Non-sequential PDF parser + PATCH
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Key: PDFBOX-1199
URL: https://issues.apache.org/jira/browse/PDFBOX-1199
Project: PDFBox
Issue Type: Improvement
Components: Parsing
Affects Versions: 1.6.0
Reporter: Timo Boehme
Currently PDF parsing is done in sequential manner resulting in problems with
stream parsing and skipping unused content. The solution is a conforming parser
which first reads XREF tables and uses this information to only parse required
objects and uses length information for stream parsing. A completely new
implementation of such a parser is currently worked on in PDFBOX-1000. While
this parser will be the long term solution a short term solution based on
existing code would be desirable. A first incomplete solution was presented in
PDFBOX-1104.
Starting from PDFBOX-1104 I have implemented an 'as much as possible'
conforming parser, called 'non-sequential parser', which handles all PDF
documents (even inlined, with object streams etc.). The parser can be used as a
drop-in-replacement for PDFParser (subclass of PDFParser). It overwrites method
parse and getPage method. The only restriction is currently the need to specify
a file instead of an input stream. In order to efficiently read the file and
use it with the existing object parsing code I developed a
RandomAccessBufferedFileInputStream which allows InputStream operations in
combination with seek operations and cached read data.
In order to use NonSequentialPDFParser small changes and additions on existing
classes are needed. This includes changing some methods/fields from private to
protected in PDFParser, add parsing of stream object information from XREF
streams, store and get this information from XrefTrailerResolver (object ids
are stored negated in order to distinguish them from offsets) and allow
resetting offset in PushBackInputStream. All these changes do not change
behavior of current parser. Another requirement is the long offset patch
(PDFBOX-1196) which is excluded from the patch set provided here.
The provided parser currently works in a forceParsing=false mode resulting in
an IOException if a parsing error occurs. In most cases this shouldn't be a
problem since in my use cases exceptions typically occur trying to parse unused
content or streams which with this new parser are no problems anymore. In my
setup I use the new parser first and if a parsing error occurs, fall back to
the sequential parser (a bit like Acrobat does it if XREF information is buggy):
try {
// ---- try first with (mostly) standard conform parsing
doc = PDDocument.loadNonSeq( PDF_FILE, raBuf );
handleDocument(doc);
} catch ( IOException ioe ) {
// ---- retry with sequential parser and force parsing
doc = PDDocument.load( new FileInputStream(PDF_FILE), raBuf, true );
handleDocument(doc);
}
For me this new parser works very well on large document collections and is a
large step forward to parse all documents also accepted by common PDF tools.
While its behavior is nearly 'conform' there is nevertheless a need for a clean
'real' conforming parser. For instance since the underlying object structure
has no access to the parser it is necessary to first parse all objects before
they can be used. This includes objects that might not be needed at all.
Another normally not needed step is copying the content of a stream. Since we
work on a file with random access there would be no need for it. However this
parser should fill the hole until a full featured and clean conforming parser
is available.
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