Question

I would like to parse a document using SAX, and create a subdocument from some of the elements, while processing others purely with SAX. So, given this document:

  <DOC>     <small>       <element />     </small>     <entries>       <!-- thousands here -->     </entries>   </DOC> 

I would like to parse the DOC and DOC/entries elements using the SAX ContentHandler, but when I hit <small> I want to create a new document containing just the <small> and its children.

Is there an easy way to do this, or do I have to build the DOM myself, by hand?

Answer 1

One approach is to create a ContentHandler that watches for events that signal the entry or exit from a <small> element. This handler acts as a proxy, and in ‘normal’ mode passes the SAX events straight through to the ‘real’ ContentHandler.

However, when entry into a <small> element is detected, the proxy is responsible for the creation of a TransformerHandler (with a no-op, ‘null’ transform), plumbed up to a DOMResult. The TransformerHandler expects all the events that a complete, well-formed document would produce; you cannot immediately send it a startElement event. Instead, simulate the beginning of a new document by invoking setDocumentLocator, startDocument, and other necessary events on the TransformerHandler instance first.

Then, until the end of the <small> element element is detected by the proxy, all events are forwarded to this TransformerHandler instead of the ‘real’ ContentHandler. When the closing </small> tag is encountered, the proxy simulates the end of a document by invoking endDocument on the TransformerHandler. A DOM is now available as the result of the TransformerHandler, which contains only the <small /> fragment.