Java Dom 解析器报告子节点数量错误

我有以下 xml 文件：

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

然后我尝试用 java 解析它，但 getchildnodes 报告子节点数错误。

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

另外，我还获得了用于访问节点属性的 NPE，所以我猜测出现了严重错误。

子节点由元素和空白文本节点组成。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javax.xml.xpath从 Java SE 5 开始，JDK/JRE 中提供了 API。

实施例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"));
    }

}

实施例2

以下示例演示了如何针对某个对象发出 XPath 语句InputSource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"));
    }

}