We Keep Coding

How to write XPath to get node attribute value from a "Name Space XML" in Java

INPUT_XML:
<?xml version="1.0" encoding="UTF-8">
<root xmlns:ns1="http://path1/schema1" xmlns:ns2="http://path2/schema2">
<ns1:abc>1234</ns1:abc>
<ns2:def>5678</ns2:def>
</root>
In Java, I am trying to write XPath expression which will get the value corresponding to this attribute "xmlns:ns1" from the above INPUT_XML string content.
I've tried the following:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
DocumentBuilder builder = factory.newDocumentBuilder();
Document document = builder.parse(INPUT_XML);
String xpathExpression = "/root/xmlns:ns1";
// Create XPathFactory object
XPathFactory xpathFactory = XPathFactory.newInstance();
// Create XPath object
XPath xpath = xpathFactory.newXPath();
// Create XPathExpression object
XPathExpression expr = xpath.compile(xpathExpression);
// Evaluate expression result on XML document
NodeList nodes = (NodeList) expr.evaluate(document, XPathConstants.NODESET);
for (int i = 0; i < nodes.getLength(); i++) {
System.out.println(nodes.item(i).getNodeValue());
}
But the above code is not giving the expected value of the specified attribute i.e. xmlns:ns1. I heavily suspect the xPathExpression is wrong. Please suggest with the right XPath expression or the right approach to tackle this issue.

If you're using an XPath 1.0 processor, or a XPath 2.0 processor with XPath 1.0 compatibility mode turned on, you can use the namespace axis to select the namespace value.
You will need to make the following change in your code:
String xpathExpression = "/root/namespace::ns1"

The xmlns:ns1="http://path1/schema1" and xmlns:ns2="http://path2/schema2" are not attributes, but namespace declarations. You cannot retrieve them with an XPath declaration so easily (there is XPath function namespace-uri() for this purpose, but root element does not have any namespace, it only defines them for future use).
When using DOM API you could use method lookupNamespaceURI():
System.out.println("ns1 = " + doc.getDocumentElement().lookupNamespaceURI("ns1"));
System.out.println("ns2 = " + doc.getDocumentElement().lookupNamespaceURI("ns2"));
When using XPath you could try following expressions:
namespace-uri(/*[local-name()='root']/*[local-name()='abc'])
namespace-uri(/*[local-name()='root']/*[local-name()='def'])

Why does getLocalName() return null?

I'm loading some XML string like this:
Document doc = getDocumentBuilder().parse(new InputSource(new StringReader(xml)));
Later, I extract a node from this Document:
XPath xpath = getXPathFactory().newXPath();
XPathExpression expr = xpath.compile(expressionXPATH);
NodeList nodeList = (NodeList)expr.evaluate(doc, XPathConstants.NODESET);
Node node = nodeList.item(0);
Now I want to get the local name of this node but I get null.
node.getLocalName(); // return null
With the debugger, I saw that my node has the following type: DOCUMENT_POSITION_DISCONNECTED.
The Javadoc states that getLocalName() returns null for this type of node.
Why node is of type DOCUMENT_POSITION_DISCONNECTED and not ELEMENT_NODE?
How to "convert" the type of the node?

As the documentation https://docs.oracle.com/javase/7/docs/api/org/w3c/dom/Node.html#getLocalName() states:
for nodes created with a DOM Level 1 method, [...] this is always null
so make sure you use a namespace aware DocumentBuilderFactory with setNamespaceAware(true), that way the DOM is supporting the namespace aware DOM Level 2/3 and will have a non-null value for getLocalName().
A simple test program
String xml = "<root/>";
DocumentBuilderFactory db = DocumentBuilderFactory.newInstance();
Document dom1 = db.newDocumentBuilder().parse(new InputSource(new StringReader(xml)));
System.out.println(dom1.getDocumentElement().getLocalName() == null);
db.setNamespaceAware(true);
Document dom2 = db.newDocumentBuilder().parse(new InputSource(new StringReader(xml)));
System.out.println(dom2.getDocumentElement().getLocalName() == null);
outputs
true
false
so (at least) the local name problem you have is caused by using a DOM Level 1, not namespace aware document (builder factory).

Java xPath - extract subdocument from XML

I have an XML document as follows:
<DocumentWrapper>
<DocumentHeader>
...
</DocumentHeader>
<DocumentBody>
<Invoice>
<Buyer/>
<Seller/>
</Invoice>
</DocumentBody>
</DocumentWrapper>
I would like to extract from it the content of DocumentBody element as String, raw XML document:
<Invoice>
<Buyer/>
<Seller/>
</Invoice>
With xPath it could be simple to get by:
/DocumentWrapper/DocumentBody
Unfrotunatelly, my Java code doesn't want to work as I want. It returns empty lines instead of expected result. Is there any chance to do that, or I have to return NodeList and then genereate xml document from them?
My Java code:
XPathFactory xPathFactoryXPathFactory.newInstance();
XPath xPath xPathFactory.newXPath();
XPathExpression xPath.compile(xPathQuery);
String result = expression.evaluate(xmlDocument);

Calling this method
String result = expression.evaluate(xmlDocument);
is the same as calling this
String result = (String) expression.evaluate(xmlDocument, XPathConstants.STRING);
which returns the character data of the result node, or the character data of all child nodes in case the result node is an element.
You should probably do something like this:
Node result = (Node) expression.evaluate(xmlDocument, XPathConstants.NODE);
TransformerFactory.newInstance().newTransformer()
.transform(new DOMSource(result), new StreamResult(System.out));

how to get a node value in Xpath - Java

I've got a section of XML that looks like this:
<entry>
<id>tag:example.com,2005:Release/343597</id>
<published>2012-04-10T11:29:19Z</published>
<updated>2012-04-10T12:04:41Z</updated>
<link type="text/html" href="http://example.com/projects/example1" rel="alternate"/>
<title>example1</title>
</entry>
I need to grab the link http://example.com/projects/example1 from this block. I'm not sure how to do this. To get the title of the project I use this code:
String title1 = children.item(9).getFirstChild().getNodeValue();
where children is the getChildNodes() object for the <entry> </entry> block. But I keep getting NullPointerExceptions when I try to get the node value for the <link> node in a similar way. I see that the XML code is different for the <link> node, and I'm not sure what it's value is.... Please advise!

The xpath expression to get that node is
//entry/link/#href
In java you can write
Document doc = ... // your XML document
XPathExpression xp = XPathFactory.newInstance().newXPath().compile("//entry/link/#href");
String href = xp.evaluate(doc);
Then if you need to get the link value of the entry with a specific id you can change the xpath expression to
//entry[id='tag:example.com,2005:Release/343597']/link/#href
Finally if you want to get all the links in the documents, if the document has many entry elements you can write
Document doc = ... // your XML document
XPathExpression xp = XPathFactory.newInstance().newXPath().compile("//entry/link/#href");
NodeList links = (NodeList) xp.evaluate(doc, XPathConstants.NODESET);
// and iterate on links

Here is the complete code:
DocumentBuilderFactory domFactory = DocumentBuilderFactory
.newInstance();
domFactory.setNamespaceAware(true);
DocumentBuilder builder = domFactory.newDocumentBuilder();
Document doc = builder.parse("test.xml");
XPath xpath = XPathFactory.newInstance().newXPath();
XPathExpression expr = xpath.compile("//entry/link/#href");
Object result = expr.evaluate(doc, XPathConstants.NODESET);
NodeList nodes = (NodeList) result;
for (int i = 0; i < nodes.getLength(); i++) {
System.out.println(nodes.item(i));
}

Get an attribute of a dom node

I am trying to get an attribute of an xml node example:
<Car name="Test">
</Car>
I want to grab the name attribute of the car node.
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(configFile);
doc.getDocumentElement().normalize();
NodeList layerConfigList = doc.getElementsByTagName("CAR");
Node node = layerConfigList.item(0);
// get the name attribute out of the node.
this is where i get stuck because the only method that looks like i can use is getAttributes() with returns a NamedNodeMap and im not sure how to extract it from that.

Your node is an Element so you just have to
Element e = (Element)node;
String name = e.getAttribute("name");

you can do it without using elements, like this:
//HtmlTag represents any arbitrary node that you are trying to get its "car" attribute
if("HtmlTag".equals(node.getNodeName()))
String nodeContent=node.getAttributes().getNamedItem("car").getNodeValue()