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怎樣快速從一個XML文件中查找信息
編輯:XML詳解     
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。
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