class ErrorLogListener implements LogListener {
    FileOutputStream _outFile;

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

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

        try {
            if (msg.getType() != LogMessage.ERROR) {
                //this class deals exclusively with errors
                return;
            }

            user = msg.getUser();
            if (user == null) {
                user = "NULL";
            }

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

ServerContext serv_context;
FileOutputStream outFile
serv_context.addErrorListener(new ErrorLogListener(outFile));