Servlet question

Posted on 2006-06-25
Last Modified: 2012-05-05
I have a question about an example of servlet that illustrates the use of session.

This servlet forwards the user first to the url "urlPage0" where the user needs to introduce a name.
So urlPage0 is a JSP with a form, with textfield asking for a name.
The value of the action of the form is the servlet and it gives a hidden field "étape" with the value 1.
Then the servlet forwards the user to "urlPage1" where the name is printed and asks for an age.
A hidden parameter is given again "étape" with value 2.
Then finally the user gets forwarded to "urlPage2" where name and age are printed.

I don't understand the use of the line request.setAttribute("nom",""); in étape0 method
like request.setAttribute("age",""); in étape1.

and what is the use of
followed by

public class main extends HttpServlet{
String msgErreur=null;
String urlPage0=null;
String urlPage1=null;
String urlPage2=null;
String urlErreur=null;

//-------- INIT
public void init(){
      ServletConfig config=getServletConfig();
      if(urlPage0==null || urlPage1==null || urlPage2==null){
            msgErreur="Configuration incorrecte";

//-------- GET
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException{
      String étape=request.getParameter("etape");
      HttpSession session=request.getSession();
      if(étape==null) étape0(request,response,session);
      if(étape.equals("1")) étape1(request,response,session);
      if(étape.equals("2")) étape2(request,response,session);

//-------- POST
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException{

//-------- étape0
public void étape0(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{

//-------- étape1
public void étape1(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{
      String nom=request.getParameter("nom");
      if(nom==null) étape0(request,response,session);
            ArrayList erreurs=new ArrayList();
            erreurs.add("Nous n'avez pas indiqué de nom");

//-------- étape2
public void étape2(HttpServletRequest request, HttpServletResponse response, HttpSession session)
throws IOException, ServletException{
      String nom=(String)session.getAttribute("nom");
      if(nom==null) étape0(request,response,session);
      String age=request.getParameter("age");
      if(! Pattern.matches("^\\s*\\d+\\s*$",age)){
            ArrayList erreurs=new ArrayList();
            erreurs.add("Age invalide");

Question by:matthew016
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Accepted Solution

pablomorales earned 375 total points
ID: 16988182
In a Servlet/JSP you can store information in the request and in the session. The following code in etape0:


Is storing the variable "nom" with a value of "" in the request. This will (I guess) be read by the JSP code in ulrPage0 (using request.getAttribute("nom")). This would probably allow you to specify a default name (blank in this case) to be shown in the form.

In etape1 the code is reading the value that the user entered in the form from the request (using nom=request.getParameter("nom")) and then storing the value in the request (using request.setAttribute("nom",nom)) so that it can be read the JSP code contained in urlPage1. It is also storing it in the session (session.setAttribute("nom",nom)) so that it can be read later.

In etape2 the "nom" is read from the session and stored in the request so that it can be used by the JSP code served by urlPage2.

I hope this helps you.


Author Comment

ID: 16988794
Thank you, I just have one more question about what u said :
storing the value in the request (using request.setAttribute("nom",nom)) so that it can be read the JSP code contained in urlPage1. It is also storing it in the session (session.setAttribute("nom",nom)) so that it can be read later.
I don't understand why in this example "nom" is stored in the request + in the session.
Why not only in the session and then JSP retrieve it from the session ?
LVL 16

Assisted Solution

suprapto45 earned 125 total points
ID: 16990821

Attributes in the request are removed automatically and hence request is good if you only would like the parameter to be read only in the subsequent JSP. Session, unlike request, will retain the attributes until it is invalidated or you removed it manually. Session is useful if you want the parameter to be available in every JSP but you must bear in mind that your session may get bigger and bigger if you do not remove them accordingly.


Expert Comment

ID: 16993956
My guess is that the designer was trying to keep the interface between the JSPs and the servlet consistant and to separate concerns and responsabilities. Kind of like in the MVC programming pattern ( The JSP is the view and the Servlet acts as the controller and also stores the data (more or less what the model normaly does). There seems to be no independant model in this example. Ideally the data should be stored by the model (Javabean) and the view (JSP) and controller (Servlet) should read the data from the model. I agree with you in that is not as efficient as just reading the data from the session for such a small example, but  for larger proyects efficiency may need to be sacrificied.

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