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Servlets

Session attribute keeps reverting to the declared value

Ray mann

Ranch Hand

Posts: 48

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posted 2 years ago

Hi there guys,
being new to java I am a bit lost as to why my session attribute for this banking app wont add the deposits to the session var... it just keeps going back to the amount I set it to originally - so if I set the beginning balance to 3000 then deposit 100 it becomes 3100, but if I then try deposit another amount eg. a extra 200 it becomes 3200 not 3300 like it should be !! I must be dong something wrong can any one help me figure out what it is I have done wrong and how I can fix it ?

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.net.*;
import java.text.DecimalFormat;

public class SessionBank extends HttpServlet
{
    //Class Variable
   // double accBalance = 3000.00;

DecimalFormat myFormat = new DecimalFormat("$#,000.00");

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

public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
    {
        //Makes instance variable a local variable

//double accBalance = 0.00;

//HttpSession session = request.getSession();

//session.setAttribute("accBalance",new Double(accBalance));

//Set MIME type of content returned to browser

response.setContentType("text/html");

PrintWriter out = response.getWriter();

//Starts outputting the HTML Form

out.println("<html>");
        out.println("<head>");
        out.println("<title>Online Bank Simulator</title>");
        out.println("</head>");
        out.println("<body>");
		out.println("<hr color=\"#808000\">");
        out.println("<center><h1>Banking Simulation</h1></center>");
        out.println("<form method=\"POST\" action=\"../servlet/SessionBank\">");
        out.println("<center>Amount: <input type=\"text\" name=\"Amount\" size=\"20\"></center>");

//Decides which action to take
          double accBalance = 3000.00;

String action = request.getParameter("act");

if(action != null)
        {
            if(action.equals("Deposit"))
            {
             	double amount;
			    String strAmount = request.getParameter("Amount");
				try
				{
				amount = Double.parseDouble(strAmount);

if(amount <= 0.00)
                  {
                  out.println("<h2>Error: The amount is either null or a Negative</h2>");
              	  }
              	  else
              	  {
                   accBalance = accBalance + amount;

double amount;
				String strAmount = request.getParameter("Amount");
				try
				{
					amount = Double.parseDouble(strAmount);

if(amount <= 0.00)
                {
                   out.println("<h2>Error: The amount is either null or a Negative</h2>");
                }
               else
                {
                    accBalance = accBalance - amount;

out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
                }
                }
								catch(NumberFormatException ex)
								{
									out.println("<h2>Error: The amount is either null or a Negative...exception thrown and caught</h2>");
				}
            }
            else if(action.equals("Balance"))
            {
                out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
            }
            else
            {
                out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
            }
        }
        else
        {
            out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
        }

//current session Attribute (get&set)

HttpSession session = request.getSession(true);

session.setAttribute("accBalance",new Double(accBalance));

//HTML

out.println("<table width=\"35%\" align=\"center\">");
        out.println("<tr><td width=\"33%\" align=\"center\">");
        out.println("<input type=\"submit\" name=\"act\" value=\"Balance\">");
        out.println("<td width=\"33%\" align=\"center\">");
       	out.println("<input type=\"submit\" name=\"act\" value=\"Deposit\">");
        out.println("</td>");
        out.println("<td width=\"33%\" align=\"center\">");
        out.println("<input type=\"submit\" name=\"act\" value=\"Withdraw\">");
        out.println("</td>");
        out.println("<td width=\"33%\" align=\"center\">");
        out.println("</td>");
        out.println("</tr>" );
        out.println("</table>");
        out.println("<br>");
        out.println("</form>");
        out.println("<hr color=\"#80800\">");
        out.println("</body>");
        out.println("</html>");
    }

}//end class

thanks in advanced

Stefan Evans

Bartender

Posts: 1836

posted 2 years ago

Well you correctly store the value into session:

I don't see anywhere where you retrieve that value back again though.
Something like

Ray mann

Ranch Hand

Posts: 48

I like...

posted 2 years ago

ok cool thanks that helped it works almost as desired now.....

Why is it that when open 2 windows of my browser of this servlet they share the same balance ? I was under the impression that it should not behave this way - they (the separate windows) should be separate and have independent session values ? So the if I deposit 100 into the first Window then deposit 400 in the second Window, the second Widow should have a balance of 400 not 500 like it is doing now. Hope this makes sense to you .

How do i get it to not react in that way, my guide lines in my course work say that if I open a second instance of my browser the 2 threads should not interfere with each other. Also I should not use synchronization only sessions to do this, I am not to sure if that is even possible, if it is how would I go about doing this ?

My code so far:

import java.lang.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.net.*;
import java.text.DecimalFormat;

public class SessionBank extends HttpServlet //throws ServletException, IOException
{
    //Class Variable
   // double accBalance = 3000.00;

DecimalFormat myFormat = new DecimalFormat("$#,000.00");

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

public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
    {
        //Makes instance variable a local variable

//Set MIME type of content returned to browser
        HttpSession session = request.getSession();

response.setContentType("text/html");

PrintWriter out = response.getWriter();

//Starts outputting the HTML Form

out.println("<html>");
        out.println("<head>");
        out.println("<title>Online Bank Simulator</title>");
        out.println("</head>");
        out.println("<body>");
		out.println("<hr color=\"#808000\">");
        out.println("<center><h1>Banking Simulation</h1></center>");
        out.println( session.getId());
        if(session.getAttribute("accBalance")!=null)
        out.println(session.getAttribute("accBalance"));
        out.println("<form method=\"POST\" action=\"../servlet/SessionBank\">");
        out.println("<center>Amount: <input type=\"text\" name=\"Amount\" size=\"20\"></center>");

//Decides which action to take
        //

double accBalance = 0.0;
		Double savedBalance = (Double)session.getAttribute("accBalance");

if (savedBalance != null)
		{
		    accBalance = savedBalance.doubleValue();
		}
//////////
        String action = request.getParameter("act");

double amount;
				String strAmount = request.getParameter("Amount");
				try
				{
					amount = Double.parseDouble(strAmount);

out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
                }
                }
				catch(NumberFormatException ex)
				{
					out.println("<h2>Error: The amount is either null or a Negative...exception thrown and caught</h2>");
				}
            }
            else if(action.equals("Balance"))
            {
                out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
            }
            else
            {
                out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
            }
        }
        else
        {
            out.println("<br><center>Balance:"+myFormat.format(accBalance)+" </center> <br>");
        }

session.removeAttribute("accBalance");

session.setAttribute("accBalance",new Double(accBalance));

//HTML

}//end class

Thanks

Stefan Evans

Bartender

Posts: 1836

posted 2 years ago

What you are seeing is the expected behaviour. Servlet sessions are maintained by a browser session cookie. By default, most browsers share the same process/cookies among all of its windows. This is normally desirable so if you open a new window from your current one via a link, it retains the same session.

You can create multiple sessions in Internet Explorer, via the "File: New Session" option which opens up a new browser that does not share existing values.

Also as a suggestion, a lot of this servlet would be better off as a JSP page. All of those 'out.println' calls outputting HTML are the entire reason JSP was invented in the first place!

Ray mann

Ranch Hand

Posts: 48

I like...

posted 2 years ago

ok so you telling me that the text book is wrong ? I see how what you saying makes sense (PHP works like that). Yes I know that I only did it this way because the next task I must submit states that I should update this code to use a separate .JSP file for the display of the User Interface (Is it referred to as Front End and back end in Java ? - when I was learning PHP we used terms like that)
Thanks for all the help!

Stefan Evans

Bartender

Posts: 1836

posted 2 years ago

It is possible the textbook is outdated. "if I open a second instance of my browser the 2 threads should not interfere with each other. "
That sounds right to me. The only question is how do you open up a second (independent) instance? It is no longer as easy as just clicking on a shortcut (though it used to be)
How old is the textbook?

If you know the PHP concepts, many of them are the same. and yes referring to the browser as the front end and the server as the back end is the standard approach.
Understanding which bits of code run where is a fundamental concept.

Bear Bibeault

Author and ninkuma
Marshal

Posts: 66143

141

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posted 2 years ago

It is very dependent upon browser and platform.

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Bear Bibeault

Author and ninkuma
Marshal

Posts: 66143

141

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posted 2 years ago

Stefan Evans wrote:and yes referring to the browser as the front end and the server as the back end is the standard approach.

Clarification: which means that server-side templating engines like JSP and PHP are part of the backend. Lots of people refer to these as front-end technologies because they equate "front-end" with the GUI layer.

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Ray mann

Ranch Hand

Posts: 48

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posted 2 years ago

Thanks Guys i get it ... so yes the copy i have is 2006 print ....I found a digital copy of the book...(Text Book Question 6 on this page)
its possible I misunderstood misinterpreted the question?
Thanks again you guys rock!!

It is sorta covered in the JavaRanch Style Guide.

Session attribute keeps reverting to the declared value

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