Java中的XML XPath解析

Question

這是以下標准代碼，用於使用Java中的XPath解析XML。 我無法調試為什么我得到空值。 我已經附加了java文件，xml文件和輸出。 如果有人可以解釋我哪里出錯了，將不勝感激。 提前致謝！ :)

XPathParser.java

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.NodeList;

public class XPathParser {
    public static void main(String args[]) throws Exception {
        //loading the XML document from a file
        DocumentBuilderFactory builderfactory = DocumentBuilderFactory.newInstance();
        builderfactory.setNamespaceAware(true);

        //XML read
        DocumentBuilder builder = builderfactory.newDocumentBuilder();
        Document xmlDocument = builder.parse("Stocks.xml");

        // Creates a XPath factory
        XPathFactory factory = javax.xml.xpath.XPathFactory.newInstance();

        //Creates a XPath Object
        XPath xPath = factory.newXPath();

        //Compiles the XPath expression
        //XPathExpression xPathExpression_count = xPath.compile("count(//stock)");
        XPathExpression xPathExpression = xPath.compile("//stock");

        //Run the query and get a nodeset
        Object result = xPathExpression.evaluate(xmlDocument,XPathConstants.NODESET);

        //Cast the result into a DOM nodelist
        NodeList nodes = (NodeList) result;
        System.out.println(nodes.getLength());
        System.out.println(nodes.item(0));
        for (int i=0; i<nodes.getLength();i++){
          System.out.println(nodes.item(i).getNodeValue());
        }
    }
}

Stocks.xml

<?xml version="1.0" encoding="UTF-8"?>
<stocks>
       <stock>
              <symbol>ABC</symbol>
              <price>10</price>
              <quantity>50</quantity>
       </stock>
       <stock>
              <symbol>XYZ</symbol>
              <price>20</price>
              <quantity>1000</quantity>
       </stock>
</stocks>

輸出：

2
[stock: null]
null
null

Answer 1

您嘗試在Stock節點上調用getNodeValue方法-這沒有意義，因為它們沒有值，它們是父節點。

您可以遍歷Stock的子節點並查找信息：

final Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new ByteArrayInputStream(xml.getBytes()));
final XPathExpression expression = XPathFactory.newInstance().newXPath().compile("//stock");
final NodeList nodeList = (NodeList) expression.evaluate(document, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); ++i) {
    final NodeList childList = ((Element) nodeList.item(i)).getChildNodes();
    for (int j = 0; j < childList.getLength(); ++j) {
        final Node node = childList.item(j);
        if (node.getNodeType() == Node.ELEMENT_NODE) {
            System.out.println(node.getNodeName() + "=" + node.getTextContent());
        }
    }
}

輸出：

symbol=ABC
price=10
quantity=50
symbol=XYZ
price=20
quantity=1000

請注意，您必須按類型過濾子Node ，否則將遍歷子節點和該節點的文本值（作為文本節點出現）的組合。 這是以這種方式遍歷XML的常見陷阱。

您還可以遍歷Stock所有子文本節點：

final Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new ByteArrayInputStream(xml.getBytes()));
final XPathExpression expression = XPathFactory.newInstance().newXPath().compile("//stock/*/text()");
final NodeList nodeList = (NodeList) expression.evaluate(document, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); ++i) {
    final Node node = nodeList.item(i);
    System.out.println(node.getNodeValue());
}

輸出：

ABC
10
50
XYZ
20
1000

在這種情況下，您將遍歷Stock子級的子級的所有文本節點-這意味着您會丟失有關節點名的信息。 但是您可以通過遍歷不是文本節點的所有Stock子級來重新創建第一種方法：

final Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new ByteArrayInputStream(xml.getBytes()));
final XPathExpression expression = XPathFactory.newInstance().newXPath().compile("//stock/*");
final NodeList nodeList = (NodeList) expression.evaluate(document, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); ++i) {
    final Node node = nodeList.item(i);
    System.out.println(node.getNodeName() + "=" + node.getTextContent());
}

輸出：

symbol=ABC
price=10
quantity=50
symbol=XYZ
price=20
quantity=1000

或者，如果需要更具體的信息，則可以在XPath中選擇一個特定的子節點：

final Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new ByteArrayInputStream(xml.getBytes()));
final XPathExpression expression = XPathFactory.newInstance().newXPath().compile("//stock/symbol/text()");
final NodeList nodeList = (NodeList) expression.evaluate(document, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); ++i) {
    final Node node = nodeList.item(i);
    System.out.println(node.getNodeValue());
}

輸出：

ABC
XYZ