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https://issues.apache.org/jira/browse/PDFBOX-2592?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14270227#comment-14270227
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John Hewson commented on PDFBOX-2592:
-------------------------------------
I took at look at SVN and I wasn't able to find any evidence that the close()
method was modified due to users having problems with memory usage after
close() due to objects not being GC'd.
I see that you were the author of that code in [r1553017
|http://svn.apache.org/r1553017], and that you added a patch proposed in
PDFBOX-1777. That issue was about resource leaks due to not calling close() on
the COSStream objects which belonged to the COSDocument - which is perfectly
valid.
However, I see that you added to the patch, so that instead of closing all
streams, you also clear() the contents of all COSDictionary and COSArray
objects, i.e. they get clobbered. But the JIRA issue makes no mention of this,
the user didn't request it, and no reason was given for its presence. I assume
that you weren't ware that it wasn't necessary to call those clear() methods? -
as they don't close any streams.
So this really does look like a mistake - I see no indication that users ever
complained about this.
> Allow sharing of COS objects between different documents
> --------------------------------------------------------
>
> Key: PDFBOX-2592
> URL: https://issues.apache.org/jira/browse/PDFBOX-2592
> Project: PDFBox
> Issue Type: Improvement
> Components: PDModel
> Affects Versions: 2.0.0
> Reporter: John Hewson
> Assignee: John Hewson
>
> A number of users on the mailing list have asked about how to import pages
> from other PDFs as forms, our current solution is LayerUtility, which is
> depends on PDFCloneUtility. Both these classes are surprisingly complex for
> what should be a simple task.
> The two main tasks which these classes perform is copying the page's
> COSStream and cloning every relevant COS object. However, there seems to be
> no real need to do any of this copying and cloning - there's nothing about
> any of the COS objects which is specific to a given document. While a
> COSStream can share the same backing file as the COSDocument, this isn't a
> problem for COSWriter, even then we need only make sure that an exception is
> thrown if a COSStream is used after its parent COSDocument is closed.
> Note that there *is* one artificial dependency between COSDictionary and
> COSArrays and their parent COSDocument, that is that calling close() on the
> COSDocument clears the contents of all child COSDictionary and COSArrays.
> However, there's no need for this, it seems to have come about due to some
> long past confusion regarding how garbage collection works in Java - we all
> know that it's not necessary to set objects to null or clear lists when we
> are done with them.
> I propose that we get rid of the unnecessary object and list clearing in
> COSDocument#close() and add some checks to COSStream to throw user-friendly
> exceptions when reading from a closed backing stream. This will allow us to
> directly share COS objects between different COSDocuments, allowing simple "x
> = y" copying and making LayerUtility and PDFCloneUtility unnecessary. Instead
> of:
> {code}
> COSStream pageStream = (COSStream)page.getStream().getCOSObject();
> PDStream newStream = new PDStream(targetDoc,
> pageStream.getUnfilteredStream(), false);
> PDFormXObject form = new PDFormXObject(newStream);
> PDResources pageRes = page.getResources();
> PDResources formRes = new PDResources();
> PDFCloneUtility cloner = new PDFCloneUtility(document);
> cloner.cloneMerge(pageRes, formRes);
> form.setResources(formRes);
> {code}
> We could have:
> {code}
> PDFormXObject form = new PDFormXObject(page.getStream());
> form.setResources(page.getResources());
> {code}
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