Charles Chen wrote:
> Contact: Tel: 2726 - New Media Systems, 1st Floor South, Queens Walk
> [...]
> Bo Xu,
>
> I can see during the weekend you have learnt a lot, even changed your [...]
> style to *...*.
>
> I don't think anyone will be lucky to get the CD you promised, because I know by
> the fact that servlet alone (meaning without persistency) CAN NOT provide a true
> persistent counter. This is because the obvious reason that web server can be
> shut down and your servlet can be reloaded. Your counter variable (no matter if
> it is an instance variable or class variable) only has the maximum life span
> between the time the class is being loaded into memory, to the time the instance
> is garbage collected.
>
> However, if you are opt for the persistent solution, the safest way is to update
> the counter directly from the persistent media (ideally a database of some
> sort). That is, every time you need to increase the counter, increase it from
> the persistent media (with proper locking of course). This will GUARANTEE that
> the counter reflects the true value. This solution is designed for those who
> does not trust (or maybe just not sure) that a servlet has one and only one
> inctance.
>
> Another more elegent solution is to implement a counter JavaBean and load it
> into memory when the web server is started. Your servlet will simply access this
> bean instance to read/update the counter. This way you don't even need to worry
> about anything that you are not sure about servlet. There are many ways to
> ensure that the JavaBean instance is unique during the life span of the web
> server, and obviously you still need to implement the bean as a persistent
> object.
>
> Cheers,
>
> Charles
> [...]
Thanks for your email :-) and after I sent a email to reply Milt's email
yesterday,
I also guess perhaps the safest way is:
# persisting counter everytime JUST after it has been updated.
# and I guess it is the ONLY safe way to load it in *static/non-static
initialization block* in MyServlet class.
the following is my updated code, if it is wrong, please correct me :-)
public class MyServlet extends HttpServlet
{
private static Object sync =new Object();
private static int counter =0;
{ // or use static initialization block --> static {
synchronized(sync){
counter=...; //load counter from persistent state
sync.notifyAll();
}
}
...
//service/doGet/doPost {
...
synchronized(sync){
counter++;
... //save counter into persistent state
sync.notifyAll();
}
...
}
Thanks in advance! and I will continue to try my best to reduce
the number of *...* in my email :-) :-)
Bo
Nov.06, 2000
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