CharacterTransformerFactory.java

public class CharacterTransformerFactory implements TransformerFactory {

    @Override
    public Transformer createTransformer() {
        return new CharacterTransformer();
    }

    private class CharacterTransformer extends AbstractTransformer {

        @Override
        public void characters(char[] ch, int start, int length) throws SAXException {
            String str = new String(ch, start, length);
            if (str.contains("<!--") && str.contains("-->")) {
                str = str.replace("<!--", "<!-- HELLO");
                super.characters(str.toCharArray(), 0, str.length());
            } else {
                super.characters(ch, start, length);
            }
        }
    }
}