[jdom-interest] Converting elements from a SAX stream to JDOM elements

[Colin Horne]

Hello,

I have a long (infinite) XML stream, which I intend to parse with SAX.
Each individual element in the stream is small, and should be parsed
with JDOM:

<stream>
<element>...</element>
<element>...</element>
</stream>

So each <element> (and their children) are parsed with JDOM, but the
<stream> as a whole is parsed with SAX. It would be preferable if each
<element> does not have to be serialized to an encoded string, and if
elements are not processed twice (e.g., using SAX to echo the
<element>'s XML to a stream, which is then read by JDOM).

I've found several references to this problem from the past, but could
not find a complete solution.

My initial approach was to use the SAXHandler, like so:

        jdomHandler = new SAXHandler() {
            public void endElement(String uri, String localName, String qName) {
                if (qName.equals("an element which I want JDOM to parse")) {
                    // change the SAX handler to myHandler
                } else {
                    super.endElement(uri, localName, qName);
                }
            }
        };


        myHandler = new DefaultHandler() {
            public void startElement(String uri, String localName,
String qName, Attributes attributes) {
                if (qName.equals("an element which I want JDOM to parse")) {
                    // change the SAX handler to jdomHandler
                }
            }
        };


(Ignoring for now that the endElement() method needs to keep track of
its nesting level)

However, trivially doing the above does not work, and fails after
calling jdomHandler.getDocument():

Exception in thread "main" java.lang.IllegalStateException: Root element not set
	at org.jdom.Document.getRootElement(Document.java:218)


I've looked at the initalization code that JDOM is normally doing in
the SAXBuilder.build() method, and am reluctant to copy/modify the
code, because I suspect it will break with future releases, and can't
help but wonder if it would be over-complicating things.

Is there a Right Way(TM) to do this? If so, I might also suggest that
it's referenced from the FAQ.

Many thanks,
  Colin