Code in sun.net.www.protocol.mailto.Handler#parseURL can be simplified.
Manual cycle could be replaced with String.isBlank

        boolean nogood = false;
        if (file == null || file.isEmpty())
            nogood = true;
        else {
            boolean allwhites = true;
            for (int i = 0; i < file.length(); i++)
                if (!Character.isWhitespace(file.charAt(i)))
                    allwhites = false;
            if (allwhites)
                nogood = true;
        }
        if (nogood)
            throw new RuntimeException("No email address");