what is the purpose of javax StreamSource

Question

I am parsing an xml document being read in as an InputStream, and an example I've seen first stages the stream in a javax.xml.transform.stream.StreamSource. Why do this when I can parse the stream as it's read in? The Java API's description isn't helpful: "Acts as a holder for a transformation Source in the form of a stream of XML markup."

Example with StreamSource:

    XMLInputFactory xif = XMLInputFactory.newFactory();
    StreamSource reportStream = 
              new StreamSource(new URL("file:///myXmlDocURL.xml").openStream());
    XMLStreamReader xmlReader = xif.createXMLStreamReader(reportStream);
    xmlReader.nextTag();
    while (xmlReader.hasNext()) {
        if (xmlReader.getLocalName().equals("attributeICareAbout")) {
            String tempTagValue = xmlReader.getText();
            xmlReader.nextTag();
        }
    }
    xmlReader.close();

Example without StreamSource:

    XMLInputFactory xif = XMLInputFactory.newFactory();        
    XMLStreamReader xmlReader = 
      xif.createXMLStreamReader(new URL("file:///myXmlDocURL.xml").openStream());
    xmlReader.nextTag();
    while (xmlReader.hasNext()) {
        if (xmlReader.getLocalName().equals("attributeIcareAbout")) {
            String tempTagValue = xmlReader.getText();
            xmlReader.nextTag();
        }
    }
    xmlReader.close();

Answer 1

It is an abstraction so that the same parsing code can be used for a variety of sources (note: StreamSource implements Source):

• XMLInputFactory.createXMLStreamReader(Source)
• Validator.validate(Source)
• Transformer.transfrom(Source, Result)
• Unmarshaller.unmarshal(Source, Class)

Getting XML from a file is just one possibility. There are also implementations of Source for DOM (DOMSource), SAX (SAXSource), StAX (StAXSource), and JAXB (JAXBSource).