my first session bean

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ian ward - 21 Jun 2005 11:14 GMT

I am exploring what I can do with my first page of jsp. I have included
a useBean action which allows me to access the state of a class so that
I can display it.
If the bean has session scope then, when I call the page several times,
I get repetitions of the displayed data. It is as if the page is not
'reinitialised'. In fact, it seems that, with each transformation into
html, the previous transformations of java code are retained rather
than the page being regenerated from scratch. This doesn't happen if
the bean has request scope.
Is this the normal behaviour for a jsp page which uses a session bean?
I had understood that the scope of a bean determines its availability
but wouldn't normally impact the construction of a page which
declares/uses it.

Thanks
Ian

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Wendy Smoak - 21 Jun 2005 15:12 GMT

> If the bean has session scope then, when I call the page several times,
> I get repetitions of the displayed data. It is as if the page is not
[quoted text clipped - 6 lines]
> but wouldn't normally impact the construction of a page which
> declares/uses it.

Sounds normal to me. If you put something in session scope, it stays there.
What is this "reinitialization" that you think should be happening? Why
don't you post some of the JSP, the output, and explain what you [thought
would|wanted to] happen.

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Wendy Smoak

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ian ward - 21 Jun 2005 19:53 GMT

OK, well here's the page.......

<%@ page import = "beans.SystemBean, businessservices.Programme,
java.util.List, java.util.ListIterator" %>
<jsp:useBean id="dialogerSystem" class="beans.SystemBean"
scope="request"/>

<HTML>
<BODY BGCOLOR="white">

<% String s_userid=request.getParameter("userid");
String s_password=request.getParameter("password");
%>

The user id is <%= s_userid %>
<br>

<% List theProgrammes;
Programme aProgramme;
String did_it_work = new String();

if (!dialogerSystem.logOn(s_userid,s_password)) {
    did_it_work = "failed";
}
else
    did_it_work = "succeeded";
session.setAttribute("logonStatus",did_it_work);

if (did_it_work == "failed") {
%>

    <jsp:forward page="/dialoger/jsp/logon.jsp"/>

<% }
else {
%>

logon ok - we're going to list the programmes available to this user
<br><br>

<%     theProgrammes = dialogerSystem.getTheProgrammes();
    for (ListIterator l = theProgrammes.listIterator();l.hasNext();) {
        aProgramme = (Programme)l.next();
%>

<a href="/dialoger/jsp/programmeContents.jsp">

<%        out.println(aProgramme.getName());
%>

</a>
<br>

<%
    }
}
%>
clicking on one of these above will normally display its contents on
the next page

</BODY>
</HTML>

...in this version the scope is 'request'but the rest of the code is
unchanged....I think the critical line is the out.println where a name,
having been retrieved from a list belonging to the bean, is displayed.

It seems logical to me to expect that a page, if rerun without having
been changed, should do the same thing. The bean, in session scope
stays there of course but it shouldn't change just 'cos you've accessed
some property or other.
I want it to have session scope 'cos I might use it elsewhere on
another page, just like the session attribute I've created.
Can you explain what you mean by putting something in session scope?

thanks
Ian

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Andrew Thompson - 21 Jun 2005 20:20 GMT

...

> <jsp:useBean id="dialogerSystem" class="beans.SystemBean"
> scope="request"/>

...

> Can you explain what you mean by putting something in session scope?

Well, I'm not Wendy, but..

<jsp:useBean id="dialogerSystem" class="beans.SystemBean"
scope="session"/>

See -
<http://java.sun.com/products/jsp/tags/10/syntaxref10.fm14.html>

HTH

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Andrew Thompson
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ian ward - 21 Jun 2005 22:00 GMT

I think I'm finding your second contribution a bit more useful than
your first.
Having read it through I'm wondering if it implies, on repeated
reloading of the page, the creation of several variables of the same
name pointing at the same instantiated object (I had thought before
reading this that the object may be duplicated) - and then, for some
strange reason, the 'use' of all of these when the object's attribute
is accessed/displayed via the variable(s). Seems unlikely? My problem
seems to go a bit further than just understanding the meaning of
'session scope' - as I first asked, does it have something to do with
the way the html page is generated? Maybe I expressed myself badly.

Ian

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Andrew Thompson - 22 Jun 2005 06:04 GMT

> I think I'm finding your second contribution a bit more useful than
> your first.

Note that two people have each contributed a single reply to your post..
(before this post - obviously)

> ..My problem
> seems to go a bit further than just understanding the meaning of
> 'session scope' ..

Aahh.. I probably led the thread off on a tangent then.

I will bow out of this thread now and 'hand the reigns'
back to the servlet guru's - like Wendy!

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Wendy Smoak - 22 Jun 2005 17:38 GMT

> Having read it through I'm wondering if it implies, on repeated
> reloading of the page, the creation of several variables of the same
> name pointing at the same instantiated object [...] and then, for some
> strange reason, the 'use' of all of these when the object's attribute
> is accessed/displayed via the variable(s). Seems unlikely?

I'm having trouble figuring out what your question is. Can you restate it?

I suggest that you read the Servlet and JSP specifications. They will tell
you exactly what the container is required to do for you, (and some things
it may/must not do also.) You also need a good handle on the HTTP
request/response cycle. No offense intended if you already know this
stuff...

http://www.servlets.com/ is a good reference page.

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Wendy Smoak

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ian ward - 22 Jun 2005 21:15 GMT

Yes, I'm being a bit intuitive about this, trying to avoid a mountain
of reading for what is a reasonably simple try-out.

I have tried to ask my question as clearly as possible. I've also tried
to make some semi-intelligent guesses about what seems to be happening
based on what (little) I do know. If what I've described doesn't seem
odd to you then I think I probably do need to go into this a bit more
as it doesn't seem very logical to me.

Thanks anyway
Ian

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John C. Bollinger - 24 Jun 2005 04:23 GMT

> Yes, I'm being a bit intuitive about this, trying to avoid a mountain
> of reading for what is a reasonably simple try-out.
[quoted text clipped - 4 lines]
> odd to you then I think I probably do need to go into this a bit more
> as it doesn't seem very logical to me.

1) You do not need to use beans to use JSP.

2) If you do use beans in your JSP, you need to understand what the
various values of the scope attribute mean:

+ application: A bean instance associated with the specified id is
sought in the application (via its ServletContext). If found, that
instance is used in the page, otherwise a new instance is created,
stored in the application and used in the page as it services the
current request. The same instance may later be used by any page or
servlet in the same webapp.

+ session: A bean instance associated with the specified id is sought
in the session (via its HttpSession). If found, that instance is used
in the page, otherwise a new instance is created, stored in the session
and used in the page as it services the current request. The same
instance may later be used by any page or servlet in the same webapp,
while servicing another request in the same session. Other distinct
bean instances may be associated with the same name in other sessions.

+ request: A bean instance associated with the specified id is sought in
the request (via its HttpServletRequest). If found, that instance is
used in the page, otherwise a new instance is created, stored in the
request and used in the page as it services the current request. The
same instance may later be used by any page or servlet in the same
webapp, while servicing the same HTTP request (which can happen when a
JSP forward or include is used -- but do not confuse that with an HTTP
redirect). Other distinct bean instances may be associated with the
same name in other requests.

+ page: A bean instance created and associated with the specified id in
the context of the JSP in which the declaration appears for the duration
of that page's handling of the current request. Every invocation of the
page will cause a distinct bean instance to be created and used, and the
bean instances will not be visible to other pages.

3) I personally find it helpful to keep in mind that JSP pages are
internally translated into Java source for servlets, then compiled to
classes for use by the container. Understanding how JSP syntax and
features map to Java code guides me through some of the intricacies of
JSP. At least some knowledge of these correspondences is essential if
you use JSPs and servlets together in one web application.

4) The ultimate reference material for JSP is the JSP specification,
available free from Sun. The most recent version is 2.0. You may also
want to obtain the Servlet spec, also available from Sun; its most
recent version is 2.4. The size of these specs is a bit daunting, but
they contain a great deal of material that you can skip over if you are
just trying to orient yourself. For servlets, you may also want to get
the HTML version of their API specification; it is part of the J2EE API
spec.

Signature

John Bollinger
jobollin@indiana.edu

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ian ward - 24 Jun 2005 08:14 GMT

John,
Many thanks for your clear explanations. As I said to Wendy, I have an
understanding of these concepts which is perhaps less precise than both
the full spec (which I appreciate is the final reference) and what you
have given me here. My original query was based on that understanding
and I was reasonably sure, according to my current grasp of JSP, that
the behaviour I was seeing was not normal. I understand why those who
replied to my query didn't seem to connect with the issue because, as
it turns out, it was not in the JSP, nor, I believe, in the scope
attribute used (although this may or may not 'reveal' the underlying
problem). I developed my Java code with certain scenarios in mind. The
use of that code from another level of program control (ie the browser)
introduces possibilities which I will have to allow for. In this
particular case, code was being rerun which 'shouldn't be' and 'state'
was being reloaded into my bean. I had thought that perhaps I was
seeing something to do with your point 3 but in fact it was back in the
bean and I simply needed to ensure that the state of my bean is
'always' is it should be when I call the code concerned.
Ian
PS You don't fancy having a stab at my 'totally unexplainable tomcat
mystery' do you?!:) (warning, the issue changes halfway through the
thread!)

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my first session bean

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