Java - XPath - 如何以相同的方式管理不同的表达式

Question

我有这个代码示例：

package com.test.xml;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl;
import net.sf.saxon.xpath.XPathFactoryImpl;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
import javax.xml.parsers.DocumentBuilderFactory;

import static javax.xml.xpath.XPathConstants.NODESET;
import static org.w3c.dom.Node.ATTRIBUTE_NODE;

public class MyMainClass {

    public static void main(String[] args) throws XPathExpressionException, IOException, SAXException, ParserConfigurationException {
        Arrays.stream(args).forEach(System.out::println);

        DocumentBuilderFactory factory = new DocumentBuilderFactoryImpl();
        DocumentBuilder documentBuilder = factory.newDocumentBuilder();
        Document document = documentBuilder.parse(new File(MyMainClass.class.getClassLoader().getResource("demo.xml").getFile()));
        Node root = document.getFirstChild();

        String expression = "//TestAttribute/@value";
        XPathFactory xpathFactory = new XPathFactoryImpl();
        XPath xpath = xpathFactory.newXPath();
        XPathExpression xpathExpression = xpath.compile(expression);
        Object evaluation = xpathExpression.evaluate(root, NODESET);
        List<String> list = toStringList((NodeList)evaluation);

        list.forEach(System.out::println);
    }

    private static List<String> toStringList(NodeList nodeList) {
        final int size = nodeList.getLength();
        List<String> strings = new ArrayList<>(size);
        for (int i = 0; i < size; i++) {
            Node item = nodeList.item(i);
            if (item.getNodeType() == ATTRIBUTE_NODE) {
                strings.add(item.getTextContent());
            }
        }
        return strings;
    }
}

'Demo.xml'的内容是：

<?xml version="1.0"?>
<test:Demo xmlns:test="http://java.sun.com/xml/ns/jaxb">
    <TestAttribute value="something" />
    <TestAttribute value="other" />
</test:Demo>

执行我得到结果：

something
other

一切正常，我没有任何问题。现在我想支持另一种表达方式，例如：namespace-uri(/*)

如果我替换这个表达式：

        String expression = "namespace-uri(/*)";
        XPathFactory xpathFactory = new XPathFactoryImpl();
        XPath xpath = xpathFactory.newXPath();
        XPathExpression xpathExpression = xpath.compile(expression);
        Object evaluation = xpathExpression.evaluate(root, NODESET);
        List<String> list = toStringList((NodeList)evaluation);

我收到此错误：

Exception in thread "main" net.sf.saxon.trans.XPathException: Cannot convert XPath value to Java object: required class is org.w3c.dom.NodeList; supplied value has type xs:anyURI
    at net.sf.saxon.dom.DOMObjectModel.convertXPathValueToObject(DOMObjectModel.java:422)
    at net.sf.saxon.dom.DOMObjectModel$7.convert(DOMObjectModel.java:205)
    at net.sf.saxon.xpath.XPathExpressionImpl.evaluate(XPathExpressionImpl.java:242)
    at com.test.xml.MyMainClass.main(MyMainClass.java:41)
--------------- linked to ------------------
javax.xml.xpath.XPathExpressionException: net.sf.saxon.trans.XPathException: Cannot convert XPath value to Java object: required class is org.w3c.dom.NodeList; supplied value has type xs:anyURI
    at net.sf.saxon.xpath.XPathExpressionImpl.evaluate(XPathExpressionImpl.java:247)
    at com.test.xml.MyMainClass.main(MyMainClass.java:41)
Caused by: net.sf.saxon.trans.XPathException: Cannot convert XPath value to Java object: required class is org.w3c.dom.NodeList; supplied value has type xs:anyURI
    at net.sf.saxon.dom.DOMObjectModel.convertXPathValueToObject(DOMObjectModel.java:422)
    at net.sf.saxon.dom.DOMObjectModel$7.convert(DOMObjectModel.java:205)
    at net.sf.saxon.xpath.XPathExpressionImpl.evaluate(XPathExpressionImpl.java:242)
    ... 1 more

我的问题是：

• 如何管理这些不同的表达式？
• 如何评估表达式然后检查类型？

谢谢！

Answer 1

您在 saxon-help 论坛上提出了同样的问题。作为记录，这是我的答案。今后请避免交叉发布。

这是 JAXP XPath API 的弱点之一，当您迁移到具有更丰富类型系统的 XPath 2.0 时，该弱点变得尤为突出：当您执行 XPath 表达式时，您必须说明预期的结果类型。

我建议迁移到 s9api API。在 s9api 中，您不必说出您期望的结果类型。此表达式的 XPathSelector.evaluate() 将返回 XdmAtomicValue。您可以使用 getTypeName() 方法确定其 XDM 类型。

请注意，除非您有非常充分的理由使用 DOM 作为树模型，否则我不会推荐它。这是一个设计不佳的 API，与 XDM 没有很好的对齐（尤其是在命名空间领域）并且不是线程安全的。优先使用 Saxon 的默认 TinyTree：它可以快 10 倍。