Memory-mapping Files in Java Causes Problems

Today, we released PDFTextStream v2.0.1— a minor patch release that contains a workaround for an interesting and unfortunate bug: on Windows, if one accesses a PDF file on disk using PDFTextStream, then closes the PDFTextStream instance (using PDFTextStream.close()), the PDF file will still be locked. It can’t be moved or deleted.

This is actually not a bug in PDFTextStream, but in Java, documented as Sun bug #4724038. In short, any file that is memory-mapped cannot reliably be “closed” (i.e. the `DirectByteBuffer` (or some native proxy, perhaps) that holds the OS-level file handle does not release those resources, even when the `FileChannel` is closed that was used to create the `DirectByteBuffer`). Reading the comments on that bug report show a great deal of frustration, and rightly so: regardless of the technical reasons for the behavior, memory-mapping files isn’t rocket science (or, hasn’t been for 20 years or somesuch), and this kind of thing shouldn’t happen.

Since we can’t fix the bug, we devised a workaround: if you set the `pdfts.mmap.disable` system property to `Y`, then PDFTextStream won’t memory-map PDF files. Simple enough fix. FYI, there appears to be no performance degredation associated with using PDFTextStream in this mode.

Of course, this is only a problem on Windows, which does not allow files to be moved or deleted while a process has an open file handle. We have a number of customers that deploy on Windows Server (although that number is much smaller than those that deploy on a variety of *nix), but until last week, they hadn’t reported any problems. Our best guess is that, given the systems we know those customers are running, they are probably using PDFTextStream’s in-memory mode (where PDF data is in memory, and provided to PDFTextStream as a `ByteBuffer`). Of course, in that case, no file handles are ever opened, so all is well.

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2 Responses to Memory-mapping Files in Java Causes Problems

  1. Tom Palmer says:

    Wow. Thanks for the info. Sun should at least @Deprecate memory mapped files if they aren’t going to fix this bug. JNI time I suppose for those that really need memory mapped files.

    • Chas Emerick says:

      That’s a cruel thing to do to such a new API, but I understand the sentiment. I do think a lot of confusion could have been avoided if they didn’t use map as the function name that returns the mapped DirectByteBuffer, so as to indicate that the operation’s behaviour is not quite what one would expect.

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