----- Original Message -----
From: "David Smith" <[EMAIL PROTECTED]>
To: "Tomcat Users List" <users@tomcat.apache.org>
Sent: Wednesday, September 17, 2008 3:40 PM
Subject: Re: Tomcat 5.5, JNDI Connection Pooling, Active connections keep
increasing....
Comments inline ...
Johnny Kewl wrote:
>> So, what exactly does it mean when we say that Tomcat is thread
safe >> for
>> requests.
>> Tomcat creates a new thread for each request, so somehow my static
>> methods
>> are then thread safe (incdirectly, since it is managed by Tomcat).
I actually searched all over to try find something for you.... this
looks ok
http://www.codestyle.org/java/servlets/faq-Threads.shtml
But here my un-scientific way of thinking about it...
When tomcat sends a thread into your doGet method...
All those local variables in that thread are ok...
As long as you mean variables defined inside of the doGet() method.
Class instance variables are not cool in servlets.
So I think about it as each thread being a lane on a big
freeway/highway...
But objects are not thread local unless they are setup that way
implementing something like ThreadLocal. Only the references to the
object are local.
No... ouch mention 2 words in a sentence and association kicks in.... ha ha
I'm saying...
These local variables are completely private; there is no way for one thread
to access the local variables of another thread.
Nothing at all to do with ThreadLocal...
So yes you right in a way... Tomcat isnt allowing those local method
variables to get mixed up...
but the second you decalare a static method or static variable... you
had better be sure its thread safe.
Let's not confuse static methods and static variables. Static variables
should be avoided unless they are true constants. Static methods on the
other hand are fine, but need to be careful when those static methods
interact with object instances that may not be thread safe.
All true... but damn hard to see when you plugged into someone elses
engine...
... in your own code in a static method, only have to worry about your
globals...
But someone elses code... not easy... unless they explicitly say thread
safe.
What happens to that highway is that all the lanes merge into one, and
every car has to go through it...
Now your are talking about a singleton -- a whole different ball of wax.
No... I'm not... its actually your C analogy
When you add synchronized... your 30 lane high way becomes a single
lane and its one car at a time...
Still in singleton land with the one lane analogy. Syncs do add a huge
performance penalty though, so even outside of singletons, syncs should
be avoided as much as possible.
You dont want to add synchronized because here you have the TC guys
gone thru all the hassle of making sure you can have a 30 lane
highway... and you bang it into one lane.... so new is better because
each lane gets its own engine... and java is pretty damn good at
garbage collecting, that little bit of memory is a blip on the radar
screen...
You got to really know what your code is doing when its static... you
got to know its thread safe, and it can be really hard to see that 30
car pile up coming on the highway ;)
My way of thinking about this stuff..... mad science - chapter 1 ;)
Johnny -- You might find the java language reference an interesting
read. It describes all the rules regarding threading, access, value vs
reference variables, etc., ...
Maybe you want to tell us why it is his code is leaking connections?
What is it exactly in his code thats jumping a connection?
Do you know?
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