Hi Bo, thanks for your reply
I think I follow you, and you seem to be saying the same things as Hans,
apart from this part:
> no, but please notice that this local/method can not refrence to a
> instance/static
> field of MyServlet, or refrence to a object shared by several
> Threads, for
> example:
> doGet(...){
> ...
> HttpSession session=req.getSession();
> session.getAttibute(...)/setAttibute(...);
> ...
> }
>
> session is a local variable, but because it references to
> req.getSession(), so
> I think "synchronized" need to be used here
>
- isn't the point of the getSession method is that a new instance of the
Session object is created for each new request, so each new thread spawned
by a request would have its own instance of Session?
Hope I havn't misunderstood you.
thanks,
Paul
> -----Original Message-----
> From: Bo Xu [mailto:[EMAIL PROTECTED]]
> Sent: 16 July 2001 16:43
> To: [EMAIL PROTECTED]
> Subject: Re: concurrent access to variables with class or block scope
>
>
> Hi :-) please see intermix
>
>
> Paul Foxton wrote:
>
> > Hi all,
> >
> > I've been wondering about the fact that concurrent requests
> to a servlet
> > will result in a new thread being created to access the
> same instance of the
> > servlet as long as singleThreadModel is not implemented
> with reference to
> > the following:
> >
> > If you have a servlet which accesses a database, and
> request parameters from
> > an HTML form are used to build the SQL where clause, if
> these request
> > parameters are stored in class instance variables (i.e.
> declared at the
> > beginning of the class and outside of any methods, so class
> scope), would
> > this mean that there is a possibility of data being
> overwritten in the
> > middle of 2 or more concurrent requests and therefore
> compromising the
> > integrity of the data retrieved from the database?
> >
>
> it is possible: several Threads(made with the same instance
> of MyServlt)
> will "share"-(for example, make loacal working copy of) the instance
> variables(which is in heap-memory) of MyServlet. with the
> similar reason,
> static fields in MyServlet will also be "shared". from several emails,
> now it is possible that the different clients of MyServlet get
> "mixed- responses".
>
>
> >
> > If so, I would have thought that the way around this is to
> store the values
> > from the request parameters in variables declared inside
> methods, which
> > would then have block scope within that method.
> >
>
> yes, if a static/instance method is "used" by sevaral
> Threads, I guess several
> "working
> copies" of this method will be made in "stack-memory", so the
> loacal/method
> variables
> will not be "shared" by several Threads.
>
>
> >
> > But surely, if two requests (threads) run through this
> block at the same
> > time there is still a possibility of a problem with data integrity?
> >
>
> no, but please notice that this local/method can not refrence to a
> instance/static
> field of MyServlet, or refrence to a object shared by several
> Threads, for
> example:
> doGet(...){
> ...
> HttpSession session=req.getSession();
> session.getAttibute(...)/setAttibute(...);
> ...
> }
>
> session is a local variable, but because it references to
> req.getSession(), so
> I think "synchronized" need to be used here
>
>
> >
> > This doesn't seem quite right to me tho, because then you
> would have to
> > synchronize access to any method or variable which was used
> to build an sql
> > query or update.
> >
> > Can anyone clarify?
> >
> > Sorry if this is an obvious question. I've read through the
> documentation on
> > threads in servlets at Suns java site, but am still a
> little confused.
> >
> > Thanks,
> >
> > Paul
> > [...]
>
> Bo
> July 16, 2001
>
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