在网络时代,XML文件起到了一个保存和传输数据的作用。Soap协议通过Xml交流信息，数据库通过Xml文件存取等等。那么怎样快速的从一个XML文件中取得所需的信息呢？

我们知道，JAVA的JAXP中和Microsoft.Net都有Xml分析器，Microsoft.Net是边读边分析，而JAXP是读到内存中然后才进行分析（还有一种是事件机制去读），总而言之，是不利于快速读取。基于此，Microsoft.Net 和JAXP都提供了XPATH机制，来快速定位到XML文件中所需的节点。

例如有一个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>

如果我们想快速查找”last-name”等于”Austen”的所有标题名，可以通过以下方法可以得到：

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);

}

}

}

运行这个程序，结果为：

Book title: Pride And Prejudice

Book title: Emma

Book title: Sense and Sensibility


可以看到查找正确。

利用XPATH中的一些功能，也可以实现简单的排序和简单运算。如在数据库中经常要对数据进行汇总，就可用XPATH实现。

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>


我们可以对该XML文件中的price求和，以得到价格总数。

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());

}

}


}

运行这个程序，结果如下：

30.97

36.9

36.9

我们可以看到，30.97是books.xml中所有price值的总和，而36.9则是order.xml中所有price值的总和。通过XPAH不仅可以快速查找信息，而且还可以对信息进行一些基本的处理。