@Override
    public void startDocument() throws SAXException {
        if (!startedDocument) {
            try {
                init();
                handler.startDocument();
            } finally {
                startedDocument = true;
                super.startDocument();
            }
        }
    }
Buffers a startDocument event if buffer is set to true.

@see stroom.pipeline.filter.AbstractXMLFilter#startDocument()
stroom-pipeline/src/main/java/stroom/pipeline/filter/TinyTreeBufferFilter.java