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https://issues.apache.org/jira/browse/PDFBOX-5602?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17723219#comment-17723219
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Tilman Hausherr commented on PDFBOX-5602:
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It's not surprising when you merge 1000 of files. IIRC for each merge a new
hierarchy level is created. This means the tree grows. This is because the
"append" code doesn't know that there are 1000s of files that are equal in the
hierarchy. You can look at the files before and after merge with PDFDebugger to
see what really happens. This happens in mergeKEntries(). I looked at the
commit comments and the main changes were 2 years ago in PDFBOX-5198 to align
with what Adobe is doing.
> Consider adding support for PDF files Concatenation in addition to the full
> Merge
> ----------------------------------------------------------------------------------
>
> Key: PDFBOX-5602
> URL: https://issues.apache.org/jira/browse/PDFBOX-5602
> Project: PDFBox
> Issue Type: New Feature
> Components: Utilities
> Affects Versions: 3.0.0 PDFBox
> Reporter: Zbigniew Minciel
> Priority: Major
> Attachments: CapturePdfDebugger.PNG, Large527MbytesPDF.PNG,
> cpu-20-middle.PNG, cpu-hot-spots-3.0.0-SNAPSHOT-1.PNG,
> cpu-hot-spots-3.0.0-SNAPSHOT.PNG, cpu-hot-spots-3.0.0-alpha3-1.PNG,
> cpu-hot-spots-3.0.0-alpha3.PNG
>
>
> I decided to evaluate pdfbox 3.0.0-alpha3 limits on merging large number of
> PDF files.
> I attempted to merge 7500 mails in separate PDF files on Windows. Given the
> limitation on the max size of the command line arguments, I was merging
> subsets of files. I ended up with 5 large PDF files, each around
> 500-600MBytes. I tried to merge these 5 files but eventually merge failed
> after running more than 6 hours. See error log at the bottom. I have large
> RAM 48GBytes. PDFBox was using up 13GB of memory max. Usage was changing
> between 600MB and 13Gb.
> I am wondering whether PDFBox could support Concatenation mode in addition to
> the full Merge mode. No need to create index table, etc. It could work as
> follow I suppose given my total lack of understanding how PDF works:
> # Read first file, process and append to the target PDF file. Delete PDF
> data and related meta data for this file except perhaps the last page number.
> # Read the second file and process in similar fashion as in the step 1
> # etc
> If Concatenation is possible, it would greatly reduce the cpu and memory
> overhead and reduce processing time.
> I admit merging of such large number of PDF files is not typical but the
> issue is valid.
> ^CException in thread "main" java.lang.OutOfMemoryError: Java heap space
> at java.base/java.util.Hashtable.rehash(Hashtable.java:419)
> at java.base/java.util.Hashtable.addEntry(Hashtable.java:441)
> at java.base/java.util.Hashtable.put(Hashtable.java:493)
> at
> org.apache.pdfbox.pdfwriter.COSWriter.doWriteBodyCompressed(COSWriter.java:481)
> at
> org.apache.pdfbox.pdfwriter.COSWriter.visitFromDocument(COSWriter.java:1260)
> at org.apache.pdfbox.cos.COSDocument.accept(COSDocument.java:402)
> at org.apache.pdfbox.pdfwriter.COSWriter.write(COSWriter.java:1542)
> at org.apache.pdfbox.pdfwriter.COSWriter.write(COSWriter.java:1418)
> at org.apache.pdfbox.pdmodel.PDDocument.save(PDDocument.java:1018)
> at org.apache.pdfbox.pdmodel.PDDocument.save(PDDocument.java:963)
> at org.apache.pdfbox.pdmodel.PDDocument.save(PDDocument.java:982)
> at
> org.apache.pdfbox.multipdf.PDFMergerUtility.legacyMergeDocuments(PDFMergerUtility.java:476)
> at
> org.apache.pdfbox.multipdf.PDFMergerUtility.mergeDocuments(PDFMergerUtility.java:355)
> at
> org.apache.pdfbox.multipdf.PDFMergerUtility.mergeDocuments(PDFMergerUtility.java:339)
> at org.apache.pdfbox.tools.PDFMerger.call(PDFMerger.java:76)
> at org.apache.pdfbox.tools.PDFMerger.call(PDFMerger.java:37)
> at picocli.CommandLine.executeUserObject(CommandLine.java:1953)
> at picocli.CommandLine.access$1300(CommandLine.java:145)
> at
> picocli.CommandLine$RunLast.executeUserObjectOfLastSubcommandWithSameParent(CommandLine.java:2358)
> at picocli.CommandLine$RunLast.handle(CommandLine.java:2352)
> at picocli.CommandLine$RunLast.handle(CommandLine.java:2314)
> at
> picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:2179)
> at picocli.CommandLine$RunLast.execute(CommandLine.java:2316)
> at picocli.CommandLine.execute(CommandLine.java:2078)
> at org.apache.pdfbox.tools.PDFBox.main(PDFBox.java:76)
> Respectfully,
> Zbigniew
>
>
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