question about session?

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Posts: 35

posted 19 years ago

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Problem

Consider the following JSP code (See exhibit).
What will it print for the very first request to this page as well as the web application that contains this page?

Code

<html><body>
<%
Integer count = (Integer) request.getSession(false).getAttribute("count");
if(count != null )
{
out.println(count);
}
else request.getSession(false).setAttribute("count", new Integer(1));
%>
Hello!
</body></html>

(Select 1 correct option.)

A. It will print Hello!
B. It will print Hello and will set the count attribute in the session.
C. It will throw a NullPointerException at request time.
D. It will not compile.

I will choose C, but the answer is B.
Why?

Baiju Mehta

Greenhorn

Posts: 18

posted 19 years ago

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Hi Romy,

In JSP, session tracking is true by default (<%@ page session=�true� %>

. So when you load this JSP page, request.getSession(false) will return an existing session. Now, since the count attribute is not set in the session scope, getAttribute(�count�) will return null which means else clause is executed.

Hope this helps.

Ritu varada

Ranch Hand

Posts: 119

posted 19 years ago

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Your answer would be true for a servlet but JSP's operate a little differently. I will post a sample code which shows a _jspService method. There you can see that we already have a session object.

public void _jspService(HttpServletRequest request, 
   HttpServletResponse  response) 
     throws IOException, ServletException {
 
     JspFactory _jspxFactory = null;
     PageContext pageContext = null;
     HttpSession session = null;
     ServletContext application = null;
     ServletConfig config = null;
     JspWriter out = null;
     Object page = this;
     String  _value = null;
     try {
          if (_jspx_inited == false) {
             _jspx_init();
             _jspx_inited = true;
         }
         _jspxFactory = JspFactory.getDefaultFactory();
         response.setContentType("text/html");
         pageContext = _jspxFactory.getPageContext(this, 
       request,response, "", true, 8192, true);
 
         application = pageContext.getServletContext();
         config = pageContext.getServletConfig();
         session = pageContext.getSession();
         out = pageContext.getOut();
         // begin
         out.write("\r\n<html>\r\n<body>\r\n");
        // end
        // begin [file="E:\\jsp\\jsptest.jsp";from=(3,2);to=(5,0)]
        Date d = new Date();
        String today =  DateFormat.getDateInstance().format(d);
        // end
        // begin
        out.write("\r\nToday is: \r\n<em> ");
        // end
........

Hope this makes it clear!

SCJP,SCWCD(1.4)

Mikalai Zaikin

Saloon Keeper

Posts: 3929

posted 19 years ago

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Originally posted by Romy Huang:
Problem

Consider the following JSP code (See exhibit).
What will it print for the very first request to this page as well as the web application that contains this page?

Code

<html><body>
<%
Integer count = (Integer) request.getSession(false).getAttribute("count");
if(count != null )
{
out.println(count);
}
else request.getSession(false).setAttribute("count", new Integer(1));
%>
Hello!
</body></html>

(Select 1 correct option.)

A. It will print Hello!
B. It will print Hello and will set the count attribute in the session.
C. It will throw a NullPointerException at request time.
D. It will not compile.

I will choose C, but the answer is B.
Why?

Hi Romy !

The correct answer is "B".

JSP sessions are configured via 'page' directive and 'session' attribute.
By default sessions are always *enabled*. This is equal if you would have :

in your JSP.

So, when you call 'getAttribute("count")' the session already exists - it was *implicitly* created by container for you. No NullPointerException thrown.

If you put the following code in your JSP :

then you will get a NullPointerException in server's log file and 500 error in browser as you expected.

regards,
MZ

Romy Huang

Ranch Hand

Posts: 35

posted 19 years ago

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