Java Dom 解析器报告错误的子节点数

Question

我有以下 xml 文件:

<?xml version="1.0" encoding="UTF-8"?>
<users>
<user id="0" firstname="John"/>
</users>

然后我试图用 java 解析它，但是 getchildnodes 报告错误的子节点数。

Java代码:

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document document = builder.parse(this.file);
document.getDocumentElement().normalize();
Element root = document.getDocumentElement();
NodeList nodes = root.getChildNodes();
System.out.println(nodes.getLength());

结果:3

此外，我还收到了访问节点属性的 NPE，所以我猜是出了什么大问题。

Answer 1

子节点由空白元素和文本节点组成。在处理属性之前，您需要检查节点类型。您可能还想考虑使用从 Java SE 5 开始的 JDK/JRE 中可用的 javax.xml.xpath API。

示例 1

此示例演示如何针对 DOM 发出 XPath 语句。

package forum11649396;

import java.io.StringReader;
import javax.xml.parsers.*;
import javax.xml.xpath.*;
import org.w3c.dom.*;
import org.xml.sax.InputSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        String xml = "<?xml version='1.0' encoding='UTF-8'?><users><user id='0' firstname='John'/></users>";

        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        Document document = db.parse(new InputSource(new StringReader(xml)));

        XPathFactory xpf = XPathFactory.newInstance();
        XPath xpath = xpf.newXPath();
        Element userElement = (Element) xpath.evaluate("/users/user", document, XPathConstants.NODE);
        System.out.println(userElement.getAttribute("id"));
        System.out.println(userElement.getAttribute("firstname"));
    }

}

示例 2

以下示例演示了如何针对 InputSource 发出 XPath 语句以获取 DOM 节点。这使您不必自己将 XML 解析为 DOM。

package forum11649396;

import java.io.StringReader;
import javax.xml.xpath.*;
import org.w3c.dom.*;
import org.xml.sax.InputSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        String xml = "<?xml version='1.0' encoding='UTF-8'?><users><user id='0' firstname='John'/></users>";

        XPathFactory xpf = XPathFactory.newInstance();
        XPath xpath = xpf.newXPath();
        InputSource inputSource = new InputSource(new StringReader(xml));
        Element userElement = (Element) xpath.evaluate("/users/user", inputSource, XPathConstants.NODE);
        System.out.println(userElement.getAttribute("id"));
        System.out.println(userElement.getAttribute("firstname"));
    }

}