java - 为什么只有某些 XPath 表达式在 xml 具有 namespace 前缀时找到节点

Question

在下面的示例代码中，当源 xml 具有命名空间前缀时，形式为 '//elementName' 的任何 XPath 都返回 null(请参阅 testWithNS()底部的代码)。

当源 xml 没有命名空间前缀时，所有列出的 XPath 表达式都返回一个节点(参见 testNoNS())。

我知道我可以通过设置 NamespaceContext(如 testWithNSContext())、将 xml 解析为命名空间感知文档并在 XPath 中使用命名空间前缀来解决此问题。但是我不想这样做，因为我的实际代码需要处理带有和不带有命名空间前缀的 xml。

我的问题是为什么只有:

• //测试
• // child 1
• //孙子1
• // child 2

返回 null，而 testWithNS() 中的所有其他示例都返回节点？

输出

testNoNS()
test = found
/test = found
//test = found
//test/* = found
//test/child1 = found
//test/child1/grandchild1 = found
//test/child2 = found
//child1 = found
//grandchild1 = found
//child1/grandchild1 = found
//child2 = found

testWithNS()
test = found
/test = found
//test = *** NOT FOUND ***
//test/* = found
//test/child1 = found
//test/child1/grandchild1 = found
//test/child2 = found
//child1 = *** NOT FOUND ***
//grandchild1 = *** NOT FOUND ***
//child1/grandchild1 = found
//child2 = *** NOT FOUND ***

testWithNSContext()
ns1:test = found
/ns1:test = found
//ns1:test = found
//ns1:test/* = found
//ns1:test/ns1:child1 = found
//ns1:test/ns1:child1/ns1:grandchild1 = found
//ns1:test/ns1:child2 = found
//ns1:child1 = found
//ns1:grandchild1 = found
//ns1:child1/ns1:grandchild1 = found
//ns1:child2 = found

代码

import java.io.StringReader;
import java.util.Iterator;

import javax.xml.XMLConstants;
import javax.xml.namespace.NamespaceContext;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathFactory;

import org.junit.Test;
import org.w3c.dom.Document;
import org.xml.sax.InputSource;

public class XPathBugTest {

    private String xmlDec = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?>";
    private String xml = xmlDec + 
        "<test>" +
        "  <child1>" +
        "    <grandchild1/>" +
        "  </child1>" +
        "  <child2/>" +
        "</test>";
    private String xmlNs = xmlDec + 
        "<ns1:test xmlns:ns1=\"http://www.wfmc.org/2002/XPDL1.0\">" +
        "  <ns1:child1>" +
        "    <ns1:grandchild1/>" +
        "  </ns1:child1>" +
        "  <ns1:child2/>" +
        "</ns1:test>";

    final XPathFactory xpathFactory = XPathFactory.newInstance();
    final XPath xpath = xpathFactory.newXPath();

    @Test
    public void testNoNS() throws Exception {
        System.out.println("\ntestNoNS()");
        final Document doc = getDocument(xml);

        isFound("test", xpath.evaluate("test", doc, XPathConstants.NODE));
        isFound("/test", xpath.evaluate("/test", doc, XPathConstants.NODE));
        isFound("//test", xpath.evaluate("//test", doc, XPathConstants.NODE));
        isFound("//test/*", xpath.evaluate("//test/*", doc, XPathConstants.NODE));
        isFound("//test/child1", xpath.evaluate("//test/child1", doc, XPathConstants.NODE));
        isFound("//test/child1/grandchild1", xpath.evaluate("//test/child1/grandchild1", doc, XPathConstants.NODE));
        isFound("//test/child2", xpath.evaluate("//test/child2", doc, XPathConstants.NODE));
        isFound("//child1", xpath.evaluate("//child1", doc, XPathConstants.NODE));
        isFound("//grandchild1", xpath.evaluate("//grandchild1", doc, XPathConstants.NODE));
        isFound("//child1/grandchild1", xpath.evaluate("//child1/grandchild1", doc, XPathConstants.NODE));
        isFound("//child2", xpath.evaluate("//child2", doc, XPathConstants.NODE));
    }

    @Test
    public void testWithNS() throws Exception {
        System.out.println("\ntestWithNS()");
        final Document doc = getDocument(xmlNs);

        isFound("test", xpath.evaluate("test", doc, XPathConstants.NODE));
        isFound("/test", xpath.evaluate("/test", doc, XPathConstants.NODE));
        isFound("//test", xpath.evaluate("//test", doc, XPathConstants.NODE));
        isFound("//test/*", xpath.evaluate("//test/*", doc, XPathConstants.NODE));
        isFound("//test/child1", xpath.evaluate("//test/child1", doc, XPathConstants.NODE));
        isFound("//test/child1/grandchild1", xpath.evaluate("//test/child1/grandchild1", doc, XPathConstants.NODE));
        isFound("//test/child2", xpath.evaluate("//test/child2", doc, XPathConstants.NODE));
        isFound("//child1", xpath.evaluate("//child1", doc, XPathConstants.NODE));
        isFound("//grandchild1", xpath.evaluate("//grandchild1", doc, XPathConstants.NODE));
        isFound("//child1/grandchild1", xpath.evaluate("//child1/grandchild1", doc, XPathConstants.NODE));
        isFound("//child2", xpath.evaluate("//child2", doc, XPathConstants.NODE));
    }

    @Test
    public void testWithNSContext() throws Exception {
        System.out.println("\ntestWithNSContext()");
        final Document doc = getDocumentNS(xmlNs);

        xpath.setNamespaceContext(new MyNamespaceContext());

        isFound("ns1:test", xpath.evaluate("ns1:test", doc, XPathConstants.NODE));
        isFound("/ns1:test", xpath.evaluate("/ns1:test", doc, XPathConstants.NODE));
        isFound("//ns1:test", xpath.evaluate("//ns1:test", doc, XPathConstants.NODE));
        isFound("//ns1:test/*", xpath.evaluate("//ns1:test/*", doc, XPathConstants.NODE));
        isFound("//ns1:test/ns1:child1", xpath.evaluate("//ns1:test/ns1:child1", doc, XPathConstants.NODE));
        isFound("//ns1:test/ns1:child1/ns1:grandchild1", xpath.evaluate("//ns1:test/ns1:child1/ns1:grandchild1", doc, XPathConstants.NODE));
        isFound("//ns1:test/ns1:child2", xpath.evaluate("//ns1:test/ns1:child2", doc, XPathConstants.NODE));
        isFound("//ns1:child1", xpath.evaluate("//ns1:child1", doc, XPathConstants.NODE));
        isFound("//ns1:grandchild1", xpath.evaluate("//ns1:grandchild1", doc, XPathConstants.NODE));
        isFound("//ns1:child1/ns1:grandchild1", xpath.evaluate("//ns1:child1/ns1:grandchild1", doc, XPathConstants.NODE));
        isFound("//ns1:child2", xpath.evaluate("//ns1:child2", doc, XPathConstants.NODE));
    }

    private void isFound(String xpath, Object object) {
        System.out.println(xpath + " = " + (object == null ? "*** NOT FOUND ***" : "found"));
    }

    private Document getDocument(final String xml) throws Exception {
        final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        return factory.newDocumentBuilder().parse(new InputSource(new StringReader(xml)));        
    }

    private Document getDocumentNS(final String xml) throws Exception {
        final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(true);
        return factory.newDocumentBuilder().parse(new InputSource(new StringReader(xml)));
    }

    public class MyNamespaceContext implements NamespaceContext {
        @Override
        public String getNamespaceURI(String prefix) {
            if ("ns1".equals(prefix)) {
                return "http://www.wfmc.org/2002/XPDL1.0";
            }
            return XMLConstants.NULL_NS_URI;
        }
        @Override
        public String getPrefix(String uri) {
            throw new UnsupportedOperationException();
        }
        @Override
        public Iterator getPrefixes(String uri) {
            throw new UnsupportedOperationException();
        }
    }
}

在 Saxon 测试之后更新

我现在已经使用 Saxon 将 XPahtFactory 行更改为此来测试相同的代码

final XPathFactory xpathFactory = new net.sf.saxon.xpath.XPathFactoryImpl();

在 testWithNS() 中使用 Saxon 所有行返回 *** NOT FOUND *** 而不仅仅是像 '//elementName' 与默认的 Xalan 实现一样。

鉴于我正在使用非命名空间感知文档生成器工厂来解析 xml，为什么这些 xpath 都不起作用，而只有一些与 Xalan 一起工作？

Answer 1

如果要忽略命名空间，可以使用local-name XPath 函数:

//*[local-name()='grandchild1']