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From Oyvind.Bakk...@Sun.COM
Subject Re: Question about using URLClassLoader and Derby
Date Thu, 10 Nov 2005 11:30:37 GMT
David W. Van Couvering wrote:
> Hi, Kathey.  At first glance it looks good to me.  I'm assuming *all* 
> classes your app needs are available to the from the URL you specify, 
> because you are not specifying a parent classloader when you create it. 

Not specifying a parent classloader does not mean that there will not be 
one. When using this constructor, the parent classloader will be the 
default classloader at the invocation point, e.g. the same classloader 
that would be used if you wrote Class.forName() at that spot.

>  Also, you'll need to make sure you give the class that runs this code 
> the permission to load classes (I'm not sure exactly how this is done, I 
> just know it's an issue).
> 
> But I am no classloader guru.  I am forwarding this to folks I know 
> within Sun who have done a *lot* of work within classloaders as part of 
> the app server effort.  I'll get back to you with any information I get.
> 
> David
> 
> Kathey Marsden wrote:
> 
>> My goal is:
>> I want to use a specific version of Derby which I ship with my app and I
>> don't  want to interfere with any other derby versions loaded  in the
>> same JVM or have them interfere with me.   I am creating a new
>> datasource in a separate URLClassLoader and using that for creating all
>> my connections.  Are there other things I need to do to meet my goal?  

Your URLClassLoader will delegate loading to its parent before it tries 
to load anything itself. You need to be sure that your URLClassLoader's 
parents have no Derby system to load - if they do, the result is not 
what you want.

If you cannot assume anything about what's in the parent classloaders' 
search path, you will need to create your own custom classloader which 
"cuts off" the delegation for classes whose names follow a certain 
pattern - for instance, start with "org.apache.derby." I have done this 
successfully in other projects.

>> I have a feeling it all must be more complex than it looks to me right 
>> now.

Just a little. :)

-- 
Oyvind Bakksjo
Sun Microsystems, Database Technology Group
Trondheim, Norway
http://weblogs.java.net/blog/bakksjo/

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