State management servlet

State management
Http protocol that is used for communication between client and web server
is a stateless protocol that is for each request new HTTP connection is
established between client and server.
The disadvantage of stateless protocol is that server fails to recognize that a
series of request submitted by a user or logically related and represent a
single task form end user prospective.
Problem of state management deals with the maintenance of state over
the stateless protocol.
Following use case explains the problem of state management:

Example 1- program ServletContext object is used for state management.
Program save with Name: index.html
<html>
<head><title>Inter Application Forwarding..</title></head>
<body>
<form method="get" action="welcomeServlet">
Name :<input type="text" name="txtName"><br>
<input type="submit" value="submit">
</form>
</body>
</html>
Program save with Name: WelcomeServlet.java
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class WelcomeServlet extends HttpServlet
{
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
response.setContentType("text/html");
String name=request.getParameter("txtName");
PrintWriter out=response.getWriter();
out.println("<b> Welcome, "+name+"</b>");
ServletConfig cnf=getServletConfig();
ServletContext ctx=cnf.getServletContext();
ctx.setAttribute("user",name);
out.println("<br> <form action=tourServlet method=get>");

out.println("<br><input type=submit value="Take a Tour "></form>" );
out.close();
}
}
Program save with Name: TourServlet.java
import java.io.*;
public class TourServlet extends HttpServlet
{
{
ServletConfig cnf=getServletConfig();
ServletContext ctx=cnf.getServletContext();
String name=(String)ctx.getAttribute("user");
out.println("<b> Sorry, "+name+"</b>");
out.println("<br>Site is down for routine maintenance, vist again later..");
out.close();
}
}
Program save with Name: web.xml
<web-app>
<servlet>
<servlet-name>s1</servlet-name>
<servlet-class>WelcomeServlet</servlet-class>
</servlet>
<servlet-mapping>
<url-pattern>welcomeServlet</url-pattern>
</servlet-mapping>
<servlet>
<servlet-class>TourServlet</servlet-class>
</servlet>
<servlet-mapping>
<url-pattern>tourServlet</url-pattern>
</servlet-mapping>
</web-app>

Output :
Run program on two browser and test with both users one by one
Yes This is not complete solution of state management so there is requirement of another
technique.
Problem with ServletContext object when we use it for state management:
Name of user is user specific which must not be store in ServletContext because other user
is override it.
Note: In ServletContext store only application specific data not store user specific data.
Following solution is device to solve the problem of State
management:
1- Cookies
2- Hidden form fields
3- URL rewriting
4- HttpSession object
1- Cookies
A cookie represent information in the form of key value pair which is send by the
server as part of response and is submitted by the browser to the server as part of
the sub sequent request.
Cookie provides a simple mechanism of maintaining user information between
multiple requests.
Cookie can be of 2 types:
1- Persistent cookies
2- Non Persistent cookies
1- Persistent cookies:

Persistent cookies remain valid for multiple sessions.
They are store by the browser in a text file on the client machine to be use
again and again.
2- Non Persistent cookies:
Non persistence cookies remain valid only for a
single session, they are store in the browser cache during the session and
discarded by the browser when the session is complete.
Note: By default all cookies are non persistent.
Advantage:
1- Simplicity is main advantage of this approach.
Disadvantage:
1- This method of state maintenance is browser dependent.
2- Only textual information can be persisted between request (Object Map,
File are not persisted in cookies).
3- Persistent cookies do not differentiate users on the machine.
Note: Netscape first introduce cookies.
javax.servlet.http.Cookie, class provides object representation of cookies.
Cookies object can be created using following constructor:
public Cookie(String name, String value);

Method of Cookie class:
1- getName():
Is use to obtain the name of the cookie.
public String getName();
2- getValue():
Is use to obtain the value of the cookie.
public String getValue();
3- setMaxAge():
Is use to specify the time for which cookie remain valid.
This method makes cookies persistent.
public void setMaxAge(int seconds);
etc…
addCookie():
Method of HttpServletResponse interface is used to add
the cookie as a part of the response.
public void addCookie(Cookie ck);
getCookies():
Method of HttpServletRequest interface is used to
obtain the cookies which are submitted as part of the request.
public Cookie[] getCookies();
Example 1- program for state management using Cookies.
<html>
<body>
</form>
</body>
</html>
import java.io.*;

{
{
Cookie ck=new Cookie("user",name);
response.addCookie(ck);
ck.setMaxAge(60); // for making Persisted Cookies
out.println("<br><input type=submit value="Take a Tour"></form>" );
out.close();
}
}
import java.io.*;
{
{
String name="Gust";
Cookie ch[]=request.getCookies();
if(ch!=null)
name=ch[0].getValue();
out.close();
}
}
<web-app>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>

<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
</web-app>
Output :
Yes, now state is managed it will give correct output.
2- Hidden form field
Hidden form field provide the browser independent
method of maintaining state between requests. In this method information is
to persistence between one request to another is contain in invisible text
field which are added to the response page whenever a new request is
submitted using the response page value of invisible field are posted as
request parameter.
Limitation:
1- Only textual information can be persisted between requests.
2- This approach can only be used when request is submitted using the form
of response page that is this approach does not work with hyperlink.
Example 1- program for state management using Hidden form field.

<html>
<body>
</form>
</body>
</html>
import java.io.*;
{
{
out.println("<br><input type=hidden name=txtHidden
value=""+name+"">");
out.close();
}
}
import java.io.*;
{
{
String name=request.getParameter("txtHidden");
out.close();
}

}
<web-app>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
</web-app>
Output :
3- URL rewriting (mostly used)
In this approach information to be persisted between request is dynamically
appended to the URL in response page, whenever a request is submitted
using these URL, append information is submitted as request parameters.
Syntax of appending:
URL?paramName=value & paramName=value
Limitation:
1- Only textual information can be persisted using this approach.
2- In case of input form this approach work only if post request is submitted
(Because get request changed the URL).
Example 1- program for state management using URL rewriting (Using Form
Field).

<html>
<body>
</form>
</body>
</html>
import java.io.*;
{
{
out.println("<br> <form method=post action="tourServlet?userName="+name+"">");
out.close();
}
}
import java.io.*;
{
public void doPost(HttpServletRequest request, HttpServletResponse response)
{
String name=request.getParameter("userName");
if(name.equals(""))
name="Gust";
out.close();
}
}

<web-app>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
</web-app>
Output :
In this program we are using post method for submitting request on tourServlet that help
URL is overridden that is specified in program and if we using get request then new URL is
created instead of specified in program.
If we try to use get method then NullPointerException occure.
Example 1- program for state management using URL rewriting (Using Hyperlink).
<html>
<body>
</form>
</body>
</html>
import java.io.*;

{
{
out.println("<br><a href="tourServlet?userName="+name+"">Take a
Tour</a>");
out.close();
}
}
import java.io.*;
{
{
String name=request.getParameter("userName");
if(name.equals(""))
name="Gust";
out.close();
}
}
<web-app>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
<servlet>

</servlet>
<servlet-mapping>
</servlet-mapping>
</web-app>
Output :
In this program use get method because Hyperlink is generate get request.
4- HttpSession
javax.servlet.http.HttpSession is an interface of Servlet API.
Implementation of which is provided by the vendors. An object of type
HttpSession can be get created by the web server per user. This object is
used by the application developer to store user information between
requests.
Commonly used method of HttpSession interface:
1- setAttribute():
Is used to store an attribute in the session scope.
public void setAttribute(String name, Object value);
2- getAttribute():
Is used to obtain an attribute from the session scope.
public Object getAttribute(String name);
3- getAttributeNames():
Is used to find out the names of attributes saved in
session scope.
public Enumeration getAttributeNames();
4- removeAttribute():
Is used to remove an attributes from the session scope.
public boolean removeAttribute(String name);
Methods used For management Session Object:
5- isNew():
Is used to find out whether session is created for the current
request or not.
public boolean isNew();

6- setMaxInactiveInterval():
Is used to specify the time for which a session remains valid
even if no request is received from the client.
public void setMaxInactiveInterval(int second);
7-invalidate():
Is used to release a session.
public void invalidate();
etc…..
getSession():
getSession() method of HttpServletRequest interface is used
to obtain a HttpSession object.
public HttpSession getSession();
public HttpSession getSession(boolean createFlag);
Example 1- program for state management using HttpSession object.
<html>
<body>
</form>
</body>
</html>
import java.io.*;
{
{
HttpSession session=request.getSession();
session.setAttribute("uname",name);

session.setMaxInactiveInterval(3); // If user is ideal state max(3 second)
then
// Session object is destroyed.
out.println("<br><a href=" tourServlet ">Take a Tour</a>");
out.close();
}
}
import java.io.*;
{
{
HttpSession session=request.getSession();
String name=(String)session.getAttribute("uname");
// session.invalidate(); // Use for destroy Session object.
if(name==null)
name="Guest";
out.close();
}
}
<web-app>
<servlet>
</servlet>
<servlet-mapping>
</servlet-mapping>
<servlet>
</servlet>
<servlet-mapping>

</servlet-mapping>
</web-app>
Output :
Run program and test it frequently submitting request and wait 3 second then submit
request.
In order to manage session object web server create unique id for each
session. Session object are store in a map by the server using the id as key.
Unique identifier for each session is sent with the response in the form of
cookies or request parameter using URL rewriting.
When subsequent request are submitted session id is made available which
is used by the server to identify the session.

State management servlet

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