我得到：
javax.xml.transform.TransformerException: Unable to evaluate expression using this context
at com.sun.org.apache.xpath.internal.XPath.execute(Unknown Source)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.eval(Unknown Source)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(Unknown Source)
at XPathImplementation.evaluate(XPathImplementation.java:136)
at Main.main(Main.java:23)
Caused by: java.lang.RuntimeException: Unable to evaluate expression using this context
at com.sun.org.apache.xpath.internal.axes.NodeSequence.setRoot(Unknown Source)
at com.sun.org.apache.xpath.internal.axes.LocPathIterator.execute(Unknown Source)
在尝试使用InputStreamobject 评估 Xpath 表达式时，我尝试对其进行调试，但没有发现任何错误（当然我错过了一些东西..）。这是代码：
从主要：
XPathProject m = new XPathImplementation();
m.loadXML("books.xml");
String q = "inventory/book/chapter[3]/preceding-sibling::chapter//title";
Object ob = m.evaluate(q, null, XPathConstants.NODESET);
我们使用这种evaluate方法：
public Object evaluate(String expression, Node source, QName returnType) throws XPathExpressionException,IllegalArgumentException,NullPointerException, TransformerException
{
...
InputStream is = nodeToInputStream(source);
Object returnedObject= xpath.evaluate(expression, is, returnType); // it happens here !!
... more code
}
辅助方法nodeToInputStream：
/*
* Convert Node object into InputStream object
*/
private InputStream nodeToInputStream(Node node) throws TransformerException {
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
StreamResult outputTarget = new StreamResult(outputStream);
Transformer t = TransformerFactory.newInstance().newTransformer();
t.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
t.transform(new DOMSource(node), outputTarget);
return new ByteArrayInputStream(outputStream.toByteArray());
}
知道我哪里出错了吗？10倍！