Chapter 8. SAX Filters
In all of XML, I have found nothing quite so hard to understand yet easy to do as writing SAX filters. For a long time, it felt as though I had a mental block preventing me from grokking just how filters worked, and yet every time I wrote one it almost always worked on the first try. In fact, even when I was convinced that the code I had written could not possibly work, it did. I can't decide whether this is an example of wonderful or awful API design.
The basic idea of filters is that an XMLReader, instead of receiving XML text directly from a file, socket, or other source, receives already parsed events from another XMLReader. It can change these events before passing them along to the client application through the usual methods of ContentHandler and the other callback interfaces. For example, it can add a unique ID attribute to every element or delete all elements in the SVG namespace from the input stream.
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