DOM and SAX APIs

Using the DOM API

Question

How do I get the number of elements in a particular tag using the parser?

Answer

You can use the getElementsByTagName() method that returns a NodeList of all descent elements with a given tag name. You can then find out the number of elements in that NodeList to determine the number of the elements in the particular tag.

How DOM Parser Works

Question

How does the XML DOM parser work?

Answer

The parser accepts an XML formatted document and constructs in memory a DOM tree based on its structure. It will then check whether the document is well-formed and optionally whether it complies with a DTD. It also provides methods to support DOM Level 1.

Creating a Node With Value to be Set Later

Question

How do I create a node whose value I can set later?

Answer

If you check the DOM spec referring to the table discussing the node type, you will find that if you are creating an element node, its nodeValue is to be null and hence cannot be set. However, you can create a text node and append it to the element node. You can put the value in the text node.

Traversing the XML Tree

Question

How to traverse the XML tree

Answer

You can traverse the tree by using the DOM API. Or alternately, you can use the selectNodes() method which takes XPath syntax to navigate through the XML document. selectNodes() is part of oracle.xml.parser.v2.XMLNode.

Extracting Elements from XML File

Question

How do I extract elements from the XML file?

Answer

If you're using DOM, the getElementsByTagName() method can be used to get all of the elements in the document.

Does a DTD Validate the DOM Tree?

Question

If I add a DTD to an XML Document, does it validate the DOM tree?

Answer

No, we do not do any validation while creating the DOM tree using the DOM APIs. So setting the DTD in the Document will not help in validating the DOM tree that is constructed. The only way to validate an XML file is to parse the XML document using the DOMParser or SAXParser.

First Child Node Element Value

Question

How do I efficiently obtain the value of first child node of the element without going through the DOM Tree?

Answer

If you do not need the entire tree, use the SAX interface to return the desired data. Since it is event-driven, it does not have to parse the whole document.

Creating DocType Node

Question

How do I create a DocType Node?

Answer

The only current way of creating a doctype node is by using the parseDTD functions. For example, emp.dtd has the following DTD:

<!ELEMENT employee (Name, Dept, Title)>
 <!ELEMENT Name (#PCDATA)> 
<!ELEMENT Dept (#PCDATA)>
 <!ELEMENT Title (#PCDATA)> 

You can use the following code to create a doctype node:

parser.parseDTD(new FileInputStream(emp.dtd), "employee"); 
dtd = parser.getDocType();

XMLNode.selectNodes() Method

Question

How do I use the selectNodes() method in XMLNode class?

Answer

The selectNodes() method is used in XMLElement and XMLDocument nodes. This method is used to extract contents from the tree/subtree based on the select patterns allowed by XSL. The optional second parameter of selectNodes, is used to resolve Namespace prefixes (return the expanded namespace URL given a prefix). XMLElement implements NSResolver, so it can be sent as the second parameter. XMLElement resolves the prefixes based on the input document. You can implement the NSResolver interface, if you need to overide the namespace definitions. The following sample code uses selectNodes

public class SelectNodesTest  {
public static void main(String[] args) throws Exception {
String pattern = "/family/member/text()";
String file    = args[0];

if (args.length == 2)
  pattern = args[1];

DOMParser dp = new DOMParser();

dp.parse(createURL(file));  // Include createURL from DOMSample
XMLDocument xd = dp.getDocument();
XMLElement e = (XMLElement) xd.getDocumentElement();
NodeList nl = e.selectNodes(pattern, e);
for (int i = 0; i < nl.getLength(); i++) {
   System.out.println(nl.item(i).getNodeValue());
    }
  }
}

> java SelectNodesTest family.xml
Sarah
Bob
Joanne
Jim

> java SelectNodesTest family.xml //member/@memberid
m1
m2
m3
m4

Using SAX API to Get the Data Value

Question

I am using SAX to parse an XML document. How does it get the value of the data?

Answer

During a SAX parse the value of an element will be the concatenation of the characters reported from after the startElement event to before the corresponding endElement event is called.

SAXSample.java

Question

Inside the SAXSample program, I did not see any line that explicitly calls setDocumentLocator and some other methods. However, these methods are 'run'. Can you explain when they are called and from where

Answer

SAX is a standard interface for event-based XML parsing. The parser reports parsing events directly through callback functions such as setDocumentLocator() and startDocument(). The application, in this case, the SAXSample, implements handlers to deal with the different events. Here is a good place to help you start learning about the event-driven API, SAX: http://www.megginson.com/SAX/index.html

Does DOMParser implement Parser interface

Question

Does the XML Parser DOMParser implement org.xml.sax.Parser interface at all ? The documentation says it implements XMLConstants and the API does not include that class at all.

Answer

You'll want oracle.xml.parser.v2.SAXParser to work with SAX and to have something that implements the org.xml.sax.Parser interface.

Creating an New Document Type Node Via DOM

Question

I am trying to create a XML file on the fly. I use the NodeFactory to contruct a document (createDocument()). I have then setStandalone("no") and setVersion("1.0"). when I try to add a DOCTYPE node via appendChild( new XMLNode("test", Node.DOCUMENT_TYPE_NODE)), I get a ClassCastException. What is the mechanism to add a node of this type? I noticed that the NodeFactory did not have a machanism for creating a DOCTYPE node.

Answer

There is no mechanism to create a new DOCUMENT_TYPE_NODE object via DOM APIs. The only way to get a DTD object is to parse theDTD file or the XML file using the DOMParser, and then use the getDocType() method.

Note that new XMLNode("test",Node.DOCUMENT_TYPE_NODE) does not create a DTDobject. It creates an XMLNode object with the type set toDOCUMENT_TYPE_NODE, which infact should not be allowed. TheClassCastException is raised because appendChild expects a DTDobject (based on the type).

Also, we do not do any validation while creating the DOM tree using the DOM APIs. So setting the DTD in the Document will not help in validating the DOM tree that is constructed. The only way to validate an XML file is to parse the XML document using DOMParser or SAXParser.

Querying for First Child Node's Value of a Certain Tag

Question

I am using the XML Parser for Java v2. Given a XML document containing the following Calculus Math Jim Green Jack Mary Paul, I want to obtain the value of first child node of whose tag is . I could not find any method that can do that efficiently. The nearest match is method getElementsByTag("Name"), which traverses the entire tree under .

Answer

Your best bet, if you do not need the entire tree, is to use the SAX interface to return the desired data. Since it is event driven it does not have to parse the whole document.

XML Document Generation From Data in Variables

Question

Is there an example of XML document generation starting from information contained in simple variables? An example would be: A client fills a Java form and wants to obtain an XML document containing the given data.

Answer

Here are two possible interpretations of your question and answers to both. Let's say you have two variables in Java:

String firstname = "Gianfranco";
String lastname = "Pietraforte";

The two ways that come to mind first to get this into an XML document are as follows:

1. Make an XML document in a string and parse it.

   String xml = "<person><first>"+firstname+"</first>"+
   

"<last>"+lastname+"</last></person";

 DOMParser d = new DOMParser();
 d.parse( new StringReader(xml));
 Document xmldoc = d.getDocument();

Document xmldoc = new XMLDocument();
Element e1 = xmldoc.createElement("person");
xmldoc.appendChild(e1);
Element e2 = xmldoc.createElement("first");
e1.appendChild(e2);
Text t = xmldoc.createText(firstname);
e2.appendChild(t);
// and so on

<name>macy</name>

String value = myElement.getFirstChild().getNodeValue();

<key>value</key><name>george</name><zip>20000</zip>'

  Document doc1 = new XMLDocument();
  Element element1 = doc1.creatElement("foo");
  Document doc2 = new XMLDocument();
  Element element2 = doc2.createElement("bar");
  element1.appendChild(element2);  

oracle.xml.parser.XMLDOMException: Node cannot be modifiedwhile trying to set 
the value of a newly created node as below:     String eName="Mynode";     
XMLNode aNode = new XMLNode(eName, Node.ELEMENT_NODE);     
aNode.setNodeValue(eValue);