InputStream的異常而計算XPath

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) 

雖然試圖用InputStream對象對XPath表達式，我試圖調試它但沒有發現任何錯誤（當然我錯過了什麼。 ）。下面的代碼：

從主營：

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); 

我們用這個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 
} 

輔助方法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()); 
} 

任何想法是我有什麼錯？ 10倍！

Answer 1

好m.evaluate(q, null, XPathConstants.NODESET);所以你創建new DOMSource(null)我覺得這似乎沒有任何意義，我也可能會導致該錯誤後，其中一個相關的XPath inventory/book/chapter[3]/preceding-sibling::chapter//title評估傳入一個空引用的評估方法。