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java - 使用 XPath 检索 XML 元素会引发 NullPointerException

问题描述

我的代码没有什么问题。我需要将 XML 文件解析为 Java，但我需要解析某些东西……我需要解析价格超过 10 并且出版日期超过 2005 年的书籍。
我写了一个使用 XPath 的代码，但它不起作用。

这是 XML 文件：

<pre>
    <code>
        <catalog>
            <book id="bk101">
                <author>Gambardella, Matthew</author>
                <title>XML Developer's Guide</title>
                <genre>Computer</genre>
                <price>41.95</price>
                <publish_date>2000-10-01</publish_date>
                <description>An in-depth look at creating applications 
                with XML.</description>
            </book>
            <book id="bk102">
                <author>Ralls, Kim</author>
                <title>Midnight Rain</title>
                <genre>Fantasy</genre>
                <price>5.95</price>
                <publish_date>2002-12-16</publish_date>
                <description>A former architect battles corporate zombies, 
                    an evil sorceress, and her own childhood to become queen 
                of the world.</description>
            </book>
            <book id="bk103">
                <author>Corets, Eva</author>
                <title>Maeve Ascendant</title>
                <genre>Fantasy</genre>
                <price>5.95</price>
                <publish_date>2000-11-17</publish_date>
                <description>After the collapse of a nanotechnology 
                    society in England, the young survivors lay the 
                foundation for a new society.</description>
            </book>
            <book id="bk104">
                <author>Corets, Eva</author>
                <title>Oberon's Legacy</title>
                <genre>Fantasy</genre>
                <price>5.95</price>
                <publish_date>2001-03-10</publish_date>
                <description>In post-apocalypse England, the mysterious 
                    agent known only as Oberon helps to create a new life 
                    for the inhabitants of London. Sequel to Maeve 
                Ascendant.</description>
            </book>
            <book id="bk105">
                <author>Corets, Eva</author>
                <title>The Sundered Grail</title>
                <genre>Fantasy</genre>
                <price>5.95</price>
                <publish_date>2001-09-10</publish_date>
                <description>The two daughters of Maeve, half-sisters, 
                    battle one another for control of England. Sequel to 
                Oberon's Legacy.</description>
            </book>
            <book id="bk106">
                <author>Randall, Cynthia</author>
                <title>Lover Birds</title>
                <genre>Romance</genre>
                <price>4.95</price>
                <publish_date>2003-09-02</publish_date>
                <description>When Carla meets Paul at an ornithology 
                conference, tempers fly as feathers get ruffled.</description>
            </book>
            <book id="bk107">
                <author>Thurman, Paula</author>
                <title>Splish Splash</title>
                <genre>Romance</genre>
                <price>4.95</price>
                <publish_date>2004-11-02</publish_date>
                <description>A deep sea diver finds true love twenty 
                thousand leagues beneath the sea.</description>
            </book>
            <book id="bk108">
                <author>Knorr, Stefan</author>
                <title>Creepy Crawlies</title>
                <genre>Horror</genre>
                <price>4.95</price>
                <publish_date>2005-12-06</publish_date>
                <description>An anthology of horror stories about roaches,
                centipedes, scorpions  and other insects.</description>
            </book>
            <book id="bk109">
                <author>Kress, Peter</author>
                <title>Paradox Lost</title>
                <genre>Science Fiction</genre>
                <price>6.95</price>
                <publish_date>2006-11-02</publish_date>
                <description>After an inadvertant trip through a Heisenberg
                    Uncertainty Device, James Salway discovers the problems 
                of being quantum.</description>
            </book>
            <book id="bk110">
                <author>O'Brien, Tim</author>
                <title>Microsoft .NET: The Programming Bible</title>
                <genre>Computer</genre>
                <price>36.95</price>
                <publish_date>2006-12-09</publish_date>
                <description>Microsoft's .NET initiative is explored in 
                detail in this deep programmer's reference.</description>
            </book>
            <book id="bk111">
                <author>O'Brien, Tim</author>
                <title>MSXML3: A Comprehensive Guide</title>
                <genre>Computer</genre>
                <price>36.95</price>
                <publish_date>2007-12-01</publish_date>
                <description>The Microsoft MSXML3 parser is covered in 
                    detail, with attention to XML DOM interfaces, XSLT processing, 
                SAX and more.</description>
            </book>
            <book id="bk112">
                <author>Galos, Mike</author>
                <title>Visual Studio 7: A Comprehensive Guide</title>
                <genre>Computer</genre>
                <price>49.95</price>
                <publish_date>2008-04-16</publish_date>
                <description>Microsoft Visual Studio 7 is explored in depth,
                    looking at how Visual Basic, Visual C++, C#, and ASP+ are 
                    integrated into a comprehensive development 
                environment.</description>
            </book>
        </catalog>

    </code>
</pre>

这是我的 Java 代码：

XPathFactory factory = XPathFactory.newInstance();
XPath path = factory.newXPath();
XPathExpression xpe = path.compile("//book[price>10]/author  | //book[publish_date>2005]/publish_date");

File xmlDocument = new File("boo.xml");
InputSource inputSource = new InputSource(new FileInputStream(xmlDocument));

Object result = xpe.evaluate(inputSource, XPathConstants.NODESET);



NodeList nodeList = (NodeList)result;

for (int i = 0; i < nodeList.getLength(); i++) {
    Node nNode = nodeList.item(i);
    System.out.println("\nCurrent Element :" + nNode.getNodeName());

if (nNode.getNodeType() == Node.ELEMENT_NODE) {
       Element eElement = (Element) nNode;
       System.out.println("Author : " 
          + eElement
          .getElementsByTagName("author")
          .item(0)
          .getTextContent());
       System.out.println("Title : " 
          + eElement
          .getElementsByTagName("title")
          .item(0)
          .getTextContent());
       System.out.println("Genre : " 
          + eElement
          .getElementsByTagName("genre")
          .item(0)
          .getTextContent());
       System.out.println("Price : " 
          + eElement
          .getElementsByTagName("price")
          .item(0)
          .getTextContent());
         System.out.println("Publish date : " 
          + eElement
          .getElementsByTagName("publish_date")
          .item(0)
          .getTextContent());
         System.out.println("Description : " 
          + eElement
          .getElementsByTagName("description")
          .item(0)
          .getTextContent());
     }
}

所以如果你能帮助我，我将不胜感激

标签： javaxml

解决方案

问题出在您的 XPath 上。“|” 运算符基本上允许您选择多个路径。因此，在您的代码中，这将只返回所有 author 和 publish_date 节点。那不是你想要的。您似乎想要一个符合特定标准的书籍列表。在这种情况下，您只想使用以下 XPath：

XPathExpression xpe = path.compile("//book[price>10 and number(translate(publish_date,'-','')) >20050101]");

注意日期比较。我只是将 2005 年 1 月 1 日用于大于 2005 年的日期。如果这不是您想要的，请进行相应的调整。

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