教你怎样快速从一个XML文件中查找信息的详细介绍

互联网 17-3-6

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：