JAVA: gathering byte offsets of xml tags using an XmlStreamReader

Question

Is there a way to accurately gather the byte offsets of xml tags using the XMLStreamReader?

I have a large xml file that I require random access to. Rather than writing the whole thing to a database, I would like to run through it once with an XMLStreamReader to gather the byte offsets of significant tags, and then be able to use a RandomAccessFile to retrieve the tag content later.

XMLStreamReader doesn't seem to have a way to track character offsets. Instead people recommend attaching the XmlStreamReader to a reader that tracks how many bytes have been read (the CountingInputStream provided by apache.commons.io, for example)

e.g:

CountingInputStream countingReader = new CountingInputStream(new FileInputStream(xmlFile)) ;
XMLStreamReader xmlStreamReader = xmlStreamFactory.createXMLStreamReader(countingReader, "UTF-8") ;


while (xmlStreamReader.hasNext()) {
    int eventCode = xmlStreamReader.next();

    switch (eventCode) {
        case XMLStreamReader.END_ELEMENT :
            System.out.println(xmlStreamReader.getLocalName() + " @" + countingReader.getByteCount()) ;
    }

}
xmlStreamReader.close();

Unfortunately there must be some buffering going on, because the above code prints out the same byte offsets for several tags. Is there a more accurate way of tracking byte offsets in xml files (ideally without resorting to abandoning proper xml parsing)?

Answer 1

You could use getLocation() on the XMLStreamReader (or XMLEvent.getLocation() if you use XMLEventReader), but I remember reading somewhere that it is not reliable and precise. And it looks like it gives the endpoint of the tag, not the starting location.

I have a similar need to precisely know the location of tags within a file, and I'm looking at other parsers to see if there is one that guarantees to give the necessary level of location precision.