Dom4j and XPath

In this, the latest installment of obscure gotchas in Java development, I'm going to discuss an interesting behavior of Dom4j, definitely something to beware of: when you use '/' or '//' to start an XPath search expression in conjunction with the instance method Node.select{Nodes | SingleNode}, the search does not start at that node! In fact, it will always start at the actual document root, contrary to what one may expect from looking at the code / API.

Allow me to illustrate with an example. Lets say you are working with this simplified XML document:

 <Account>
   <Owner>
     <ContactInfo>
       <Name>Tom Jones</Name>
       ...
     </ContactInfo>
     ...
   </Owner>
   <Cosigner>
     <ContactInfo>
       <Name>Jim Johnson</Name>
       ...
     </ContactInfo>
     ...
   </Cosigner>
 </Account>

Dom4j makes it easy to find the Cosigner node:

Node cosignerNode = document.selectSingleNode("/Account/Cosigner");

and at first glance, I thought the following code snippet would return the Cosigner's name:

cosignerNode.selectSingleNode("//ContactInfo/Name") => "Tom Jones"

Counter intuitively, this code returns 'Tom Jones'. This is because when you start an XPath query with '/' or '//', Dom4j will traverse the DOM back up to the root node to begin its search. By removing the leading slashes, it works as expected:

cosignerNode.selectSingleNode("ContactInfo/Name") => "Jim Johnson"

So in conclusion, be careful whenever you use selectSingleNode; make sure that you understand that whenever you use // relative XPath queries, the result will come from the root of the entire document, and will not be limited to children of the node on which you invoke it.