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From "Cox, Charlie" <c...@cincom.com>
Subject RE: static,synchronized and classloaders
Date Wed, 13 Jun 2001 12:40:53 GMT
I'm not sure what you mean by abc.MyClass and xyz.MyClass as the class in
question is in the same package in both contexts.

This archive link from Craig McClanahan(thank you Craig) gives a good
explaination of classloaders:
http://www.mail-archive.com/tomcat-user@jakarta.apache.org/msg13260.html
 
Based upon this, the individual web app classloader does not know about the
other webapp classloaders or the classes that they have loaded. That's why I
think MyObject is treated as two classes since it is loaded by two different
loaders at the same level in the class-loader heirarchy. I am currently
using the same servlet class under two class loaders. It is not throwing any
errors. The difference in my contexts is in my properties file that is
context-specific and loaded by the servlet. 

Actually now that I think about it, I do have a static field that is
remaining context-specific in my servlet, therefore it must be two different
Class objects representing my class; therefore two different instances of my
static members(confusing isn't it?).

I guess I answered my own question. ;) And since I already typed this email,
I'll send it anyway and hopefully someone else will find it useful. 

you can synchonize on the class by using the 'this' object(I am doing it).
I'm just not sure if 'this' is available for static classes.I would think
that it is.

Charlie

-----Original Message-----
From: Luba Powell [mailto:luba999@bellatlantic.net]
Sent: Tuesday, June 12, 2001 3:50 PM
To: tomcat-user@jakarta.apache.org
Subject: Re: static,synchronized and classloaders(was:Java Question)


Also, look if identical files are generated based on 2 different context:
abc.MyClass is not the same as xyx.MyClass.  If 2 class loaders 
load from identical class files - JVM verifier should throw the exception
before a second object is created. If this doesn't happen - this is a 
serious bug.
 
Also it is not possible to synchronize on the class - only on the method
and a variable that is an object
----- Original Message ----- 
From: Cox, Charlie 
To: 'tomcat-user@jakarta.apache.org' 
Sent: Tuesday, June 12, 2001 3:35 PM
Subject: RE: static,synchronized and classloaders(was:Java Question)


Now is this true for when I have multiple contexts mapped to the same
docbase?  I have seen on this list that Tomcat loads the classes(in
web-inf/classes) independently for each context and passing MyObject class
instance between contexts is not the same 'MyObject' class in the second
context even though it is the same code. Since it is not the same class I
would assume that I would have two instances of a static method/field. And
if it is synchronized on a static member(or the class itself) it would only
be synchonized for this context.
I'm really not sure what I would have if the class is static. 
Thoughts? 
Charlie 
-----Original Message----- 
From: Pae Choi [mailto:paechoi@earthlink.net] 
Sent: Tuesday, June 12, 2001 2:42 PM 
To: tomcat-user@jakarta.apache.org 
Subject: Re: Java Question 


When you access the 'synchronized' static method, it locks its class. 
so this will ensure the thread-safe access. Otherwise, the result is 
unknown. 


Pae 


> I have looked all over and can't find the answer to this simple question. 
> If you use a static method, do you have to synchronize it in case other 
> people may access it at the same time.  For example, I have a static
Utility 
> class to do date calculations.  The method Utility.getMonth(String date) 
> takes in a full date string, parses it, and returns just the month value. 
> If 5 different people all using the website attempt to use 
> Utility.getMonth(String date) at the same time for different dates, will
it 
> return the right results?  If not, do I have to synchronize it or
something 
> in case multiple users attempt to access it? 
> 
> I know this is not really related to tomcat, but since I am using tomcat, 
> and everyone else using tomcat is also a java developer, I figured this is

> the best place I can ask. 
> 
> Thanks for any help!!! 
> 
> Brandon 
> 

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