Is there any XPath processor for SAX model?

I'm looking for an XPath evaluator that doesn't rebuild the whole DOM document to look for the nodes of a document: actually the object is to manage a large amount of XML data (ideally over 2Gb) with SAX model, which is very good for memory management, and give the possibility to search for nodes.

Thank you all for the support!

For all those who say it's not possible: I recently, after asked the question, found a project named "saxpath" (http://www.saxpath.org/), but I can't find any implementing project.

My current list (compiled from web search results and the other answers) is:

• http://code.google.com/p/xpath4sax/
• http://spex.sourceforge.net/
• https://github.com/santhosh-tekuri/jlibs/wiki/XMLDog (also contains a performance chart)
• http://www.cs.umd.edu/projects/xsq/ (uniersity project, dead since 10 years, GPL)
• MIT-Licensed approach http://softwareengineeringcorner.blogspot.com/2012/01/conveniently-processing-large-xml-files.html
• Other parsers/memory models supporting fast XPath:
  • http://vtd-xml.sourceforge.net/ ("The world's fastest XPath 1.0 implementation.")
  • http://jaxen.codehaus.org/ (contains http://www.saxpath.org/)
  • http://www.saxonica.com/documentation/sourcedocs/streaming/streamable-xpath.html

The next step is to use the examples of XMLDog and compare the performance of all these approaches. Then, the test cases should be extended to the supported XPath expressions.

We regularly parse 1GB+ complex XML files by using a SAX parser which extracts partial DOM trees that can be conveniently queried using XPath. I blogged about it here: http://softwareengineeringcorner.blogspot.com/2012/01/conveniently-processing-large-xml-files.html - Sources are available on github - MIT License.

XPath DOES work with SAX, and most XSLT processors (especially Saxon and Apache Xalan) do support executing XPath expressions inside XSLTs on a SAX stream without building the entire dom.

They manage to do this, very roughly, as follows :

1. Examining the XPath expressions they need to match
2. Receiving SAX events and testing if that node is needed or will be needed by one of the XPath expressions.
3. Ignoring the SAX event if it is of no use for the XPath expressions.
4. Buffering it if it's needed

How they buffer it is also very interesting, cause while some simply create DOM fragments here and there, others use very optimized tables for quick lookup and reduced memory consumption.

How much they manage to optimize largely depends on the kind of XPath queries they find. As the already posted Saxon documentation clearly explain, queries that move "up" and then traverse "horizontally" (sibling by sibling) the document obviously requires the entire document to be there, but most of them require just a few nodes to be kept into RAM at any moment.

I'm pretty sure of this because when I was still making every day webapp using Cocoon, we had the XSLT memory footprint problem each time we used a "//something" expression inside an XSLT, and quite often we had to rework XPath expressions to allow a better SAX optimization.

SAX is forward-only, while XPath queries can navigate the document in any direction (consider parent::, ancestor::, preceding:: and preceding-sibling:: axis). I don't see how this would be possible in general. The best approximation would be some sort of lazy-loading DOM, but depending on your queries this may or may not give you any benefit - there is always a worst-case query such as //*[. != preceding::*].

Sorry, a slightly late answer here - it seems that this is possible for a subset of XPath - in general it's very difficult due to the fact that XPath can match both forwards and backwards from the "current" point. I'm aware of two projects that solve it to some degree using state machines: http://spex.sourceforge.net & http://www.cs.umd.edu/projects/xsq. I haven't looked at them in detail but they seem to use a similar approach.

I'll toss in a plug for a new project of mine, called AXS. It's at https://code.google.com/p/annotation-xpath-sax/ and the idea is that you annotate methods with (forward-axis-only) XPath statements and they get called when the SAX parser is at a node that matches it. So with a document

<doc>
<nodes>
  <node name="a">text of node 1</node>
  <node name="b">text of node 2</node>
  <node otherattr="I have attributes!">text of node 3</node>
</nodes>
</doc>

you can do things like

@XPath("/nodes/node")
void onNode(String nodeText)
{
  // will be called with "text of node [123]"
}

or

@XPathStart("//node[@name='']")
void onNode3(Attrs node3Attrs) { ... }

or

@XPathEnd("/nodes/node[2]")
void iDontCareAboutNode3() throws SAXExpression
{
  throw new StopParsingExpression();
}

Of course, the library is so new that I haven't even made a release of it yet, but it's MIT licensed, so feel free to give it a try and see if it matches your need. (I wrote it to do HTML screen-scraping with low enough memory requirements that I can run it on old Android devices...) If you find bugs, please let me know by filing them on the googlecode site!

There are SAX/StAX based XPath implementations, but they only support a small subset of XPath expressions/axis largely due to SAX/StAX's forward only nature.. the best alternative I am aware of is extended VTD-XML, it supports full xpath, partial document loading via mem-map.. and a max document size of 256GB, but you will need 64-bit JVM to use it to its full potential

What you could do is hook an XSL transformer to a SAX input source. Your processing will be sequential and the XSL preprocessor will make an attempt to catch the input as it comes to fiddle it into whatever result you specified. You can use this to pull a path's value out of the stream. This would come in especially handy if you wanted to produce a bunch of different XPATH results in one pass.

You'll get (typically) an XML document as a result, but you could pull your expected output out of, say, a StreamResult with not too much hassle.

Have a look at the streaming mode of the Saxon-SA XSLT-processor.

http://www.saxonica.com/documentation/sourcedocs/serial.html

"The rules that determine whether a path expression can be streamed are:

• The expression to be streamed starts with a call on the document() or doc() function.

• The path expression introduced by the call on doc() or document must conform to a subset of XPath defined as follows:

•        any XPath expression is acceptable if it conforms to the rules for path expressions appearing in identity constraints in XML Schema. These rules allow no predicates; the first step (but only the first) can be introduced with "//"; the last step can optionally use the attribute axis; all other steps must be simple Axis Steps using the child axis.
  

•      In addition, Saxon allows the expression to contain a union, for example doc()/(*/ABC | */XYZ). Unions can also be expressed in abbreviated form, for example the above can be written as doc()/*/(ABC|XYZ).
  

•      The expression must either select elements only, or attributes only, or a mixture of elements and attributes.
  

•        Simple filters (one or more) are also supported. Each filter may apply to the last step or to the expression as a whole, and it must only use downward selection from the context node (the self, child, attribute, descendant, descendant-or-self, or namespace axes). It must not be positional (that is, it must not reference position() or last(), and must not be numeric: in fact, it must be such that Saxon can determine at compile time that it will not be numeric). Filters cannot be applied to unions or to branches of unions. Any violation of these conditions causes the expression to be evaluated without the streaming optimization.
  

• These rules apply after other optimization rewrites have been applied to the expression. For example, some FLWOR expressions may be rewritten to a path expression that satisfies these rules.

•   The optimization is enabled only if explicitly requested, either by using the saxon:stream() extension function, or the saxon:read-once attribute on anXSLT xsl:copy-of instruction, or the XQuery pragma saxon:stream. It is available only if the stylesheet or query is processed using Saxon-SA."
  

Note: It is most likely in the commercial version this facility is available. I've used Saxon extensively earlier, and it is a nice piece of work.

Mmh I don't know if I really understand you. As far as I know, the SAX model is event oriented. That means, you do something if a certain node is encountered during the parsing. Yeah, it is better for memory but I don't see how you would like to get XPath into it. As SAX does not build a model, I don't think that this is possible.

I don't think xpath works with SAX, but you might take a look at StAX which is an extended streaming XML API for Java.

http://en.wikipedia.org/wiki/StAX

The standard javax xpath API technically already works with streams; javax.xml.xpath.XPathExpression can be evaluated against an InputSource, which in turn can be constructed with a Reader. I don't think it constructs a DOM under the covers.

Did you have tried also QuiXPath https://code.google.com/p/quixpath/ ?