
         </html>"
      );
   }
}

编译上面的 servlet SessionTrack 并创建 web.xml 文件中适当的条目。现在运行 [role="bare"] [role="bare"]http://localhost:8080/SessionTrack ，当您第一次运行时，将显示以下结果 −

Compile the above servlet SessionTrack and create appropriate entry in web.xml file. Now running [role="bare"]http://localhost:8080/SessionTrack would display the following result when you would run for the first time −

Welcome to my website
Session Infomation


Session info
value


id
0AE3EC93FF44E3C525B4351B77ABB2D5


Creation Time
Tue Jun 08 17:26:40 GMT+04:00 2010


Time of Last Access
Tue Jun 08 17:26:40 GMT+04:00 2010


User ID
ABCD


Number of visits
0

现在尝试第二次运行相同的 servlet，它将显示以下结果。

Now try to run the same servlet for second time, it would display following result.

Welcome Back to my website
Session Infomation


info type
value


id
0AE3EC93FF44E3C525B4351B77ABB2D5


Creation Time
Tue Jun 08 17:26:40 GMT+04:00 2010


Time of Last Access
Tue Jun 08 17:26:40 GMT+04:00 2010


User ID
ABCD


Number of visits
1

Deleting Session Data

完成用户的会话数据时，您有几种选择 −

When you are done with a user’s session data, you have several options −

  1. Remove a particular attribute − You can call public void removeAttribute(String name) method to delete the value associated with a particular key.

  2. Delete the whole session − You can call public void invalidate() method to discard an entire session.

  3. Setting Session timeout − You can call public void setMaxInactiveInterval(int interval) method to set the timeout for a session individually.

  4. Log the user out − The servers that support servlets 2.4, you can call logout to log the client out of the Web server and invalidate all sessions belonging to all the users.

  5. web.xml Configuration − If you are using Tomcat, apart from the above mentioned methods, you can configure session time out in web.xml file as follows.

<session-config>
   <session-timeout>15</session-timeout>
</session-config>

超时时间表示为分钟数，并覆盖 Tomcat 中的默认超时时间（30 分钟）。

The timeout is expressed as minutes, and overrides the default timeout which is 30 minutes in Tomcat.

servlet 中的 getMaxInactiveInterval( ) 方法以秒为单位返回该会话的超时时间。因此，如果在 web.xml 中为您的会话配置 15 分钟，则 getMaxInactiveInterval( ) 返回 900。

The getMaxInactiveInterval( ) method in a servlet returns the timeout period for that session in seconds. So if your session is configured in web.xml for 15 minutes, getMaxInactiveInterval( ) returns 900.