在網絡時代,XML文件起到了一個保存和傳輸數據的作用。Soap協議通過XML交流信息,數據庫通過Xml文件存取等等。那麼怎樣快速的從一個XML文件中取得所需的信息呢?
我們知道,Java的JAXP中和Microsoft.Net都有XML分析器,Microsoft.Net是邊讀邊分析,而JAXP是讀到內存中然後才進行分析(還有一種是事件機制去讀),總而言之,是不利於快速讀取。基於此,Microsoft.Net 和JAXP都提供了XPATH機制,來快速定位到XML文件中所需的節點。
例如有一個XML文件:booksort.XML:
<?XML version="1.0"?>
<!-- a fragment of a book store inventory database -->
<bookstore XMLns:bk="urn:samples">
<book genre="novel" publicationdate="1997" bk:ISBN="1-861001-57-8">
<title>Pride And Prejudice</title>
<author>
<first-name>Jane</first-name>
<last-name>Austen</last-name>
</author>
<price>24.95</price>
</book>
<book genre="novel" publicationdate="1992" bk:ISBN="1-861002-30-1">
<title>The Handmaid's Tale</title>
<author>
<first-name>Margaret</first-name>
<last-name>Atwood</last-name>
</author>
<price>29.95</price>
</book>
<book genre="novel" publicationdate="1991" bk:ISBN="1-861001-57-6">
<title>Emma</title>
<author>
<first-name>Jane</first-name>
<last-name>Austen</last-name>
</author>
<price>19.95</price>
</book>
<book genre="novel" publicationdate="1982" bk:ISBN="1-861001-45-3">
<title>Sense and Sensibility</title>
<author>
<first-name>Jane</first-name>
<last-name>Austen</last-name>
</author>
<price>19.95</price>
</book>
</bookstore>
如果我們想快速查找”last-name”等於”Austen”的所有標題名,可以通過以下方法可以得到:
XMLReaderSample.cs
//Corelib.Net/System.XML.Xsl/XPathDocument Class
//Author :Any
using System;
using System.IO;
using System.XML;
using System.XML.XPath;
public class XMLReaderSample
{
public static void Main()
{
XmlTextReader myxtreader = new XmlTextReader("booksort.XML");
XMLReader myxreader = myxtreader;
XPathDocument doc = new XPathDocument(myxreader);
XPathNavigator nav = doc.CreateNavigator();
XPathExpression expr;
expr = nav.Compile("descendant::book[author/last-name='Austen']");
//expr.AddSort("title", XmlSortOrder.Ascending, XmlCaSEOrder.None, "", XMLDataType.Text);
XPathNodeIterator iterator = nav.Select(expr);
while (iterator.MoveNext())
{
XPathNavigator nav2 = iterator.Current;
nav2.MoveToFirstChild();
Console.WriteLine("Book title: {0}", nav2.Value);
}
}
}
運行這個程序,結果為:
Book title: Pride And Prejudice
Book title: Emma
Book title: Sense and Sensibility
可以看到查找正確。
利用XPATH中的一些功能,也可以實現簡單的排序和簡單運算。如在數據庫中經常要對數據進行匯總,就可用XPATH實現。
如:
order.XML
<!--Represents a customer order-->
<order>
<book ISBN='10-861003-324'>
<title>The Handmaid's Tale</title>
<price>19.95</price>
</book>
<cd ISBN='2-3631-4'>
<title>Americana</title>
<price>16.95</price>
</cd>
</order>
和:books.XML
<?XML version="1.0"?>
<!-- This file represents a fragment of a book store inventory database -->
<bookstore>
<book cc="dd" xmlns:bk="urn:sample" XMLns:ns="http://www.Any.com" genre="autobiography" publicationdate="1981" ISBN="1-861003-11-0">
<title>The Autobiography of Benjamin Franklin</title>
<ns:author>
<first-name>Benjamin</first-name>
<last-name>Franklin</last-name>
</ns:author>
<price>8.99</price>
</book>
<book genre="novel" publicationdate="1967" ISBN="0-201-63361-2">
<title>The Confidence Man</title>
<author>
<first-name>Herman</first-name>
<last-name>Melville</last-name>
</author>
<price>11.99</price>
</book>
<book genre="philosophy" publicationdate="1991" ISBN="1-861001-57-6">
<title>The Gorgias</title>
<author>
<name>Plato</name>
</author>
<price>9.99</price>
</book>
</bookstore>
我們可以對該XML文件中的price求和,以得到價格總數。
Evaluate.cs
//Corelib.Net/System.XML.Xsl/XPathNavigator Class
//Author :Any
using System;
using System.IO;
using System.XML;
using System.XML.XPath;
public class EvaluateSample
{
public static void Main()
{
EvaluateSample myEvaluateSample = new EvaluateSample();
myEvaluateSample.test("books.XML");
}
public void test(String args)
{
try
{
//test Evaluate(String);
XPathDocument myXPathDocument = new XPathDocument(args);
XPathNavigator myXPathNavigator = myXPathDocument.CreateNavigator();
Console.WriteLine(myXPathNavigator.Evaluate("sum(descendant::book/price)"));
//testEvaluate(XPathExpression);
XmlDocument doc = new XMLDocument();
doc.Load("order.XML");
XPathNavigator nav = doc.CreateNavigator();
XPathExpression expr = nav.Compile("sum(//price/text())");
Console.WriteLine(nav.Evaluate(expr));
//testEvaluate(XPathExpression);
XPathNodeIterator myXPathNodeIterator = nav.Select("descendant::book/title");
expr = nav.Compile("sum(//price/text())");
Console.WriteLine(nav.Evaluate(expr,myXPathNodeIterator));
}
catch (Exception e)
{
Console.WriteLine ("Exception: {0}", e.ToString());
}
}
}
運行這個程序,結果如下:
30.97
36.9
36.9
我們可以看到,30.97是books.xml中所有price值的總和,而36.9則是order.XML中所有price值的總和。通過XPAH不僅可以快速查找信息,而且還可以對信息進行一些基本的處理。