The real solution that preserves linebreaks should be like this:

public static String br2nl(String html) {
    if(html==null)
        return html;
    Document document = Jsoup.parse(html);
    document.outputSettings(new Document.OutputSettings().prettyPrint(false));//makes html() preserve linebreaks and spacing
    document.select("br").append("\\n");
    document.select("p").prepend("\\n\\n");
    String s = document.html().replaceAll("\\\\n", "\n");
    return Jsoup.clean(s, "", Whitelist.none(), new Document.OutputSettings().prettyPrint(false));
}

It satisfies the following requirements:

  1. if the original html contains newline(\n), it gets preserved
  2. if the original html contains br or p tags, they gets translated to newline(\n).