class WarningLogListener implements LogListener {
    FileOutputStream _outFile;

    public WarningLogListener(FileOutputStream outFile) {
        _outFile = outFile;
    }

    public void messageLogged(ServerContext sc, LogMessage msg) {
        String user;

        try {
            if (msg.getType() != LogMessage.WARNING) {
                // This class deals exclusively with warnings.
                return;
            }
            user = msg.getUser();

            if (user == null) {
                user = "NULL";
            }

            _outFile.write(("Caught warning"
                + " user=" + user
                + " text=" + msg.getText()
                + "\n").getBytes()
            );
            _outFile.flush();
        }
        catch(Exception e) {
            // Print some warning output to the MobiLink log.
            e.printStackTrace();
        }
    }
}

ServerContext serv_context;
FileOutputStream outFile
serv_context.addWarningListener(new WarningLogListener(outFile));