怎样快速从一个XML文件中查找信息

网络编程 2021-07-05 11:35www.168986.cn编程入门
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./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 ="dd" xmlns:bk="urn:sample" xmlns:ns="http://.Any." 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 Gias</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./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。

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