XML: Finding namespace uri from xsi:type attribute in SAX content handler

Question

I have an xml document as

<fr:frame xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xmlns:fr="http://mebigfatguy.com/ds/frame" 
      xmlns:comp="http://mebigfatguy.com/ds/component"
      xmlns:cont="http://mebigfatguy.com/ds/container"
      xmlns:b="http://mebigfatguy.com/ds/button">
    <comp:preferredSize>500,300</comp:preferredSize>
    <cont:childComponent>
        <cont:name>CENTER</cont:name>
        <cont:component xsi:type="b:Button">
            <comp:name>Click Me</comp:name>
        </cont:component>
    </cont:childComponent>
    <fr:title>Example</fr:title>
</fr:frame>

where b:Button is an xml extension type of cont:component

In my startElement call, i receive a uri of http://mebigfatguy.com/ds/container and qname of cont:component as expected. The xsi:type="b:Button" is found in the attributes, also as expected.

The question I have is how does one lookup the namespace uri of b:Button as retrieved from the xsi:type attribute. Do i have to manage the xmlns attributes manually myself? or is there a built in way to resolve what the uri is?

Answer 1

Yes. you have to do lookup namespace uri for b:Button and you have to manage xmlns attributes using org.xml.sax.helpers.NamespaceSupport. It is little tricky to populate NamespaceSupport properly.

Below is the sample code which prints xsi:type value with uri and localname:

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;
import org.xml.sax.helpers.NamespaceSupport;

import javax.xml.XMLConstants;
import javax.xml.parsers.SAXParserFactory;

/**
 * @author Santhosh Kumar Tekuri
 */
public class XSIHandler extends DefaultHandler{
    private boolean needNewContext;
    private NamespaceSupport nsSupport;

    @Override
    public void startDocument() throws SAXException{
        nsSupport = new NamespaceSupport();
        needNewContext = true;
        super.startDocument();
    }

    @Override
    public void startPrefixMapping(String prefix, String uri) throws SAXException{
        if(needNewContext){
            nsSupport.pushContext();
            needNewContext = false;
        }
        nsSupport.declarePrefix(prefix, uri);
    }

    @Override
    public void startElement(String uri, String localName, String qName, Attributes atts) throws SAXException{
        if(needNewContext)
            nsSupport.pushContext();
        needNewContext = true;
        String xsiType = atts.getValue(XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI, "type");
        if(xsiType!=null){
            String prefix, suffix;
            int colon = xsiType.indexOf(':');
            if(colon==-1){
                prefix = "";
                suffix = xsiType;
            }else{
                prefix = xsiType.substring(0, colon);
                suffix = xsiType.substring(colon+1);
            }
            System.out.println("xsi:type for " + qName + " is uri: " + nsSupport.getURI(prefix) + " localName: " + suffix);
        }
    }

    @Override
    public void endElement(String uri, String localName, String qName) throws SAXException{
        nsSupport.popContext();
    }
}

here is the driver function:

public static void main(String[] args) throws Exception{
    SAXParserFactory factory = SAXParserFactory.newInstance();
    factory.setNamespaceAware(true);
    factory.newSAXParser().parse("test.xml", new XSIHandler());
}

the output is:

xsi:type for cont:component is uri: http://mebigfatguy.com/ds/button localName: Button