Question

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InputStream对象评估Xpath表达式时，我尝试调试它，但发现没有错（当然我错过了一些东西......）。这是代码：

来自Main：

    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);

我们使用此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！

Answer 1

好m.evaluate(q, null, XPathConstants.NODESET);传入一个对evaluate方法的null引用，所以你创建new DOMSource(null)我认为这对我来说似乎没有意义，并且可能会导致稍后的相对XPath {{ 1}}被评估。

评估Xpath时的InputStream异常

1 个答案: