ibm xml專區中對XPATH的一個好文

  ibm xml專區中對XPATH的一個好文,http://www.ibm.com/developerworks/cn/xml/x-xpathjava/

主要小結如下:
1 JDK 1.5中已經內建了很好的import javax.xml.xpath.XPathFactory;
了,用1.5吧

2 核心代碼
  public XPathEvaluator(String xmlFilename) throws IOException {
    try {
      // Convert filename into a DOM tree
      DocumentBuilderFactory domFactory =
        DocumentBuilderFactory.newInstance();
      domFactory.setNamespaceAware(true);
      DocumentBuilder builder = domFactory.newDocumentBuilder();
      this.domTree = builder.parse(xmlFilename);
    } catch (SAXException e) {
      throw new IOException("Error in document parsing: " + e.getMessage());
    } catch (ParserConfigurationException e) {
      throw new IOException("Error in configuring parser: " + e.getMessage());
    }
  }

  public NodeList evaluateXPath(String xpathString) throws IOException {
    try {
      XPathFactory factory = XPathFactory.newInstance();
      XPath xpath = factory.newXPath();
      return (NodeList)xpath.evaluate(
        xpathString, domTree, XPathConstants.NODESET);
    } catch (XPathExpressionException e) {
      throw new IOException("Error evaluating XPath: " + e.getMessage());
    }
  }

  public static void main(String[] args) {
    try {
      if (args.length != 1) {
        System.err.println("Usage: java ibm.dw.xpath.XPathEvaluator " +
          "[XML filename]");
        System.exit(1);
      }
      XPathEvaluator evaluator = new XPathEvaluator(args[0]);
      String xpathString = "//target[@name='init']/property[" +
                           "starts-with(@name, 'parser')]";
      NodeList results = evaluator.evaluateXPath(xpathString);
      for (int i=0; i<results.getLength(); i++) {
        Node node = results.item(i);
        System.out.print("Result: ");
        switch (node.getNodeType()) {
          case Node.ELEMENT_NODE: System.out.println("Element node named " +
                                    node.getNodeName());
                                  break;
          case Node.ATTRIBUTE_NODE: System.out.println(
                                     "Attribute node named " +
                                       node.getNodeName() + " with value '" +
                                       node.getNodeValue() + "'");
                                  break;
          case Node.TEXT_NODE:    System.out.println("Text: '" +
                                    node.getNodeValue() + "'");
                                  break;
          default: System.out.println(node);
        }
      }
  

  可見其基本步驟為,1 讀入一個XML

                         2 建立DOMTREE,即:
                             DocumentBuilderFactory domFactory =
         DocumentBuilderFactory.newInstance();
       domFactory.setNamespaceAware(true);
       DocumentBuilder builder = domFactory.newDocumentBuilder();
       this.domTree = builder.parse(xmlFilename);

                        3 String xpathString = "//target[@name='init']/property[" +
            "starts-with(@name, 'parser')]";
     NodeList results = evaluator.evaluateXPath(xpathString);

    這裡向該方法傳入xpath.其中evaluatexpath方法的功能為把domtree中用xpath解析後,返回一個nodelist,然後再解析.

             4 建立XPATH工廠
                 XPathFactory factory = XPathFactory.newInstance();
      XPath xpath = factory.newXPath();
      return (NodeList)xpath.evaluate(
        xpathString, domTree, XPathConstants.NODESET);

    
XPathFactory是一個抽象工廠.

抽象工廠設計模式使得這一種 API 能夠支援不同的物件模型,如 DOM、JDOM 和 XOM.

為了選擇不同的模型,需要向XPathFactory.newInstance()方法傳遞標識物件模型的統一資源識別項(URI).
目前只支援DOM.

    之後建立xpath對象.

   之後應用xpath運算式
   其中evaluate() 方法被聲明為返回 Object,實際返回什麼依賴於 XPath 運算式的結果以及要求的類型

一般來說,XPath與Java的映射關係是:

number 映射為 java.lang.Double
string 映射為 java.lang.String
boolean 映射為 java.lang.Boolean
node-set 映射為 org.w3c.dom.NodeList

在 Java 中計算 XPath 運算式時,第二個參數(XPathConstants常量)指定需要的傳回型別.有五種可能,都在 javax.xml.xpath.XPathConstants 類中命名了常量:

XPathConstants.NODESET
XPathConstants.BOOLEAN
XPathConstants.NUMBER
XPathConstants.STRING
XPathConstants.NODE

XPathConstants.NODE提示:

最後一個 XPathConstants.NODE 實際上沒有匹配的 XPath 類型.只有知道 XPath 運算式只返回一個節點或者只需要一個節點時才使用它.如果 XPath 運算式返回了多個節點並且指定了 XPathConstants.NODE,則 evaluate() 按照文檔順序返回第一個節點.如果 XPath 運算式選擇了一個空集並指定了 XPathConstants.NODE,則 evaluate() 返回 null.

注意:如果不能完成要求的轉換,evaluate()將拋出 XPathException

  最後,可以轉換為DOM的NodeList進行解析.

一個更典型的小例子如下:

        DocumentFactory factory=DocumentFactory.newInstance();

  factory.setNamespaceAware(true); 

        DocumentBuilder builder=factory.newDocumentBuilder();

         Document doc=builder.parse("src/books.xml");

     XpathFactory xpathfactory=XpathFactory.newInstance();

    Xpath xpath=xpathfactory.newXpath();

   XPathExpression pathExpression = xpath   

                                .compile("//book[author='TEST']/title/text()");   

   Object result=pathExpression.evalucate(doc,XPathConstants.NODESET);

  NodeList nodes=(NodeList)result;

 for (i=0;i<=nodes.getLenth();i++)

   System.out.println(nodes.item(i).getNodevalue):