JAVA：使用 XmlStreamReader 收集 xml 标签的字节偏移量

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)

eg:

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.

Answer 2

Unfortunatly Aalto doesn't implement the LocationInfo interface.

The last java VTD-XML ximpleware implementation, currently 2.11 on sourceforge or on github provides some code maintaning a byte offset after each call to the getChar() method of its IReader implementations.

IReader implementations for various caracter encodings are available inside VTDGen.java and VTDGenHuge.java

IReader implementations are provided for the following encodings

ASCII;
ISO_8859_1
ISO_8859_10
ISO_8859_11
ISO_8859_12
ISO_8859_13
ISO_8859_14
ISO_8859_15
ISO_8859_16
ISO_8859_2
ISO_8859_3
ISO_8859_4
ISO_8859_5
ISO_8859_6
ISO_8859_7
ISO_8859_8
ISO_8859_9
UTF_16BE
UTF_16LE
UTF8;   
WIN_1250
WIN_1251
WIN_1252
WIN_1253
WIN_1254
WIN_1255
WIN_1256
WIN_1257
WIN_1258

Updating IReader with a getCharOffset() method and implementing it by adding a charCount member along to the offset member of the VTDGen and VTDGenHuge classes and by incrementing it upon each getChar() and skipChar() call of each IReader implementation should give you the start of a solution.

Answer 3

您可以在实际输入流周围使用包装输入流，只需将实际 I/O 操作推迟到包装流，但保留带有各种代码的内部计数机制来检索当前偏移量？

Answer 4

I think I've found another option. If you replace your switch block with the following, it will dump the position immediately after the end element tag.

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

This solution also would require that the actual start position of the end tags would have to be manually calculated, and would have the advantage of not needing an external JAR file.

I was not able to track down some minor inconsistencies in the data management (I think it has to do with how I initialized my XMLStreamReader ), but I always saw a consistent increase in the location as the reader moved through the content.

Hope this helps!

Answer 5

I recently worked out a solution for a similar question on How to find character offsets in big XML files using java? . I think it provides a good solution based on a ANTLR generated XML-Parser.

Answer 6

I just burned a day long weekend on this, and arrived at the solution partially thanks to some clues here. Remarkably I don't think this has gotten much easier in the 10 years since the OP posted this question.

TL;DR Use Woodstox and char offsets

The first problem to contend with is that most XMLStreamReader implementations seem to provide inaccurate results when you ask them for their current offsets. Woodstox however seems to be rock-solid in this regard.

The second problem is the actual type of offset you use. Unfortunately it seems that you have to use char offsets if you need to work with a multi-byte charset, which means the random-access retrieval from the file is not going to be very efficient - you can't just set a pointer into the file at your offset and start reading, you have to read through until you get to the offset, then start extracting. There may be a more efficient way to do this that I haven't though of, but the performance is acceptable for my case. 500MB files are pretty snappy.

[edit] So this turned into one of those splinter-in-my-mind things, and I ended up writing a FilterReader that keeps a buffer of byte offset to char offset mappings as the file is read. When we need to get the byte offset, we first ask Woodstox for the char offset, then get the custom reader to tell us the actual byte offset for the char offset. We can get the byte offset from the beginning and end of the element, giving us what we need to go in and surgically extract the element from the file by opening it as a RandomAccessFile.

I created a library for this, it's on GitHub and Maven Central . If you just want to get the important bits, the party trick is in the ByteTrackingReader . [/edit]

There is another similar question on SO about this (but the accepted answer frightened and confused me), and some people commented about how this whole thing is a bad idea and why would you want to do it? XML is a transport mechanism, you should just import it to a DB and work with the data with more appropriate tools. For most cases this is true, but if you're building applications or integrations that communicate via XML (still going strong in 2020), you need tooling to analyze and operate on the files that are exchanged. I get daily requests to verify feed contents, having the ability to quickly extract a specific set of items from a massive file and verify not only the contents, but the format itself is essential.

Anyhow, hopefully this can save someone a few hours, or at least get them closer to a solution. God help you if you're finding this in 2030, trying to solve the same problem.