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** Created from <tt>VTDNav</tt> instance
 
** Created from <tt>VTDNav</tt> instance
 
** Allows for XPath execution
 
** Allows for XPath execution
** Allows for iterating over  
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** Allows for iterating over elements selected by XPath
  
 
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Revision as of 13:56, 14 December 2012

VTD-XML Investigation

VTD-XML (http://vtd-xml.sourceforge.net/) is a high-performance XML processing model that deals with XML in a binary form, instead of the traditional text form. VTD stands for Virtual Token Descriptor.

VTD-XML parses an XML document and builds an internal data structure representing the entire XML document in byte[] form. Each "token" of the XML document is represented as the following 64-bit integer:

Vtd layout.jpg

  • Big endian
  • Starting offset: 30 bits (b29 ~ b0) maximum value is 2^30 -1 = 1G -1
  • Length: 20 bits (b51 ~ b32) maximum value is 2^20-1 = 1M -1
    • For some token type
      • Prefix length: 9 bits (b51~ b43) max value 511
      • Q-name length: 11 bits (b42 ~ b 32) max value 1023
  • Depth: 8 bits (b59~b52) max value is 2^8-1 = 255
  • Token type: 4 bits (b63~b60)
  • Reserved bit: 2 bits (b31: b30)


VTD-XML Core Concepts

Generating a VTD-XML Representation of the XML Document (Unmarshal)

Instantiate a new VTDGen object, and parse a byte[]:

VTDGen vg = new VTDGen();
 
// from existing byte[]
// could be either VTD-XML bytes, or bytes from a regular File, etc. (?)
// true indicates namespace aware
byte[] bytes = ...
vg.setDoc(bytes);
vg.parse(true);
 
// - or - 
 
// from file
vg.parseFile("old.xml", false);


Navigating a VTD-XML Document

  • VTDNav
    • Obtained from VTDGen instance
    • Allows for basic "walking" through elements
    • Maintains a single cursor into the byte[] representation of XML
VTDGen vg = new VTDGen();
vg.parseFile("old.xml", false);
 
VTDNav vn = vg.getNav();


  • AutoPilot
    • Created from VTDNav instance
    • Allows for XPath execution
    • Allows for iterating over elements selected by XPath
VTDGen vg = new VTDGen();
vg.parseFile("old.xml", false);
 
VTDNav vn = vg.getNav();


Writing a VTD-XML Document (Marshal)

VTDGen vg = new VTDGen();
vg.parseFile("old.xml", false);
VTDNav vn = vg.getNav();
 
XMLModifier xm = new XMLModifier();
xm.bind(vn);
 
// ...
 
// Write to OutputStream
xm.output(new FileOutputStream("new.xml"));


Generating a VTD-XML Index File for Faster Parsing