Processing Xml With Java - A Guide To Sax, Dom, Jdom, Jaxp, And Trax

Queries

XPath can be thought of as a query language like SQL. Rather than extracting information from a database, however, XPath extracts information from an XML document. An example should help make this more concrete. Consider the simple weather report document in Example 16.1.

Example 16.1 Weather Data in XML

<?xml version="1.0" encoding="ISO-8859-1"?>
<weather time="2002-06-06T15:35:00-05:00">
  <report latitude="41.2° N" longitude="71.6° W">
    <locality>Block Island</locality>
    <temperature units="°C">16</temperature>
    <humidity>88%</humidity>
    <dewpoint units="°C">14</dewpoint>
    <wind>
      <direction>NE</direction>
      <speed units="km/h">16.1</speed>
      <gust units="km/h">31</gust>
    </wind>
    <pressure units="hPa">1014</pressure>
    <condition>overcast</condition>
    <visibility>13 km</visibility>
  </report>
  <report latitude="34.1° N" longitude="118.4° W">
    <locality>Santa Monica</locality>
    <temperature units="°C">19</temperature>
    <humidity>79%</humidity>
    <dewpoint units="°C">16</dewpoint>
    <wind>
      <direction>WSW</direction>
      <speed units="km/h">14.5</speed>
    </wind>
    <pressure units="hPa">1010</pressure>
    <condition>hazy</condition>
    <visibility>5 km</visibility>
  </report>
</weather>

Here are some XPath expressions that identify particular parts of this document:

• /weather/report is an XPath expression that selects the two report elements.

• /weather/report[1] is an XPath expression that selects the first report element.

• /weather/report/temperature is an XPath expression that selects the two temperature elements.

• /weather/report[locality="Santa Monica"] is an XPath expression that selects the second report element.

• //report[locality="Block Island"]/attribute::longitude is an XPath expression that selects the longitude attribute of the first report element.

• /child::weather/child::report/child::wind/child::* is an XPath expression that selects all of the direction, speed, and gust elements.

• 9 * number(/weather/report[locality="Block Island"]/temperature) div 5 + 32 is an XPath expression that returns the temperature on Block Island in degrees Fahrenheit.

• /descendant::* is an XPath expression that selects all of the elements in the document.

Like SQL, XPath expressions are used in many different contexts, including the following:

• Dedicated query tools such as Alex Chaffee's XPath Explorer. Figure 16.1 shows this tool evaluating the expression /weather/report/temperature against Example 16.1.

  Figure 16.1. XPath Explorer

  

• Native XML databases such as the Apache XML Project's XIndice and Software AG's Tamino.

• As a component of other, broader languages such as XSLT and XQuery.

• Last but certainly not least, as a search component for your own Java programs that read XML documents.