Question

I am trying to extract image metadata (and the image) for images embedded in pdfs, using pdfbox. I have the following problem,

for each page i:
 for each image j in page i:
  extract metadata, output
  create the image file in separate thread

Now, I have the following code for creating the image file, which is included in a method called generate_image() in FileWritingclass implementing Runnable. This method is called from run(). The code follows:

try {
    File F=new File(figurename);
    item.getImage().write2file( F );


} catch (Exception e) {
    e.printStackTrace();
} 

where item.getImage() returns a PDXObjectImage object. If I do this without creating a separate thread, it works fine, but when I create a thread to do this task, it shows following error:

java.lang.IndexOutOfBoundsException: Index: 5, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:604)
at java.util.ArrayList.get(ArrayList.java:382)
at org.apache.pdfbox.io.RandomAccessBuffer.seek(RandomAccessBuffer.java:84)
at org.apache.pdfbox.io.RandomAccessFileInputStream.read(RandomAccessFileInputStream.java:96)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:235)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:275)
at java.io.BufferedInputStream.read(BufferedInputStream.java:334)
at java.io.FilterInputStream.read(FilterInputStream.java:107)
at org.apache.pdfbox.pdmodel.graphics.xobject.PDCcitt$TiffWrapper.read(PDCcitt.java:468)
at org.apache.pdfbox.io.IOUtils.copy(IOUtils.java:68)
at org.apache.pdfbox.pdmodel.graphics.xobject.PDCcitt.write2OutputStream(PDCcitt.java:184)
at org.apache.pdfbox.pdmodel.graphics.xobject.PDXObjectImage.write2file(PDXObjectImage.java:165)
at extractor.FileWriting.generate_image(FileWriting.java:136)

Can anyone please point out where I am going wrong?

Answer 1

I suspect that your problem is that you are using the same pdfbox classes in multiple threads.

Older versions of PDFBox had this in their FAQ page:

Is PDFBox thread safe?

No! Only one thread may access a single document at a time. You can have multiple threads each accessing their own PDDocument object.

However, in 2.0+ (maybe?) the PDFBox FAQ page now says:

PDFBox has experimental support for read-only operations on the same PDDocument from different threads.

Given the discussion right now (June 2019), this doesn’t seem promising however. So, unless you want to live on the edge, you still should consider using a ThreadLocal and keeping one PDDocument or whatever class you are using. Something like:

private ThreadLocal<PDDocument> localPdDocument = new ThreadLocal<PDDocument>() {
    @Override
    protected PDDocument initialValue() {
        return new PDDocument();
    }
};

...
PDDocument doc = localPdDocument.get();
...