XPath select element with an attribute

Question

I have a xml file which looks like:

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?> 
 <alarm-response-list xmlns="http://www.ca.com/spectrum/restful/schema/response"   
 error="EndOfResults" throttle="277" total-alarms="288">
 <alarm-responses> 
  <alarm id="53689bf8-6cc8-1003-0060-008010186429">
   <attribute id="0x11f4a" error="NoSuchAttribute" /> 
   <attribute id="0x12b4c">UPS DIAGNOSTIC TEST FAILED</attribute> 
   <attribute id="0x10b5a">IDG860237, SL3-PL4, US, SapNr=70195637,</attribute> 
 </alarm>
 <alarm id="536b8c9a-28b3-1008-0060-008010186429">
  <attribute id="0x11f4a" error="NoSuchAttribute" /> 
  <attribute id="0x12b4c">DEVICE IN MAINTENANCE MODE</attribute> 
  <attribute id="0x10b5a">IDG860237, SL3-PL4, US, SapNr=70195637,</attribute> 
</alarm>
</alarm-responses>
</alarm-response-list>

There a lot of these alarms. Now I want save for every alarm tag the attribute with the id = 0x10b5a in a String. But I haven't a great clue. In my way it doesn't do it. I get only showed the expression.

My idea:

FileInputStream file = new FileInputStream(
                new File(
                        "alarms.xml"));

        DocumentBuilderFactory builderFactory = DocumentBuilderFactory
                .newInstance();

        DocumentBuilder builder = builderFactory.newDocumentBuilder();

        Document xmlDocument = builder.parse(file);

        XPath xPath = XPathFactory.newInstance().newXPath();

        System.out.println("*************************");
        String expression = "/alarm-responses/alarm/attribute[@id='0x10b5a'] ";
        System.out.println(expression);
        NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(
                xmlDocument, XPathConstants.NODESET);
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.println(nodeList.item(i).getFirstChild()
                    .getNodeValue());
        }

Since <alarm-responses> is not the root element, try the following xpath expression: "//alarm-responses/alarm/attribute[@id='0x10b5a'] " — terrywb
– terrywb, Commented Sep 12, 2014 at 19:00
System.out.println(nodeList.item(i) .getNodeValue()); Depending on where the null pointer is occurring, this might help — terrywb
– terrywb, Commented Sep 12, 2014 at 19:07

Ian Roberts · Accepted Answer · 2014-09-12 22:19:37Z

1

There are several different problems here that are interacting to mean that your XPath expression doesn't match anything. Firstly the alarm-responses element isn't the root of the document - you need an extra step on the front of the path to select the alarm-response-list element. But more importantly you have namespace issues.

XPath only works when the XML has been parsed with namespaces enabled, which for some reason is not the default for DocumentBuilderFactory. You need to enable namespaces before you do newDocumentBuilder.

Now your XML document has xmlns="http://www.ca.com/spectrum/restful/schema/response", which puts all the elements in this namespace, but unprefixed node names in an XPath expression always refer to nodes that are not in a namespace. In order to match namespaced nodes you need to bind a prefix to the namespace URI and then use prefixed names in the path.

For javax.xml.xpath this is done using a NamespaceContext, but annoyingly there is no default implementation of this interface available by default in the Java core library. There is a SimpleNamespaceContext implementation available as part of Spring, or it's fairly simple to write your own. Using the Spring class:

DocumentBuilderFactory builderFactory = DocumentBuilderFactory
        .newInstance();

// enable namespaces
builderFactory.setNamespaceAware(true);

DocumentBuilder builder = builderFactory.newDocumentBuilder();

Document xmlDocument = builder.parse(file);

XPath xPath = XPathFactory.newInstance().newXPath();

// Set up the namespace context
SimpleNamespaceContext ctx = new SimpleNamespaceContext();
ctx.bindNamespaceUri("ca", "http://www.ca.com/spectrum/restful/schema/response");
xPath.setNamespaceContext(ctx);

System.out.println("*************************");

// corrected expression
String expression = "/ca:alarm-response-list/ca:alarm-responses/ca:alarm/ca:attribute[@id='0x10b5a']";

System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(
        xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
    System.out.println(nodeList.item(i).getTextContent());
}

Note also how I'm using getTextContent() to get the text under each matched element. The getNodeValue() method always returns null for element nodes.

answered Sep 12, 2014 at 22:19

Ian Roberts

123k17 gold badges177 silver badges191 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

LittleProgrammer Over a year ago

thank you a lot! one problem is still there. how i can include SimpleNamespaceContext? with which import command? eclipse doesn't sugget one- ah i understand have I write it myself? I doesn't know how :(

Ian Roberts Over a year ago

@LittleProgrammer the SimpleNamespaceContext class I used is part of the Spring framework, you need the spring-core JAR (if you're not using anything else from Spring you just need that JAR, not the other ones it depends on). Or just copy the implementation into your own project, it's pretty simple and released under the Apache licence: github.com/spring-projects/spring-framework/blob/master/…

Collectives™ on Stack Overflow

XPath select element with an attribute

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related