Session Tracking Techniques:
There are four techniques used in Session tracking:
1)Cookies:
You can use HTTP cookies to store information. Cookies will be
stored at browser side.
2)URL rewriting:
With this method, the information is carried through url as
request parameters. In general added parameter will be sessionid,
userid.
3)HttpSession:
Using HttpSession, we can store information at server side. Http
Session provides methods to handle session related information.
4)Hidden form fields:
By using hidden form fields we can insert information in the webpages
and these information will be sent to the server. These fields are not
visible directly to the user, but can be viewed using view source
option from the browsers. The hidden form fields are as given below:
<input type='hidden' name='siteName' value='java2novice'/>
1)Cookies in Servlet:
A cookie is a small piece of information that is persisted between the multiple
client requests.
A cookie has a name, a single value, and optional attributes such as a comment,
path and domain qualifiers, a maximum age, and a version number.
How Cookie works:
By default, each request is considered as a new request. In cookies technique,
we add cookie with response from the servlet. So cookie is stored in the cache
of the browser. After that if request is sent by the user, cookie is added with
request by default. Thus, we recognize the user as the old user.
Types of Cookie:
There are 2 types of cookies in servlets.
1)Non-persistent cookie
2)Persistent cookie
1)Non-persistent cookie:
It is valid for single session only. It is removed each time when user
closes the browser.
Persistent cookie:
It is valid for multiple session . It is not removed each time when user closes
the browser. It is removed only if user logout or signout.
Advantage of Cookies:
Simplest technique of maintaining the state.
Cookies are maintained at client side.
Disadvantage of Cookies
It will not work if cookie is disabled from the browser.
Only textual information can be set in Cookie object.
Cookie class:
javax.servlet.http.Cookie class provides the functionality of using
cookies. It provides a lot of useful methods for cookies.
Constructor of Cookie class
Constructor Description
Cookie() constructs a cookie.
Cookie(String name, String value) constructs a cookie with a specified
name and value.
Useful Methods of Cookie class:
There are given some commonly used methods of the Cookie class.
| Method | Description |
| public void setMaxAge(int expiry) | Sets the maximum age of the cookie in seconds.\ |
| public String getName() | Returns the name of the cookie. The name cannot be changed after creation. |
| public String getValue() | Returns the value of the cookie. |
| public void setName(String name) | changes the name of the cookie. |
| public void setValue(String value) | changes the value of the cookie. |
Other methods required for using Cookies:
For adding cookie or getting the value from the cookie, we need some methods
provided by other interfaces. They are:
public void addCookie(Cookie ck):method of HttpServletResponse
interface is used to add cookie in response object.
public Cookie[] getCookies():method of HttpServletRequest interface is
used to return all the cookies from the browser.
How to create Cookie:
Let's see the simple code to create cookie.
Cookie ck=new Cookie("user","alok sharma");//creating cookie object
response.addCookie(ck);//adding cookie in the response
How to delete Cookie:
Let's see the simple code to delete cookie. It is mainly used to logout or
signout the user.
Cookie ck=new Cookie("user","");//deleting value of cookie
ck.setMaxAge(0);//changing the maximum age to 0 seconds
response.addCookie(ck);//adding cookie in the response
How to get Cookies:
Let's see the simple code to get all the cookies.
Cookie ck[]=request.getCookies();
for(int i=0;i<ck.length;i++){
out.print("<br>"+ck[i].getName()+" "+ck[i].getValue());//printing name and value
of cookie
}
Simple example of Servlet Cookies:
In this example, we are storing the name of the user in the cookie object and
accessing it in another servlet. As we know well that session corresponds to the
particular user. So if you access it from too many browsers with different
values, you will get the different value.
index.html:
|
<form action="servlet1" method="post"> Name:<input type="text" name="userName"/><br/> <input type="submit" value="go"/> </form> |
FirstServlet.java
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class FirstServlet extends HttpServlet { public void doPost(HttpServletRequest request, HttpServletResponse response){ try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); String n=request.getParameter("userName"); out.print("Welcome "+n); Cookie ck=new Cookie("uname",n);//creating cookie object response.addCookie(ck);//adding cookie in the response //creating submit button out.print("<form action='servlet2'>"); out.print("<input type='submit' value='go'>"); out.print("</form>"); out.close(); }catch(Exception e){System.out.println(e);} } } |
SecondServlet.java
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SecondServlet extends HttpServlet { public void doPost(HttpServletRequest request, HttpServletResponse response){ try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); Cookie ck[]=request.getCookies(); out.print("Hello "+ck[0].getValue()); out.close(); }catch(Exception e){System.out.println(e);} }} |
web.xml
<web-app>
<servlet>
<servlet-name>s1</servlet-name>
<servlet-class>FirstServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s1</servlet-name>
<url-pattern>/servlet1</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>s2</servlet-name>
<servlet-class>SecondServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s2</servlet-name>
<url-pattern>/servlet2</url-pattern>
</servlet-mapping>
</web-app>
2)Hidden Form Field:
In case of Hidden Form Field a hidden (invisible) textfield is used for
maintaining the state of an user.
In such case, we store the information in the hidden field and get it from
another servlet. This approach is better if we have to submit form in all the
pages and we don't want to depend on the browser.
Let's see the code to store value in hidden field.
<input type="hidden" name="uname" value="alok sharma">
Here, uname is the hidden field name and alok sharma is the hidden field value.
Real application of hidden form field:
It is widely used in comment form of a website. In such case, we store page id
or page name in the hidden field so that each page can be uniquely identified.
Advantage of Hidden Form Field:
It will always work whether cookie is disabled or not.
Disadvantage of Hidden Form Field:
It is maintained at server side.
Extra form submission is required on each pages.
Only textual information can be used.
Example of using Hidden Form Field:
In this example, we are storing the name of the user in a hidden textfield
and getting that value from another servlet.
index.html
|
<form action="servlet1"> Name:<input type="text" name="userName"/><br/> <input type="submit" value="go"/> </form> |
FirstServlet.java:
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class FirstServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response){ try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); String n=request.getParameter("userName"); out.print("Welcome "+n); //creating form that have invisible textfield out.print("<form action='servlet2'>"); out.print("<input type='hidden' name='uname' value='"+n+"'>"); out.print("<input type='submit' value='go'>"); out.print("</form>"); out.close(); }catch(Exception e){System.out.println(e);} }} |
SecondServlet.java:
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SecondServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); //Getting the value from the hidden field String n=request.getParameter("uname"); out.print("Hello "+n); out.close(); }catch(Exception e){System.out.println(e);} } } |
web.xml:
<web-app>
<servlet>
<servlet-name>s1</servlet-name>
<servlet-class>FirstServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s1</servlet-name>
<url-pattern>/servlet1</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>s2</servlet-name>
<servlet-class>SecondServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s2</servlet-name>
<url-pattern>/servlet2</url-pattern>
</servlet-mapping>
</web-app>
3)URL Rewriting:
In URL rewriting, we append a token or
identifier to the URL of the next Servlet or the next resource. We can send
parameter name/value pairs using the following format:
url?name1=value1&name2=value2&??
A name and a value is separated using an equal = sign, a parameter name/value
pair is separated from another parameter using the ampersand(&). When the user
clicks the hyperlink, the parameter name/value pairs will be passed to the
server. From a Servlet, we can use getParameter() method to obtain a
parameter value.
Advantage of URL Rewriting:
It will always work whether cookie is
disabled or not (browser independent).
Extra form submission is not required on each pages.
Disadvantage of URL Rewriting:
It will work only with links.
It can send Only textual information.
Example of using URL Rewriting:
In this example, we are maintaning the state of the user using link. For this
purpose, we are appending the name of the user in the query string and getting
the value from the query string in another page.
index.html
|
<form action="servlet1"> Name:<input type="text" name="userName"/><br/> <input type="submit" value="go"/> </form> |
FirstServlet.java:
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class FirstServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response){ try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); String n=request.getParameter("userName"); out.print("Welcome "+n); //appending the username in the query string out.print("<a href='servlet2?uname="+n+"'>visit</a>"); out.close(); }catch(Exception e){System.out.println(e);} } |
SecondServlet.java:
|
import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SecondServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) try{ response.setContentType("text/html"); PrintWriter out = response.getWriter(); //getting value from the query string String n=request.getParameter("uname"); out.print("Hello "+n); out.close(); }catch(Exception e){System.out.println(e);} }} |
web.xml:
<web-app>
<servlet>
<servlet-name>s1</servlet-name>
<servlet-class>FirstServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s1</servlet-name>
<url-pattern>/servlet1</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>s2</servlet-name>
<servlet-class>SecondServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s2</servlet-name>
<url-pattern>/servlet2</url-pattern>
</servlet-mapping>
</web-app>
4) HttpSession interface:
In such case, container creates a
session id for each user.The container uses this id to identify the particular
user.An object of HttpSession can be used to perform two tasks:
bind objects
view and manipulate information about a session, such as the session identifier,
creation time, and last accessed time.
How to get the HttpSession object ?
The HttpServletRequest interface provides two methods to get the object
of HttpSession:
public HttpSession getSession():Returns the current session associated
with this request, or if the request does not have a session, creates one.
public HttpSession getSession(boolean create):Returns the current
HttpSession associated with this request or, if there is no current session and
create is true, returns a new session.
Commonly used methods of HttpSession interface
public String getId():Returns a string containing the unique identifier
value.
public long getCreationTime():Returns the time when this session was
created, measured in milliseconds since midnight January 1, 1970 GMT.
public long getLastAccessedTime():Returns the last time the client sent a
request associated with this session, as the number of milliseconds since
midnight January 1, 1970 GMT.
public void invalidate():Invalidates this session then unbinds any
objects bound to it.
Example of using HttpSession:
In this example, we are setting the attribute in the session scope in one
servlet and getting that value from the session scope in another servlet. To set
the attribute in the session scope, we have used the setAttribute() method of
HttpSession interface and to get the attribute, we have used the getAttribute
method.
index.html
<form action="servlet1">
Name:<input type="text" name="userName"/><br/>
<input type="submit" value="go"/>
</form>
FirstServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class FirstServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response){
try{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String n=request.getParameter("userName");
out.print("Welcome "+n);
HttpSession session=request.getSession();
session.setAttribute("uname",n);
out.print("<a href='servlet2'>visit</a>");
out.close();
}catch(Exception e){System.out.println(e);}
}
}
SecondServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class SecondServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
try{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
HttpSession session=request.getSession(false);
String n=(String)session.getAttribute("uname");
out.print("Hello "+n);
out.close();
}catch(Exception e){System.out.println(e);}
}}
web.xml:
<web-app>
<servlet>
<servlet-name>s1</servlet-name>
<servlet-class>FirstServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s1</servlet-name>
<url-pattern>/servlet1</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>s2</servlet-name>
<servlet-class>SecondServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>s2</servlet-name>
<url-pattern>/servlet2</url-pattern>
</servlet-mapping>
</web-app>