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From Raiden <rai...@wonko.inow.com>
Subject RE: How to synchronize based on session? (Prevent multiple submissions of forms)
Date Tue, 01 Jul 2003 22:01:36 GMT
Here's a thread that's discussing the same problem.  Though it's referring
to the use made in the Struts code, and I'm just talking about a small
synchronize call, it's essentially the same issue.  If the best way is to
synchronize on an object that's in the session, how do you properly get
that object in the session in the first place?  (It's almost a circular
problem).

Here's the thread:
http://www.mail-archive.com/tomcat-dev@jakarta.apache.org/msg41219.html

Thanks again for your help!

-Raiden



On Tue, 1 Jul 2003, Raiden wrote:

> Hi Stefan,
>
> Thanks for the explanation.  That makes sense, except how can you make
> sure that the object is added to the session only once, when the session
> is created?
>
> It would seem like you would need to synchronize on something else for
> adding the object to the session, because there might be a race
> condition (unless there is a special bit of code that is only ever called
> once, when the session is created).  Otherwise, two different threads
> might each have their own instance of the lock object.
>
> How do I guarantee that the lock object is only created once, at the time
> the session is created?
>
> Thanks,
> Raiden
>
>
> On Tue, 1 Jul 2003, Stefan Radzom wrote:
>
> > For security reasons your are not handed the internal
> > org.apache.catalina.session.StandardSession . Instead, the implicit session
> > you are trying to synchronize holds a reference to
> > org.apache.catalina.session.StandardSessionFacade which is backed by the
> > actual StandardSession to which all method calls are delegated. The facade
> > object can therefore change between different requests, thus making it not
> > appropriate for synchronisation.
> >
> > Try putting a dummy object into the session when creating a new session:
> > session.setAttribute("lock", new Object()) . On subsequent requests you can
> > get the attribute and obtain a lock on this object.
> >
> > Object lock = session.getAttribute("lock");
> > synchronized (lock) { ; }
> >
> > -Stefan
> >
> >
> > > -----Original Message-----
> > > From: Raiden [mailto:raiden@wonko.inow.com]
> > > Sent: Tuesday, July 01, 2003 8:19 PM
> > > To: tomcat-user@jakarta.apache.org
> > > Subject: How to synchronize based on session? (Prevent
> > > multiple submissions of forms)
> > >
> > >
> > > Hello,
> > >
> > > We are trying to prevent the "multiple submission of a form problem",
> > > that can result when a user double-clicks the submit button.
> > > We have a
> > > process in place, but we have been unable to get the session based
> > > synchronization to work correctly.
> > >
> > > Our order page submits to a "meta-refresh" page that checks to see the
> > > current status of the order processing each time it loads.
> > > If there is
> > > an error, it sends the user back to the order page.  If the order was
> > > successful, the user is sent to the success page.  If the
> > > order is still
> > > processing, it refreshes in 5 seconds (and meanwhile, the user has a
> > > pretty 5 second progress bar to watch).  However, The
> > > relevant snippet of
> > > code is below:
> > >
> > > String nextUrl = null;
> > > synchronized (session) {
> > >   BigDecimal status =
> > >     (BigDecimal)session.getAttribute(SiteProps.PROCESS_STATUS);
> > >
> > >   if (status != null) {
> > >     System.out.println("  Processing has already begun.  Checking
> > > status.");
> > >
> > >     if (status.equals(SiteProps.PROCESS_SUCCESS)) {
> > >       System.out.println("  We already know it was approved.
> > > Just showing
> > > them the success page.");
> > >       nextUrl = "/success.jsp";
> > >       response.sendRedirect(response.encodeRedirectUrl(nextUrl));
> > >       return;  // will remove status object on next page
> > >     } else if (status.equals(SiteProps.PROCESS_ERROR)) {
> > >       System.out.println("  We know the credit card charge
> > > failed.  Send
> > > them back to the order form.");
> > >       nextUrl = "/order.jsp";
> > >       response.sendRedirect(response.encodeRedirectUrl(nextUrl));
> > >       return;  // will remove status object on next page
> > >     } else if (status.equals(SiteProps.PROCESS_UNKNOWN)) {
> > >       System.out.println("  Still processing.  Not sure what
> > > the result
> > > will be.  This page will refresh in 5 seconds.");
> > >       nextUrl = "/order_check.jsp"; // refresh back to this same page
> > >     }
> > >   } else {
> > >     // we don't have any record of a previous attempt to process
> > >     System.out.println("  First time trying to process order page.
> > > Starting processing, scheduling a refresh.");
> > >     session.setAttribute(SiteProps.PROCESS_STATUS,
> > >       SiteProps.PROCESS_UNKNOWN);
> > >   }
> > >   %>
> > >   <html>
> > >     <!-- progress bar animated gif html code here -->
> > >   </html>
> > >   <%
> > >   if (nextUrl == null) {
> > >     // first time to page
> > >
> > >     // order processing code here
> > >   }
> > > }
> > >
> > > The problem is, that there isn't any real synchronization
> > > here.  This is
> > > because the "session" object is a different object each time this page
> > > loads.  In fact, printing out the "session" object each time the page
> > > loads results in:
> > >
> > > org.apache.catalina.session.StandardSessionFacade@24ea85
> > > org.apache.catalina.session.StandardSessionFacade@10275fa
> > > org.apache.catalina.session.StandardSessionFacade@18706f6
> > > org.apache.catalina.session.StandardSessionFacade@f1f34a
> > > org.apache.catalina.session.StandardSessionFacade@fb6763
> > >
> > > Since the session object keeps changing, what is the proper way to
> > > synchronize based on the user's session, to make this
> > > anti-double-process
> > > code work properly?
> > >
> > > This middle page with the progress bar helps to remove most of our
> > > double-click problems, but occassionally a user is able to
> > > get multiple
> > > copies of this "progress page" opened, and then if the timing
> > > is right,
> > > they both enter that snippet of code above at the same time,
> > > both think
> > > it's the first time this page is being processed, and both
> > > run the order
> > > processing code.
> > >
> > > Thank you,
> > > Raiden
> > >
> > >
> > > ---------------------------------------------------------------------
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> > >
> > >
> >
> >
> >
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> >
>
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