Traditionally DOM or SAX-based enterprise applications have to repeat CPU-intensive XML parsing when accessing the same documents multiple times. VTD-XML 2.0 introduces a simple general-purpose XML index called VTD+XML (http://vtd-xml.sourceforge.net/persistence.html) that eliminates the need for repetitive parsing of those applications.

This article combines various examples and the latest benchmark reports to show you how to get started with this indexing. This article also discusses various scenarios and use cases where you may find VTD+XML useful.

Avoid Repetitive XML Parsing with VTD-XML
As discussed in "Simplify XML processing with VTD-XML," to date one of underlying assumptions in XML application development is that an XML document must be parsed before anything else can be done with it. In other words, the processing logic of XML applications can't start without parsing. Frequently considered a threat to database performance, XML parsing is usually many times slower than other XML operations such as XPath evaluation. When those applications perform multiple read-only access to XML data that don't change very often, wouldn't it be nice to able to eliminate the overhead of associated repetitive parsing?

With the native XML indexing feature introduced in version 2.0 of VTD-XML, you can do precisely that. VTDGen, the class encapsulating various parsing routines, now adds "readIndex(...)" and "writeIndex(...)." VTD-XML 2.0 also introduces two new exceptions: indexWriteException and indexReadException.

Let me put those new methods into action and show you how to turn on the indexing capability in your application. Consider the following XML document:

   <purchaseOrder orderDate="1999-10-21">
     <item partNum="872-AA">
       <productName>Lawnmower</productName>
       <quantity>1</quantity>
       <USPrice>148.95</USPrice>
     </item>
   </purchaseOrder>

Below is a simple pre-2.0 VTD-XML code named "printPrice.java" that prints out the content of the element "USPrice." Notice that it parses the XML file and then uses XPath to filter out the target nodes.

import com.ximpleware.*;
import com.ximpleware.xpath.*;
public class printPrice{
   public static void main(String args[]){
     VTDGen vg = new VTDGen();
     try{
       if (vg.parseFile("po.xml",true)){
         VTDNav vn = vg.getNav();
         AutoPilot ap = new AutoPilot(vn);
         ap.selectXPath("/purchaseOrder/item/USPrice/text()");
         int i=-1;
         while((i=ap.evalXPath())!=-1){
           System.out.println(" USPrice ==> "+vn.toString(i));
         }
       }
     }catch(Exception e){

     }
   }
}

A few changes are needed to add VTD-XML's new indexing capability to the Java code above. First, you need to read in the XML document, parse it, and then write out the indexed version of the same XML document. From that point onward, your application can run XPath query or processing logic directly on top of the index, saving the CPU cycles of parsing the XML document again. The following code snippets (named "genIndex.java" and "accessIndex.java" respectively) show you how to generate and access the index. Notice that, when executed sequentially, both applications produce the identical output as "printPrice.java."
The first application (genIndex.java) reads in "po.xml" and produces "po.vxl."

import com.ximpleware.*;
import com.ximpleware.xpath.*;
public class genIndex{
   public static void main(String args[]){
     VTDGen vg = new VTDGen();
     try{
       if (vg.parseFile("po.xml",true)){
         vg.writeIndex("po.vxl");
       }
     }catch(Exception e){
     }
   }
}

The second application (accessIndex.java) loads "po.vxl" and filters the document using XPath expression "/purchaseOrder/item/USPrice/text()."

import com.ximpleware.*;
import com.ximpleware.xpath.*;
public class accessIndex{
   public static void main(String args[]){
     VTDGen vg = new VTDGen();
     try{
       VTDNav vn = vg.loadIndex("po.vxl");
       AutoPilot ap = new AutoPilot(vn);
       ap.selectXPath("/purchaseOrder/item/USPrice/text()");
       int i=-1;
       while((i=ap.evalXPath())!=-1){
         System.out.println(" USPrice ==> "+vn.toString(i));
       }
     }catch(Exception e){
     }

   }
}