Question

So I have an XML file here, and I want to get a value from it using XPath. The file in question is an XML file in a zip that you can download here:

I believe that it’s valid XML.

My current goal is to parse out the last (newest) LMP_PRC value in the document.

My current code looks like this and returns an empty string every time:

XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();
XPathExpression expr =
  xpath.compile("//m:OASISReport/m:MessagePayload/m:RTO/m:REPORT_ITEM[3]/m:REPORT_DATA[/m:DATA_ITEM='LMP_PRC'][last()]/m:VALUE/text()") ;
String result = (String) expr.evaluate(doc, XPathConstants.STRING);

The m:REPORT_ITEM[3] has been an area of confusion for me so I’ve tried it with just about every number and have had no luck with it.

Answer 1

The m:REPORT_ITEM[3] just means the 3rd one that matches. You’re not getting any matches because of:

m:REPORT_DATA[/m:DATA_ITEM='LMP_PRC'][last()]

You want the relative path from there, not the absolute path:

m:REPORT_DATA[m:DATA_ITEM='LMP_PRC'][last()]

So the whole thing would look like:

//m:OASISReport/m:MessagePayload/m:RTO/m:REPORT_ITEM[3]/m:REPORT_DATA[m:DATA_ITEM='LMP_PRC'][last()]/m:VALUE/text()

The other thing to deal with is the namespace resolution. For a good summary of what is happening in your case, see http://blog.davber.com/2006/09/17/xpath-with-namespaces-in-java/. So you should add something like:

private static NamespaceContext getNamespaceContext() {
    NamespaceContext namespaceContext = new NamespaceContext() {
        public String getNamespaceURI(String prefix) {
            String uri = null;
            if (prefix == null) {
                throw new IllegalArgumentException("No prefix provided!");
            } else if(prefix.equals("m")) {
                uri = "http://oasis.caiso.com/mrtu-oasis/xsd/OASISReport.xsd";
            }
            return uri;
        }

        @SuppressWarnings("rawtypes")
        @Override
        public Iterator getPrefixes(String namespaceURI) {
            // TODO Auto-generated method stub
            return null;
        }

        @Override
        public String getPrefix(String namespaceURI) {
            // TODO Auto-generated method stub
            return null;
        }
    };

    return namespaceContext;
}

And then use that method to set the namespace context on your xpath object:

xpath.setNamespaceContext(getNamespaceContext());