In a servlet world, the database handles concurrency (at least, any database
that supports transactions - mySQL does not). If the database does not
support transactions, you must either synchronize the database calling
methods or the instance variables holding the update values. If you use
entity CMP EJBs, the EJB container will take care of transactions for you,
or if you use BMP EJBs (either entity or stateful session), the database
must handle transactions.
Cheers!
Mark
----- Original Message -----
From: "Paul Foxton" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, July 16, 2001 10:21 AM
Subject: concurrent access to variables with class or block scope
> 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?
>
> 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.
>
> 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?
>
> 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
>
>
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