Use Swing API to convert a simple RTF-format file to HTML-format.

rtf2html_swing.groovy

import javax.swing.JEditorPane


(args[0] as File).withReader {
    def p = new JEditorPane()
    p.contentType = 'text/rtf'
    def kitRtf = p.getEditorKitForContentType p.contentType
    kitRtf.read it, p.document, 0
    def kitHtml = p.getEditorKitForContentType 'text/html'
    def out = new StringWriter()
    kitHtml.write out, p.document, 0, p.document.length
    println out
}