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From Stephan Schuster <st...@wilken.de>
Subject Re: [vfs] Why doesn't AbstractFileObject override equals?
Date Thu, 12 Jun 2008 09:54:15 GMT
hi mario,

thank you for your answer.

> The current VFS implementation (if you do not use anything else then the 
> only-working SoftRefFilesCache) ensures that two resolveFile will return 
> the same object if you ask for the same filename, thus, in terms of VFS 
> there is nothing bad with the object comparison.

that is what i was hoping as i saw the code for the first time. however 
i always got NPEs somewhere further up in stack. to me it seemed that 
the original cause was always the AbstractFileObject.getParent() method 
or more precisely the comparison

   if (this == fs.getRoot()) { ... };

which never evaluated to true, even if it should! so i tracked down the 
problem a bit further. the reason is the following: under certain 
circumstances my custom filesystemmanager uses CacheStrategy.ON_CALL. on 
resolving a file via this filesystem i therefore get an instance of a 
DecoratedFileObject, in fact an instance of OnCallRefreshFileObject. now 
when calling getParent() on a DecoratedFileObject the call is forwarded 
to the actual implementation of the file object, lets say 
LocalFileObject. then, in the above comparison the "this" pointer is an 
instance of LocalFileObject. however, "fs.getRoot()" which internally 
calls "resolveFile(rootName)" uses the same filesystem and therefore 
returns an instance of OnCallRefreshFileObject. comparing both, 
LocalFileObject and OnCallRefreshFileObject, via identity will never 
work. that's the problem!


there a several solutions:

1. in AbstractFileObject.getParent():

replace

   if (this == fs.getRoot()) { ... };

with

   FileObject root = fs.getRoot();
   if (root instanceof DecoratedFileObject) {
     root = ((DecoratedFileObject) root).getDecoratedFileObject();
   }
   if (this == root) { ... }


2. use equals for the above comparison and overwrite equals in 
AbstractFileObject in such way that the identity comparison respects 
decorated file objects.


3. use the file object's path for determinig equality (as in my last 
posting)



what do think is the best way?


regards,
stephan



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