Java5中为XPath添加扩展函数

Java5中增加了XPath API，并且可以通过Java添加XPath的扩展函数。下面是个例子:
//函数用来计算打折信息
public class DiscountFunction implements XPathFunction {
    public Object evaluate(List args) throws XPathFunctionException {
        if (args.size() != 1) {
            throw new XPathFunctionException("Wrong number of arguments to discount function()");
        }

        Object o = args.get(0);
        double value = 0;
        // perform conversions
        if (o instanceof Double) {
            value = (Double) o;
        } else if (o instanceof NodeList) {
            NodeList list = (NodeList) o;
            Node node = list.item(0);
            value = Double.parseDouble(node.getTextContent());

        } else {
            throw new XPathFunctionException("Could not convert argument type");
        }
        if (value < 50) return value * 0.9;
        else if (value < 100) return value * 0.95;
        else return value;
    }
}

//
public class DiscountFunctionResolver implements XPathFunctionResolver {
    public XPathFunction resolveFunction(QName fname, int arity) {
        if (fname == null)
              throw new NullPointerException("The function name cannot be null.");

            // We only recognize one function, i.e. ex:addFunc().
            if (fname.equals(new QName("http://ext.com", "discount", "ext")))
              /**
               * Return a customized implementation of XPathFunction. We need
               * to implement the evaluate(List) method.
               */
              return new DiscountFunction();
            else
              return null;
          }
}

//namespace context
public class ExtensionFuncContext implements NamespaceContext
      {
          public String getNamespaceURI(String prefix)
          {
              if (prefix == null)
                throw new IllegalArgumentException("The prefix cannot be null.");

              if (prefix.equals("ext"))
                  return "http://ext.com";
              else
                  return null;
          }

          public String getPrefix(String namespace)
          {
              if (namespace == null)
                throw new IllegalArgumentException("The namespace uri cannot be null.");
              if (namespace.equals("http://ext.com"))
                return "ext";
              else
                return null;
          }

          public Iterator getPrefixes(String namespace)
          {
              return null;
          }
      }

//
public class XPathSample {
    public static void main(String[] args) throws Exception {
        XPathSample test = new XPathSample();

        test.xpathtest();
    }

    private void xpathtest() throws ParserConfigurationException, IOException, SAXException, XPathExpressionException {
        DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
        domFactory.setNamespaceAware(true); // never forget this!
        DocumentBuilder builder = domFactory.newDocumentBuilder();
        String filename="D://DEV//codebase//xip//trunk//" +
                "TestCenter//CaseCenter//testData//CommonTest//bible//bib.xml";
        Document doc = builder.parse(filename);

        XPathFactory factory = XPathFactory.newInstance();
        XPath xpath = factory.newXPath();
        xpath.setNamespaceContext(new ExtensionFuncContext());
        xpath.setXPathFunctionResolver(new DiscountFunctionResolver());
        XPathExpression expr
                = xpath.compile("ext:discount(/bib/vendor[name='Amazon']/book[1]/price/text())");

        Object result = expr.evaluate(doc, XPathConstants.NUMBER);
        if(result instanceof Double){
            System.out.println("result:"+result);
        }
    }
}