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https://issues.apache.org/jira/browse/PDFBOX-2592?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14270127#comment-14270127
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John Hewson commented on PDFBOX-2592:
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Those issues are not relevant to the current discussion, we were deliberately
caching images over the lifetime of a document, so of course we eventually ran
out of memory.
The garbage collector *never* fails to work correctly, it always does its job
if you use it properly. There's no magic involved. I've actually hacked on
.NET's runtime code and can tell you that dispose() is for releasing native
resources such as sockets and window handles and is not connected to GC at all.
If you can explain your specific concerns about changing the behaviour of
COSDocument's close() then I'd be happy to discuss them.
> 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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