@Override
    public void startDocument() throws SAXException {
        try {
            if (validator != null) {
                validator.startDocument();
            }
        } catch (final RuntimeException e) {
            unexpectedError("startDocument()", e);
        } finally {
            super.startDocument();
        }
    }
This method tells filters that a stream is about to be parsed so that
they can complete any setup necessary.
stroom-pipeline/src/main/java/stroom/pipeline/filter/SchemaFilter.java