Parsing XML file with DOM (Java)

Question

I want to parse the following url: http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary.fcgi?db=nucleotide&id=224589801

As a result I came up with the following method:

public void parseXml2(String URL) {
    DOMParser parser = new DOMParser();

    try {
        parser.parse(new InputSource(new URL(URL).openStream()));
        Document doc = parser.getDocument();

        NodeList nodeList = doc.getElementsByTagName("Item");
        for (int i = 0; i < nodeList.getLength(); i++) {
            Node n = nodeList.item(i);
            Node actualNode = n.getFirstChild();
            if (actualNode != null) {
                System.out.println(actualNode.getNodeValue());
            }
        }

    } catch (SAXException ex) {
        Logger.getLogger(TaxMapperXml.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
        Logger.getLogger(TaxMapperXml.class.getName()).log(Level.SEVERE, null, ex);
    }
}

With this method I can take the values of the Item nodes but I can't take any of their attributes. I tried experimenting with getAttribute() with NamedNodeMap but still to no avail.

1. Why do I have to do n.getFirstChild().getNodeValue(); to get the actual value? n.getNodeValue() returns just null? Isn't this counter-intuitive - obviously in my case node's doesn't have subnodes?

2. Is there some more robust and widely accepted way of parsing XML files using DOM? My files aren't gonna be big 15-20 lines at most, so SAX isn't necessary (or is it?)

Answer 1

1. Text value that is surrounded by XML tag are also considered as Node in DOM. That's why you have to get the text Node before getting the value. If you try to count the number of node in an <Item>, you will see that whenever there is a text, there is a node.

2. XOM has more intuitive interface but it doesn't have org.w3c.dom.* interface.

If you want to use the build-in parser, you should look at http://www.java-samples.com/showtutorial.php?tutorialid=152

The DOMParser you tried to use are propriety and it's not portable.

Answer 2

Text inside xml elements are in text nodes because subelements can be mixed with text. For instance:

...
<A>blah<B/>blah</A>
...

Element A has three children: a text node, element B, another text node.

Answer 3

import java.io.IOException;
import java.net.URL;
import org.apache.xerces.parsers.DOMParser;

import org.w3c.dom.Document;
import org.w3c.dom.NamedNodeMap;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.InputSource;

public class XMLParser {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        parseXml2("http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary.fcgi?db=nucleotide&id=224589801");
    }

    public static void parseXml2(String URL) {
        DOMParser parser = new DOMParser();

        try {
            parser.parse(new InputSource(new URL(URL).openStream()));
            Document doc = parser.getDocument();

            NodeList nodeList = doc.getElementsByTagName("Item");
            for (int i = 0; i < nodeList.getLength(); i++) {
                System.out.print("Item "+(i+1));
                Node n = nodeList.item(i);
                NamedNodeMap m = n.getAttributes();
                System.out.print(" Name: "+m.getNamedItem("Name").getTextContent());
                System.out.print(" Type: "+m.getNamedItem("Type").getTextContent());
                Node actualNode = n.getFirstChild();
                if (actualNode != null) {
                    System.out.println(" "+actualNode.getNodeValue());
                } else {
                    System.out.println(" ");                    
                }
            }

        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Completed the sample code and added a few lines to get the attributes.

This should get you started, although I feel that you need to get yourself up to date with the basic notions of DOM. This site (and many others) can help you with that. Most importantly is understanding the different kinds of nodes there are.