ibm xml专区中对XPATH的一个好文
ibm xml专区中对XPATH的一个好文,http://www.ibm.com/developerworks/cn/xml/x-xpathjava/主要小结如下:
1 JDK 1.5中已经自带了很好的import javax.xml.xpath.XPathFactory;
了,用1.5吧
2 核心代码
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 " +
"");
System.exit(1);
}
XPathEvaluator evaluator = new XPathEvaluator(args);
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);
}
}
可见其基本步骤为,1 读入一个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);
这里向该方法传入xpath.其中evaluatexpath方法的功能为把domtree中用xpath解析后,返回一个nodelist,然后再解析.
4 建立XPATH工厂
XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
return (NodeList)xpath.evaluate(
xpathString, domTree, XPathConstants.NODESET);
XPathFactory是一个抽象工厂.
抽象工厂设计模式使得这一种 API 能够支持不同的对象模型,如 DOM、JDOM 和 XOM.
为了选择不同的模型,需要向XPathFactory.newInstance()方法传递标识对象模型的统一资源标识符(URI).
目前只支持DOM.
之后创建xpath对象.
之后应用xpath表达式
其中evaluate() 方法被声明为返回 Object,实际返回什么依赖于 XPath 表达式的结果以及要求的类型
一般来说,XPath与Java的映射关系是:
number 映射为 java.lang.Double
string 映射为 java.lang.String
boolean 映射为 java.lang.Boolean
node-set 映射为 org.w3c.dom.NodeList
在 Java 中计算 XPath 表达式时,第二个参数(XPathConstants常量)指定需要的返回类型.有五种可能,都在 javax.xml.xpath.XPathConstants 类中命名了常量:
XPathConstants.NODESET
XPathConstants.BOOLEAN
XPathConstants.NUMBER
XPathConstants.STRING
XPathConstants.NODE
XPathConstants.NODE提示:
最后一个 XPathConstants.NODE 实际上没有匹配的 XPath 类型.只有知道 XPath 表达式只返回一个节点或者只需要一个节点时才使用它.如果 XPath 表达式返回了多个节点并且指定了 XPathConstants.NODE,则 evaluate() 按照文档顺序返回第一个节点.如果 XPath 表达式选择了一个空集并指定了 XPathConstants.NODE,则 evaluate() 返回 null.
注意:如果不能完成要求的转换,evaluate()将抛出 XPathException
最后,可以转换为DOM的NodeList进行解析.
一个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/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):
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