Vincent Hennebert commented on FOP-2211:

Hi Alexios,

thanks for your patches. It is true that the current situation is less than 
optimal. Maybe temporary resources shouldn't be handled through URIs at all. 
However, do we always want to store them in a temp file? Referring to the email 
thread you mention above, I think Peter has a point in saying that we may want 
to just store them in memory, and reduce the I/O load on the server. Shouldn't 
we leave that configurable?

As to the patches: I'm not sure we want to add a delete method to the 
ResourceResolver interface. That method applies to temporary resources only and 
the scope of that interface is broader than that. I'd rather have that method 
on some sub-class implementing that interface.

That said, there seems to be a discrepancy in the code: if I take 
PSDocumentHandler as an example, there is a TempResourceURIGenerator that is 
used to create tmp URIs, that are then passed on to the userAgent's 
ResourceResolver, but nothing indicates that that ResourceResolver can handle 
tmp URIs. In fact, that may well not be the case if the user configured the 
FopFactory with a custom ResourceResolver.

In light of this, it's probably best to remove the tmp URI facility altogether 
and switch back to the createTempFile method for now. I guess we can always 
think about a RAM version of temp files later on, if the OS' own caching 
facility proves insufficient.

What do people think?
> [PATCH] Fix & improve the handling of temporary files using the new URI 
> resource resolvers
> ------------------------------------------------------------------------------------------
>                 Key: FOP-2211
>                 URL: https://issues.apache.org/jira/browse/FOP-2211
>             Project: Fop
>          Issue Type: Bug
>          Components: general
>    Affects Versions: trunk
>            Reporter: Alexis Giotis
>             Fix For: trunk
>         Attachments: fop.patch, xgc.patch
> As written in http://markmail.org/message/zelumstxxsdyvkcz , after the merge 
> of the Temp_URI_Resolution branch (Sept 2012), the actual pattern of using 
> temp files has changed from:
> {code}
> File tmpFile = File.createTempFile(....);
> // Write and read from the file
> tmpFile.delete();
> {code}
> to:
> {code}
> File tmpFile = new File(System.getProperty("java.io.tmpdir"), 
> counterStartingFrom1AsString);
> tmpFile.deleteOnExit();
> // Write and read from the file
> {code}
> This is fine when FOP is executed from the command line (which I guess this 
> is how most people use it) but it introduces a number of bad side effects for 
> long running processes that use FOP embedded.
> 1. Different FOP processes can't be executed in parallel on the same system 
> because creating the same temp file fails.
> 2. If the JVM is not normally terminated, the temp files are never deleted 
> and the next invocation of the JVM fails to run.
> 3. deleteOnExit() keeps for the life of the JVM an unknown number of temp 
> files both on disk and a reference in memory.
> There should not be a need to implement a custom resource resolver when using 
> FOP embedded in order to fix those issues. The default implementation should 
> work at least as good as it worked in FOP 1.1 or earlier. 
> Attached are 2 patches, one for XGC and one for FOP that should fix and 
> improve the handling of at least the temporary files.
> For reference, [1] lists some reasons for implementing the new URI resource 
> resolvers.
> [1] http://wiki.apache.org/xmlgraphics-fop/URIResolution

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