NamespaceContext并使用带有XPath的命名空间 [英] NamespaceContext and using namespaces with XPath

问题描述

解析包含Java中命名空间的xpath似乎需要使用 NamespaceContext 对象，将前缀映射到命名空间URL，反之亦然。但是，除了自己实现之外，我找不到获取 NamespaceContext 的机制。这似乎违反直觉。

Resolving an xpath that includes namespaces in Java appears to require the use of a NamespaceContext object, mapping prefixes to namespace urls and vice versa. However, I can find no mechanism for getting a NamespaceContext other than implementing it myself. This seems counter-intuitive.

问题：有没有简单的方法从文档中获取 NamespaceContext ，或创建一个，或者失败，完全放弃前缀并指定具有完全限定名称的xpath？

The question: Is there any easy way to acquire a NamespaceContext from a document, or to create one, or failing that, to forgo prefixes altogether and specify the xpath with fully qualified names?

推荐答案

它是可以在不编写自己的类的情况下获取 NamespaceContext 实例。其类使用页面显示您可以使用获取一个javax.xml.stream 包。

It is possible to get a NamespaceContext instance without writing your own class. Its class-use page shows you can get one using the javax.xml.stream package.

String ctxtTemplate = "<data xmlns=\"http://base\" xmlns:foo=\"http://foo\" />";
NamespaceContext nsContext = null;

XMLInputFactory factory = XMLInputFactory.newInstance();
XMLEventReader evtReader = factory
    .createXMLEventReader(new StringReader(ctxtTemplate));
while (evtReader.hasNext()) {
  XMLEvent event = evtReader.nextEvent();
  if (event.isStartElement()) {
    nsContext = ((StartElement) event)
        .getNamespaceContext();
    break;
  }
}

System.out.println(nsContext.getNamespaceURI(""));
System.out.println(nsContext.getNamespaceURI("foo"));
System.out.println(nsContext
    .getNamespaceURI(XMLConstants.XMLNS_ATTRIBUTE));
System.out.println(nsContext
    .getNamespaceURI(XMLConstants.XML_NS_PREFIX));

完全放弃前缀可能会导致表达式含糊不清 - 如果你想删除名称空间前缀，你就是' d需要更改文档格式。从文档创建上下文不一定有意义。前缀必须与XPath表达式中使用的前缀匹配，而不是与任何文档中的前缀相匹配，如下代码所示：

Forgoing prefixes altogether is likely to lead to ambiguous expressions - if you want to drop namespace prefixes, you'd need to change the document format. Creating a context from a document doesn't necessarily make sense. The prefixes have to match the ones used in the XPath expression, not the ones in any document, as in this code:

String xml = "<data xmlns=\"http://base\" xmlns:foo=\"http://foo\" >"
    + "<foo:value>"
    + "hello"
    + "</foo:value>"
    + "</data>";
String expression = "/stack:data/overflow:value";
class BaseFooContext implements NamespaceContext {
  @Override
  public String getNamespaceURI(String prefix) {
    if ("stack".equals(prefix))
      return "http://base";
    if ("overflow".equals(prefix))
      return "http://foo";
    throw new IllegalArgumentException(prefix);
  }

  @Override
  public String getPrefix(String namespaceURI) {
    throw new UnsupportedOperationException();
  }

  @Override
  public Iterator<String> getPrefixes(
      String namespaceURI) {
    throw new UnsupportedOperationException();
  }
}
XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
xpath.setNamespaceContext(new BaseFooContext());
String value = xpath.evaluate(expression,
    new InputSource(new StringReader(xml)));
System.out.println(value);

StAX API返回的实现和上面的实现都没有实现完整的 class / 方法按文档中的定义进行合同。您可以在这里获得完整的基于地图的实施方案。

Neither the implementation returned by the StAX API nor the one above implement the full class/method contracts as defined in the doc. You can get a full, map-based implementation here.

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